Heinz Welsch
email: welsch@uni-oldenburg.de

Udo Ebert

Klaus W Schüler

The Welfare Costs of Corruption

Keywords: D61 -Allocative Efficiency|Cost Benefit Analysis corruption, welfare measurement, subjective Welfare Costs of Corruption corruption, welfare measurement, subjective well-being, happiness JEL classification: D73, I31, D61

Corruption has been shown to affect a variety of economic indicators, especially GDP per capita. However, as GDP is not a genuine indicator of welfare, it may reflect the welfare costs of corruption only in an incomplete way. This paper uses self-rated subjective well-being as an empirical approximation to general welfare and shows that cross-national welfare -operationalized in this way -is affected by corruption not only indirectly, through GDP, but also directly, through non-material factors. The paper estimates the size of these effects as well as their monetary equivalent. The direct effect -not previously investigated in the corruption literature -is found to be substantially larger than the indirect effect.

Introduction

Corruption involves behavior on the part of public officials in which they unlawfully enrich themselves by the misuse of the power entrusted to them (Transparency International 2000).

Levels of corruption have undergone considerable changes in recent years. While some countries succeeded in reducing corruption, others experienced an increase. In the period 2001-2004, for instance, Transparency International's 10-point Corruption Perceptions Index improved by as much as 1.1 points in the case of Bangladesh while deteriorating by no less than 1.3 points in the case of Namibia. 1 What are the effects of changes in corruption, and how are they to be evaluated? Empirical research has shown that corruption affects a variety of economic indicators, such as government expenditures, total investment, capital flows and foreign direct investment, international trade, foreign aid, and GDP per capita (see Lambsdorff 1999 for an overview).

Among these indicators, per capita GDP can be seen as a condensed measure that aggregates several of the economic effects of corruption. However, as GDP is hardly viewed as a genuine indicator of welfare, it probably reflects the welfare costs of corruption only in a crude and incomplete way. This paper pursues a different approach to studying the welfare effects of corruption.

Following a growing literature in economics (see Frey and Stutzer 2002), it uses self-rated subjective well-being ('happiness', life satisfaction) elicited in surveys as an empirical approximation to general welfare and explores several channels through which welfareoperationalized in this way -is affected by corruption.

Since subjective well-being (SWB) prevailing in a country is linked to average income (Di Tella et al. 2003, Hagerty and Veenhoven 2003), one channel through which corruption affects SWB involves corruption's effect on per capita GDP. In addition to this indirect effect, however, corruption may affect SWB in a more immediate way. As noted by Lambsdorff (2003), corruption includes many different types of behavior. Some of these, like artificial bottlenecks created by public officials, may entail substantial time and effort to be expended by citizens in order to attain public services. Moreover, there may be psychological costs, associated with a general climate of unlawfulness, and similar intangibles.

Based on these considerations the paper hypothesizes that corruption affects SWB not only indirectly, through income, but also directly, through non-material factors. The aim of the paper is to disentangle these potentially distinct effects and to measure their relative size. In Technically, the basic approach of the paper is to run a cross-national happiness regression with income and corruption as explanatory variables, jointly with an income regression with corruption among the explanatory variables. Given this system of equations, the total effect of corruption on welfare can be decomposed into a direct effect, and an indirect effect via income.

Measuring the welfare costs of corruption requires to translate these effects into monetary units. With respect to marginal changes in corruption, this involves dividing the derivative of happiness with respect to corruption by the derivative with respect to income, thus obtaining the marginal rate of substitution. As regards infra-marginal changes in corruption, a counterpart to the Hicksian compensating surplus will be computed which measures the compensating change in factor costs (rather than income) when corruption levels change and welfare is to remain constant. This measure accommodates the fact that not only less income is required to attain a given welfare level at lower corruption (due to the direct effect), but also that the lower required income level can be produced at lower factor costs (due to the indirect effect).

An intriguing result from our analysis is that the direct welfare effect of corruption is much larger than the indirect effect via income. This suggests that previous studies, which have focused on the income-related effect, have neglected a significant dimension of the problem of corruption. In addition, we find that there is a considerable monetary value to these effects.

The changes in corruption in 2001 to 2004, though rather modest on average, are equivalent to capital cost savings (median value) of more than 3 percent of GDP.

Corruption is one of several aspects of the institutional quality prevailing in a country. There have been several previous studies on how happiness is linked to institutional conditions. Frey and Stutzer (2000) showed that happiness in 26 Swiss cantons is positively related to the degree of direct democracy (referenda) and decentralization. In a cross-section study of 27 countries, Bjørnskov (2003) found a sizeable and significant association between happiness and a social capital composite that includes trust, social participation, and the control of corruption. Helliwell (2003) employed panel data from 49 countries to show the existence of substantial well-being benefits from improved quality of governance (a composite measure that includes accountability, effectiveness and stability of government, the rule of law, and control of corruption). While both of these studies confirm the role of per capita income as a predictor of cross-national happiness, they suggest that the effects flowing from the quality of institutions may be larger than the effects of economic conditions. However, they disregard the indirect linkage of happiness to institutions via the institutions' effect on income.

In relation to the previous literature, the contribution of the present paper is as follows. First, it focuses explicitly on corruption, rather than on aggregate measures of institutional quality.

Second, it offers a unified theoretical framework that allows for a joint investigation of the direct and indirect linkages of happiness to corruption. Third, it provides a monetary valuation of corruption's welfare effects. Finally, the paper not only confirms the importance of controlling corruption, but provides a separation and quantification of direct versus indirect costs of corruption.

The paper is structured as follows. Section 2 presents the methodological framework (general approach, model, welfare measurement, data, empirical approach). Section 3 presents the results (estimation results, welfare effects of corruption, monetary valuation). Section 4 concludes.

Methodological Framework

The General Approach

Data from happiness surveys are successfully used in a growing literature in economics. In these studies, the emphasis may be on identifying economic determinants of happiness or on using the 'life satisfaction approach' (Frey et al. 2004) as a methodological device for the valuation of public goods (or bads). 2 In happiness surveys, people are asked to state their degree of happiness or satisfaction with life, for instance "very happy", "quite happy ", "not too happy " etc. In order to be useful for statistical work, the data elicited in this way needs to satisfy certain requirements. Especially, a basic condition for using happiness data as empirical approximations to individual utility is that they be at least ordinal in character and satisfy usual quality standards.

Whether happiness measures meet these conditions has been widely assessed in decades of validation research (see, e.g., Frey and Stutzer 2002). In these studies measures of happiness 2 An early study of the economics of happiness is Easterlin (1974). Later contributions examine the relationship between income distribution and self-rated happiness (Morawetz et al. 1977) and between unemployment and happiness (Clark and Oswald 1994, Winkelmann and Winkelmann 1998). Di Tella et al. (2001) use data from happiness surveys to estimate the trade-off between inflation and unemployment in the framework of a macroeconomic social welfare function. The 'life satisfaction approach' as a valuation technique has been applied to air pollution (Welsch 2002, 2006), aircraft noise (van Praag and Baarsma 2005), climate (Rehdanz and Maddison 2005), terrorism (Frey et al. 2004) and fear of crime (Moore 2006). In this literature, as in the present paper, the terms happiness, life satisfaction, and subjective well-being are used interchangeably. To economists the terminology "experienced utility" may sound more familiar, see Kahnemann et al. (1997). are generally found to have a high scientific standard in terms of internal consistency, reliability and validity, and a high degree of stability over time (Diener et al. 1999). Different happiness measures correlate well with each other and, according to factor analyses, represent a single unitary construct. Happiness responses are correlated with physical reactions that can be thought of as describing true, internal happiness: People reporting to be happy tend to smile more and show lower levels of stress responses (heart rate, blood pressure). They are more frequently described by others as being happy and they are less likely to commit suicide.

In addition to these properties, using happiness data for welfare analysis requires to presume some sort of cardinality. While psychologists and sociologists usually interpret happiness scores as cardinal, economists are inclined to be skeptical about this claim. However, this may be less problematic at a practical level than at a theoretical level (Kahneman 1999), and recent analysis has produced evidence that assuming ordinality or cardinality of happiness scores has little effect on empirical results (Di Tella et al. 2001, Ferrer-i-Carbonell and Frijters 2004). In what follows, we therefore assume that cardinal numbers can be assigned to the verbal happiness categories used in survey questions, yielding an interval-scale measurable variable.

As a consequence, we can compute average happiness, and this has methodological advantages, as will be seen immediately.

In specifying a happiness function, there are basically two varieties: a function with individual happiness (micro approach) or a function with aggregate (average) happiness as the dependent variable (macro approach). The micro approach does not require to cardinalize happiness scores (since estimators for ordered variables can be used), but it requires to control for heterogeneity in the individuals' characteristics. As far as socio-demographic and socioeconomic characteristics are concerned, these are usually available in the survey data. In addition, however, there is unobserved heterogeneity across individuals (especially with respect to personality traits). This may affect results substantially (Bertrand and Mullainathan 2001, Ravallion and Lokshin 2001, Ferrer-i-Carbonnell and Frijters 2004) and should be controlled for, if possible. In this regard, the macro approach has an advantage since -if based on representative surveys -unobserved heterogeneity at the micro level can be expected to even out. 3 Below, we will therefore follow the macro approach, using average happiness by country from representative surveys as the dependent variable in the happiness regression. Cross-F o r P e e r R e v i e w 6 national heterogeneity will be captured by controls that reflect major cultural, demographic and institutional circumstances. Explanatory variables of interest are per capita income and levels of corruption. The partial derivatives with respect to these explanatory variables can be interpreted as the marginal utility of income and the marginal disutility of corruption, respectively. Their ratio yields the marginal rate of substitution between income and corruption, that is, the marginal monetary value of a change in corruption levels. 4 Moreover, because the indifference curve over income and corruption is estimated directly, the relevant welfare measures (compensating surplus, equivalent surplus) can be computed even for inframarginal changes in corruption levels.

The Model

The model is designed to capture the material and non-material effects of corruption on SWB, as hypothesized in the Introduction. The model consists of two equations, the happiness equation and the income equation.

The happiness equation accounts for previous findings (see Diener et al. 1999, Frey and Stutzer 2002) saying that happiness is influenced by material, social, and cultural factors. 5 While the relevant material factor is per capita income and the social factor of interest is corruption, cultural differences across countries are captured by a variable referred to as "rationality", that is, the prevalence of scientific and rational attitudes. The rationality variable differentiates more modern from more traditional societies and provides an effective control to account for culture-specific heterogeneity (Welsch 2003). Another source of cross-national heterogeneity may be the age structure. 6 We therefore include the median age as an additional control. The happiness equation can be written as follows:

) , , ( z c y f v = (1)
where v = average happiness in a country, y = per capita income, c = corruption measure, z = vector of control variables (rationality, median age). The partial derivative with respect to y is expected to be positive, while the partial derivative with respect to c is expected to be negative. Since the socio-emotional support offered by traditional spiritual institutions has been found to enhance happiness (Veenhoven 1995) and may be lacking in more rationalist environments, the derivative with respect to rationality is expected to be negative. Consistent with previous literature, the derivative with respect to age is likely to be negative over much of the domain (see, e.g., Helliwell 2003).

The income equation is a standard per capita production function familiar from the economics of growth (see, e.g., Hall and Jones 1999, Senhadji 2000), extended to account for corruption (Welsch 2004):

) , , ( c h k g y = (2)
where k = physical capital per capita, h = human capital per capita. The material input is thus physical capital. Its productivity is assumed to be influenced by human capital on the one hand and corruption on the other. 7 The partial derivatives with respect to k and h are expected to be positive, while the partial derivative with respect to c is expected to be negative. By substituting (2) into (1) we obtain

) , ), , , ( ( z c c h k g f v =
(3)

as the reduced form of the model.

Welfare Measurement

This subsection presents the welfare measures to be employed below for evaluating marginal and infra-marginal changes in corruption.

Taking average happiness prevailing in a country as a measure of welfare, the marginal welfare effect (MWE) of corruption can be stated as follows:

c f c g y f dc dv MWE + = = : (4)
In this formulation, dv/dc represents the total, ( f/ y) ( g/ c) the indirect and f/ c the direct marginal welfare effect of corruption.

The marginal welfare effects of corruption can be converted into monetary units by dividing by the marginal welfare effect of income, f/ y. This yields the monetized marginal welfare effect (MMWE) of corruption:

7 The influence of corruption on capital productivity has been studied by Lambsdorff (2003). Consistent with the above framework, he established a negative impact. (5)

The MMWE as defined above thus consists of the marginal product of corruption ( g/ c) and the marginal rate of substitution (MRS) of income for corruption ( f/ c / f/ y), both expected to be negative.

As stated in subsection 2.1, an advantage of the life satisfaction approach is that it permits to value not only marginal but also infra-marginal changes in the level of public goods or bads.

The welfare measures to be used in the empirical part of the paper will be logically similar to the compensating surplus (CS) associated with a change in public good supply. The CS is the amount by which the expenditure on private goods -or equivalently, income -can be reduced when public goods supply increases by a given amount and welfare is to remain unchanged.

A difficulty with the CS or similar measures in the present case is that income available for expenditure is itself affected by the public good (bad) in question, an effect unaccounted for by CS. To accommodate this problem, an analogue measure will be defined which follows the same general logic as the CS but captures both effects of corruption in a comprehensive way.

It relies on the circumstance that income is produced by means of capital, and measures the overall effect of reduced corruption in terms of the amount by which the expenditure on capital can be reduced when corruption drops by a given amount and welfare is to remain unchanged.

Formally, let the (conditional) capital expenditure function be defined as follows:

} ) , ), , , ( ( : min{ :
) ; , ( v z c c h k g f k w c w v e = = , (6)
where w denotes the remuneration rate of capital. 8 The capital expenditure function gives the capital cost to be expended to attain a predefined welfare level v, given c and w (as well as h and z). Given our assumptions on the partial derivatives of f(.) and g(.), the capital cost will be higher (at given w and v) the higher is the level of corruption.

When corruption changes from c o to c 1 , the capital cost saved (CCS) at constant welfare v 0 is: ) ; , ( ) ; , ( :

1 0 0 0 0 0 c w v e c w v e CCS = (7)
8 For simplicity, the notation e(v,w;c) omits the variables h and the controls. where the subscript "0" refers to the initial situation and the subscript "1" to the final situation.

CCS is positive when corruption drops and negative when corruption increases.9

The actual way of computing CCS proceeds according to the following set of equations:

1 0 0 0 k w k w CCS = , ( 7a
) ) , ), , , ( ( ) ,
), , , ( (

0 1 1 0 1 0 0 0 0 0 0 z c c h k g f z c c h k g f v = , ( 7b)
) , , (

0 0 0 0 c h k g w k = . ( 7c
)
Equation ( 7a) states that CCS is the difference between the actual capital cost and the hypothetical capital cost at altered corruption and constant welfare. (7b) implicitly determines k 1 , the (hypothetical) capital input required to maintain welfare v 0 when corruption changes from c 0 to c 1 . (7c) determines the remuneration rate (price) of capital, where g k := g/ k denotes the marginal product of capital.

Below, CCS will be employed to evaluate the changes in corruption levels that have occurred from 2001 to 2004.

Data

Our basic data set comprises data for 146 countries around the turn of the millenium. Since not all required variables are available for all countries, the number of countries actually included in the empirical analysis is smaller (see below).

An overview of the data and how they relate to the theoretical variables of the preceding subsections is provided in Table 1. servants, the independence of the civil service from political pressure, and the credibility of the government's commitment to policies. The rule of law index is designed to measure the extent to which agents have confidence in and abide by the rules of society and includes the incidence of crime, the effectiveness and predictability of the judiciary, and the enforceability of contracts. Both, governmental effectiveness and the rule of law are broader measures of institutional quality and social capital. They will be included as additional covariates to test model specification and whether or not results concerning corruption come from spurious correlations.

The summary statistics and the countries included in the sample are given in Appendix A.

Empirical Approach

The happiness and income equations from subsection 2.2 will be specified as Cobb-Douglas functions. With respect to the income equation, this specification is consistent with the finding of Lambsdorff (2003) that corruption has an effect on capital productivity (which would be prevented by a linear specification). Moreover, the Cobb-Douglas function is the standard specification in growth economics. The Cobb-Douglas form for happiness avoids, in particular, an unrealistic linear relationship between corruption and happiness. 14 Taking logarithms, the core estimating equations can be written as follows (where i denotes the countries and e 1i and e 2i are error terms): log(HAPPY i ) = a 0 + a 1 log (GDPPC i ) + a 2 log(CORR i ) + a 3 log(RESEARCH i ) + a 4 log(AGE i )+ e 1i ( 8)

log(GDPPC i ) = b 0 + b 1 log(CAPPC i ) + b 2 log(RESEARCH i ) + b 3 log(CORR i ) + e 2i (9)
In addition to these core specifications, we estimated versions of the happiness equation ( 8) in which government effectiveness and the rule of law are included as additional controls.

The equations ( 8) and ( 9) will be estimated as a system. It should be noted that this system of equations is not an interdependent one, since income is assumed to affect happiness, but not the other way around. With such a recursive system it is appropriate to use the method of seemingly unrelated regressions (SUR), which is equivalent to Maximum Likelihood estimation in the present circumstances. 15 In contrast to Ordinary Least Squares (OLS), SUR 14 Though unrealistic, we tested linear versions of the happiness equation and found the negative and significant linkage between happiness and corruption confirmed. 15 For the model specified in equations ( 8) and ( 9) the matrix of coefficients of the endogenous variables is triangular, implying that its determinant is 1. Thus the Jacobian term in the loglikelihood function for the system With respect to the happiness equation ( 8), it is clear that correlations do not establish causation. It may be that happier societies hold lower levels of corruption. 16 To control for the endogeneity arising in such a scenario, instrumental variable techniques would be appropriate.

However, this requires that there are valid instrumental variables that are highly correlated with corruption but not directly related to happiness. It is not clear what instruments might serve this purpose. Therefore, the issue of endogeneity of corruption must be disregarded in what follows.

With respect to the income equation, the issue arises whether to estimate it in differences, rather than levels. In this regard, it should be noted that our inference builds on cross-sectional variation in corruption. As the year-to-year variation in institutional factors, such as corruption, is typically small, differencing would eliminate much of the required variation from which to draw inference. Similarly, the growth rate of GDP usually varies much more than does the growth rate of capital (both physical and human); thus the link between GDP growth and input growth is likely to be very weak. For these and related reasons, levels estimation has gained considerable popularity in macroeconomics (see, e.g. Hall and Jones 1999, Senhadji 2000). In addition, time series data for capital are unavailable for most countries. For these reasons the income equation will be estimated in levels.

Results

Estimation Results

Table 2 presents the SUR estimates of the model of subsection 2.5. It can be seen from the adjusted R 2 that the happiness regression is able to explain about 65 percent of the crossnational variation in happiness. Not surprisingly, this is an explanatory power greater than that usually obtained in micro-regressions of happiness (typically below 30 percent), since the between-country macro variation in happiness is much smaller than the within-country micro variation. With respect to per capita income, the model explains about 85 percent of the variation. Though the income equation (production function) includes only three explanatory variables (physical and human capital per person, level of corruption), this is also not (8), ( 9) vanishes, and the loglikelihood function has the same form as the loglikelihood function for a set of linear seemingly unrelated regressions (Davidson and McKinnon 1993, 644-645). 16 I owe this observation to a referee. 13 surprising. Comparable level estimates of per capita income with physical and human capital alone yield R 2 values that range up to 99 percent for some of the OECD countries and rarely fall below 40 percent for developing countries (Senhadji 2000). 17

The coefficients all have the expected sign and are significant. The results thus suggest that social welfare, as captured by average happiness, is positively and significantly linked to per capita income and negatively and significantly to corruption. Per capita income, in turn, is also negatively and significantly linked to corruption. 18 In quantitative terms, the elasticity of happiness with respect to income is relatively low, which is a common finding in empirical happiness research (see, e.g., Frey et al 2004). The production elasticity of capital is quite close to the usual consensus value of 1/3 derived from income shares. In addition to these core estimates, we tested model versions in which the happiness equation is augmented by additional indicators of institutional quality and social capital, namely government effectiveness and the rule of law (see Appendix B). Given that these additional controls take the form of standardized scores (z-scores), some of their values are negative by 17 Consistent with the discussion at the end of subsection 2.5, the corresponding estimates in first differences yield much smaller coefficients of determination (Senhadji 2000). 18 The marginal significance level of corruption in the income equation is 1.29 percent, whereas it is 0.43 in the happiness equation. construction. They are therefore included as levels rather than logarithms. However, additional regressions include rescaled, strictly positive versions of these indicators in logarithmic form (see the note in Appendix B). Inclusion of these institutional controls, in whatever way, left corruption negative and significant, and raised rather than reduced the absolute magnitude of the coefficient on corruption. The additional institutional indicators themselves turn out to be insignificant and/or tend to have the wrong sign. Among all institutional variables considered, corruption is the only one that displays a robust and significant negative linkage to happiness. It is thus to be conjectured that much of the detrimental effect to be expected from unlawfulness and poor public services may be related to corruption. 19 Overall, these estimates support the basic hypothesis that corruption has both indirect (income-related) and direct (income-independent) effects on welfare. The next subsection addresses the size of these partial effects as well as of the total effect, using the coefficients from the core estimates reported in Table 2.

Marginal Welfare Effects

Given the estimates presented above, we are able to compute the direct, indirect and total marginal effects of corruption on welfare, as defined in subsection 2.3. More specifically, we compute the direct, indirect and total derivatives of happiness with respect to corruption and their monetary equivalents.

We first note that due to the specification given in subsection 2.5, the direct marginal effect can be written as a 2 *(HAPPY i /CORR i ) whereas the indirect effect can be written as a 1 *b 3 *(HAPPY i /CORR i ) This implies that for any single country the direct and indirect effects are proportional to a 2 = -0.039 and a 1 *b 3 = 0.121*-0.180 = -0.022, respectively. We thus find that the direct effect is almost 1.8 times larger than the indirect effect.

Table 3 presents numerical values of the marginal effects of corruption in terms of percentiles of the total effect. The median value (50 th percentile) of the total effect is -0.033. This means that in 50 percent of the cases an increase in corruption by 1 point (on the 10-point scale) reduces happiness by more than 0.033 (on the 4-point happiness scale). There is considerable dispersion in the total effect, as indicated by the 10 th and 90 th percentile. The monetary equivalents are quite substantial. The median value for the total effect is about $900 per capita per year. The indirect effect amounts to more than $300 and the direct effect to almost $600 at the median. When considering these marginal effects, it should be clear, however, that "marginal change of corruption" in fact refers to a change by 1 point on a 10-point scale. In practice, this amounts to a relatively strong change, at least in the intermediate term. The next subsection will therefore address the monetary valuation of those changes in corruption which actually occurred in the recent past.

Infra-marginal Welfare Effects

In this subsection we present the valuation of the changes in corruption that have taken place Table 4 shows the economic value of these changes for the countries that improved and those that deteriorated, respectively. The measure shown is the capital cost savings at constant welfare defined in subsection 2.3. Cost savings are expressed as fractions of GDP in 2001.

In the countries that succeeded in reducing corruption, the median value of compensating capital cost savings (CCS) is 3.6 percent of GDP. The 10 th and 90 th percentiles indicate a considerable dispersion of these values. Similar results apply to the countries in which corruption increased. The median value of CCS in these countries is -3.5 percent of GDP.

Conclusions

Empirical research has shown that corruption affects a variety of economic indicators, especially GDP per capita. However, as GDP is not a genuine indicator of welfare, it reflects the welfare costs of corruption only in an incomplete way.

Motivated by this shortcoming, the present paper has used self-rated subjective well-being as an empirical approximation to general welfare to explore several channels through which welfare, operationalized in this way, is affected by corruption. It was found that corruption affects cross-national welfare not only indirectly, through GDP, but also directly, due to, e.g., the time and effort required to cope with corrupt behavior, or psychological costs associated with a general climate of unlawfulness. Interestingly, the direct effect -which has not previously been investigated in the corruption literature -was found to be substantially larger than the indirect effect and to imply a considerable welfare loss.

As regards the relatively small size of the indirect effect, it should be noted that this effect is the effect of corruption on income times the effect of income on subjective well-being. As the latter is typically small (see, e.g., Frey et al. 2004), so is the indirect effect of corruption on welfare.

In monetary terms, the changes in corruption in 2001-2004 are equivalent to a change in the stock of physical capital worth more than 3 percent of GDP (median). The welfare costs of corruption, as measured by the monetized effect of corruption on subjective well-being, are thus of a considerable magnitude.

F

  paper estimates the welfare costs of corruption by converting the effects of corruption on SWB into monetary units.

  and correlation of errors across equations. Similar to OLS, it does not require to assume normality of residuals.

Figure 1 :

 1 Figure 1: Changes in Corruption 2001-2004

Page 12 of 21 Editorial Office, Dept of Economics, Warwick University, Coventry CV4 7AL, UK

	Submitted Manuscript
	F o r
	P
	e
	e r
	R e v
	i e
	w

Table 2 :

 2 Estimation Results

	Dependent variable: log(HAPPY)
	Log(GDPPC)	0.121
		(6.58)
	Log(RESEARCH)	-0.036
		(2.50)
	Log(AGE)	-0.230
		(3.43)
	Log(CORR)	-0.039
		(2.93)
	Observations	47
	Adjusted R 2	0.646
	Dependent variable: log(GDPPC)
	Log(CAPPC)	0.357
		(4.59)
	Log(RESEARCH)	0.189
		(3.51)
	LOG(CORR)	-0.180
		(2.54)
	Observations	50
	Adjusted R 2	0.843
	Method: SUR; t-statistics in parenthesis

Table 3 :

 3 Marginal Effects of CorruptionThe percentile values for direct and indirect effects do not necessarily add up to the percentile values for the total effect, because they may correspond to different countries.

			Marginal welfare effect		Monetized marginal welfare effect
	percentile 10 th	total -0.021	indirect -0.007	direct -0.014	total -127	indirect -45	direct -81
	25 th	-0.025	-0.009	-0.016	-310	-111	-199
	50 th	-0.033	-0.012	-0.021	-899	-322	-577
	75 th	-0.069	-0.025	-0.043	-4098	-1468	-2629
	90 th	-0.188	-0.068	-0.111	-13085	-4689	-8396
	Note:

Table 4 :

 4 Capital Cost Savings due to Changes inCorruption 2001-2004

	percentile	decrease in corruption (n=41)	increase in corruption (n=37)
	10 th	0.007	-0.008
	25 th	0.015	-0.021
	50 th	0.036	-0.035
	75 th	0.085	-0.055
	90 th	0.129	-0.150
	Note: Entries are the capital cost savings at constant welfare (see equ. (7)) as fractions of GDP in 2001.
	It can thus be stated that the changes in corruption 2001-2004 represent a considerable
	monetary value.

An alternative way of dealing with unobserved micro-heterogeneity is based on surveys in which a constant set of individuals is surveyed over time. This allows to use dummy variables for each individual as controls. The problem with this approach is that a fixed set of survey respondents will not remain representative across time.

Editorial Office, Dept of Economics, Warwick University, Coventry CV4 7AL, UK Submitted Manuscript

Note that the marginal rate of substitution is an ordinal concept, i.e., it is invariant with respect to the cardinalization of utility (happiness) as long as alternative cardinalizations are monotonically increasing transformations of each other.

Since we follow the macro approach (using data on average happiness from representative surveys), personal and demographic variables (age, gender, marital status etc.) play no role in this model.

Age is an important predictor of individual happiness (see, e.g.,[START_REF] Helliwell | How's life? Combining individual and national variables to explain subjective well-being[END_REF].

Note that the expenditure function could be extended to comprise more than just one production input, capital. The capital cost savings would then generalize to "factor cost savings". In this paper we deliberately treat labor

With respect to transition economies,[START_REF] May | Does Governance Explain Unofficial Activity?[END_REF] report that corruption significantly enhances unofficial activity, whereas government effectiveness and the rule of law display no such influence.

I3 -Welfare and Poverty < I -Health, Education, and Welfare, D73 -Bureaucracy; Public Admin.

 11The original data actually indicates absence of corruption, that is, 10 indicates a very clean and 0 a very corrupt country. In the present paper the scaling has been reversed such that 0 refers to the minimum and 10 to the maximum of corruption.

12 See http://emlab.berkeley.edu/users/chad/HallJones400.asc. 13 Note that this variable plays a twin role: In the happiness equation it serves as a control that proxies the degree of rationality (modernity) of a society, whereas in the income equation it represents the human capital variable.

Appendix A: Data

Summary statistics: List of countries:


Laura-Isabel Arranz

Magda Rafecas
email: magdarafecas@ub.edu

Miguel-Ángel Canela
email: mcanela@iese.edu

FIBROMYALGIA AND NUTRITION, WHAT DO WE KNOW?

Keywords: fibromyalgia, nutrition, diet, antioxidants, nutritional supplementation

Objective: Many people suffer from fibromyalgia (FM) without an effective treatment. They do not have a good quality of life and cannot maintain normal daily activity. Among the different hypotheses for its ethiopathophysiology, oxidative stress is one of the possibilities. Non-scientific information addressed to patients regarding the benefits of nutrition is widely available, and they are used to trying non-evidenced strategies. The aim of this paper is to find out what we know right now from scientific studies regarding fibromyalgia disease and nutritional status, diets and food supplements. Design: A systematic search has been performed on Medline with a wide range of terms about these nutritional issues. Setting: the search has been made during 2009, for articles published between 1998 and 2008. Subjects: people suffering from FM. Results: Vegetarian diets could have some beneficial effects probably due to the increase in antioxidant intake. There is a high prevalence of obesity and overweight in patients, and weight control seems to be an effective tool to improve the symptoms. Some nutritional deficiencies have been described, it is not clear whether they are directly related to this disease or not. About the usefulness of some food supplements we found very little data, and it seems that more studies are needed to prove which ones could be of help. Conclusions: Dietary advice is necessary to these patients to improve their diets and maintain normal weight. It would be interesting to investigate more in the field of nutrition and FM to reveal any possible relationships.

Introduction

Over the 1970s fibromyalgia syndrome (FM) began to be identified as a distinct clinical syndrome, different from other rheumatic illnesses. The WHO and all international medical organisations did not recognise it as a disease until 1992. Nowadays it is classified as a rheumatic disease with an unknown aetiology and without an effective medical treatment. FM is a condition characterised by widespread pain in 11 of 18 tender points experienced for at least 3 months [START_REF] Wolfe | The American College of Rheumatology 1990 criteria for the classification of fibromyalgia: report of the multicenter criteria committee[END_REF]. The 1990 American College of Rheumatology classification criteria are widely used for the diagnosis of fibromyalgia and provide a sensitivity and specificity of nearly 85% differentiating FM from other forms of chronic musculoskeletal pain. As well as pain, patients commonly report some other symptoms such as fatigue, sleep disorders, depression, anxiety, cognitive difficulties, headache, low back pain, and illnesses like irritable bowel syndrome, chronic fatigue syndrome, rheumatoid arthritis, systemic lupus erythematosus, and osteoarthritis. FM has an enormous impact on the quality of life (QOL) of patients who experience a reduced functionality or capacity to carry on the activities of daily living; every day activity becomes more difficult, more time consuming, or simply impossible.

Fibromyalgia syndrome and rheumatoid arthritis are among the most common causes of musculoskeletal pain and disability. The prevalence of fibromyalgia has not been determined using a large or international population base, but it is commonly estimated to affect 1 to 2% of the population [START_REF] Weir | The incidence of fibromyalgia and its associated comorbidities: a population-based retrospective cohort study based on International Classification of Diseases, 9th revision codes[END_REF][START_REF] Maquet | Fibromyalgia in the year 2000[END_REF][START_REF] Lawrence | Estimates of the prevalence of artritis and selected músculoskeletal disorders in the United States[END_REF][START_REF] Wolfe | The prevalence and characteristics of fibromyalgia in the general population[END_REF], although some studies show higher figures, even up to 5% [START_REF] White | The London Fibromyalgia Epidemiology Study: the prevalence of fibromyalgia syndrome in London, Ontario[END_REF][START_REF] White | Classification, epidemiology, and natural history of fibromyalgia[END_REF]. In Spain it is calculated to affect 2.4% of the general population [START_REF] Valverde | Prevalencia de la fibromialgia en la población española. Estudio EPISER[END_REF][START_REF] Carmona | EPISER Study Group. The burden of musculoskeletal diseases in the general population of Spain: results from a national survey[END_REF][START_REF] Mas | EPISER Study Group. Prevalence and impact of fibromyalgia on function and quality of life in individuals from the general population results from a nationwide study in Spain[END_REF], although in other studies the Spanish prevalence seems to be around 4% or more [START_REF] Gamero | Pain in Spanish rheumatology outpatient offices: EPIDOR epidemiological study[END_REF]. The results in an Italian survey study show a prevalence of 2.2% [START_REF] Salaffi | Pain Prevalence, Investigation Group (MAPPING) study. Prevalence of musculoskeletal conditions in an Italian population sample: results of a regional community-based study. I. The MAPPING study[END_REF]. Also, a very recent study estimates that the prevalence of fibromyalgia in the French population is 1.4% [START_REF] Bannwarth | Fibromyalgia síndrome in the general population of France: A prevalence study[END_REF]. Females are more likely than males to have fibromyalgia and patients with fibromyalgia are more likely to have one or more comorbid conditions such as depression, anxiety, headache, irritable bowel syndrome, chronic fatigue syndrome, systemic lupus erythematosus and rheumatoid arthritis [START_REF] Weir | The incidence of fibromyalgia and its associated comorbidities: a population-based retrospective cohort study based on International Classification of Diseases, 9th revision codes[END_REF]. The absence of fibromyalgia in China is an interesting issue, which could be explained by genetic differences and, maybe, by socio-cultural differences. It could be worthwhile to study and compare the prevalence gradients from China to the ones in the western world, which might provide important insights into what causes this disease [START_REF] Felson | Comparing the prevalence of rheumatic diseases in China with the rest of the world[END_REF].

Since FMS aetiology and physiopathology are unknown, it is difficult to find an effective and curative treatment, either pharmacological or non-pharmacological. There is an interesting hypothesis about the pathophysiologic mechanism, which is widespread in the scientific literature, the oxidative stress hypothesis. Some studies try to explain how oxidative stress (OE) may be involved in the development and maintenance of this syndrome. For instance, it has been observed that some FM patients had lower levels of some antioxidant nutrients like magnesium and selenium [START_REF] Eisinger | Selenium and magnesium status in fibromyalgia[END_REF][START_REF] Reinhard | Selenium status in fibromyalgia[END_REF]. They also seem to have an insufficient antioxidant status [START_REF] Ozgocmen | Antioxidant status, lipid peroxidation and nitric oxide in fibromyalgia etiologic and therapeutic concerns[END_REF], a predominance of a pro-oxidative status [START_REF] Bagis | Free radicals and antioxidants in primary fibromyalgia: an oxidative stress disorder?[END_REF], and a lower antioxidant capacity [START_REF] Altindag | Total antioxidant capacity and the severity of the pain in patients with fibromyalgia[END_REF]. The results indicate that these patients have a high level of oxidative stress and perhaps the high levels of free radicals in their bodies may be involved in the development of the disease. Some studies have investigated thoroughly the mechanisms of cellular oxidation. Nitric oxide (NO) and its derivative peroxinitrite (ONOO -) are often said to have a relevant role in this process. ONOO -is a powerful oxidant agent which is the result of the combination of the NO and the superoxide radical (O 2 -). These agents seem to act as initiators of an oxidation reactions cycle which is not totally counteracted by the antioxidant defences. This may be a potential cause of the development of FM and also of chronic fatigue syndrome (CFS) [START_REF] Pieczenik | Mitochondrial dysfunction and molecular pathways of disease[END_REF][START_REF] Ozgocmen | Current concepts in the pathophysilogy of fibromyalgia: the potential role of oxidative stress and nitric oxide[END_REF][START_REF] Pall | Common etiology of posttraumatic stress disorder, fibromyalgia, chronic fatigue syndrome and multiple chemical sensitivity via elevated nitric oxide/peroxynitrite[END_REF]. Some of the studies indicated above consider that an antioxidant supplementation might be beneficial in the treatment of FM, because of its high oxidative stress status as well as its inflammation level. In relation to this, there are data from nutritional interventions in which antioxidant intake was increased through a vegetarian diet, with positive results on rheumatic and FMS patients [START_REF] Hänninen | Antioxidants in vegan diet and rheumatic disorders[END_REF][START_REF] Donaldson | Fibromyalgia syndrome improved using a mostly raw vegetarian diet: An observational study[END_REF]. However this is a matter that would need more evaluation.

The case that in CFS patients as well as FM patients there is a high level of oxidative stress seems widely proved. However, it is unknown whether it is a cause or a consequence. The musculoskeletal symptoms are related to an increase in oxidative stress but also to a decrease of the antioxidant defences [START_REF] Vecchiet | Relationship between musculoskeletal symptoms and blood markers of oxidative stress in patients with chronic fatigue syndrome[END_REF]. OE causes cellular damage, affecting a wide range of levels and functions like the lipidic peroxidation, cell membrane functionality and prostaglandin synthesis [START_REF] Kennedy | Oxidative stress levels are raised in chronic fatigue syndrome and are associated with clinical symptoms[END_REF]. Another study shows that the higher level of OE or prooxidant substances in erithrocytes damages them and alters their normal functionality, contributing to the CFS symptoms [START_REF] Richards | Erythrocyte oxidative damage in chronic fatigue syndrome[END_REF].

Since the discovery of mythocondrial disfunction, medicine has advanced in the understanding of the roles of these cellular organelles in some diseases and in aging. Some illnesses, like FM and CFS, seem to have common pathophysiological mechanisms, which involve free radicals, their accumulation in mitochondria and DNA damage [START_REF] Pieczenik | Mitochondrial dysfunction and molecular pathways of disease[END_REF]. More than 90% of cell energy is produced in mitochondria, through oxidative phosphorylation and through the coordination of two close processes, the tricarboxylic acid cycle (or Krebs cycle) and the electron transport chain, involving different reactions, enzymes, coenzymes and nutrients acting as cofactors. In the Krebs cycle cysteine, iron, niacin, magnesium, manganese, thyamine, riboflavin, pantothenic acid, lipoic acid and L-carnitine are all involved. In the electron transport chain, coenzym Q10, flavines, copper and iron among others, are of a high importance. The correct functioning of all these reactions depends on the presence of high enough quantities of these nutrients. During the oxidative phosphorylation process, consumed oxygen is converted into superoxide radicals, but the mitochondria have mechanisms to eliminate this in aqueous form, with the help of the superoxide dismutase (SOD-Mn) and the glutathione peroxidase (GPX). These antioxidant enzymes are very important because mitochondrial lipids, proteins, enzymes and DNA are especially vulnerable to free radical action. The mitochondria may lose functionality when their components are damaged, and this could lead to a feed-back process that causes additional injuries. When the superoxide radical is not sufficiently neutralised by these protective mitochondrial processes, it combines with nitric oxide leading to the more oxidant free radical, peroxinitrite, and therefore to an oxidative stress situation in cells.

Apart from oxidative stress, another theory for the pathophysiologic mechanism of FM is the immunological theory, involving inflammatory cytokines. Although very few studies have shown elevated cytokine levels in FM, the data available suggest that a high chemokine level may play a causative role in this disease [START_REF] Zhang | High plasma levels of MCP-1 and Eotaxin provide evidence for an immunological basis of fibromyalgia[END_REF][START_REF] Wallace | Cytokines paly an aetiopathogenetic role in fibromyalgia: a hypotesis and pilot study[END_REF]. Cytokines are known to play a role in diverse clinical processes and phenomena such as fatigue, fever, sleep, pain, stress and aching. A review of the fibromyalgia literature and related studies suggest that IL-1, IL-6 and IL-8 are deregulated in the syndrome. Therefore, therapies directed against these cytokines may be of potential importance in the management of fibromyalgia, and could be more closely studied in the future [START_REF] Wallace | Is there a role for cytokine based therapies in fibromyalgia[END_REF].

In relation to the management of the disease, recent reviews show which interventions are more effective by reducing the symptoms of the disease, mainly pain but also fatigue and cognitive damage, and also reveal the fact that multidimensional approaches are the most appropriate to these patients. Some drugs, physical activity, relaxation techniques, and cognitive behaviour therapy are the most useful tools [START_REF] Chakrabarty | Fibromyalgia[END_REF][START_REF] Carville | EULAR evidence-based recommendations for the management of fibromyalgia syndrome[END_REF]. Although various pharmacological treatments have been studied for treating FMS, no single drug or group of drugs has proved to be useful in treating the disease as a whole. Several agents, including serotoninorepinephrine reuptake inhibitors (ie., duloxetine and milnacipran), opioids (ie., tramadol), and the ligand pregabalin, have recently received the U.S. Food and Drug Administration approval for treating the syndrome in the United States. They have been evaluated in clinical trials, showing a positive effect in terms of pain reduction and improvement in core symptoms such as fatigue and sleep disturbance [START_REF] Perrot | Fibromyalgia: Harmonizing Science with Clinical Practice Considerations[END_REF][START_REF] Abeles | Update on Fibromyalgia Therapy[END_REF].

Regarding non-pharmacological therapeutic approaches there are some data from different reviews. Some have demonstrated strong evidence for efficacy, like cardiovascular exercise, cognitive behavioural therapy, patient education, and multidisciplinary therapy. In addition other adjunctive treatments show moderate evidence of efficacy, like strength training, acupuncture, hypnotherapy, biofeedback and balneotherapy [START_REF] Goldenberg | Management of fibromyalgia síndrome[END_REF]. For other non-pharmacological approaches, widely used and potentially beneficial, such as ozone therapy and some food supplements, more investigation is needed [START_REF] Nishishinya | Intervenciones no farmacológicas y tratamientos alternativos en la fibromialgia[END_REF].

In summary, at present there is no cure for FM and no universally effective treatment, so physicians are searching for the best way to manage fibromyalgia. There are some aspects that could need more investigation like, for example, how nutrition may affect the clinical symptoms of these patients. Since this disease affects a significant proportion of the population, with various degrees of impact on disability and quality of life, it results in a significant number of physician visits, work disability, and medication use. Therefore it is of a crucial relevance to review what is known and to investigate what else could be done to improve health and quality of life in these patients.

The primary purpose of this review is to find out what we know about the possible relationship between nutrition and FM, and to look at the main studies carried out on nutritional evaluation and intervention. In this report we have analysed the current knowledge related to this disease and diet, nutritional status and nutritional supplementation.

Experimental methods

Studies in humans were identified through a systematic and computerised search of MEDLINE. The languages of the studies selected were English as well as Spanish, and the range of publication date from 1998 to 2008. The main terms for the systematic search were fibromyalgia in combination with nutrients, food intake, dietary patterns, nutritional supplementation, complementary medicine, vegan diet, antioxidants, dietary modification, obesity, body mass index, and others indicated in table 1. Only those papers which studied directly or indirectly the relationship we looked for were selected. Reference lists of the included articles were also scrutinized to search for relevant studies, and in some cases led to a new search to get new papers, or to contact authors for further details of their trials, in particular when the full study report was not available.

Results

Fibromyalgia, diet and nutrition

Fibromyalgia, as a rheumatoid disease where chronic inflammation and oxidative stress are main concerns, may have some benefits from diets rich in antioxidants, like those based on uncooked vegan products called living food (LF) [START_REF] Hänninen | Antioxidants in vegan diet and rheumatic disorders[END_REF]. Patients eating LF showed higher levels of beta and alfa-carotenes, lycopen and lutein, vitamin C and vitamin E in their serum, than the controls eating a normal diet. Also, levels of polyphenolic compounds like quercetin, myricetin and kaempherol were much higher than in the omnivorous controls. Moreover, the clinical outcomes of this intervention for fibromyalgic subjects were an improvement in their joint stiffness and pain as well as an improvement in their self-experienced health.

Due to this possibility of benefit from the vegan diet rich in antioxidants in these patients, some other studies evaluated the effects of vegan diets on the symptoms of FM. Two of them compared the effect of changing an omnivorous diet for a vegetarian diet, without making any change in the medical treatment of the patients. The third compared the effect of the vegetarian diet against the effect of a pharmacological treatment with amitriptyline.

The first one [START_REF] Kaartinen | Vegan diet alleviates fibromyalgia symptoms[END_REF], a non-randomised and controlled study, evaluated the effect of a strict, low-salt, uncooked vegan diet rich in lactobacteria, on symptoms in 18 fibromyalgia patients, during a 3-month intervention period. A 15-patient control group continued their omnivorous diet during the same time. The results were evaluated with a Visual Analogue Scale (VAS) of pain, joint stiffness and quality of sleep, and with a Health assessment questionnaire (HAQ), General Health Questionnaire (GHQ), and the rheumatologist's own questionnaire, indicating an improvement in the outcomes. Patients showed a reduction in body mass index (BMI), total cholesterol and urine sodium, indicating good diet compliance. In conclusion, the results suggested that a vegan diet has beneficial effects on fibromyalgia symptoms, at least in the short run.

In the second study [START_REF] Donaldson | Fibromyalgia syndrome improved using a mostly raw vegetarian diet: An observational study[END_REF], 30 people participated in a dietary intervention using a mostly raw, pure vegetarian diet, consisting of raw fruits, salads, carrot juice, tubers, grain products, nuts, seeds, and a dehydrated barley grass juice product. The outcomes measured were dietary intake and different disease/health scales such as the Fibromyalgia Impact Questionnaire (FIQ), the short form health survey (SF-36), a quality of life survey, and a physical performance measurement. The intervention was of seven months and after this period all the respondents, 19 out of 30 subjects, had a significant improvement in all measured outcomes, except for body pain. At the end of the study the scores for all the scales, except body pain, were no longer statistically different from healthy women aged 45-54. To carry on the dietary intervention, all the participants were informed at the beginning of the study, but they did not receive regular support meetings for motivational encouragement because the researchers considered that this might make the results less applicable to the general population. This study concluded that this sort of dietary intervention can help fibromyalgia patients.

Considering Donaldson's study, Bennett concluded that many FM patients could be helped by recommending a mostly raw vegetarian diet, but he encouraged the researchers to repeat the study with a control group. Although it is difficult to design a double-blind placebo control for dietary studies, it would be interesting and possible to use a different diet control group, distinct from the raw vegetarian one [START_REF] Bennet | A raw vegetarian diet for patients with fibromyalgia[END_REF].

For the third study, since it was known that brain tryptophan is low in FM, and intake of protein rich in large neutral amino acids has been reported to lower it, an open, randomised and controlled study [START_REF] Azad | Vegetarian diet en the treatment of fibromyalgia[END_REF], was undertaken to assess whether any reduction of such proteins by exclusion of animal protein from the diet reduced pain and morbidity in FM patients compared with the reduction by a pharmacological agent. 37 subjects with FM were enrolled in a vegetarian diet and 41 in a pharmacological amitriptyline treatment group. The outcome was assessed with the help of a tender point count, and VAS scale of frequencies of fatigue, insomnia, non-restorative sleep and pain. The differences were significant at 6 weeks of treatment, showing improvement in the amitryptyline group. In the vegetarian diet group all of them were insignificant, except in pain score. However, the decrease in the pain score, though significant, was much smaller than in the amitriptyline group. As a result the authors concluded that due to the differences between both groups the vegetarian diet exclusively is a poor option in the treatment of fibromyalgia.

Composition and alterations in the intestinal bacterial flora are believed to be contributing factors to many chronic inflammatory and degenerative diseases, since the flora act as an important immunologic protection barrier. However, the relationship between possible changes in intestinal flora, secretion of immunoglobulin A, and specific illnesses is unknown. One study [START_REF] Michalsen | Mediterranean diet or extended fasting's influence on changing the intestinal microflora, immunoglobulin A secretion and clinical outcome in patients with rheumatoid artritis and fibromyalgia: an observational study[END_REF] was performed to test whether a Mediterranean diet and an 8-day fasting period led, in patients with rheumatoid arthritis or fibromyalgia, to some changes in faecal flora and in clinical outcome. 51 patients participated and none of the two dietary interventions, in the way they were carried out, led to change, neither in intestinal microflora nor in the clinical parameters tested. So the impact of dietary interventions on the human intestinal flora and the role of this in rheumatic diseases still need to be clarified.

Monosodium glutamate (MSG) and aspartame could act as excitotoxin molecules in organisms, acting as excitatory neurotransmitters, and may lead to neurotoxicity when used in excess [START_REF] Smith | Relief of Fibromyalgia Symptoms Following Discontinuation of Dietary Excitotoxins[END_REF]. The elimination of MSG and aspartame from the diets of 4 fibromyalgia patients with multiple co-morbidities produced a resolution of their symptoms within months after the intervention. It is known that intramuscular injection of glutamate into the human masseter muscle evokes muscle pain, which is stronger in women. However, the effect of dietary glutamate consumption on these patients is not well known.

This suggests that research with larger number of patients with similar conditions is needed to confirm the effectiveness of this dietary treatment [START_REF] Geenen | Dietary Glutamate Will Not Affect Pain in Fibromyalgia[END_REF][START_REF] Cairns | Dietary glutamate and fibromyalgia[END_REF].

Nowadays the internet has become a major source of information, and lots of patients with FM use some treatments that mostly lack scientific evidence. This is the case of the 'elimination diet', based on identifying and avoiding irritant foods for which these patients may have a subclinical allergy. It consists of a rotation of foods with close attention to the effects; specific foods are introduced into the diet at the rate of 1 per 5 days. An "oligoantigenic" diet is another method of food therapy without scientific basis, in which modern manufactured foods are removed from the diet and replaced with the palaeolithic equivalents [START_REF] Cymet | A Practical Approach to Fibromyalgia[END_REF]. Moreover, in some websites, journals and books addressed to FM patients, information regarding the potential benefit of macrobiotic diets and foods is frequently found. Despite this, in our search we have not found any article that deals with these issues. On a practical basis some patients are recommended to avoid those foods which cause them subjective worsening of their symptoms.

Mood, anxiety and eating disorders are of a higher prevalence in FM patients. There is a significant co-occurrence of FM with some psychiatric disorders and eating disorders, such us nervous anorexia and bulimia which have a relevant incidence among this people [START_REF] Arnold | Comorbidity of Fibromyalgia and psychiatric disorders[END_REF]. It has been observed that binge eating disorders co-occurred significantly with FM disease [START_REF] Javaras | Co-occurrence of Binge Eating Disorder With Psychiatric and Medical Disorders[END_REF]. Therefore, on a practical level, if FM patients are more likely to have these eating disorders, special attention should be given to that when evaluating and treating them.

Fibromyalgia and nutritional status

A study based on an Internet survey addressed to people with fibromyalgia, developed by the National Fibromyalgia Association (NFA) of the United States, and responded to by about 2500 participants, identified several issues for further research such as the prescribing habits of FM health care providers and the impact of obesity among others. In comparison with United States National Census figures, respondents were moderately overweight, 70% had a BMI > 25, and 43% had a BMI > 30; higher figures comparable with the general population [START_REF] Bennett | An internet survey of 2,596 people with fibromyalgia[END_REF]. Several other studies have reported obesity problems in FM [START_REF] Patucchi | Prevalence of fibromyalgia in diabetes mellitus and obesity[END_REF][START_REF] Yunus | Relationship between body mass index and fibromyalgia features[END_REF][START_REF] Mengshoel | Health status in fibromyalgiaa followup study[END_REF] and others had reported that weight reduction provides improvement to these patients [START_REF] Shapiro | A pilot study of the effects of behavioral weight loss treatment on fibromyalgia symptoms[END_REF]. In fact, overweight and obesity are highly frequent in these people, and body mass index has a negative correlation with quality of life and tenderness threshold, and a positive correlation with physical dysfunction and pain point count. Obese FM patients show higher pain sensitivity and lower levels of quality of life [START_REF] Neumann | A cross-sectional study of the relationship between body mass index and clinical characteristics, tenderness measures, quality of life, and physical functioning in fibromyalgia patients[END_REF]. The high prevalence of overweight and obesity was showed also in a health telephonic survey made only on female participants, with and without FM. However, up to now, it does not seem possible from available current data to determine if weight gain, experimented in these persons, especially in women with FM, is due to the disease and its accompanying low activity levels, or if those women are more prone to premorbid obesity than healthy ones [START_REF] Shaver | Women's Health Issues with Fibromyalgia Syndrome[END_REF]. Nevertheless, sound research has offered us results on the improvement for multiple musculoskeletal complaints and function, usually measured as quality of life by a health questionnaire like SF-36, through weight loss in obese people [START_REF] Hooper | Musculoskeletal findings in obese subjects before and after weight loss following bariatric surgery[END_REF]. It is known that obesity is associated with a range of disabling musculoskeletal conditions, like FM, especially in adults. As the prevalence of obesity increases, the social burden of these chronic musculoskeletal conditions, in terms of disability, health-related quality of life, and health-care costs, also increases. Therefore weight reduction is an important tool in ameliorating some of the manifestations of musculoskeletal diseases and improving function and patients' quality of life [START_REF] Ananda | The impact of obesity on the musculoskeletal system[END_REF].

Another characteristic found in FM patients is a higher resting metabolic rate (RMR) compared with healthy controls. In both groups RMR was measured by indirect calorimetry, and predicted RMR, by fat-free weight, sex, age, height and weight. This high measured RMR was not correlated with thyroid hormones TSH, FT4 and FT3 [START_REF] Lowe | Female fibromyalgia patients: lower resting metabolic rates than matched healthy controls[END_REF].

This study began with the hypothesis that FM patients could be hypometabolic due to hypothyroidism, because of some beneficial results found in some thyroid hormone treatment trials. However TSH, T3 and T4 levels did not significantly differ between patients and the control group.

Patients with fibromyalgia commonly have a high body mass index and are physically inactive, which are two main risk factors for metabolic syndrome. Although there is not enough sound knowledge about the relationship between FM and metabolism we found one study conducted on women with chronic pain from FM which concluded that they have an increased risk of suffering from metabolic syndrome. So it recommended that these people should be educated about the benefits of physical exercise and weight loss, to reduce their metabolic risk as well as improve of pain and general health [START_REF] Loevinger | Metabolic syndrome in women with chronic pain[END_REF].

Another important aspect of nutritional status is the possibility of some nutritional deficiencies. It has been proposed that some of them are involved in FM patient symptoms, although there is no study that globally evaluates them. We have found in this revision isolated studies that focus on the potential role in the pathogenesis of FM of some possible nutritional deficiencies such as for magnesium, iodine, iron, vitamin D, melatonin, and branched chain amino acids.

Iodine deficiency is hypothesised to give rise to subtle impairment of thyroid function leading to clinical syndromes, like those in fibromyalgia, without either biochemical alterations in T3, T4 and TSH or clinical manifestations such as goiter and cretinism [START_REF] Loevinger | Metabolic syndrome in women with chronic pain[END_REF]. It has also been pointed that selenium, zinc, iron or retinoic acid disbalance could lead to the clinical effects of iodine deficiency due to their role in optimal thyroid function. It is known that the more symptoms of hypothyroidism are present, the more likely the presence of FM becomes [START_REF] Verheesen | Iodine deficiency, more than cretinism and goiter[END_REF]. There is not a total evidence of iodine deficiency in FM, but this issue deserves some attention because this nutritional impairment is a worldwide health problem.

Regarding iron, an increased frequency of FM in iron deficiency anaemia (IDA) and in thalassemia minor (TM) was diagnosed in a study. Some nonspecific symptoms of IDA and TM, like impaired cognitive function, fatigue or inability to concentrate, may not be explained by only anaemia, and consequently may not respond to therapy. In these cases considering the possibility of a FM diagnosis is recommended, since in this study the results show higher prevalence of FM in IDA and TM patients [START_REF] Pamuk | An increased prevalence of fibromyalgia in iron deficiency anemia and thalassemia minor and associated factors[END_REF]. A second step will be to understand the role that iron replacement therapy could play in some cases suffering from FM and IDA or TM at the same time.

A pilot study concluded that FM was frequently associated with osteoporosis, and thus early detection and implementation of appropriate nutritional supplementation with calcium and vitamin D could be useful [START_REF] Swezey | Fibromyalgia: a risk factor for osteoporosis[END_REF]. Vitamin D deficiency was found to be common in FM women, although they were not at an increased risk of developing osteoporosis or osteomalacia, since bone density in them was comparable with that in controls [START_REF] Mole | Bone health in patients with fibromyalgia[END_REF]. On the contrary, the highest prevalence of hypovitaminosis D, over 92.7%, was found in a group of people with nonspecific musculoskeletal pain [START_REF] Plotnikoff | Prevalence of severe hipovitaminosis D in patients with persistent nonspecific musculoskeletal pain[END_REF]. Although this study recommended urgent general screenings of FM patients and treatment with vitamin D for replenishment, it was criticised by other authors, which have obtained different and contradictory results [START_REF] Block | Vitamin D deficiency is not associated with nonspecific musculoskeletal pain syndromes including fibromyalgia[END_REF]. In those, low vitamin D levels were not associated with diffusing musculoskeletal pain, and treatment with this nutrient did not reduce pain [START_REF] Warner | Diffuse musculoskeletal pain is not associated with low vitamin D levels or improved by treatment with vitamin D[END_REF]. Recent studies have confirmed that FM patients have vitamin D deficiencies or insufficient levels compared with local healthy references. Even within those with hypovitaminosis D there is a higher prevalence of anxiety and depression. Therefore more clinical studies are needed to fully assess the impact of vitamin D in these patients, in order to reach a consensus and improved guidelines for rheumatologists [START_REF] Armstrong | Vitamin D deficiency is associated with anxiety and depression in fibromyalgia[END_REF][START_REF] Mouyis | Hypovitaminosis D among rheumatology outpatients in clinical practice[END_REF]. Some trace elements have been supposed to have a relevant role in the pathophysiology of FM, especially those related to the redox balance in cells, like selenium, zinc and magnesium. An examination of serum levels of these three elements in FM patients showed that magnesium and zinc levels were decreased, compared with controls, and there was no considerable difference in selenium levels. Moreover, serum magnesium and zinc levels were associated with clinical parameters, indicating a possible role of these two trace elements in FM ethiopathogenesis [START_REF] Sendur | The relationship between serum trace elements levels and clinical parameters in patients with fibromyalgia[END_REF]. However, this study did not evaluate whether there was a nutritional deficiency or a low magnesium or zinc intake from the diet. In our search we have not found any study on the possible deviation of nutrient intake in patients with FM, compared with recommendations for general population. On the other hand, another recent study could not conclude that there are abnormal levels of copper, iron, magnesium, selenium and zinc, among others, in blood or urine of FM patients. Although there were some differences between them and controls, they were of no clinical significance. Therefore, these results gave no support to the hypothesis that trace elements unbalance could play a significant role in the development of this disease [START_REF] Rosborg | Trace element pattern in patients with fibromyalgia[END_REF]. Otherwise, some other data indicated that this is not a clear issue and contradictory results appeared regarding some trace element levels in this disease, showing some normal levels for Se and Mg [START_REF] Eisinger | Selenium and magnesium status in fibromyalgia[END_REF] and others lower levels for Se [START_REF] Reinhard | Selenium status in fibromyalgia[END_REF].

Nevertheless, these trace elements are highly important in maintaining the oxidants/antioxidants balance in the body. When the total antioxidant capacity is measured in these patients, it seems that they are exposed to a higher oxidative stress than healthy controls, reinforcing its possible role in etiopathogenesis and the possible positive effectiveness of therapy with antioxidant vitamins such as vitamins C and E [START_REF] Altindag | Total antioxidant capacity and the severity of the pain in patients with fibromyalgia[END_REF].

Based on the hypothesis of possible tryptophan deficiency or metabolism disorders, one study investigated this issue by means of a nutritional intervention consisting of tryptophan depletion in patients with FM. The results showed some differences between FM patients and healthy subjects: IL-6 levels markedly increased during tryptophan depletion in FM patients but not in the control group. This increase seemed to be caused by the general setting of the amino acid beverage and not by the tryptophan depletion, as it was detected under this condition and also the sham depletion. Since both are objectively unpleasant, therefore, the IL-6 elevation may reflect the effect of acute stress [START_REF] Schwarz | Evidence for an altered tryptophan metabolism in fibromyalgia[END_REF]. An altered response of IL-6 to a physiologic stressor supported the notion that FM may represent a primary disorder of the stress system [START_REF] Torpy | Responses of the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis to interleukin-6. A pilot study in fibromyalgia[END_REF]. Otherwise we have not found any study comparing the nutritional status with the inflammatory status of these patients.

FM pain has been related to a deficiency of serotoninergic function, with a lower total plasma tryptophan concentration, which is known to be its precursor, as well as a lower ratio of plasma tryptophan to the amino acids valine, leucine, isoleucine, phenylalanine and tyrosine, which compete with tryptophan for the same cerebral uptake mechanism. However, these results have not been confirmed by other studies which evaluated them as well as plasma concentrations of the branched chain amino acids (BCCAs) [START_REF] Maes | Serotoninergic markers and lowered plasma branched-chain-amino acid concentrations in fibromyalgia[END_REF]. Due to the possible malfunction of energy metabolism of the muscle fibers of FM patients, it has been shown that a reduced plasma concentration of the branched chain amino acids, valine, leucine and phenylalanine, exists in these people and could potentially contribute to muscle energy depletion. Although these results are very interesting, more studies are needed to confirm that this deficiency might play a role in the pathophysiology of fibromyalgia, and to clarify whether it is due to a low dietary intake or not, and whether supplementation with BCCAs could help these patients.

Lower levels of melatonin were also found in FM patients compared to normal controls and, although circadian disturbances cannot be the cause of FM, melatonin replacement reduced complaints. Because of the good safety profile of melatonin, without either hang-over or tolerance development, the authors recommended considering using it as treatment, even more in cases when a deficiency may occur, such as in people suffering from FM [START_REF] Rohr | Melatonin deficiencies in women[END_REF].

Finally, we would like to highlight that although there is no a sound evidence of possible nutritional deficiencies in FM, there is enough data to encourage more investigation in this matter.

Fibromyalgia and nutritional supplementation

Since there is no therapeutic solution for the treatment of fibromyalgia, a substantial amount of non-scientific literature has arisen concerned with the potential benefits of some products based on nutritional ingredients or botanicals. However when scientific information is searched in databases such as Medline, very few studies are found.

Our search was based on the most popular nutritional remedies, and the results are summarised in table 2. The reports found were about the use of Chlorella pyreinoidosa, 5-Hydroxytryptophan (5-HTP), acetyl-L-carnitine, Coenzyme Q10, Ginkgo biloba, collagen hydrolysate, ascorbigen, and S-adenosyl-L-methionine. All of them reported possible benefits for FM patients, ameliorating symptoms such as pain, fatigue, morning stiffness, and quality of life, measured all with similar, but not equal, tests. Regarding 5-HTP we have included three articles dated before 1998, which are out of the time range of this review because they were used in Birdsall's review in 1998. At the moment, these reports are difficult to compare because of the differences in some aspects such as assessment tools, clinical variables evaluated, doses, time for supplementation, among others.

Although there is not enough evidence for all the nutritional supplements mentioned above, these studies point to the possibility of using them to reduce FM symptoms. Therefore, it is generally observed that when some nutritional supplementation is used, whatever it is, patients frequently report positive effects, mainly in pain, fatigue and capacity for carrying on their daily activities, although there is never a total recovery [START_REF] Dykman | The effects of nutritional supplements on the symptoms of fibromyalgia and chronic fatigue syndrome[END_REF]. Positive effects on chronic fatigue syndrome, but not directly on FM, have been described in some studies on different nutritional supplements [START_REF] Werbach | Nutritional strategies for treating chronic fatigue syndrome[END_REF].

Due to the possible relationship between a high oxidative stress and fibromyalgia, some of the reports reviewed in this paper have recommended more investigation about the potential effects of nutritional antioxidants supplementation. Nevertheless in our search we have not found enough studies to thoroughly evaluate this hypothesis.

Discussion

Fibromyalgia reduces the quality of life of many people in the world and is an important current public health issue. It is a chronic rheumatic disease, but its cause is unknown. There is no an effective treatment for this illness, and at the moment all the studies point to a multidimensional approach, recommending some pharmacological remedies as well as physical activity, relaxation techniques, and other tools. In response to the large amount of non-scientific information regarding the possible benefits of some diets, and food supplements for FM, the purpose of this review was to evaluate this hypothesised relationship between fibromyalgia and nutritional factors.

The data reviewed suggest that some nutritional interventions, such as those focused on the maintenance of weight within normal ranges, could help on FM symptoms, even more so when it is known that within this population the prevalence of overweight and obesity, as well as some eating disorders, are higher than in the general population. Vegetarian diets also seem to have a positive effect, but more complete studies are needed to confirm this. Regarding the use and benefits of elimination diets, based for example on identifying and avoiding potentially irritant food for these patients, we have not found any article dealing with these issues. Some deficiencies have been proposed to be involved in FM patients, although we have not found any study that globally evaluates them. We have found isolated studies which point to possible nutritional deficiencies for magnesium, selenium, zinc, iodine, iron, vitamin D, melatonin, branched chain amino acids, etc. Moreover FM is hypothesised to be associated with osteoporosis, hypothyroidism, anaemia, hyposerotoninergic function, and other metabolism disorders, although for none of them has a clear relationship been established.

The usefulness of some nutritional supplements has been evaluated in some studies, in general terms with positive results. Those we have found propose the use of some botanicals like Chlorella pyreinoidosa algae and gingko biloba, some antioxidants like coenzyme Q10 and ascorbigen, and other substances like acetyl-carnitine, collagen and S-adenosyl-L-methionine. However, none of the studies found are enough to generally recommend the use of a concrete food supplement in FM patients in order to reduce their symptoms. With the current data available it is reasonable to recommend carrying on more clinical trials to evaluate the benefits of a nutritional supplementation on this disease.

Since there is no a whole solution for the treatment of FM, and some of the studies reviewed show possible benefits of diet, it seems reasonable to manage the dietary habits of these patients, to ensure an optimal nutrient intake, avoiding possible deficiencies, and to help them maintain a healthy weight. In summary, this review reveals a possible benefit of improving dietary behaviour on normal weight maintenance in this population, and even using some nutritional supplementation to achieve an optimal nutritional status. This is encouraging, and therefore we suggest that more investigation is required in order to understand more details about the potential benefits for FM from nutrition. We conclude that, at the moment, it is necessary to give dietary advice to these patients to improve their diets and maintain a normal weight as well as good health.

We would like also to note that due to our search being restricted to only the Medline database, there could be other relevant studies not evaluated in this paper.

Table 1 . Search criteria. Search Results Medline

 1

	Subjects	Search criteria	Total	Selected
	Common key word	Fibromyalgia	4224	87
		plus
	Other key words
	Nutritional status	obesity	159	44
		nutritional status
		body mass index
		weight loss
		eating disorders
		deficiency
		depletion
		osteroporosis
		trace elements
	Nutrition and Diets		52	23
		nutrition
		diet
		food intake
	Nutritional		82	20
	supplementation
		nutritional
		supplementation
		complementary and

Table 2 . Search results for nutritional supplements and fibromyalgia.

 2

	Study	Type of	Nutritional	Dose and time n	Results or	Assessment tools
	references	study	supplemen-			Conclusions	or clinical
			tation				variables
							evaluated
	Caruso I,	Double-	5-HTP	100 mg of 5-	50 Significant	Number of tender
	et al. 1990	blind,		HTP three times		improvement	points, subjective
	[75]	placebo-		daily for 30		in symptoms,	pain severity,
		controlle		days.		including	morning stiffness,
		d study				pain,morning	sleep patterns,
						stiffness,	anxiety ratings,
						anxiety, and	fatigue ratings
						fatigue.
	Puttini PS,	Long-	5-HTP	100 mg of 5-	49 Significant	Number of tender
	et al. 1992	term,		HTP three times		improvement	points, pain
	[76]	open		daily for 90		in symptoms,	intensity, sleep
		study		days.		including	quality, morning
						pain,	stiffness, anxiety,
						morning	fatigue.
						stiffness,
						anxiety, and
						fatigue.
	Nicolodi	No data 5-HTP	400 mg/day and	200 Significant	Daily pain diary by
	M, et al.			200		pain	a Visual Analogue
	1996 [77]			mg/day+IMAO		improvement	Scale (VAS).
				for 12 months		in 5-HTP
						group and in
						the 5-
						HTP+IMAO

Acknowledgments

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

There are no conflict of interests.

Author contributions: LIA did criteria search selection, compilation, revision and selection of papers, wrote drafts and final document. MR did criteria search selection, selection of papers and document scientific revision. MAC did document methodological analysis and linguistic revision.

We thank to Professor Casimiro Javierre and Ramón Segura, from University of Barcelona, their help and advise during all the revision process to write this article. We are very grateful also to Rebecca Jeffree for her help in the linguistic revision of the text.