Influence of mild cold on 24 h energy expenditure, resting metabolism and diet-induced thermogenesis

High daily energy expenditure of Tuvan nomadic pastoralists living in an extreme cold environment

Research investigating thermoregulatory energy costs in free-living humans is limited. We determined the total energy expenditure (TEE) of Tuvan pastoralists living in an extreme cold environment and explored the contribution of physical activity and cold-induced thermogenesis. Twelve semi-nomadic pastoralists (47 ± 8 years, 64 ± 8 kg) living under traditional circumstances, in Tuva, south-central Siberia, Russia, were observed during two consecutive 6-day periods in winter. TEE was measured via the doubly labelled water technique. Skin and ambient temperatures, and physical activity were continuously monitored. The outdoor temperature during the observation period was - 27.4 ± 5.4 °C. During the daytime, the participants were exposed to ambient temperatures below 0 °C for 297 ± 131 min/day. The Tuvan pastoralists were more physically active compared to western populations (609 ± 90 min/day of light, moderate, and vigorous physical activity). In addition, TEE was 13.49 ± 1.33 MJ/day (3224 ± 318 kcal/day), which was significantly larger by 17% and 31% than predicted by body mass, and fat-free mass, respectively. Our research suggests the daily cold exposure combined with high levels of physical activity contributed to the elevated TEE. Future research should reconsider the assumption that energy costs due to thermoregulation are negligible in free-living humans.

Effect of temperature and/or sweetness of beverages on body composition in rats

Sweetened beverages are mainly consumed cold and various processes are activated in response to external temperature variations. However, the effect of internal temperature variations through the ingestion of cold beverages is far from clear. Two experiments were conducted to investigate the effect of beverage temperature on body composition. Sprague-Dawley rats (5-6-week-old males) had free access to food and beverage for 8 weeks. Energy intake, body weight and body composition were monitored. In Expt 1, two groups of rats (n 9) consumed water at room temperature (NW about 22°C) or cold (CW about 4°C). In Expt 2, rats were offered room-temperature (N) or cold (C) sweetened water (10 % sucrose CSu (n 7) and NSu (n 8); or 0·05 % acesulfame K CAk (n 6) and NAk (n 8)) for 12 h, followed by plain water. Our results show that in Expt 1, CW had higher lean body mass (P < 0·001) and lower body fat gain (P = 0·004) as compared with NW. In Expt 2, body weight (P = 0·013) and fat (P ≤ 0·001) gains were higher in the non-energetic sweetened groups, while lean body mass was not affected by the type of sweeteners or temperature. In conclusion, cold water ingestion improved lean body mass gain and decreased fat gain because of increased energy expenditure, while non-energetic sweetener (acesulfame K) increased body fat gain due to improved energy efficiency. Internal cold exposure failed to increase energy intake in contrast to that of external cold exposure.

The Effect of the Temperature of Plain or Sweetened Water on Body Composition in Rats

INTRODUCTION

Humans are known to adapt to external temperature variations by altering energy intake, expenditure, and body fat storage for insulation [<xref ref-type="bibr" rid="ref1">1</xref>, <xref ref-type="bibr" rid="ref2">2</xref>]. However, it is not clear whether the temperature of ingested water would induce such effects. Similarly, the involvement of the temperature of the ingested beverage has not been addressed in terms of body weight changes [<xref ref-type="bibr" rid="ref3">3</xref>].

OBJECTIVES

This study was to investigate the effect of the ingestion of plain or sweetened water with varied temperatures on growth measures of rats.

METHODS

Approval was obtained from the Institutional Animal Care and Use Committee of the American University of Beirut. After a 1-week adaptation period, 5- to 6-week-old male Sprague-Dawley rats were randomly divided into their respective experimental groups, housed individually (22 ± 1°C, reverse light cycle 12:12 h dark/light, light off at 10:00 a.m.) with free access to food and beverage for 8 weeks. Experiment 1 (Plain Water): Two groups of rats (n = 9) consumed room-temperature [∼22°C] (NW) or cold [∼5°C] (CW) water. Experiment 2 (Sweetened Water): Four groups of rats were offered sweetened water for 12 h, followed by plain water; (1) 10% sucrose + cold temperature (CS, n = 7), (2) 10% sucrose + room temperature (NS, n = 8), (3) 0.05% acesulfame K + cold temperature (CA, n = 7), and 4) 0.05% acesulfame K + room temperature (NA, n = 8). Food and beverage intake, body weight, and body composition were monitored using NMR minispec (LF110 Body Composition Analyzer, Bruker, USA) and energy expenditure was calculated based on the equation developed by Ravussin et al. [<xref ref-type="bibr" rid="ref4">4</xref>]. Significance was set at a p value <0.05.

RESULTS

Experiment 1: Body weight changes were similar between groups (Fig. <xref ref-type="fig" rid="f01">1</xref>-Exp 1a). In the CW group, lean body mass (%) was significantly higher, while body fat (%) was lower than the NW (Fig. <xref ref-type="fig" rid="f01">1</xref>-Exp 1b, c). These changes may relate to the calculated total energy expenditure [NW: 66.73 ± 4.49 kcal/day and CW: 73.75 ± 3.92 kcal/day) (p value = 0.003) since energy intake (NW: 89.97 ± 7.63 kcal/day vs. CW: 93.29 ± 6.26 kcal/day, p value = 0.329) was similar between groups. Experiment 2: Body weight of the CA group was higher than that of the other groups (Fig. <xref ref-type="fig" rid="f01">1</xref>-Exp 2a). Lean body mass (%) of the sucrose-sweetened water groups (Fig. <xref ref-type="fig" rid="f01">1</xref>-Exp 2b, c) was significantly higher, while body fat (%) was lower than that of the non-caloric sweetened water groups; these were not affected by the temperature of the beverage. Those variations are mostly explained by the differences in energy expenditure (p value temperature × sweetener = 0.015), as energy intake was not significantly different between groups.

CONCLUSION

Cold plain water decreased body fat and increased lean body mass with no effect on total body weight. Sucrose-sweetened water had a better impact on body composition irrespective of the temperature of the beverage. The beneficial effects are mainly due to increased energy expenditure rather than variations in energy intake. Thus, the energy cost of warming the water seems to have been derived from an increase in fat oxidation.

The influence of a moderate temperature drift on thermal physiology and perception

Humans spend approximately 80-90% of their time indoors. In current practice, indoor temperatures in many buildings are controlled very tightly. However, allowing more variation in indoor temperature results in more energy-efficient buildings and could potentially improve human metabolic and cardiovascular health. Therefore, this study aimed to evaluate the effect of a drifting ambient temperature versus a fixed ambient temperature on thermal physiological parameters and subjective perception. A cross-over intervention design was conducted in 16 healthy men (age 26 ± 4 y; BMI 23.0 ± 1.7 kg/m²) between July 2018 and May 2019. All participants underwent two whole-day (8:30-17:00) experimental sessions, during which they were exposed to a drifting (17-25°C with a morning ramp of 2.58°C/h and afternoon ramp of -2.58°C/h) or constant ambient temperature (21°C) in randomized order. The experiments took place in respiratory chambers, which simulated a typical office environment and in which temperature conditions can be controlled accurately. Throughout the experimental sessions core and skin temperature, heart rate, blood pressure, energy expenditure as well as activity levels were measured. Subjective thermal perception, such as thermal comfort and sensation, was assessed by questionnaires every 30 min. Results reveal that energy expenditure was higher in the morning during the drifting session, which was accompanied by an increase in activity levels. Both drifting and fixed sessions were judged as comfortable although during the drift thermal comfort was lower in the morning and afternoon and higher during midday. The results indicate that a drifting ambient temperature can be applied in practice, and as such, can contribute to a healthier and more sustainable built environment. More research is needed to understand the role of a drifting temperature on the long term.

Accuracy of resting metabolic rate prediction equations among healthy adults in Trinidad and Tobago

BACKGROUND

Over 50% of adults in Latin America and the Caribbean have a body mass index (BMI) ≥ 25 suggesting excess energy intakes relative to energy expenditure. Accurate estimation of resting metabolic rate (RMR), the largest component of total energy requirements, is crucial to strategies aimed at reducing the prevalence and incidence of overweight and obesity.

AIM

We evaluated the accuracies of established and locally developed RMR prediction equations (RMR_P) among adults.

METHODS

Four hundred adult volunteers ages 20 to 65 years had RMR measured (RMR_M) with a MedGem® indirect calorimeter according to recommended procedures. RMR_P were compared to RMR_M with values ± 10% of RMR_M deemed accurate. Anthropometry was measured using standard procedure. Linear regression with bootstrap analyses was used to develop local RMR_P equations based on anthropometric and demographic variables. The University of the West Indies Ethics Committee approved the study.

RESULTS

Males had higher mean absolute RMR (p < 0.001) but similar mean age-adjusted measured RMR per kg of body (20.9 vs. 21.5 kcals/day; p = 0.1) to females. The top performing established anthropometry-based RMR_P among participants by sex, physical activity (PA) level and BMI status subgroups were Mifflin-St Jeor, Owen, Korth, Harris-Benedict, and Livingston, while Johnstone, Cunningham, Müller (body composition (BC)), Katch and McArdle, Mifflin-St Jeor (BC) were the most accurate BC-based RMR_P. Locally developed RMR_P had accuracies comparable to their top-ranked established RMR_P counterparts.

CONCLUSIONS

Accuracies of established RMR_P depended on habitual PA level, BMI status, BC and sex. Furthermore, locally developed RMR_P provide useful alternatives to established RMR_P.

Does global warming contribute to the obesity epidemic?

Endotherms must expend more energy to digest colder food, so they acquire more calories by eating food at a higher temperature than eating the identical food cold. A recent study shows that ownership of a microwave is associated with a small increase in BMI and obesity. The same logic applies to other substances that endotherms introduce into their bodies, like air. An analysis of the National Longitudinal Study of Adolescent to Adult Health (Add Health) shows that, net of sex, age, race, education, earnings, neighborhood characteristics, and exercise activities, atmospheric temperature is associated with small but statistically significant increases in BMI, weight, overweight, and obesity. Atmospheric temperature is more strongly associated than most exercise activities, and as strongly associated as age and population density. An average American might reduce weight by 15.1 lbs, BMI by 2.52 (half the difference between normal weight and obesity), and the odds of obesity by 54% by moving from Phoenix, AZ, to Barrow, AK, or, less dramatically, 5.7 lbs in weight, .95 in BMI (a fifth of the difference between normal weight and obesity), and 25% in the odds of obesity by moving mere 150 miles north to Flagstaff, AZ. Global warming under the worst-case scenario might produce an increase of 2.2 lbs in weight, .37 in BMI, and 12% in odds of obesity from 1961 to 2081.

Humans in the cold: Regulating energy balance

For humans to maintain a stable core temperature in cold environments, an increase in energy expenditure (EE) is required. However, little is known about how cold stimulus impacts energy balance as a whole, as energy intake (EI) has been largely overlooked. This review focuses on the current state of knowledge regarding how cold exposure (CE) impacts both EE and EI, while highlighting key gaps and shortcomings in the literature. Animal models clearly reveal that CE produces large increases in EE, while decreasing environmental temperatures results in a significant negative dose-response effect in EI (r=-.787, P<.001), meaning animals eat more as temperature decreases. In humans, multiple methods are used to administer cold stimuli, which result in consistent yet quantitatively small increases in EE. However, only two studies have measured ad libitum food intake in combination with acute CE in humans. Chronic CE (i.e., cold acclimation) studies have been shown to produce minimal changes in body weight, with an average compensation of ~126%. Although more studies are required to investigate how cold impacts EI in humans, results presented in this review warrant caution before presenting or considering CE as a potential adjunct to weight loss strategies.

The effects of exercise and ambient temperature on dietary intake, appetite sensation, and appetite regulating hormone concentrations

BACKGROUND

It is not clear whether the frequently reported phenomenon of exercise-induced anorexia is exacerbated or blunted in warm or cold environments. Therefore, this study investigated the effects of exercise in three different environmental temperatures vs. rest, on perceptions of appetite, appetite regulating hormones, and food intake.

METHODS

In a randomized repeated-measures design, 18 Canadian Armed Forces members (14 male, 4 female) completed four 8-h trials in a thermally-controlled chamber: one 8-h resting trial at 21 °C (Sedentary); and three trials where participants completed two 2-h circuits of standardized military tasks interspersed with two 2-h rest periods, once at 30 °C (Hot), once at 21 °C (Temperate), and once at - 10 °C (Cold). Participants consumed military field rations ad libitum and had their appetite assessed with visual analogue scales. Plasma concentrations of GLP-1, PYY, acylated ghrelin, and leptin were also determined.

RESULTS

Appetite was perceived as being suppressed in the heat compared to the cold (p < 0.05). While neither exercise nor environmental temperature altered circulating GLP-1 levels, exercise in all environments increased blood concentrations of PYY (p < 0.05). Leptin concentrations were elevated in the heat and diminished in the cold (p < 0.05), and acylated ghrelin concentrations were affected by both exercise and ambient temperature resulting in Sedentary = Cold>Temperate = Hot (p < 0.05). Contrary to the changes in appetite perceptions and hormonal concentrations, dietary intake was not different between conditions (p > 0.05). Relative energy intake (total 24 h energy intake minus 24 h energy expenditure) on the other hand, was significantly higher during the Sedentary condition than it was during any of the active conditions (p < 0.05). Most (83%) of the participants were in a positive energy balance during the Sedentary condition, whereas during most (80%) of the active conditions (Hot, Temperate, Cold) participants were in a negative energy balance.

CONCLUSIONS

In this study where food was freely available, variations in ambient temperature, exercise vs. rest, appetite-regulating hormone concentrations, and subjective appetite sensation were not associated with any changes in dietary intake within 24-h of acute, prolonged exercise.

Compensation for cold-induced thermogenesis during weight loss maintenance and regain

Prevalence of obesity is exacerbated by low rates of successful long-term weight loss maintenance (WLM). In part, relapse from WLM to obesity is due to a reduction in energy expenditure (EE) that persists throughout WLM and relapse. Thus, interventions that increase EE might facilitate WLM. In obese mice that were calorically restricted to reduce body weight by ~20%, we manipulated EE throughout WLM and early relapse using intermittent cold exposure (ICE; 4°C, 90 min/day, 5 days/wk, within the last 3 h of the light cycle). EE, energy intake, and spontaneous physical activity were measured during the obese, WLM, and relapse phases. During WLM and relapse, the ICE group expended more energy during the light cycle because of cold exposure but expended less energy in the dark cycle, which led to no overall difference in total daily EE. The compensation in EE appeared to be mediated by activity, whereby the ICE group was more active during the light cycle because of cold exposure but less active during the dark cycle, which led to no overall effect on total daily activity during WLM and relapse. In brown adipose tissue of relapsing mice, the ICE group had greater mRNA expression of Dio2 and protein expression of UCP1 but lower mRNA expression of Prdm16. In summary, these findings indicate that despite robust increases in EE during cold exposures, ICE is unable to alter total daily EE during WLM or early relapse, likely due to compensatory behaviors in activity.

Translational Pharmacology and Physiology of Brown Adipose Tissue in Human Disease and Treatment

Human brown adipose tissue (BAT) is experimentally modeled to better understand the biology of this important metabolic tissue, and also to enable the potential discovery and development of novel therapeutics for obesity and sequelae resulting from the persistent positive energy balance. This chapter focuses on translation into humans of findings and hypotheses generated in nonhuman models of BAT pharmacology. Given the demonstrated challenges of sustainably reducing caloric intake in modern humans, potential solutions to obesity likely lie in increasing energy expenditure. The energy-transforming activities of a single cell in any given tissue can be conceptualized as a flow of chemical energy from energy-rich substrate molecules into energy-expending, endergonic biological work processes through oxidative degradation of organic molecules ingested as nutrients. Despite the relatively tight coupling between metabolic reactions and products, some expended energy is incidentally lost as heat, and in this manner a significant fraction of the energy originally captured from the environment nonproductively transforms into heat rather than into biological work. In human and other mammalian cells, some processes are even completely uncoupled, and therefore purely energy consuming. These molecular and cellular actions sum up at the physiological level to adaptive thermogenesis, the endogenous physiology in which energy is nonproductively released as heat through uncoupling of mitochondria in brown fat and potentially skeletal muscle. Adaptive thermogenesis in mammals occurs in three forms, mostly in skeletal muscle and brown fat: shivering thermogenesis in skeletal muscle, non-shivering thermogenesis in brown fat, and diet-induced thermogenesis in brown fat. At the cellular level, the greatest energy transformations in humans and other eukaryotes occur in the mitochondria, where creating energetic inefficiency by uncoupling the conversion of energy-rich substrate molecules into ATP usable by all three major forms of biological work occurs by two primary means. Basal uncoupling occurs as a passive, general, nonspecific leak down the proton concentration gradient across the membrane in all mitochondria in the human body, a gradient driving a key step in ATP synthesis. Inducible uncoupling, which is the active conduction of protons across gradients through processes catalyzed by proteins, occurs only in select cell types including BAT. Experiments in rodents revealed UCP1 as the primary mammalian molecule accounting for the regulated, inducible uncoupling of BAT, and responsive to both cold and pharmacological stimulation. Cold stimulation of BAT has convincingly translated into humans, and older clinical observations with nonselective 2,4-DNP validate that human BAT's participation in pharmacologically mediated, though nonselective, mitochondrial membrane decoupling can provide increased energy expenditure and corresponding body weight loss. In recent times, however, neither beta-adrenergic antagonism nor unselective sympathomimetic agonism by ephedrine and sibutramine provide convincing evidence that more BAT-selective mechanisms can impact energy balance and subsequently body weight. Although BAT activity correlates with leanness, hypothesis-driven selective β3-adrenergic agonism to activate BAT in humans has only provided robust proof of pharmacologic activation of β-adrenergic receptor signaling, limited proof of the mechanism of increased adaptive thermogenesis, and no convincing evidence that body weight loss through negative energy balance upon BAT activation can be accomplished outside of rodents. None of the five demonstrably β3 selective molecules with sufficient clinical experience to merit review provided significant weight loss in clinical trials (BRL 26830A, TAK 677, L-796568, CL 316,243, and BRL 35135). Broader conclusions regarding the human BAT therapeutic hypothesis are limited by the absence of data from most studies demonstrating specific activation of BAT thermogenesis in most studies. Additionally, more limited data sets with older or less selective β3 agonists also did not provide strong evidence of body weight effects. Encouragingly, β3-adrenergic agonists, catechins, capsinoids, and nutritional extracts, even without robust negative energy balance outcomes, all demonstrated increased total energy expenditure that in some cases could be associated with concomitant activation of BAT, though the absence of body weight loss indicates that in no cases did the magnitude of negative energy balance reach sufficient levels. Glucocorticoid receptor agonists, PPARg agonists, and thyroid hormone receptor agonists all possess defined molecular and cellular pharmacology that preclinical models predicted to be efficacious for negative energy balance and body weight loss, yet their effects on human BAT thermogenesis upon translation were inconsistent with predictions and disappointing. A few new mechanisms are nearing the stage of clinical trials and may yet provide a more quantitatively robust translation from preclinical to human experience with BAT. In conclusion, translation into humans has been demonstrated with BAT molecular pharmacology and cell biology, as well as with physiological response to cold. However, despite pharmacologically mediated, statistically significant elevation in total energy expenditure, translation into biologically meaningful negative energy balance was not achieved, as indicated by the absence of measurable loss of body weight over the duration of a clinical study.

Modulation of thermogenesis and metabolic health: a built environment perspective

Lifestyle interventions, obviating the increasing prevalence of the metabolic syndrome, generally focus on nutrition and physical activity. Environmental factors are hardly covered. Because we spend on average more that 90% of our time indoors, it is, however, relevant to address these factors. In the built environment, the attention has been limited to the (assessment and optimization of) building performance and occupant thermal comfort for a long time. Only recently well-being and health of building occupants are also considered to some extent, but actual metabolic health aspects are not generally covered. In this review, we draw attention to the potential of the commonly neglected lifestyle factor 'indoor environment'. More specifically, we review current knowledge and the developments of new insights into the effects of ambient temperature, light and the interaction of the two on metabolic health. The literature shows that the effects of indoor environmental factors are important additional factors for a healthy lifestyle and have an impact on metabolic health.

Fasting and glucose induced thermogenesis in response to three ambient temperatures: a randomized crossover trial in the metabolic syndrome

BACKGROUND/OBJECTIVES

Cold exposure increases thermogenesis and could improve insulin sensitivity. We hypothesized a blunted response in the metabolic syndrome (MetS).

SUBJECTS/METHODS

Twenty older adults 59 ± 10.4 years (with MetS, MetS+, n = 9; without MetS, MetS-, n = 11) completed a randomized crossover design of 3.5 h exposures to 20, 25 and 27 °C on three visits. After an hour's rest at the desired temperature, resting metabolic rate (RMR), respiratory quotient (RQ), forearm to fingertip gradients (FFG), and in the ear temperature (IET) were measured over 30 min. An oral glucose tolerance test followed, and serial measurements were continued for 2 h. Venous blood was sampled for clinical chemistry, irisin, and fibroblast growth factor 21(FGF21). A mixed model ANCOVA adjusted data for age, gender, fat mass, fat-free mass and seasonality.

RESULTS

There was a significant MetS×temperature interaction where adjusted RMR was significantly higher in MetS+ compared to MetS- by 12% at 20 °C and by 6% at 25 °C, but similar at 27 °C. FFG increased and IET decreased with decreasing temperature to the same extent in both groups. Fasting irisin and FGF21 did not vary with temperature but the former was significantly higher in MetS-. Adjusted postprandial RQ and insulin to glucose ratios were significantly higher at 20 °C relative to 25 °C. Partial correlation analysis of differences between 27 and 20 °C indicated significant positive relationships between fasting as well as postprandial RQ and the respective changes in irisin and FGF21.

CONCLUSIONS

There could be an upward shift of the TNZ in MetS+, but this needs reevaluation.

Cold-induced thermogenesis in humans

A basic property of endothermic thermoregulation is the ability to generate heat by increasing metabolism in response to cold ambient temperatures to maintain a stable core body temperature. This process, known as cold-induced thermogenesis (CIT), has been measured in humans as early as 1780 by Antoine Lavoisier, but has found renewed interest because of the recent 'rediscovery' of thermogenic, cold-activated brown adipose tissue (BAT) in adult humans. In this review, we summarize some of the key findings of the work involving CIT over the past two centuries and highlight some of the seminal studies focused on this topic. There has been a substantial range of variability in the reported CIT in these studies, from 0 to 280% above basal metabolism. We identify and discuss several potential sources of this variability, including both methodological (measurement device, cold exposure temperature and duration) and biological (age and body composition of subject population) discrepancies. These factors should be considered when measuring CIT going forward to better assess whether BAT or other thermogenic organs are viable targets to combat chronic positive energy balance based on their relative capacities to elevate human metabolism.

Combatting type 2 diabetes by turning up the heat

In our westernised society, the level of physical activity is low. Interventions that increase energy expenditure are generally associated with an improvement in metabolic health. Exercise and exercise training increase energy metabolism and are considered to be among the best strategies for prevention of type 2 diabetes mellitus. More recently, cold exposure has been suggested to have a therapeutic value in type 2 diabetes. At a cellular level, there is evidence that increasing the turnover of cellular substrates such as fatty acids is associated with preventive effects against lipid-induced insulin resistance. Cellular energy sensors may underlie the effects linking energy turnover with metabolic health effects. Here we review data supporting the hypothesis that increasing energy and substrate turnover has beneficial effects on insulin sensitivity and should be considered a target for the prevention and treatment of type 2 diabetes.

Contributors to Metabolic Disease Risk Following Spinal Cord Injury

Spinal cord injury (SCI) induced changes in neurological function have significant impact on the metabolism and subsequent metabolic-related disease risk in injured individuals. This metabolic-related disease risk relationship is differential depending on the anatomic level and severity of the injury, with high level anatomic injuries contributing a greater risk of glucose and lipid dysregulation resulting in type 2 diabetes and cardiovascular disease risk elevation. Although alterations in body composition, particularly excess adiposity and its anatomical distribution in the visceral depot or ectopic location in non-adipose organs, is known to significantly contribute to metabolic disease risk, changes in fat mass and fat-free mass do not fully account for this elevated disease risk in subjects with SCI. There are other negative adaptations in body composition including reductions in skeletal muscle mass and alterations in muscle fiber type, in addition to significant reduction in physical activity, that contribute to a decline in metabolic rate and increased metabolic disease risk following SCI. Recent studies in adult humans suggest cold- and diet-induced thermogenesis through brown adipose tissue metabolism may be important for energy balance and substrate metabolism, and particularly sensitive to sympathetic nervous signaling. Considering the alterations that occur in the autonomic nervous system (SNS) (sympathetic and parasympathetic) following a SCI, significant dysfunction of brown adipose function is expected. This review will highlight metabolic alterations following SCI and integrate findings from brown adipose tissue studies as potential new areas of research to pursue.

Ambient Temperature and Prevalence of Obesity: A Nationwide Population-Based Study in Korea

Background

Recent studies have suggested a possible association between outdoor or indoor temperature and obesity. We aimed to examine whether ambient temperature is associated with the prevalence of obesity or abdominal obesity in the Korean population.

Methods

Data on anthropometric, socio-demographic, laboratory and lifestyle factors were retrieved from National Health Insurance System data obtained in 2009–2010. Thirty years (1981 to 2010) of meteorological parameters for 71 observation areas were acquired from the Korea Meteorological Administration. Included in this analysis were 124,354 individuals. A body mass index (BMI) ≥ 25 kg/m² and a waist circumference (WC) ≥ 90 cm (men) or 85 cm (women) were considered to represent obesity and abdominal obesity, respectively.

Results

The mean annual temperature (MAT) ranged from 6.6°C to 16.6°C, and BMI was positively correlated with MAT (r = 0.0078, P = 0.0065). WC was positively correlated with MAT (r = 0.0165, P < 0.0001) and negatively correlated with the number of days with mean temperature < 0°C (DMT0; r = –0.0129, P = 0.0002). After adjusting for age, sex, smoking status, alcohol consumption, exercise, income, residential area and altitude, the odds ratios (95% CI) for obesity and abdominal obesity in the highest quintile MAT group were 1.045 (1.010, 1.081) and 1.082 (1.042, 1.124), respectively, compared with the lower four quintiles of the MAT group. Similarly, subjects in the area of the lowest quintile of DMT0 had significantly higher odds of abdominal obesity compared with the higher four quintile groups of DMT0.

Conclusion

This study finds an association between ambient temperature and prevalence of obesity in the Korean population when controlling for several confounding factors. Adaptive thermogenesis might be a possible explanation for this phenomenon.

Brown adipose tissue in the treatment of obesity and diabetes: Are we hot enough?

The identification of functional brown adipose tissue in human adults has intensified interest in exploiting thermogenic energy expenditure for the purpose of weight management. However, food intake and energy expenditure are tightly regulated and it is generally accepted that variation in one component results in compensatory changes in the other. In the context of weight loss, additional biological adaptations occur in an attempt to further limit weight loss. In the present review, we discuss the relationship between increasing energy expenditure and body weight in humans, including the effects of cold exposure. The data raise the possibility that some processes, particularly those involved in thermogenesis, induce less compensatory food intake for a given magnitude of additional energy expenditure, a state we term the ‘thermogenic disconnect’. Although cold exposure increases thermogenesis and can putatively be exploited to induce weight loss, there are multiple adaptive responses to cold, of which many actually reduce energy expenditure. In order to optimally exploit either cold itself or agents that mimic cold for thermogenic energy expenditure, these non‐thermogenic cold responses must be considered. Finally, the relative contribution of brown adipose tissue vs other thermogenic processes in humans remains to be defined. However, overall the data suggest that activation of cold‐induced thermogenic processes are promising targets for interventions to treat obesity and its secondary metabolic complications. (J Diabetes Invest, doi:10.1111/j.2040‐1124.2011.00158.x, 2011)

The chemical uncoupler 2,4-dinitrophenol (DNP) protects against diet-induced obesity and improves energy homeostasis in mice at thermoneutrality

The chemical uncoupler 2,4-dinitrophenol (DNP) was an effective and widely used weight loss drug in the early 1930s. However, the physiology of DNP has not been studied in detail because toxicity, including hyperthermia and death, reduced interest in the clinical use of chemical uncouplers. To investigate DNP action, mice fed a high fat diet and housed at 30 °C (to minimize facultative thermogenesis) were treated with 800 mg/liter DNP in drinking water. DNP treatment increased energy expenditure by ∼ 17%, but did not change food intake. DNP-treated mice weighed 26% less than controls after 2 months of treatment due to decreased fat mass, without a change in lean mass. DNP improved glucose tolerance and reduced hepatic steatosis without observed toxicity. DNP treatment also reduced circulating T3 and T4 levels, Ucp1 expression, and brown adipose tissue activity, demonstrating that DNP-mediated heat generation substituted for brown adipose tissue thermogenesis. At 22 °C, a typical vivarium temperature that is below thermoneutrality, DNP treatment had no effect on body weight, adiposity, or glucose homeostasis. Thus, environmental temperature should be considered when assessing an anti-obesity drug in mice, particularly agents acting on energy expenditure. Furthermore, the beneficial effects of DNP suggest that chemical uncouplers deserve further investigation for the treatment of obesity and its comorbidities.

Systemic disease sequelae in chronic inflammatory diseases and chronic psychological stress: comparison and pathophysiological model

In chronic inflammatory diseases (CIDs), the neuroendocrine-immune crosstalk is important to allocate energy-rich substrates to the activated immune system. Since the immune system can request energy-rich substrates independent of the rest of the body, I refer to it as the "selfish immune system," an expression that was taken from the theory of the "selfish brain," giving the brain a similar position. In CIDs, the theory predicts the appearance of long-term disease sequelae, such as metabolic syndrome. Since long-standing energy requirements of the immune system determine disease sequelae, the question arose as to whether chronic psychological stress due to chronic activation of the brain causes similar sequelae. Indeed, there are many similarities; however, there are also differences. A major difference is the behavior of body weight (constant in CIDs versus loss or gain in stress). To explain this discrepancy, a new pathophysiological theory is presented that places inflammation and stress axes in the middle.

The association between indoor temperature and body mass index in children: the PIAMA birth cohort study

Background

Several experimental studies showed consistent evidence for decreased energy expenditure at higher ambient temperatures. Based on this, an association between thermal exposure and body weight may be expected. However, the effect of thermal exposure on body weight has hardly been studied. Therefore, this study investigated the association between indoor temperature and body mass index (BMI) in children in real life.

Methods

This longitudinal observational study included 3 963 children from the Dutch Prevention and Incidence of Asthma and Mite Allergy (PIAMA) birth cohort that started in 1996. These children were followed from birth until the age of 11 years. Winter indoor temperature (living room and bedroom) was reported at baseline and BMI z-scores were available at 10 consecutive ages. Missing data were multiply imputed. Associations between indoor temperature and BMI were analyzed using generalized estimating equations (GEE), adjusted for confounders and stratified by gender. In a subgroup of 104 children, bedroom temperature was also measured with data loggers.

Results

Mean reported living room and bedroom temperature were 20.3°C and 17.4°C, respectively. Reported and measured bedroom temperatures were positively correlated (r = 0.42, p = 0.001).

Neither reported living room temperature (-0.03 ≤ β ≥ 0.04) and bedroom temperature (-0.01 ≤ β ≥ 0.02) nor measured bedroom temperature (-0.04 ≤ β ≥ 0.05) were associated with BMI z-score between the age of 3 months and 11 years.

Conclusions

This study in children did not support the hypothesized association between indoor temperature and BMI in a real life setting.

Brown fat activation mediates cold-induced thermogenesis in adult humans in response to a mild decrease in ambient temperature

CONTEXT

The contribution of brown adipose tissue (BAT) to the energy balance in humans exposed to sustainable cold has not been completely established, partially because of measurement limitations of both BAT activity and energy expenditure (EE).

OBJECTIVE

The objective of the study was to characterize the role of BAT activation in cold-induced thermogenesis (CIT).

DESIGN

This study was a single-blind, randomized crossover intervention.

SETTING

The study was conducted at the National Institutes of Health Clinical Center.

STUDY PARTICIPANTS

Thirty-one healthy volunteers participated in the study.

INTERVENTIONS

The intervention included mild cold exposure.

MAIN OUTCOMES

CIT and BAT activation were the main outcomes in this study.

METHODS

Overnight EE measurement by whole-room indirect calorimeter at 24 °C or 19 °C was followed by 2-[18F]-fluoro-2-deoxy-D-glucose positron emission tomography (PET) scan. After 36 hours, volunteers crossed over to the alternate study temperature under identical conditions. BAT activity was measured in a 3-dimensional region of interest in the upper torso by comparing the uptake at the two temperatures.

RESULTS

Twenty-four volunteers (14 males, 10 females) had a complete data set. When compared with 24 °C, exposure at 19 °C resulted in increased EE (5.3 ± 5.9%, P < .001), indicating CIT response and mean BAT activity (10.5 ± 11.1%, P < .001). Multiple regression analysis indicated that a difference in BAT activity (P < .001), age (P = .01), and gender (P = .037) were independent contributors to individual variability of CIT.

CONCLUSIONS

A small reduction in ambient temperature, within the range of climate-controlled buildings, is sufficient to increase human BAT activity, which correlates with individual CIT response. This study uncovers for the first time a spectrum of BAT activation among healthy adults during mild cold exposure not previously recognized by conventional PET and PET-computed tomography methods. The enhancement of cold-induced BAT stimulation may represent a novel environmental strategy in obesity treatment.

Historic Variations in Winter Indoor Domestic Temperatures and Potential Implications for Body Weight Gain

It has been argued that the amount of time spent by humans in thermoneutral environments has increased in recent decades. This paper examines evidence of historic changes in winter domestic temperatures in industrialised countries. Future trajectories for indoor thermal comfort are also explored. Whilst methodological differences across studies make it difficult to compare data and accurately estimate the absolute size of historic changes in indoor domestic temperatures, data analysis does suggest an upward trend, particularly in bedrooms. The variations in indoor winter residential temperatures might have been further exacerbated in some countries by a temporary drop in demand temperatures due to the 1970s energy crisis, as well as by recent changes in the building stock. In the United Kingdom, for example, spot measurement data indicate that an increase of up to 1.3°C per decade in mean dwelling winter indoor temperatures may have occurred from 1978 to 1996. The findings of this review paper are also discussed in the context of their significance for human health and well-being. In particular, historic indoor domestic temperature trends are discussed in conjunction with evidence on the links between low ambient temperatures, body energy expenditure and weight gain.

Brown adipose tissue functions in humans

Human adults have functionally active BAT. The metabolic function can be reliably measured in vivo using modern imaging modalities (namely PET/CT). Cold seems to be one of the most potent stimulators of BAT metabolic activity but other stimulators (for example insulin) are actively studied. Obesity is related to lower metabolic activity of BAT but it may be reversed after successful weight reduction such as after bariatric surgery. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

Ambient Temperature and Obesity

Homeotherms maintain an optimal body temperature that is most often above their environment or ambient temperature. As ambient temperature decreases, energy expenditure (and energy intake) must increase to maintain thermal homeostasis. With the wide spread adoption of climate control, humans in modern society are buffered from temperature extremes and spend an increasing amount of time in a thermally comfortable state where energetic demands are minimized. This is hypothesized to contribute to the contemporary increase in obesity rates. Studies reporting exposures of animals and humans to different ambient temperatures are discussed. Additional consideration is given to the potentially altered metabolic and physiologic responses in obese versus lean subjects at a given temperature. The data suggest that ambient temperature is a significant contributor to both energy intake and energy expenditure, and that this variable should be more thoroughly explored in future studies as a potential contributor to obesity susceptibility.

Could increased time spent in a thermal comfort zone contribute to population increases in obesity?

Domestic winter indoor temperatures in the USA, UK and other developed countries appear to be following an upwards trend. This review examines evidence of a causal link between thermal exposures and increases in obesity prevalence, focusing on acute and longer-term biological effects of time spent in thermal comfort compared with mild cold. Reduced exposure to seasonal cold may have a dual effect on energy expenditure, both minimizing the need for physiological thermogenesis and reducing thermogenic capacity. Experimental studies show a graded association between acute mild cold and human energy expenditure over the range of temperatures relevant to indoor heating trends. Meanwhile, recent studies of the role of brown adipose tissue (BAT) in human thermogenesis suggest that increased time spent in conditions of thermal comfort can lead to a loss of BAT and reduced thermogenic capacity. Pathways linking cold exposure and adiposity have not been directly tested in humans. Research in naturalistic and experimental settings is needed to establish effects of changes in thermal exposures on weight, which may raise possibilities for novel public health strategies to address obesity.

Implications of nonshivering thermogenesis for energy balance regulation in humans

The incidence of the metabolic syndrome has reached epidemic levels in the Western world. With respect to the energy balance, most attention has been given to reducing energy (food) intake. Increasing energy expenditure is an important alternative strategy. Facultative thermogenesis, which is the increase in energy expenditure in response to cold or diet, may be an effective way to affect the energy balance. The recent identification of functional brown adipose tissue (BAT) in adult humans promoted a renewed interest in nonshivering thermogenesis (NST). The purpose of this review is to highlight the recent insight in NST, general aspects of its regulation, the major tissues involved, and its metabolic consequences. Sustainable NST in adult humans amounts to 15% of the average daily energy expenditure. Calculations based on the limited available literature show that BAT thermogenesis can amount to 5% of the basal metabolic rate. It is likely that at least a substantial part of NST can be attributed to BAT, but it is possible that other tissues contribute to NST. Several studies on mitochondrial uncoupling indicate that skeletal muscle is another potential contributor to facultative thermogenesis in humans. The general and synergistic role of the sympathetic nervous system and the thyroid axis in relation to NST is discussed. Finally, perspectives on BAT and skeletal muscle NST are given.

Beta-adrenergic receptor blockade does not inhibit cold-induced thermogenesis in humans: possible involvement of brown adipose tissue

CONTEXT

Recently, brown adipose tissue (BAT) gained interest as a possible target for cold-induced thermogenesis, and therefore a target for treatment of obesity in adult humans. However, mitochondrial uncoupling takes place not only in BAT but also in skeletal muscle tissue. Both tissues may be involved in cold-induced thermogenesis, which is presumably regulated by the sympathetic nervous system.

OBJECTIVE

Here we studied whether blockade of β-adrenergic receptors using propranolol diminishes cold-induced thermogenesis and mitochondrial uncoupling in skeletal muscle tissue.

DESIGN

Ten lean subjects participated in this study and stayed twice (control and β-blockade using propranolol) for 84 h in a respiration chamber-the first 36 h for baseline measurements, followed by 48 h of mild cold exposure (16 C). Energy expenditure was measured continuously. After 36 and 84 h, muscle biopsies were taken in which mitochondrial uncoupling was studied.

RESULTS

Energy expenditure increased upon mild cold exposure (+5.0 ± 1.2 W; P < 0.005), i.e. cold-induced thermogenesis. However, contrary to our hypothesis, this cold-induced thermogenesis was not diminished after β-blockade (+4.7 ± 2.1 W for blockade vs. +5.1 ± 1.4 W for control; P = 0.59 for interaction cold blockade). Skeletal muscle mitochondrial uncoupling was significantly related to cold-induced thermogenesis in the control situation (R(2) = 0.650; P < 0.01). There was no such relation during β-blockade.

CONCLUSIONS

Our results suggest that skeletal muscle mitochondrial uncoupling may be involved in cold-induced thermogenesis and that this may be regulated by β(2)-receptors. When the β(1)- and β(2)-receptors are blocked, a β(3)-regulated process like mitochondrial uncoupling in BAT might take over the role of skeletal muscle mitochondrial uncoupling.

The role of the myosin ATPase activity in adaptive thermogenesis by skeletal muscle

Resting skeletal muscle is a major contributor to adaptive thermogenesis, i.e., the thermogenesis that changes in response to exposure to cold or to overfeeding. The identification of the “furnace” that is responsible for increased heat generation in resting muscle has been the subject of a number of investigations. A new state of myosin, the super relaxed state (SRX), with a very slow ATP turnover rate has recently been observed in skeletal muscle (Stewart et al. in Proc Natl Acad Sci USA 107:430–435, 2010). Inhibition of the myosin ATPase activity in the SRX was suggested to be caused by binding of the myosin head to the core of the thick filament in a structural motif identified earlier by electron microscopy. To be compatible with the basal metabolic rate observed in vivo for resting muscle, most myosin heads would have to be in the SRX. Modulation of the population of this state, relative to the normal relaxed state, was proposed to be a major contributor to adaptive thermogenesis in resting muscle. Transfer of only 20% of myosin heads from the SRX into the normal relaxed state would cause muscle thermogenesis to double. Phosphorylation of the myosin regulatory light chain was shown to transfer myosin heads from the SRX into the relaxed state, which would increase thermogenesis. In particular, thermogenesis by myosin has been proposed to play a role in the dissipation of calories during overfeeding. Up-regulation of muscle thermogenesis by pharmaceuticals that target the SRX would provide new approaches to the treatment of obesity or high blood sugar levels.

Significance and application of melatonin in the regulation of brown adipose tissue metabolism: relation to human obesity

A worldwide increase in the incidence of obesity indicates the unsuccessful battle against this disorder. Obesity and the associated health problems urgently require effective strategies of treatment. The new discovery that a substantial amount of functional brown adipose tissue (BAT) is retained in adult humans provides a potential target for treatment of human obesity. BAT is active metabolically and disposes of extra energy via generation of heat through uncoupling oxidative phosphorylation in mitochondria. The physiology of BAT is readily regulated by melatonin, which not only increases recruitment of brown adipocytes but also elevates their metabolic activity in mammals. It is speculated that the hypertrophic effect and functional activation of BAT induced by melatonin may likely apply to the human. Thus, melatonin, a naturally occurring substance with no reported toxicity, may serve as a novel approach for treatment of obesity. Conversely, because of the availability of artificial light sources, excessive light exposure after darkness onset in modern societies should be considered a potential contributory factor to human obesity as light at night dramatically reduces endogenous melatonin production. In the current article, the potential associations of melatonin, BAT, obesity and the medical implications are discussed.

Human brown fat and obesity: methodological aspects

Much is known about brown adipose tissue (BAT) in rodents. Its function is to generate heat in response to low environmental temperatures and to diet or overfeeding. The knowledge about BAT in humans is still rather limited despite the recent rediscovery of its functionality in adults. This review highlights the information available on the contribution of BAT in increasing human energy expenditure in relation to obesity. Besides that methodological aspects will be discussed that need special attention in order to unravel the heat producing capacity of human BAT, the recruitment of the tissue, and its functionality.

Minimal changes in environmental temperature result in a significant increase in energy expenditure and changes in the hormonal homeostasis in healthy adults

OBJECTIVE

Resting energy expenditure (EE) is a major contributor to the total EE and thus plays an important role in body weight regulation. Adaptive thermogenesis is a major component of EE in rodents, but little is known on the effects of exposure of humans to mild and sustainable reduction in environmental temperature.

DESIGN

To characterize the dynamic changes in continuously measured resting EE, substrate utilization, and hormonal axes simultaneously in response to mild reduction in environmental temperature, we performed a cross-over intervention.

METHODS

Twenty-five volunteers underwent two 12-h recordings of EE in whole room indirect calorimeters at 24 and 19 °C with simultaneous measurement of spontaneous movements and hormonal axes.

RESULTS

Exposure to 19 °C resulted in an increase in plasma and urine norepinephrine levels (P<0.0001), and a 5.96% (P<0.001) increase in EE without significant changes in spontaneous physical activity. Exposure to the lower temperature resulted in a significant increase in free fatty acid levels (P<0.01), fasting insulin levels (P<0.05), and a marginal decrease in postprandial glucose levels. A small but significant (P<0.002) increase in serum free thyroxine and urinary free cortisol (P<0.05) was observed at 19 °C.

CONCLUSIONS

Our observations indicate that exposure to 19 °C, a mild and tolerable cold temperature, results in a predictable increase in EE driven by a sustained rise in catecholamine and the activation of counter-regulatory mechanisms.

Low levels of physical activity increase metabolic responsiveness to cold in a rat (Rattus fuscipes)

Background

Physical activity modulates expression of metabolic genes and may therefore be a prerequisite for metabolic responses to environmental stimuli. However, the extent to which exercise interacts with environmental conditions to modulate metabolism is unresolved. Hence, we tested the hypothesis that even low levels of physical activity are beneficial by improving metabolic responsiveness to temperatures below the thermal neutral zone, thereby increasing the capacity for substrate oxidation and energy expenditure.

Methodology/Principal Findings

We used wild rats (Rattus fuscipes) to avoid potential effects of breeding on physiological phenotypes. Exercise acclimation (for 30 min/day on 5 days/week for 30 days at 60% of maximal performance) at 22°C increased mRNA concentrations of PGC1α, PPARδ, and NRF-1 in skeletal muscle and brown adipose tissue compared to sedentary animals. Lowering ambient temperature to 12°C caused further increases in relative expression of NRF-1 in skeletal muscle, and of PPARδ of brown adipose tissue. Surprisingly, relative expression of UCP1 increased only when both exercise and cold stimuli were present. Importantly, in sedentary animals cold acclimation (12°C) alone did not change any of the above variables. Similarly, cold alone did not increase maximum capacity for substrate oxidation in mitochondria (cytochrome c oxidase and citrate synthase activities) of either muscle or brown adipose tissue. Animals that exercised regularly had higher exercise induced metabolic rates in colder environments than sedentary rats, and temperature induced metabolic scope was greater in exercised rats.

Conclusions/Significance

Physical activity is a necessary prerequisite for the expression of transcriptional regulators that influence a broad range of physiological functions from energy metabolism to cardiovascular function and nutrient uptake. A sedentary lifestyle leads to decreased daily energy expenditure because of a lack of direct use of energy and a muted metabolic response to ambient temperature, which can be reversed even by low levels of physical activity.

Cold-induced adaptive thermogenesis in lean and obese

On entering a cold environment, people react by increasing insulation and energy expenditure (EE). However, large interindividual differences exist in the relative contribution of each mechanism. Short-term studies revealed that obese subjects increase EE (i.e., adaptive thermogenesis) less than lean subjects, which might have implications for the predisposition to obesity. In this study, we validate the differences in adaptive thermogenesis between lean and obese upon midterm mild cold exposure. Therefore, 10 lean and 10 obese subjects were exposed for 48 h to mild cold (16 degrees C) in a respiration chamber. The preceding 36 h they stayed in the same chamber at a neutral temperature (22 degrees C) for the baseline measurements. EE, physical activity, skin temperature, and core temperature have been measured for the last 24 h of both parts. Mean daytime EE increased significantly in the lean subjects (P < 0.01), but not in the obese. Physical activity decreased significantly in the lean (P < 0.01) and the obese (P < 0.001) subjects. The change in EE was related to the change in physical activity in both groups (respectively R(2) = 0.673, P < 0.01 and R(2) = 0.454, P < 0.05). Upon mild cold exposure, lean subjects decreased proximal skin temperature less, but distal skin temperature more than obese. In conclusion, the interindividual differences in cold-induced thermogenesis were related to changes in physical activity in both lean and obese, pointing at the existence of individual variation in physical activity to compensate for cold-induced thermogenesis. Furthermore, although a large part of the lean subjects counteracted the cold by increasing EE, most obese subjects changed temperature distribution, and therefore, increased insulation.

The genetics of brown adipose tissue

Brown adipose tissue is highly differentiated and has evolved as a mechanism for heat production based upon uncoupling of mitochondrial oxidative phosphorylation. Additionally, large amounts of lipid can be stored in the cells to provide fuel necessary for heat production upon adrenergic stimulation from the central nervous system, and a highly developed vascular system evolved to rapidly deliver heat to vital organs. For unknown reasons, the development of brown adipocytes has two independent pathways: one originates from muscle progenitor cells in the fetus and leads to a fully functional cell at birth (interscapular-type brown fat), while the other transiently emerges in traditional white fat depots at weaning, regresses, and then can be induced in adult mice upon adrenergic stimulation. No genetic variants have been found for interscapular fat, but naturally occurring alleles at eight genetic loci in mice lead to over 100-fold variation for brown adipocytes in white fat upon adrenergic stimulation. The ability to activate this potential for energy expenditure is of great interest in obesity research.

Ten putative contributors to the obesity epidemic

The obesity epidemic is a global issue and shows no signs of abating, while the cause of this epidemic remains unclear. Marketing practices of energy-dense foods and institutionally-driven declines in physical activity are the alleged perpetrators for the epidemic, despite a lack of solid evidence to demonstrate their causal role. While both may contribute to obesity, we call attention to their unquestioned dominance in program funding and public efforts to reduce obesity, and propose several alternative putative contributors that would benefit from equal consideration and attention. Evidence for microorganisms, epigenetics, increasing maternal age, greater fecundity among people with higher adiposity, assortative mating, sleep debt, endocrine disruptors, pharmaceutical iatrogenesis, reduction in variability of ambient temperatures, and intrauterine and intergenerational effects as contributing factors to the obesity epidemic are reviewed herein. While the evidence is strong for some contributors such as pharmaceutical-induced weight gain, it is still emerging for other reviewed factors. Considering the role of such putative etiological factors of obesity may lead to comprehensive, cause specific, and effective strategies for prevention and treatment of this global epidemic.

Recent advances in adaptive thermogenesis: potential implications for the treatment of obesity

Large inter-individual differences in cold-induced (non-shivering) and diet-induced adaptive thermogenesis exist in animals and humans. These differences in energy expenditure can have a large impact on long-term energy balance and thus body weight (when other factors remain stable). Therefore, the level of adaptive thermogenesis might relate to the susceptibility to obesity; efforts to increase adaptive thermogenesis might be used to treat obesity. In small mammals, the main process involved is mitochondrial uncoupling in brown adipose tissue (BAT), which is regulated by the sympathetic nervous system. For a long time, it was assumed that mitochondrial uncoupling is not a major physiological contributor to adaptive thermogenesis in adult humans. However, several studies conducted in recent years suggest that mitochondrial uncoupling in BAT and skeletal muscle tissue in adult humans can be physiologically significant. Other mechanisms besides mitochondrial uncoupling that might be involved are futile calcium cycling, protein turnover and substrate cycling. In conjunction with recent advances on signal transduction studies, this knowledge makes manipulation of adaptive thermogenesis a more realistic option and thus a pharmacologically interesting target to treat obesity.

Cold-induced heat production preceding shivering

Individual changes in heat production and body temperature were studied in response to cold exposure, prior to shivering. The subjects ten women (seven men) were of normal weight, had a mean age of 23 (SD 3) years and average BMI 22·2 (SD 1·6) Kg/m2. They were lying supine under thermoneutral conditions for 30 min and were subsequently exposed to air of 15°C until shivering occurred. Heat production was measured with a ventilated hood. Body composition was measured with underwater weighing and 2H dilution. Body temperatures were measured with thermistors. Heat production during cold exposure prior to shivering increased and reached a plateau. Skin temperature decreased and did not reach a plateau during the test period. The non-shivering interval (NSI) ranged from 20 to 148 min, was not related to body composition and was not significantly different between women (81 (sd 15) min) and men (84 (sd 34) min). NSI was negatively related to skin temperature (r2 0·44, P=0·004), and skin temperature was related to heat production (r2 0·39, P=0·007) In conclusion, subjects with a relatively large heat production during cold exposure maintained a relatively high skin temperature but showed a short NSI, independent of differences in body composition.

Validation of an individualised model of human thermoregulation for predicting responses to cold air

Most computer models of human thermoregulation are population based. Here, we individualised the Fiala model [Fiala et al. (2001) Int J Biometeorol 45:143-159] with respect to anthropometrics, body fat, and metabolic rate. The predictions of the adapted multisegmental thermoregulatory model were compared with measured skin temperatures of individuals. Data from two experiments, in which reclining subjects were suddenly exposed to mild to moderate cold environmental conditions, were used to study the effect on dynamic skin temperature responses. Body fat was measured by the three-compartment method combining underwater weighing and deuterium dilution. Metabolic rate was determined by indirect calorimetry. In experiment 1, the bias (mean difference) between predicted and measured mean skin temperature decreased from 1.8 degrees C to -0.15 degrees C during cold exposure. The standard deviation of the mean difference remained of the same magnitude (from 0.7 degrees C to 0.9 degrees C). In experiment 2 the bias of the skin temperature changed from 2.0+/-1.09 degrees C using the standard model to 1.3+/-0.93 degrees C using individual characteristics in the model. The inclusion of individual characteristics thus improved the predictions for an individual and led to a significantly smaller systematic error. However, a large part of the discrepancies in individual response to cold remained unexplained. Possible further improvements to the model accomplished by inclusion of more subject characteristics (i.e. body fat distribution, body shape) and model refinements on the level of (skin) blood perfusion, and control functions, are discussed.

Cold-induced metabolism

PURPOSE OF REVIEW

Cold response can be insulative (drop in peripheral temperature) or metabolic (increase in energy expenditure). Nonshivering thermogenesis by sympathetic, norepinephrine-induced mitochondrial heat production in brown adipose tissue is a well known component of this metabolic response in infants and several animal species. In adult humans, however, its role is less clear. Here we explore recent findings on the role and variability of nonshivering thermogenesis in adults.

RECENT FINDINGS

Large individual differences exist in mild cold response with respect to the relative contribution of the insulative response and the metabolic (nonshivering) response. In search for the possible explanations of this variation, recent studies on potential mechanisms of nonshivering thermogenesis in humans are presented. Emphasis is given to the role of uncoupling proteins, mitochondrial ATP-synthase, and calcium cycling. The potential contribution of human skeletal muscle to nonshivering thermogenesis is discussed. The differences in nonshivering thermogenesis can partly be attributed to factors such as age, gender, physical fitness, adaptation, and diet. There are indications that genetic variation affect cold response.

SUMMARY

The implications of the observed large individual variation in cold response is that a low metabolic response to cold can partly explain increased risk to develop obesity. Both the effect of environmental factors and genetic factors on nonshivering thermogenesis require more well controlled studies. With extended knowledge on these factors it can be ascertained if a pharmacological regimen is possible which would mimic the effects of chronic cold or elevated catecholamine levels, without attendant side effects.

Metabolic lessons from genetically lean mice

Different types of lean mice have been produced by genetic manipulation. Leanness can result from deficiency of stored energy or a lack of adipocytes to store the lipid. Mice lacking functional adipocytes are usually insulin resistant and have fatty livers, and elevated circulating triglyceride levels. Insulin resistance may result from the lack of adipocyte hormones (such as leptin) and increased metabolite (such as triglyceride) levels in nonadipose tissue. Mice with depleted adipocyte triglyceride levels typically are insulin sensitive and have normal or low liver and circulating triglycerides. Mechanisms to produce depleted adipocytes include increased energy expenditure by peripheral tissues, peripheral mechanisms to decrease food intake, and altered central regulation of these processes.

Individual variation in body temperature and energy expenditure in response to mild cold

We studied interindividual variation in body temperature and energy expenditure, the relation between these two, and the effect of mild decrease in environmental temperature (16 vs. 22 degrees C) on both body temperature and energy expenditure. Nine males stayed three times for 60 h (2000-0800) in a respiration chamber, once at 22 degrees C and twice at 16 degrees C, in random order. Twenty-four-hour energy expenditure, thermic effect of food, sleeping metabolic rate, activity-induced energy expenditure, and rectal and skin temperatures were measured. A rank correlation test with data of 6 test days showed significant interindividual variation in both rectal and skin temperatures and energy expenditures adjusted for body composition. Short-term exposure of the subjects to 16 degrees C caused a significant decrease in body temperature (both skin and core), an increase in temperature gradients, and an increase in energy expenditure. The change in body temperature gradients was negatively related to changes in energy expenditure. This shows that interindividual differences exist with respect to the relative contribution of metabolic and insulative adaptations to cold.

Energy metabolism in humans at a lowered ambient temperature

OBJECTIVE

Assessment of the effect of a lowered ambient temperature, ie 16 degrees C (61 degrees F), compared to 22 degrees C (72 degrees F), on energy intake (EI), energy expenditure (EE) and respiratory quotient (RQ) in men.

DESIGN

Randomized within-subject design in which subjects stayed in a respiration chamber three times for 60 h each, once at 22 degrees C, and twice at 16 degrees C, wearing standardized clothing, executing a standardized daily activities protocol, and were fed in energy balance (EBI): no significant difference between EE and EI over 24 h). During the last 24 h at 22 degrees C, and once during the last 24 h at 16 degrees C, they were fed ad libitum.

SUBJECTS

Nine dietary unrestrained male subjects (ages 24+/-5 y, body mass index (BMI) 22.7+/-2.1 kg/m(2), body weight 76.2+/-9.4 kg, height 1.83+/-0.06 m, 18+/-5% body fat).

RESULTS

At 16 degrees C (EB), EE (total 24 h EE) was increased to 12.9+/-2.0 MJ/day as compared to 12.2+/-2.2 MJ/day at 22 degrees C (P<0.01). The increase was due to increases in sleeping metabolic rate (SMR; the lowest EE during three consecutive hours with hardly any movements as indicated by radar): 7.6+/-0.7 vs 7.2+/-0.7 MJ/day (P<0.05) and diet-induced thermogenesis (DIT; EE-SMR, when activity induced energy expenditure as indicated by radar=0): 1.7+/-0.4 vs 1.0+/-0.4 MJ/day (P<0.01). Physical activity level (PAL; EE/SMR) was 1.63-1.68. At 16 degrees C compared to at 22 degrees C, rectal, proximal and distal skin temperatures had decreased (P<0.01). RQ was not different between the two ambient temperature situations. During ad libitum feeding, subjects overate by 32+/-12% (at 22 degrees C) and by 34+/-14% (at 16 degrees C). Under these circumstances, the decrease of rectal temperature at 16 degrees C was attenuated, and inversely related to percentage overeating (r(2)=0.7; P<0.01).

CONCLUSION

We conclude that at 16 degrees C, compared to 22 degrees C, energy metabolism was increased, due to increases in SMR and DIT. Overeating under ad libitum circumstances at 16 degrees C attenuated the decrease in rectal core body temperature.

The effect of mild cold exposure on UCP3 mRNA expression and UCP3 protein content in humans

OBJECTIVE

In rodents, adaptive thermogenesis in response to cold exposure and high-fat feeding is accomplished by the activation of the brown adipose tissue specific mitochondrial uncoupling protein, UCP1. The recently discovered human uncoupling protein 3 is a possible candidate for adaptive thermogenesis in humans. In the present study we examined the effect of mild cold exposure on the mRNA and protein expression of UCP3.

SUBJECTS

Ten healthy male volunteers (age 24.4 +/- 1.6 y; height 1.83 +/- 0.02 m; weight 77.3 +/- 3.0 kg; percentage body fat 19 +/- 2).

DESIGN

Subjects stayed twice in the respiration chamber for 60 h (20.00-8.00 h); once at 22 degrees C (72 degrees F), and once at 16 degrees C (61 degrees F). After leaving the respiration chamber, muscle biopsies were taken and RT-competitive-PCR and Western blotting was used to measure UCP3 mRNA and protein expression respectively.

RESULTS

Twenty-four-hour energy expenditure was significantly increased at 16 degrees C compared to 22 degrees C (P<0.05). At 16 degrees C, UCP3T (4.6 +/- 1.0 vs 7.7 +/- 1.5 amol/microg RNA, P=0.07), UCP3L (2.0 +/- 0.5 vs 3.5 +/- 0.9 amol/microg RNA, P=0.1) and UCP3S (2.6 +/- 0.6 vs 4.2 +/- 0.7 amol/microg RNA, P=0.07) mRNA expression tended to be lower compared with at 22 degrees C, whereas UCP3 protein content was, on average, not different. However, the individual differences in UCP3 protein content (16-22 degrees C) correlated positively with the differences in 24 h energy expenditure (r=0.86, P<0.05).

CONCLUSION

The present study suggests that UCP3 protein content is related to energy metabolism in humans and might help in the metabolic adaptation to cold exposure. However, the down-regulation of UCP3 mRNA with mild cold exposure suggests that prolonged cold exposure will lead to lower UCP3 protein content. What the function of such down-regulation of UCP3 could be is presently unknown.

Energy metabolism in women during short exposure to the thermoneutral zone

Ambient temperature has been shown to affect energy metabolism in field situations. Therefore, we assessed the effect of a short exposure to the thermoneutral zone, i.e., 27 degrees C (81 degrees F), in comparison to the usual ambient temperature of 22 degrees C (72 degrees F), on energy expenditure (EE), substrate oxidation, and energy intake (EI) in a controlled situation. Subjects, i.e., women (ages 22+/-2 years, BMI 22+/-3, 28+/-4% body fat), stayed in a respiration chamber three times for 48 h each: once at 22 degrees C, and twice at 27 degrees C in random order, wearing standardized clothing, executing a standardized daily-activities protocol, and being fed in energy balance (EB). During the last 24 h at 22 degrees C, and once during the last 24 h at 27 degrees C, they were fed ad libitum. At 27 degrees C, compared to at 22 degrees C, EE was 8.9+/-1.3 MJ/day vs. 9.9+/-1.5 MJ/day (P<.001) due to decreases in diet-induced thermogenesis (DIT) and activity-induced energy expenditure (AEE) (P<.01); respiratory quotient (RQ) had increased (P<.05); core (P<.05) and skin (P<.001) temperatures had increased. During ad lib feeding, EI was 90-91% of EE (P=.9), due to changes in energy density (ED) of the food choice (P<.01), and related to changes in body temperature and EE (P<.001). Thus, at 27 degrees C, compared to 22 degrees C, energy metabolism was reduced by reductions in DIT and in AEE, while RQ was increased. Reduction in EI was primarily related to body temperature changes and secondarily to changes in EE.