Alexandra Niclou

Philippe St-Martin

Leanne M Redman

PhD Audrey Bergouignan
email: audrey.bergouignan@iphc.cnrs.fr

NEW FRONTIERS IN THE MEASUREMENT OF ENERGY METABOLISM

This perspective highlights three key areas of current and future energy metabolism research: intergenerational health, climate change, and interplanetary exploration. We describe the recent advances in determining estimated energy requirements for a large subset of the general population using the gold standard method for free-living total daily energy expenditure estimates, the doubly labeled water method. The global rise in overweight and obesity demands particular attention to energy requirements in pregnancy and early life as accumulating evidence contributes to our understanding of intergenerational health transmission and the potential for epigenetic programming in utero. We also acknowledge some gaps in necessary guidelines and understandings of energy requirements for underrepresented populations (i.e., individuals from low and middle-income countries) or those who undergo major physiological changes in new environment (e.g., astronauts). The rising prevalence of excess weight gain, together with climate change, cumulate into a global syndemic exposing vulnerable populations to both malnutrition and the effects of unpredictable and severe weather events, emphasizing the need for varied energetic data accounting for rapid physiological and socio-economic changes.

Finally, we relate how specific estimated energy requirements are needed to account for the energetic challenges specific to extended space travel and ensure the success of interplanetary exploration.

Introduction

Body mass and composition directly depend on the dynamic balance between the amount of water, protein, fat, and carbohydrate that enter the body via food and water consumption and the amount of these molecules being used by the body through oxidation or metabolism. Because gains and losses to body weight influence health and disease, research in energy (and substrate) metabolism is critical.

Both energy balance and oxidative balance are involved in body weight and composition control.

Oxidative balance is the difference between the ingestion and oxidative rates of macronutrients (lipid, carbohydrate and protein). Energy substrate oxidation can be quantified by measuring changes in oxygen consumption and carbon dioxide production using indirect calorimetry along with measures of urinary nitrogen output. Energy balance is the state in which energy intake is equal to energy expenditure. Estimated energy requirements (EER) are used to predict the average dietary energy intake needed to maintain energy balance and avoid weight gains or losses. To establish accurate EER, the total number of calories burned within a given time is measured using the doubly labeled water (DLW) method. The DLW method provides free-living total daily energy expenditure measures over the course of multiple days, accounting for day-today variability in energy expenditure [START_REF] Schoeller | Measurement of energy expenditure in humans by doubly labeled water method[END_REF][START_REF] Schoeller | Energy expenditure by doubly labeled water: validation in humans and proposed calculation[END_REF][START_REF] Westerterp | Comparison of doubly labeled water with respirometry at low-and high-activity levels[END_REF][START_REF] Ainslie | Estimating human energy expenditure: a review of techniques with particular reference to doubly labelled water[END_REF]. This non-invasive method, requiring individuals to drink water enriched with deuterium and oxygen-18 and collect spot urine samples, was established in the early 1980s and has since become the gold-standard measure of total energy expenditure under daily conditions for its reliability and ease of data collection [START_REF] Schoeller | Measurement of energy expenditure in humans by doubly labeled water method[END_REF].

Body weight control relies on accurate EER predictions that can account for energetic variation between individuals and populations. In January 2023, the National Academies of Sciences, Engineering, and Medicine (NASEM) published a revised report of their recommended EER for the US and Canada (5). NASEM's new dietary reference intakes (DRI) for EER were derived from published studies and from an international compilation of DLW data governed by the International Atomic Energy Agency (IAEA) [START_REF] Speakman | The international atomic energy agency international doubly labelled water database: aims, scope and procedures[END_REF]. They are based on sex-specific equations to predict EER from birth to old age (5). Although the updated DRI report marks a turning point in the definition and use of EER to maintain energy balance that optimizes health and growth, they remain incomplete.

Global challenges such as the intergenerational transmission of health and disease, climate change, and human interplanetary travel call for comprehensive approaches to energetic variability in order to establish context specific EER and lay the groundwork for effective precision medicine. In this perspective, we provide a brief roadmap of the advances to the field of energy metabolism in the key research areas: intergenerational health, climate change and their interconnecting factors, as well as human interplanetary exploration (Figure 1).

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Beyond Childbirth: Intergenerational transmission of obesity

In 2005, the EER for pregnancy were based upon only four studies measuring total energy expenditure and six studies measuring energy deposition among pregnant women, all of whom had a body mass index between 18.5 and 25 kg/m 2 , omitting a large portion of the population (5).

The ongoing population shift towards a greater propensity for individuals becoming pregnant whilst living with overweight or obesity requires updated considerations for pregnant individuals of all body sizes [START_REF]Health fact sheets: Overweight and obese adults[END_REF][START_REF] Fryar | Prevalence of overweight, obesity, and severe obesity among children and adolescents aged 2-19 years: United States, 1963-1965 through 2017-2018[END_REF]. Included in the 2023 NASEM report is a prospective study in a cohort of pregnant females living with obesity (n=54) showing that to achieve the recommended weight gain in pregnancy (170-270g/week) energy intake should not exceed energy expenditure [START_REF] Most | Evidence-based recommendations for energy intake in pregnant women with obesity[END_REF]. NASEM now recommends EER for all females during pregnancy according to pre-pregnancy body mass index, physical activity level, week of gestation, and pregnancy weight gain recommendations (5). Indeed, there is an inverse association between the EER for pregnant individuals and pre-pregnancy BMI; individuals who enter pregnancy at a higher BMI need fewer additional calories in pregnancy to achieve a healthy gestational weight gain (5.0-9.0kg of total weight gain) compared to lean individuals (11.5-16kg of total weight gain).

The intergenerational transmission of obesity and diabetes and potential for epigenetic programming in utero and in early life promotes the need for additional succinct determinations of EER across different levels of obesity before, during, and post-pregnancy including all breastfeeding and partially or non-breastfeeding contexts. For the first time, the EER for postpartum females consider different levels of breastfeeding instead of assuming all lactating females are exclusively breastfeeding from birth to 6 and even 12 months postpartum (5). While maternal obesity exposure during pregnancy and gestational weight gain increases the risk for both mother and child to develop long-term obesity and type II diabetes, evidence suggests that human milk feeding has a protective role against obesity for the child [START_REF] Pettitt | Birth weight as a predictor of type 2 diabetes mellitus: the Ushaped curve[END_REF][START_REF] Mamun | Associations of excess weight gain during pregnancy with long-term maternal overweight and obesity: evidence from 21 y postpartum follow-up[END_REF][START_REF] Yan | The association between breastfeeding and childhood obesity: a meta-analysis[END_REF].

After evidence of an overestimation of energy expenditure and energy deposition in newborns, EER was updated for children between birth and 24 months [START_REF] Butte | Energy requirements of infants[END_REF]. The current EER for infants includes groups based on age: 0 to 2.99 months and 3 to 24 months, respectively. Notably the latter covers a large period of time and potentially misrepresents the variation in the effects of growth and development on energy expenditure [START_REF] Pontzer | & IAEA DLW Database Consortium. Daily energy expenditure through the human life course[END_REF]. The low EER of young children limits the range of error which can be tolerated to avoid accelerating detrimental weight gain trajectories [START_REF] Altazan | Unintentional error in formula preparation and its simulated impact on infant weight and adiposity[END_REF]. Furthermore, logistics of pregnancy and early life, such as timing of pregnancy discovery, increased medical care, and behavioral shifts associated with newborn care limit opportunities for longitudinal measures of energy metabolism from conception throughout postpartum.

Our understanding of the energetic demands of pregnancy and the postpartum period emphasizes the limitations of current findings and the need for more energetic data in pregnancy, the postpartum period, and prospectively in early life. In addition to examining the changes in energy balance during peripartum, a better understanding of the changes in substrate oxidation is needed to convey appropriate nutritional requirements during this period. Evidence shows that carbohydrate oxidation decreases, and lipid utilization increases in lean individuals between pregnancy and the first 6 months postpartum, especially in those lactating [START_REF] Butte | Adjustments in energy expenditure and substrate utilization during late pregnancy and lactation[END_REF][START_REF] Butte | Carbohydrate and lipid metabolism in pregnancy: normal compared with gestational diabetes mellitus[END_REF]. Among individuals with obesity and gestational diabetes mellitus, the increase in fat oxidation between early and late pregnancy is significantly greater compared to healthy individuals with obesity [START_REF] Okereke | Longitudinal changes in energy expenditure and body composition in obese women with normal and impaired glucose tolerance[END_REF]. To best characterize appropriate EER across and beyond pregnancy more investigations on substrate metabolism among different phenotypes and gestational weight gain variation during pregnancy and postpartum are needed. The future frontiers in the measurements of energy metabolism as it pertains to pregnancy will go beyond the individual and assess EER from mother-infant dyads to minimize the possible detrimental health effects of parental obesity on future generations [START_REF] Thompson | Intergenerational impact of maternal obesity and postnatal feeding practices on pediatric obesity[END_REF].

Beyond Borders: Underrepresented populations and the global malnutrition syndemic

The rapidly changing environments, be it extreme temperatures due to climate change or the growing rates of obesogenic environments in predominantly low-and middle-income countries, create a global syndemic, i.e., the association of malnutrition (undernutrition and obesity) and climate change, requiring rapid physiological adjustments [START_REF] Singer | Introduction to syndemics: a critical systems approach to public and community health[END_REF][START_REF] Swinburn | The global syndemic of obesity, undernutrition, and climate change: the Lancet Commission report[END_REF]. By increasing food and water insecurity, climate change along with rapid and continuous modifications in land use, agriculture, and food enhance the risk of malnutrition among vulnerable populations [START_REF] Thompson | Water, food, and the dual burden of disease in Galápagos, Ecuador[END_REF]. Policy inertia, economic burdens, and the exclusion of obesity from global concerns worsen the malnutrition syndemic, especially among disadvantaged groups such as low-income and nonwhite individuals [START_REF] Swinburn | The global syndemic of obesity, undernutrition, and climate change: the Lancet Commission report[END_REF]. Given the increased risk of becoming overweight later in life related to undernutrition of fetuses and infants, obesity and overweight prevalence is likely to rise quickly in these populations. [START_REF] Swinburn | The global syndemic of obesity, undernutrition, and climate change: the Lancet Commission report[END_REF]. The interdependence of physiological and socioeconomic factors that contribute to malnutrition and energy imbalance emphasizes the need for a holistic and comprehensive approach to EER and precision medicine, more broadly. The rapid global shift towards industrialization promotes an increase in obesogenic environments in turn contributing to weight again, especially among African Americans and Native populations [START_REF] Guo | Obesogenic environments and cardiovascular disease: a path analysis using US nationally representative data[END_REF][START_REF] Lake | Obesogenic environments: exploring the built and food environments[END_REF]. Extreme weather events (both heat and cold exposure) due to climate change, will continue to drive human migration towards urban centers exposing many to thermogenic stressors in addition to novel obesogenic environments compounding adjustments in energy requirements [START_REF] Delavari | Acculturation and obesity among migrant populations in high income countries-a systematic review[END_REF][START_REF] Minos | Economic growth, climate change, and obesity[END_REF][START_REF] Ocobock | Human cold adaptation: An unfinished agenda v2. 0[END_REF].

The speed of shifts in the global socio-economic landscape coupled with the rapid increase in novel environmental extremes limit the existing data on their effects on human energy expenditure and substrate utilization. The systematic underrepresentation of non-White populations in energetic studies further exacerbates the lack of understanding of the variation in physiological adaptations to these rapid changes. Controlled DLW and substrate utilization data, of which currently very little or none exist, are needed among populations facing urbanization along with nutrition and physical activity transitions to determine the etiology of obesity and support future EER in accounting for and addressing the global syndemic. Given their relative ease of use, the DLW method and indirect calorimetry represent safe and non-invasive ways to bridge the gap in our understanding of the interaction between the physical activity and nutrition in populations undergoing these transitions [START_REF] Popkin | Global nutrition dynamics: the world is shifting rapidly toward a diet linked with noncommunicable diseases[END_REF][START_REF] Bourdier | How interdisciplinary research at the crossroad between socio-cultural anthropology and nutritional physiology can help addressing the obesity epidemic[END_REF].

Beyond Earth: Human interplanetary exploration

The current use of the DLW equations may, however, fall short for individuals with specific requirements (e.g., astronauts). Spaceflight is characterized by systematic body mass loss and large inter-individual variability in EI and EE [START_REF] Wade | Body mass, energy intake, and water consumption of rats and humans during space flight[END_REF][START_REF] Matsumoto | Weight Loss in Humans in Space[END_REF][START_REF] Laurens | Revisiting the role of exercise countermeasure on the regulation of energy balance during space flight[END_REF]. Chronic energy deficit can lead to a cascade of health issues and exacerbate those already triggered by microgravity including cardiovascular deconditioning, loss of bone and muscle mass, and changes to intermediate metabolism [START_REF] Mulavara | Physiological and functional alterations after spaceflight and bed rest[END_REF][START_REF] Lang | Towards human exploration of space: the THESEUS review series on muscle and bone research priorities[END_REF]. Using the DLW method to measure EE during the 16-day LMS mission, an energy deficit of up to 5.7 ± 0.3 MJ/day was reported in four male astronauts [START_REF] Stein | Energy expenditure and balance during spaceflight on the space shuttle[END_REF]. The systematic body mass changes that were once acceptable for shorter-duration spaceflight represent a serious health risk (e.g., reductions in crew performance, increased risk of illness, severe dehydration, etc.) [START_REF] Douglas | Risk of performance decrement and crew illness due to an inadequate food system[END_REF] and are incompatible with a mission to Mars, where weight losses astronauts while onboard the ISS. Although muscle remodeling, transcriptional and molecular adaptations of peripheral tissues are likely involved, we unexpectedly found that dietary changes during spaceflight widely contribute to this shift in fuel selection (unpublished data, Bergouignan). Further investigations are needed to better understand the factors regulating energy and substrate metabolism in low or zero gravity.

Future Frontiers of Energy Metabolism

NASEM's new EER guidelines acknowledge and incorporate the changing metabolic landscapes of the US and Canada. Their advances also shed light on the limitations of current energy metabolism measures and their applicability. These include the use of physical activity level for late pregnancy and postpartum EER determination or the lack of energetic data in humans experiencing environmental and social stressor such as climate change and prolonged microgravity.

Future scientific pursuits will need to evaluate how the NASEM EER translates to low-and middle-income countries and across population subgroups and provide accurate EER for people worldwide. Although the IAEA dataset allows for a broad overview of different DLW measures across ages, ethnicities, geographies, and socio-economic contexts, many socio-ecological contexts remain understudied (e.g., early life, deep space, or extreme temperatures) and need to be included in future data collections to best represent the dynamic and intersecting environmental, social and physiological characteristics contributing to energetic requirements.

Research on energy metabolism is entering an exciting era as it ought to tackle the biggest and novel challenges in energy metabolism facing humankind; obesity, climate change, and human deep space exploration.

Figure 1 .

 1 Figure 1. Future frontiers in the measurement of energy metabolism and the underlying connecting factors contributing to variations in energetic requirements between and within populations.

of 15% or more are projected [START_REF] Mulavara | Physiological and functional alterations after spaceflight and bed rest[END_REF]. Thus, the space agencies' roadmaps have assigned inflight energy balance control a high priority for both operational and medical reasons [START_REF] Bergouignan | Towards human exploration of space: The THESEUS review series on nutrition and metabolism research priorities[END_REF].

The energy cost of exercise, the cornerstone of the countermeasure program developed to mitigate the microgravity-induced body alterations, was considered for the first time in a study measuring EER among 11 male astronauts during 6-month missions onboard the International Space Station (ISS) [START_REF] Bourdier | Effect of Exercise on Energy Expenditure and Body Composition in Astronauts Onboard the International Space Station: Considerations for Interplanetary Travel[END_REF]. It established that inflight body composition regulation is influenced by inflight activity energy expenditure, which is connected to both non-exercise and exercise physical activities. Astronauts with stable total energy expenditure (n=6) maintained their fatfree mass while losing fat mass. Those who were unable to maintain the preflight energy expenditure (n=5) lost fat-free mass but gained fat mass. The changes in fat mass resulted from energy intake adaptations uncoupled with inflight energy expenditures.

Given the significant inter-astronaut heterogeneity in energy balance and other underlying issues impacting astronauts' energy balance, EER for space travel cannot be determined using general population equations. Monitoring in-flight changes in body composition, calorie intake, and energy expenditure is essential for guaranteeing astronaut performance and the success of future exploratory missions. The next stages should be to identify the DRI for both male and female astronauts of various age groups for prolonged space missions and assess the energy cost and impact on the energy balance of the countermeasure, such as exercise, in the astronauts' deep space trip program. The ideal exercise parameters and other possible countermeasures, and the tolerable body mass, fat mass and fat-free mass loss thresholds will need to be defined. As a variety of factors, such as limited food availability, contribute to the loss of body mass [START_REF] Douglas | Space food for thought: challenges and considerations for food and nutrition on exploration missions[END_REF], it is essential to investigate the nutritional value of the food produced in space and how well it is assimilated if we want to consider the potential of human existence on another planet.

Ultimately, EER for humans living on planets with lower gravity (e.g., 0.38 on Mars and 0.17 on the Moon) will need to be established.

Research is also required on the mechanisms underlying inflight energy imbalance, including regulation of energy expenditures, feeding behavior and appetite, mechanical and metabolic efficiencies, and substrate use. Human ground-based studies repeatedly reported alterations in substrate oxidation during simulated microgravity, with a shift toward carbohydrate use as fuel and to the detriment of fat oxidation [START_REF] Blanc | Fuel homeostasis during physical inactivity induced by bed rest[END_REF][START_REF] Bergouignan | Physical inactivity differentially alters dietary oleate and palmitate trafficking[END_REF][START_REF] Le Roux | Physiology of physical inactivity, sedentary behaviours and non-exercise activity: insights from the space bedrest model[END_REF]. Recently, we observed such a fuel shift in

Author Contributions

A.N. and P.S. drafted manuscript; A.N. prepared figure; A.N., P.S., L.R., and A.B. edited and revised manuscript; A.N., P.S., L.R., and A.B. approved final version of manuscript.