Effects of experimental weight perturbation on skeletal muscle work efficiency in human subjects

Changes in peak fat oxidation in response to different doses of endurance training

The effect of different doses of endurance training on the capacity to oxidize fat during exercise in sedentary, overweight men and assessment of variables associated with changes in peak fat oxidation (PFO) were evaluated. Young, sedentary, overweight men were randomized to either the high-dose (HIGH, 600 kcal/day, n = 17) or moderate-dose (MOD, 300 kcal/day, n = 18) endurance training groups or controls (CON, n = 15). PFO and peak oxygen uptake (VO2 peak) were measured using indirect calorimetry, body composition using dual-energy x-ray absorptiometry, and protein levels of mitochondrial enzymes determined by Western blotting. PFO increased in both MOD [1.2 mg/kg fat-free mass (FFM)/min, 95% confidence interval (CI): 0.08:2.3, P = 0.03] and HIGH (1.8 mg/kg FFM/min, CI: 0.6:2.9, P < 0.001) compared with CON. Skeletal muscle expression of citrate synthase, β-hydroxyacyl-CoA dehydrogenase, and mitochondrial oxphos complexes II-V increased similarly in MOD and HIGH. Stepwise multiple linear regression analysis with backward elimination of individual variables correlated with changes in PFO revealed increases in cycling efficiency, FFM, and VO2 peak as the remaining associated variables. In conclusion, PFO during exercise increased with both moderate- and high-dose endurance training. Increases in PFO were mainly predicted by changes in VO2 peak, FFM, and cycling efficiency, and less with skeletal muscle mitochondrial enzymes.

Weight loss-induced reduction in physical activity recovers during weight maintenance

BACKGROUND

Weight loss due to a negative energy balance is considered to be accompanied by a decrease in physical activity.

OBJECTIVE

The aim of this study was to investigate whether a decrease in physical activity is sustained during weight maintenance.

DESIGN

Subjects were 20 men and 31 women [mean (±SD) age: 42 ± 8 y; BMI (in kg/m(2)): 31.4 ± 2.8]. Weight loss was achieved by an 8-wk very-low-energy diet period, followed by 44 wk of weight maintenance. Physical activity measures were total energy expenditure expressed as a multiple of sleeping metabolic rate (PALSMR) and resting metabolic rate (PALRMR), activity-induced energy expenditure divided by body weight (AEE/kg), and activity counts measured by a triaxial accelerometer. Measurements took place at 0, 8, and 52 wk.

RESULTS

Body mass decreased significantly during the diet period (10.5 ± 3.8%, P < 0.001), and this reduction was sustained after 52 wk (6.0 ± 5.1%, P < 0.001). PALSMR and PALRMR decreased from 1.81 ± 0.23 and 1.70 ± 0.22, respectively, before the diet to 1.69 ± 0.20 and 1.55 ± 0.19 after the diet (P < 0.001) and increased again after weight maintenance to 1.85 ± 0.27 and 1.71 ± 0.23, respectively, compared with 8-wk measurements (P < 0.001). AEE/kg decreased from 0.043 ± 0.015 MJ/kg at baseline to 0.037 ± 0.014 MJ/kg after the diet (P < 0.001) and was higher after 52 wk (0.044 ± 0.17 MJ/kg) compared with after 8 wk (P < 0.001). Activity counts decreased from 1.64 ± 0.37 megacounts/d at baseline to 1.54 ± 0.35 megacounts/d after the diet (P < 0.05) and were higher after 52 wk (1.73 ± 0.49 megacounts/d) compared with 8 wk (P < 0.01).

CONCLUSION

A weight loss-induced reduction in physical activity returns to baseline values when weight loss is maintained. This trial was registered at clinicaltrials.gov as NCT01015508.

Physical activity in obesity and metabolic syndrome

Biological aging is typically associated with a progressive increase in body fat mass and a loss of lean body mass. Owing to the metabolic consequences of reduced muscle mass, it is understood that normal aging and/or decreased physical activity may lead to a higher prevalence of metabolic disorders. Lifestyle modification, specifically changes in diet, physical activity, and exercise, is considered the cornerstone of obesity management. However, for most overweight people it is difficult to lose weight permanently through diet or exercise. Thus, prevention of weight gain is thought to be more effective than weight loss in reducing obesity rates. A key question is whether physical activity can extenuate age-related weight gain and promote metabolic health in adults. Current guidelines suggest that adults should accumulate about 60 minutes of moderate-intensity physical activity daily to prevent unhealthy weight gain. Because evidence suggests that resistance training may promote a negative energy balance and may change body fat distribution, it is possible that an increase in muscle mass after resistance training may be a key mediator leading to better metabolic control.

Effects of reduced weight maintenance and leptin repletion on functional connectivity of the hypothalamus in obese humans

Treating obesity has proven to be an intractable challenge, in part, due to the difficulty of maintaining reduced weight. In our previous studies of in-patient obese subjects, we have shown that leptin repletion following a 10% or greater weight loss reduces many of the metabolic (decreased energy expenditure, sympathetic nervous system tone, and bioactive thyroid hormones) and behavioral (delayed satiation) changes that favor regain of lost weight. FMRI studies of these same subjects have shown leptin-sensitive increases in activation of the right hypothalamus and reduced activation of the cingulate, medial frontal and parahippocampal gryi, following weight loss, in response to food stimuli. In the present study, we expanded our cohort of in-patient subjects and employed psychophysiological interaction (PPI) analysis to examine changes in the functional connectivity of the right hypothalamus. During reduced-weight maintenance with placebo injections, the functional connectivity of the hypothalamus increased with visual areas and the dorsal anterior cingulate (dorsal ACC) in response to food cues, consistent with higher sensitivity to food. During reduced-weight maintenance with leptin injections, however, the functional connectivity of the right hypothalamus increased with the mid-insula and the central and parietal operculae, suggesting increased coupling with the interoceptive system, and decreased with the orbital frontal cortex, frontal pole and the dorsal ACC, suggesting a down-regulated sensitivity to food. These findings reveal neural mechanisms that may underlie observed changes in sensitivity to food cues in the obese population during reduced-weight maintenance and leptin repletion.

The defence of body weight: a physiological basis for weight regain after weight loss

Although weight loss can usually be achieved by restricting food intake, the majority of dieters regain weight over the long-term. In the hypothalamus, hormonal signals from the gastrointestinal tract, adipose tissue and other peripheral sites are integrated to influence appetite and energy expenditure. Diet-induced weight loss is accompanied by several physiological changes which encourage weight regain, including alterations in energy expenditure, substrate metabolism and hormone pathways involved in appetite regulation, many of which persist beyond the initial weight loss period. Safe effective long-term strategies to overcome these physiological changes are needed to help facilitate maintenance of weight loss. The present review, which focuses on data from human studies, begins with an outline of body weight regulation to provide the context for the subsequent discussion of short- and long-term physiological changes which accompany diet-induced weight loss.

Changes in body composition, cardiovascular disease risk factors, and eating behavior after an intensive lifestyle intervention with high volume of physical activity in severely obese subjects: a prospective clinical controlled trial

We examined the effects of a 10-14-weeks inpatient lifestyle modification program, including minimum 90 min of physical activity (PA) five days/week, on body composition, CVD risk factors, and eating behavior in 139 obese subjects (BMI 42.6 ± 5.2 kg/m²). Completion rate was 71% (n = 71) in the intensive lifestyle intervention (ILI) group and 85% (n = 33) among waiting list controls. Compared to controls body weight (-17.0 (95% CI: -18.7, -15.3) kg, P < 0.0001), fat mass (-15.2 (95% CI: -17.4, -13.1) kg, P < 0.0001), fat free mass (-1.2 (95% CI: -2.2, -0.2) kg, P = 0.016) and visceral fat (-86.6(95% CI: -97.4, -75.7) cm², P < 0.0001) were reduced in the ILI-group after 10-14 weeks. Within the ILI-group weight loss was -23.8 (95% CI: -25.9, -21.7) kg, P < 0.0001 and -20.3 (95% CI: -23.3, -17.3) kg, P < 0.0001, after six and 12 months, respectively. Systolic BP, glucose, triglycerides, and LDL-C were reduced, and HDL-C was increased (all P ≤ 0.006) after 10-14 weeks within the ILI group. The reduction in glucose and increase in HDL-C were sustained after 12 months (all P < 0.0001). After one year, weight loss was related to increased cognitive restraint and decreased uncontrolled eating (all P < 0.05). Thus, ILI including high volume of PA resulted in weight loss with almost maintenance of fat-free mass, favorable changes in CVD risk factors, and eating behavior in subjects with severe obesity.

Effects of dietary history on energy metabolism and physiological parameters in C57BL/6J mice

Understanding body weight regulation is essential to fight obesity. Mouse studies, using different types of diets, showed conflicting results in terms of body weight persistence after changing from an ad libitum high-fat diet to an ad libitum low-fat diet. In this study, we questioned specifically whether the energy content of the diet has a lasting effect on energy balance and body weight, using multiple switches and two purified diets with a different fat-to-sugar ratio, but otherwise identical ingredients. Young-adult obesity-prone male C57BL/6J mice were fed single or double switches of semi-purified diets with either 10 energy % (en%) fat (LF) or 40en% fat (HF), with starch replaced by fat, while protein content remained equal. After none, one or two dietary changes, energy metabolism was assessed at 5, 14 and 19 weeks. We observed no systematic continuous compensation in diet and energy intake when returning to LF after HF consumption. Body weight, white adipose tissue mass and histology, serum metabolic parameters, energy expenditure and substrate usage all significantly reflected the current diet intake, independent of dietary changes. This contrasts with studies that used diets with different ingredients and showed persistent effects of dietary history on body weight, suggesting diet-dependent metabolic set points. We conclude that body weight and metabolic parameters 'settle', based on current energetic input and output. This study also highlights the importance of considering the choice of diet in physiological and metabolic intervention studies.

Addressing weight loss recidivism: a clinical focus on metabolic rate and the psychological aspects of obesity

Obesity in the United States has reached epidemic proportions and has become an unprecedented public health burden. This paper returns to the evidence for metabolic rate set points and emphasizes the clinical importance of addressing changes in metabolic rate throughout the weight loss process. In addition to the importance of clinically attending to the modulation of metabolic rate, the psychological aspects of obesity are addressed as part of the need to holistically treat obesity.

Weight loss and exercise training effect on oxygen uptake and heart rate response to locomotion

Effects of resistance and aerobic training on the ease of physical activity during and after weight loss are unknown. The purpose of the study was to determine what effect weight loss combined with either aerobic or resistance training has on the ease of locomotion (net V[Combining Dot Above]O2 and heart rate). It is hypothesized that exercise training will result in an increased ease, lowers heart rate during locomotion. Seventy-three overweight premenopausal women were assigned to diet and aerobic training, diet and resistance training, or diet only. Subjects were evaluated while overweight, after diet-induced weight loss (average, 12.5 kg loss), and 1 year after weight loss (5.5 kg regain). Submaximal walking, grade walking, stair climbing, and bike oxygen uptake and heart rate were measured at all time points. Weight loss diet was 800 kcal per day. Exercisers trained 3 times per week during weight loss and 2 times per week during 1-year follow-up. Resistance training increased strength, and aerobic training increased maximum oxygen uptake. Net submaximal oxygen uptake was not affected by weight loss or exercise training. However, heart rate during walking, stair climbing, and bicycling was reduced after weight loss. No significant differences in reduction in heart rate were observed among the 3 treatment groups for locomotion after weight loss. However, during 1-year follow-up, exercise training resulted in maintenance of lower submaximal heart rate, whereas nonexercisers increased heart rate during locomotion. Results suggest that moderately intense exercise is helpful in improving the ease of movement after weight loss. Exercise training may be helpful in increasing the participation in free-living physical activity.

Efficiency of Walking and Stepping: Relationship to Body Fatness

OBJECTIVE

To determine energetic efficiency of walking and stepping in a heterogeneous normal adult population and its association with body fatness and to assess within- and between-individual variations.

RESEARCH METHODS AND PROCEDURES

Using a combination of a whole-room indirect calorimeter and a large precision force platform, we simultaneously measured minute-by-minute energy expenditure (EE) and mechanical work during walking and stepping in healthy adult men (n = 60) and women (n = 85). Efficiency was calculated as a ratio (percentage) of mechanical work and EE of activity. Efficiency of walking and stepping performed at various intensities was compared for reproducibility within the same day (morning and afternoon) and correlated with a subject's characteristics.

RESULTS

The efficiency of walking was negatively correlated with body fatness in both men and women at 0.9 to 1.2 m/s but positively correlated with body fatness in men and not correlated in women at the slowest speed tested (0.6 m/s). Efficiency of walking and stepping of various intensities was reproducible during the same day. Compared at similar EE levels, walking was more efficient than stepping (26% to 27% vs. 18% to 22%, p < 0.01). Women were significantly (p < 0.01) more efficient than men during stepping. Age, sex, body mass, fat-free mass, fitness (maximal oxygen uptake), height, and speed variations contributed to the between-subject differences in efficiency.

DISCUSSION

Obese individuals were less efficient than lean individuals during normal-speed walking. Significant interindividual variations in efficiency of walking and stepping may be attributed to habituation and physical characteristics such as age, sex, and fitness level.

Contribution of Different Mechanisms to Compensation for Energy Restriction in the Mouse

OBJECTIVE

Restriction of energy intake produces weight loss, but the rate of loss is seldom sustained. This is presumed to be a consequence of compensatory reductions in energy expenditure, although the exact contributions of different components to the energy budget remain uncertain. We examined the compensatory responses of mice to a 20% dietary restriction.

RESEARCH METHODS AND PROCEDURES

We measured body mass, body fatness, body temperature, and the components of daily energy expenditure for 50 MF1 mice. Forty mice were then placed on a restricted diet at 80% of their ad libitum intake for 50 days. The remaining 10 mice continued to feed ad libitum. Ten days before the end of the restriction period, the same measurements were taken.

RESULTS

There were no significant differences between the control and restricted groups in any parameters before restriction. During the restriction period, body mass increased in both the control and restricted groups, but at a slower rate in the restricted mice. The control group increased in both fat and fat free mass; however, although the restricted group increased fat to the same extent as the controls, fat free mass increased to a lesser extent. The contributions of the different components of the expended energy to compensate for the reduced energy intake were energy deposition, 2.2%; resting metabolic rate, 22.3%; and activity, 75.5%.

DISCUSSION

Mice were able to compensate almost completely for the restricted energy intake that was achieved by altering the amount of energy required for each component of the energy budget except digestive efficiency.

A Weight Reduction Program Preserves Fat-Free Mass but Not Metabolic Rate in Obese Adolescents

OBJECTIVE

To determine the effects of a multidisciplinary weight reduction program on body composition and energy expenditure (EE) in severely obese adolescents.

RESEARCH METHODS AND PROCEDURES

Twenty-six severely obese adolescents, 12 to 16 years old [mean BMI: 33.9 kg/m(2); 41.5% fat mass (FM)] followed a 9-month weight reduction program including moderate energy restriction and progressive endurance and resistance training. Body composition was assessed by DXA, basal metabolic rate by indirect calorimetry, and EE by whole-body indirect calorimetry with the same activity program over 36-hour periods before starting and 9 months after the weight reduction period.

RESULTS

Adolescents gained (least-square mean +/- SE) 2.9 +/- 0.2 cm in height, lost 16.9 +/- 1.3 kg body weight (BW), 15.2 +/- 0.9 kg FM, and 1.8 +/- 0.5 kg fat-free mass (FFM) (p < 0.001). Basal metabolic rate, sleeping, sedentary, and daily EE were 8% to 14% lower 9 months after starting (p < 0.001) and still 6% to 12% lower after adjustment for FFM (p < 0.05). Energy cost of walking decreased by 22% (p < 0.001). The reduction in heart rate during sleep and sedentary activities (-10 to -13 beats/min), and walking (-20 to -25 beats/min) (p < 0.001) resulted from both the decrease in BW and physical training.

DISCUSSION

A weight reduction program combining moderate energy restriction and physical training in severely obese adolescents resulted in great BW and FM losses and improvement of cardiovascular fitness but did not prevent the decline in EE even after adjustment for FFM.

Effects of dietary composition on energy expenditure during weight-loss maintenance

CONTEXT

Reduced energy expenditure following weight loss is thought to contribute to weight gain. However, the effect of dietary composition on energy expenditure during weight-loss maintenance has not been studied.

OBJECTIVE

To examine the effects of 3 diets differing widely in macronutrient composition and glycemic load on energy expenditure following weight loss.

DESIGN, SETTING, AND PARTICIPANTS

A controlled 3-way crossover design involving 21 overweight and obese young adults conducted at Children's Hospital Boston and Brigham and Women's Hospital, Boston, Massachusetts, between June 16, 2006, and June 21, 2010, with recruitment by newspaper advertisements and postings.

INTERVENTION

After achieving 10% to 15% weight loss while consuming a run-in diet, participants consumed an isocaloric low-fat diet (60% of energy from carbohydrate, 20% from fat, 20% from protein; high glycemic load), low-glycemic index diet (40% from carbohydrate, 40% from fat, and 20% from protein; moderate glycemic load), and very low-carbohydrate diet (10% from carbohydrate, 60% from fat, and 30% from protein; low glycemic load) in random order, each for 4 weeks.

MAIN OUTCOME MEASURES

Primary outcome was resting energy expenditure (REE), with secondary outcomes of total energy expenditure (TEE), hormone levels, and metabolic syndrome components.

RESULTS

Compared with the pre-weight-loss baseline, the decrease in REE was greatest with the low-fat diet (mean [95% CI], -205 [-265 to -144] kcal/d), intermediate with the low-glycemic index diet (-166 [-227 to -106] kcal/d), and least with the very low-carbohydrate diet (-138 [-198 to -77] kcal/d; overall P = .03; P for trend by glycemic load = .009). The decrease in TEE showed a similar pattern (mean [95% CI], -423 [-606 to -239] kcal/d; -297 [-479 to -115] kcal/d; and -97 [-281 to 86] kcal/d, respectively; overall P = .003; P for trend by glycemic load < .001). Hormone levels and metabolic syndrome components also varied during weight maintenance by diet (leptin, P < .001; 24-hour urinary cortisol, P = .005; indexes of peripheral [P = .02] and hepatic [P = .03] insulin sensitivity; high-density lipoprotein [HDL] cholesterol, P < .001; non-HDL cholesterol, P < .001; triglycerides, P < .001; plasminogen activator inhibitor 1, P for trend = .04; and C-reactive protein, P for trend = .05), but no consistent favorable pattern emerged.

CONCLUSION

Among overweight and obese young adults compared with pre-weight-loss energy expenditure, isocaloric feeding following 10% to 15% weight loss resulted in decreases in REE and TEE that were greatest with the low-fat diet, intermediate with the low-glycemic index diet, and least with the very low-carbohydrate diet.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00315354.

Responses of gut microbiota to diet composition and weight loss in lean and obese mice

Maintenance of a reduced body weight is accompanied by a decrease in energy expenditure beyond that accounted for by reduced body mass and composition, as well as by an increased drive to eat. These effects appear to be due--in part--to reductions in circulating leptin concentrations due to loss of body fat. Gut microbiota have been implicated in the regulation of body weight. The effects of weight loss on qualitative aspects of gut microbiota have been studied in humans and mice, but these studies have been confounded by concurrent changes in diet composition, which influence microbial community composition. We studied the impact of 20% weight loss on the microbiota of diet-induced obese (DIO: 60% calories fat) mice on a high-fat diet (HFD). Weight-reduced DIO (DIO-WR) mice had the same body weight and composition as control (CON) ad-libitum (AL) fed mice being fed a control diet (10% calories fat), allowing a direct comparison of diet and weight-perturbation effects. Microbial community composition was assessed by pyrosequencing 16S rRNA genes derived from the ceca of sacrificed animals. There was a strong effect of diet composition on the diversity and composition of the microbiota. The relative abundance of specific members of the microbiota was correlated with circulating leptin concentrations and gene expression levels of inflammation markers in subcutaneous white adipose tissue in all mice. Together, these results suggest that both host adiposity and diet composition impact microbiota composition, possibly through leptin-mediated regulation of mucus production and/or inflammatory processes that alter the gut habitat.

Exercise efficiency of low power output cycling

Exercise efficiency at low power outputs, energetically comparable to daily living activities, can be influenced by homeostatic perturbations (e.g., weight gain/loss). However, an appropriate efficiency calculation for low power outputs used in these studies has not been determined. Fifteen active subjects (seven females, eight males) performed 14, 5-min cycling trials: two types of seated rest (cranks vertical and horizontal), passive (motor-driven) cycling, no-chain cycling, no-load cycling, cycling at low (10, 20, 30, 40 W), and moderate (50, 60, 80, 100, 120 W) power outputs. Mean delta efficiency was 57% for low power outputs compared to 41.3% for moderate power outputs. Means for gross (3.6%) and net (5.7%) efficiencies were low at the lowest power output. At low power outputs, delta and work efficiency values exceeded theoretical values. In conclusion, at low power outputs, none of the common exercise efficiency calculations gave values comparable to theoretical muscle efficiency. However, gross efficiency and the slope and intercept of the metabolic power vs mechanical power output regression provide insights that are still valuable when studying homeostatic perturbations.

Role of the hypothalamus in the neuroendocrine regulation of body weight and composition during energy deficit

Energy deficit in lean or obese animals or humans stimulates appetite, reduces energy expenditure and possibly also decreases physical activity, thereby contributing to weight regain. Often overlooked in weight loss trials for obesity, however, is the effect of energy restriction on neuroendocrine status. Negative energy balance in lean animals and humans consistently inhibits activity of the hypothalamo-pituitary-thyroid, -gonadotropic and -somatotropic axes (or reduces circulating insulin-like growth factor-1 levels), while concomitantly activating the hypothalamo-pituitary-adrenal axis, with emerging evidence of similar changes in overweight and obese people during lifestyle interventions for weight loss. These neuroendocrine changes, which animal studies show may result in part from hypothalamic actions of orexigenic (e.g. neuropeptide Y, agouti-related peptide) and anorexigenic peptides (e.g. alpha-melanocyte-stimulating hormone, and cocaine and amphetamine-related transcript), can adversely affect body composition by promoting the accumulation of adipose tissue (particularly central adiposity) and stimulating the loss of lean body mass and bone. As such, current efforts to maximize loss of excess body fat in obese people may inadvertently be promoting long-term complications such as central obesity and associated health risks, as well as sarcopenia and osteoporosis. Future weight loss trials would benefit from assessment of the effects on body composition and key hormonal regulators of body composition using sensitive techniques.

Factors predicting individual variability in diet-induced weight loss in MF1 mice

The effectiveness of caloric restriction (CR) as a treatment for obesity varies considerably between individuals. Reasons for this interindividual variation in weight loss in response to CR may lie in pre-existing individual differences and/or individual differences in compensatory responses. Here we studied the responses of 127 MF1 mice to 30% CR over four weeks, and investigated whether pre-existing differences or compensatory changes in body temperature, resting metabolic rate (RMR) and behavior explained the variation observed in body mass (BM) and fat mass (FM) changes. Mice showed considerable variation in BM loss (36-1%), and in the type of tissue lost (FM or fat free mass, FFM). About 50% of the variation in BM and FM loss could be predicted by pre-existing differences in food intake, RMR, and general activity, where BM loss was greater when food intake was lower and activity and RMR were higher. Compensatory changes in activity and body temperature together explained ~50% of the variation in BM and FM loss in both sexes. In models incorporating baseline variables and compensatory changes, food intake, and activity were the strongest predictors of weight loss in both sexes; i.e., lower baseline food intake and increased changes in activity resulted in greater BM and FM loss. Interestingly, increased baseline activity was a significant predictor of weight loss independent of compensatory changes in activity. Identifying factors involved in individual variability in weight loss may give insights into the mechanisms that underlie this variability, and is important to develop individually tailored weight-management strategies.

Leptin reverses declines in satiation in weight-reduced obese humans

BACKGROUND

Individuals who are weight-reduced or leptin deficient have a lower energy expenditure coupled with higher hunger and disinhibition and/or delayed satiation compared with never-weight-reduced control subjects. Because exogenous leptin inhibits feeding in congenitally leptin-deficient humans, reduced leptin signaling may reduce the expression of feeding inhibition in humans.

OBJECTIVE

The objective was to test the hypothesis that reduced leptin signaling may reduce the expression of feeding inhibition (ie, blunt satiation) in humans by examining the effects of leptin repletion on feeding behavior after weight loss.

DESIGN

Ten obese humans (4 men, 6 women) were studied as inpatients while they received a weight-maintaining liquid-formula diet. Satiation was studied by measuring intake and ratings of appetite-related dispositions 3 h after ingestion of 300 kcal of the liquid-formula diet. The subjects were studied at each of 3 time periods: 1) while they maintained their usual weight (Wt(initial)) and then after weight reduction and stabilization at 10% below initial weight and while they received 5 wk of either 2) twice-daily injections of placebo (Wt(-10%placebo)) or 3) "replacement doses" of leptin (Wt(-10%leptin)) in a single-blind crossover design with a 2-wk washout period between treatments. Energy expenditure was also measured at each study period.

RESULTS

Both energy expenditure and visual analog scale ratings that reflect satiation were significantly lower at Wt(-10%placebo) than at Wt(initial) and Wt(-10%leptin).

CONCLUSION

The results are consistent with the hypothesis that the absence of leptin signaling after weight loss may blunt the expression of feeding inhibition in humans.

Repletion of TNFα or leptin in calorically restricted mice suppresses post-restriction hyperphagia

The causes of post-restriction hyperphagia (PRH) represent a target for drug-based therapies to prevent obesity. However, the factors causing PRH are poorly understood. We show that, in mice, the extent of PRH was independent of the time under restriction, but depended on its severity, suggesting that PRH was driven by signals from altered body composition. Signals related to fat mass were important drivers. Circulating levels of leptin and TNFα were significantly depleted following caloric restriction (CR). We experimentally repleted their levels to match those of controls, and found that in both treatment groups the level of PRH was significantly blunted. These data establish a role for TNFα and leptin in the non-pathological regulation of energy homeostasis. Signals from adipose tissue, including but not limited to leptin and TNFα, regulate PRH and might be targets for therapies that support people engaged in CR to reduce obesity.

Set points, settling points and some alternative models: theoretical options to understand how genes and environments combine to regulate body adiposity

The close correspondence between energy intake and expenditure over prolonged time periods, coupled with an apparent protection of the level of body adiposity in the face of perturbations of energy balance, has led to the idea that body fatness is regulated via mechanisms that control intake and energy expenditure. Two models have dominated the discussion of how this regulation might take place. The set point model is rooted in physiology, genetics and molecular biology, and suggests that there is an active feedback mechanism linking adipose tissue (stored energy) to intake and expenditure via a set point, presumably encoded in the brain. This model is consistent with many of the biological aspects of energy balance, but struggles to explain the many significant environmental and social influences on obesity, food intake and physical activity. More importantly, the set point model does not effectively explain the 'obesity epidemic'--the large increase in body weight and adiposity of a large proportion of individuals in many countries since the 1980s. An alternative model, called the settling point model, is based on the idea that there is passive feedback between the size of the body stores and aspects of expenditure. This model accommodates many of the social and environmental characteristics of energy balance, but struggles to explain some of the biological and genetic aspects. The shortcomings of these two models reflect their failure to address the gene-by-environment interactions that dominate the regulation of body weight. We discuss two additional models--the general intake model and the dual intervention point model--that address this issue and might offer better ways to understand how body fatness is controlled.

Effects of weight loss and leptin on skeletal muscle in human subjects

Maintenance of a 10% or greater reduced body weight results in decreases in the energy cost of low levels of physical activity beyond those attributable to the altered body weight. These changes in nonresting energy expenditure are due mainly to increased skeletal muscle work efficiency following weight loss and are reversed by the administration of the adipocyte-derived hormone leptin. We have also shown previously that the maintenance of a reduced weight is accompanied by a decrease in ratio of glycolytic (phosphofructokinase) to oxidative (cytochrome c oxidase) activity in vastus lateralis muscle that would suggest an increase in the relative expression of the myosin heavy chain I (MHC I) isoform. We performed analyses of vastus lateralis muscle needle biopsy samples to determine whether maintenance of an altered body weight was associated with changes in skeletal muscle metabolic properties as well as mRNA expression of different isoforms of the MHC and sarcoplasmic endoplasmic reticular Ca(2+)-dependent ATPase (SERCA) in subjects studied before weight loss and then again after losing 10% of their initial weight and receiving twice daily injections of either placebo or replacement leptin in a single blind crossover design. We found that the maintenance of a reduced body weight was associated with significant increases in the relative gene expression of MHC I mRNA that was reversed by the administration of leptin as well as an increase in the expression of SERCA2 that was not significantly affected by leptin. Leptin administration also resulted in a significant increase in the expression of the less MHC IIx isoform compared with subjects receiving placebo. These findings are consistent with the leptin-reversible increase in skeletal muscle chemomechanical work efficiency and decrease in the ratio of glycolytic/oxidative enzyme activities observed in subjects following dietary weight loss.

Leptin in human physiology and pathophysiology

Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIV-related lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics.

Biology's response to dieting: the impetus for weight regain

Dieting is the most common approach to losing weight for the majority of obese and overweight individuals. Restricting intake leads to weight loss in the short term, but, by itself, dieting has a relatively poor success rate for long-term weight reduction. Most obese people eventually regain the weight they have worked so hard to lose. Weight regain has emerged as one of the most significant obstacles for obesity therapeutics, undoubtedly perpetuating the epidemic of excess weight that now affects more than 60% of U.S. adults. In this review, we summarize the evidence of biology's role in the problem of weight regain. Biology's impact is first placed in context with other pressures known to affect body weight. Then, the biological adaptations to an energy-restricted, low-fat diet that are known to occur in the overweight and obese are reviewed, and an integrative picture of energy homeostasis after long-term weight reduction and during weight regain is presented. Finally, a novel model is proposed to explain the persistence of the "energy depletion" signal during the dynamic metabolic state of weight regain, when traditional adiposity signals no longer reflect stored energy in the periphery. The preponderance of evidence would suggest that the biological response to weight loss involves comprehensive, persistent, and redundant adaptations in energy homeostasis and that these adaptations underlie the high recidivism rate in obesity therapeutics. To be successful in the long term, our strategies for preventing weight regain may need to be just as comprehensive, persistent, and redundant, as the biological adaptations they are attempting to counter.

Energy substrate partitioning and efficiency in individuals with atherogenic lipoprotein phenotype

Individuals with an atherogenic lipoprotein phenotype (ALP) characterized by increased levels of small dense low-density lipoprotein (LDL) particles tend to have greater adiposity compared to unaffected subjects. We sought to determine whether this may be related to alterations in energy substrate partitioning or efficiency. These were assessed by indirect calorimetry in men with ALP (ALP(+), n = 7) and unaffected controls (ALP(-), n = 8) during rest (30 min) and exercise (10 min). Gross, net and delta efficiencies were calculated during graded leg-cycle ergometry at workloads of 10 and 50 W. Respiratory exchange ratios (RER) were significantly (P < 0.05) higher in ALP(+) vs. ALP(-) during rest (0.86 ± 0.01 vs. 0.83 ± 0.02) and exercise at 10 W (0.88 ± 0.02 vs. 0.84 ± 0.02) and 50 W (0.92 ± 0.01 vs. 0.87 ± 0.01, respectively) (P < 0.05). Lipid oxidation (kcal/min) was lower in ALP(+) vs. ALP(-) during rest (0.56 ± 0.02 vs. 0.71 ± 0.07) and exercise at 10 W (1.52 ± 0.25 vs. 2.00 ± 0.20) and 50 W (1.28 ± 0.10 vs. 2.32 ± 0.22, respectively) (P < 0.05). Gross and net efficiencies were significantly increased (P = 0.005) in ALP(+) vs. ALP(-) at 10 W. RER was correlated positively with plasma triglyceride during exercise and inversely with high-density lipoprotein (HDL) cholesterol and LDL peak particle diameter during rest and exercise (P < 0.05). These findings suggest that increased muscular efficiency at low exercise intensity and reduced lipid oxidation during rest and exercise may contribute to both dyslipidemia and increased adiposity in individuals with ALP.

Analysis of time-dependent adaptations in whole-body energy balance in obesity induced by high-fat diet in rats

Background

High-fat (HF) diet has been extensively used as a model to study metabolic disorders of human obesity in rodents. However, the adaptive whole-body metabolic responses that drive the development of obesity with chronically feeding a HF diet are not fully understood. Therefore, this study investigated the physiological mechanisms by which whole-body energy balance and substrate partitioning are adjusted in the course of HF diet-induced obesity.

Methods

Male Wistar rats were fed ad libitum either a standard or a HF diet for 8 weeks. Food intake (FI) and body weight were monitored daily, while oxygen consumption, respiratory exchange ratio, physical activity, and energy expenditure (EE) were assessed weekly. At week 8, fat mass and lean body mass (LBM), fatty acid oxidation and uncoupling protein-1 (UCP-1) content in brown adipose tissue (BAT), as well as acetyl-CoA carboxylase (ACC) content in liver and epidydimal fat were measured.

Results

Within 1 week of ad libitum HF diet, rats were able to spontaneously reduce FI to precisely match energy intake of control rats, indicating that alterations in dietary energy density were rapidly detected and FI was self-regulated accordingly. Oxygen consumption was higher in HF than controls throughout the study as whole-body fat oxidation also progressively increased. In HF rats, EE initially increased, but then reduced as dark cycle ambulatory activity reached values ~38% lower than controls. No differences in LBM were detected; however, epidydimal, inguinal, and retroperitoneal fat pads were 1.85-, 1.89-, and 2.54-fold larger in HF-fed than control rats, respectively. Plasma leptin was higher in HF rats than controls throughout the study, indicating the induction of leptin resistance by HF diet. At week 8, UCP-1 content and palmitate oxidation in BAT were 3.1- and 1.5-fold higher in HF rats than controls, respectively, while ACC content in liver and epididymal fat was markedly reduced.

Conclusion

The thermogenic response induced by the HF diet was offset by increased energy efficiency and time-dependent reduction in physical activity, favoring fat accumulation. These adaptations were mainly driven by the nutrient composition of the diet, since control and HF animals spontaneously elicited isoenergetic intake.

Regulation of body weight: what is the regulated parameter?

Despite dramatic variations in day to day intake and energy expenditure, weight remains relatively stable in most animals and humans. There are clear physiological responses to over and underfeeding suggesting that the body strives to maintain a constant weight. Despite this, for most humans and experimental animals, there is a tendency for weight to increase slowly over the lifespan. Recent increases in the prevalence of both obesity and anorexia nervosa suggest that factors other than homeostatic physiological mechanisms are important in determining body weight. Clearly reward pathways are activated by palatable food and evidence is emerging that energy balance can modulate these reward pathways and alter the salience of food related stimuli. Significant inhibitory control of reward pathways also comes from a number of brain regions involved in regulation of behavior. Finally there is strong evidence of the important role that social and environmental factors play in modulating both food intake and physical activity behaviors which in turn result in alterations in weight over time. While some aspects of these regulatory systems are within the conscious awareness of people, many, perhaps even most are not. The evidence then would suggest that weight is controlled by several complex regulatory systems that respond to internal metabolic and hormonal signals, hedonic properties of food, internal forces of valuation and self-control, and social factors. Each of these systems is likely 'regulated' and is important in ultimately determining body weight. Experimental paradigms that test one variable in one of these interrelated systems should, where possible control or at least consider the other systems in an effort to provide an integrated picture of weight regulation.

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.

Effects of chronic weight perturbation on energy homeostasis and brain structure in mice

Maintenance of reduced body weight in lean and obese human subjects results in the persistent decrease in energy expenditure below what can be accounted for by changes in body mass and composition. Genetic and developmental factors may determine a central nervous system (CNS)-mediated minimum threshold of somatic energy stores below which behavioral and metabolic compensations for weight loss are invoked. A critical question is whether this threshold can be altered by environmental influences and by what mechanisms such alterations might be achieved. We examined the bioenergetic, behavioral, and CNS structural responses to weight reduction of diet-induced obese (DIO) and never-obese (CON) C57BL/6J male mice. We found that weight-reduced (WR) DIO-WR and CON-WR animals showed reductions in energy expenditure, adjusted for body mass and composition, comparable (-10-15%) to those seen in human subjects. The proportion of excitatory synapses on arcuate nucleus proopiomelanocortin neurons was decreased by ∼50% in both DIO-WR and CON-WR mice. These data suggest that prolonged maintenance of an elevated body weight (fat) alters energy homeostatic systems to defend a higher level of body fat. The synaptic changes could provide a neural substrate for the disproportionate decline in energy expenditure in weight-reduced individuals. This response to chronic weight elevation may also occur in humans. The mouse model described here could help to identify the molecular/cellular mechanisms underlying both the defense mechanisms against sustained weight loss and the upward resetting of those mechanisms following sustained weight gain.

Effect of calorie restriction on the free-living physical activity levels of nonobese humans: results of three randomized trials

The objective of this study was to evaluate the influence of calorie restriction (CR) on free-living physical activity levels among humans. Data were from three CALERIE phase I site-specific protocols. Participants were nonobese (body mass index = 23.5-29.9 kg/m² adults randomly assigned to 25% CR, low-calorie diet (LCD, 890 kcal/day supplement diet until 15% weight loss, then weight maintenance), or control at Pennington Biomedical Research Center (PBRC); 30% or 10% CR at Tufts University; and 20% CR or control at Washington University School of Medicine (WUSM). Activity was measured at months 0, 3, and 6 (PBRC) and at months 0, 3, 6, 9, and 12 (WUSM and Tufts). Total daily energy expenditure (TEE) by doubly labeled water and resting metabolic rate (RMR) were used to compute activity energy expenditure: AEE = TEE - RMR - 0.1 * TEE. Accelerometry and 7-day recall categorized activities by intensity. At Tufts, the 10% and 30% CR groups experienced significant decreases in AEE at months 6, 9, and 12. At month 6, a larger decrease in AEE was observed in the CR than the control group at WUSM. At months 3 and 6, larger decreases in AEE were observed in the CR and LCD groups than the control group at PBRC. Accelerometry and 7-day PAR did not consistently detect changes in activity categories. CR-associated changes in AEE were variable but, generally, reduced the energy deficit, which would reduce the expected rate of weight loss. Accelerometry and recall did not consistently explain reduced AEE, suggesting that increased muscle efficiency and/or decreased fidgeting accounted for decreased AEE. Inaccuracy of accelerometry and recall also likely negatively affected sensitivity.

Mechanical work and metabolic cost of walking after weight loss in obese adolescents

PURPOSE

This study was performed to investigate whether changes in biomechanical parameters of walking explain the reduction in net metabolic cost after weight loss in obese adolescents.

METHODS

Body composition and metabolic and mechanical energy costs of walking at 1.25 m·s(-1) were assessed in 16 obese adolescents before and after a weight loss. Center of mass (COM) and foot accelerations were measured using two inertial sensors and integrated twice to determine COM and foot velocities and displacements. Potential and kinetic energy fluctuations of the COM and the external mechanical work were calculated. Lateral leg swing was calculated from foot displacements.

RESULTS

As expected, the decrease in net metabolic cost was greater, which would have been expected on the basis of the amount of weight loss. The smaller lateral leg swing after weight loss did not explain part of the decrease in net metabolic cost. The reduced body mass required less leg muscle work to raise and accelerate the COM as well as to support body weight. The decrease in body mass seems also associated with a lesser leg muscle work required to raise the COM because of smaller vertical motions. As a result of the inverted pendulum mechanism, the decrease in vertical motions (hence in potential energy fluctuations) was probably related to the decrease in mediolateral kinetic energy fluctuations. Moreover, the lesser amount of fat mass in the gynoid region seems related to the decrease in net metabolic cost of walking.

CONCLUSIONS

The reduction in net metabolic cost of walking after weight loss in weight-reduced adolescents is associated with changes in the biomechanical parameters of walking.

Phenotypic and genetic variation in leptin as determinants of weight regain

AIMS

Over 75% of obese subjects fail to maintain their weight following weight loss interventions. We aimed to identify phenotypic and genetic markers associated with weight maintenance/regain following a dietary intervention.

SUBJECTS AND METHODS

In the 2-year Dietary Intervention Randomized Controlled Trial, we assessed potential predictors for weight changes during the 'weight loss phase' (0-6 months) and the 'weight maintenance/regain phase' (7-24 months). Genetic variation between study participants was studied using single-nucleotide polymorphisms in the leptin gene (LEP).

RESULTS

Mean weight reduction was -5.5% after 6 months, with a mean weight regain of 1.2% of baseline weight during the subsequent 7-24 months. In a multivariate regression model, higher baseline high-molecular-weight adiponectin was the only biomarker predictor of greater success in 0- to 6-month weight loss (β = -0.222, P-value = 0.044). In a multivariate regression model adjusted for 6-month changes in weight and various biomarkers, 6-month plasma leptin reduction exhibited the strongest positive association with 6-month weight loss (β = 0.505, P-value < 0.001). Conversely, 6-month plasma leptin reduction independently predicted weight regain during the following 18 months (β = -0.131, P-value < 0.013). Weight regain was higher among participants who had a greater (top tertiles) 6-month decrease in both weight and leptin (+3.4% (95% confidence interval 2.1-4.8)) as compared with those in the lowest combined tertiles (+0.2% (95% confidence interval -1.1 to 1.4)); P-value < 0.001. Weight regain was further significantly and independently associated with genetic variations in LEP (P = 0.006 for both rs4731426 and rs2071045). Adding genetic data to the phenotypic multivariate model increased its predictive value for weight regain by 34%.

CONCLUSION

Although greater reduction in leptin concentrations during the initial phase of a dietary intervention is associated with greater weight loss in the short term, plasma leptin reduction, combined with the degree of initial weight loss and with genetic variations in the LEP gene, constitutes a significant predictor of subsequent long-term weight regain.

Adaptive thermogenesis in humans

The increasing prevalence of obesity and its comorbidities reflects the interaction of genes that favor the storage of excess energy as fat with an environment that provides ad libitum availability of energy-dense foods and encourages an increasingly sedentary lifestyle. Although weight reduction is difficult in and of itself, anyone who has ever lost weight will confirm that it is much harder to keep the weight off once it has been lost. The over 80% recidivism rate to preweight loss levels of body fatness after otherwise successful weight loss is due to the coordinate actions of metabolic, behavioral, neuroendocrine and autonomic responses designed to maintain body energy stores (fat) at a central nervous system-defined 'ideal'. This 'adaptive thermogenesis' creates the ideal situation for weight regain and is operant in both lean and obese individuals attempting to sustain reduced body weights. Much of this opposition to sustained weight loss is mediated by the adipocyte-derived hormone 'leptin'. The multiple systems regulating energy stores and opposing the maintenance of a reduced body weight illustrate that body energy stores in general and obesity in particular are actively 'defended' by interlocking bioenergetic and neurobiological physiologies. Important inferences can be drawn for therapeutic strategies by recognizing obesity as a disease in which the human body actively opposes the 'cure' over long periods of time beyond the initial resolution of symptomatology.

Spontaneous activity, economy of activity, and resistance to diet-induced obesity in rats bred for high intrinsic aerobic capacity

Though obesity is common, some people remain resistant to weight gain even in an obesogenic environment. The propensity to remain lean may be partly associated with high endurance capacity along with high spontaneous physical activity and the energy expenditure of activity, called non-exercise activity thermogenesis (NEAT). Previous studies have shown that high-capacity running rats (HCR) are lean compared to low-capacity runners (LCR), which are susceptible to cardiovascular disease and metabolic syndrome. Here, we examine the effect of diet on spontaneous activity and NEAT, as well as potential mechanisms underlying these traits, in rats selectively bred for high or low intrinsic aerobic endurance capacity. Compared to LCR, HCR were resistant to the sizeable increases in body mass and fat mass induced by a high-fat diet; HCR also had lower levels of circulating leptin. HCR were consistently more active than LCR, and had lower fuel economy of activity, regardless of diet. Nonetheless, both HCR and LCR showed a similar decrease in daily activity levels after high-fat feeding, as well as decreases in hypothalamic orexin-A content. The HCR were more sensitive to the NEAT-activating effects of intra-paraventricular orexin-A compared to LCR, especially after high-fat feeding. Lastly, levels of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) in the skeletal muscle of HCR were consistently higher than LCR, and the high-fat diet decreased skeletal muscle PEPCK-C in both groups of rats. Differences in muscle PEPCK were not secondary to the differing amount of activity. This suggests the possibility that intrinsic differences in physical activity levels may originate at the level of the skeletal muscle, which could alter brain responsiveness to neuropeptides and other factors that regulate spontaneous daily activity and NEAT.

Identification of body fat mass as a major determinant of metabolic rate in mice

OBJECTIVE

Analysis of energy expenditure (EE) in mice is essential to obesity research. Since EE varies with body mass, comparisons between lean and obese mice are confounded unless EE is normalized to account for body mass differences. We 1) assessed the validity of ratio-based EE normalization involving division of EE by either total body mass (TBM) or lean body mass (LBM), 2) compared the independent contributions of LBM and fat mass (FM) to EE, and 3) investigated whether leptin contributes to the link between FM and EE.

RESEARCH DESIGN AND METHODS

We used regression modeling of calorimetry and body composition data in 137 mice to estimate the independent contributions of LBM and FM to EE. Subcutaneous administration of leptin or vehicle to 28 obese ob/ob mice and 32 fasting wild-type mice was used to determine if FM affects EE via a leptin-dependent mechanism.

RESULTS

Division of EE by either TBM or LBM is confounded by body mass variation. The contribution of FM to EE is comparable to that of LBM in normal mice (expressed per gram of tissue) but is absent in leptin-deficient ob/ob mice. When leptin is administered at physiological doses, the plasma leptin concentration supplants FM as an independent determinant of EE in both ob/ob mice and normal mice rendered leptin-deficient by fasting.

CONCLUSIONS

The contribution of FM to EE is substantially greater than predicted from the metabolic cost of adipose tissue per se, and the mechanism underlying this effect is leptin dependent. Regression-based approaches that account for variation in both FM and LBM are recommended for normalization of EE in mice.

Energy intake in weight-reduced humans

Almost anyone who has ever lost weight can attest that it is harder to sustain weight loss than to lose weight. Maintenance of a 10% or greater reduced body weight is accompanied by decreases in energy expenditure to levels significantly below what is predicted solely on the basis of weight and body composition changes. This disproportionate decline in energy expenditure would not be sufficient to account for the over 80% recidivism rate to pre-weight loss levels of body fatness after otherwise successful weight reduction if there were a corresponding reduction in energy intake. In fact, reduced body weight maintenance is accompanied by increased energy intake above that required to maintain reduced weight. The failure to reduce energy intake in response to decreased energy output reflects decreased satiation and perception of how much food is eaten and multiple changes in neuronal signaling in response to food which conspire with the decline in energy output to keep body energy stores (fat) above a CNS-defined minimum (threshold). Much of this biological opposition to sustained weight loss is mediated by the adipocyte-derived hormone "leptin."

Predicting metabolic adaptation, body weight change, and energy intake in humans

Complex interactions between carbohydrate, fat, and protein metabolism underlie the body's remarkable ability to adapt to a variety of diets. But any imbalances between the intake and utilization rates of these macronutrients will result in changes in body weight and composition. Here, I present the first computational model that simulates how diet perturbations result in adaptations of fuel selection and energy expenditure that predict body weight and composition changes in both obese and nonobese men and women. No model parameters were adjusted to fit these data other than the initial conditions for each subject group (e.g., initial body weight and body fat mass). The model provides the first realistic simulations of how diet perturbations result in adaptations of whole body energy expenditure, fuel selection, and various metabolic fluxes that ultimately give rise to body weight change. The validated model was used to estimate free-living energy intake during a long-term weight loss intervention, a variable that has never previously been measured accurately.

The 24-hour respiratory quotient predicts energy intake and changes in body mass

To define the relationship between the respiratory quotient (RQ) and energy intake (EI) and to determine the impact of spontaneous locomotor activity (LMA) in the development of diet-induced obesity (DIO), we fed C57BL/6 mice a high-fat diet (HFD) for either 4 days or 17 wk and analyzed them using indirect calorimetry. Importantly, changes in body mass during calorimetry (Δ Mb) significantly covaried with RQ and EI; adjusting the data for Δ Mb permitted an analysis of the energy-balanced state. The 24-h RQ strongly predicted 24-h EI, and the slope of this relationship was diet dependent (HFD or chow) but independent of the HFD feeding period. Early-stage DIO was characterized by dark-period hyperphagia and fat storage, offset by greater light-period lipid oxidation; later stage DIO mice had a milder hyperphagia and lower substrate flexibility. Consequently, whereas 24-h RQ equaled the food quotient of the HFD in both early- and late-stage DIO, the range of RQ values was negatively correlated with, and mostly explained by, 24-h EI only in late-stage DIO. Lean and early-stage DIO mice had similar LMA values that were reduced in late-stage DIO. However, LMA significantly explained variance in total energy expenditure (EE) in only early-stage DIO mice. This indicated that the link between LMA and EE was a transient adaptive response to early DIO, whereas the later loss of LMA did not explain body weight gain in C57BL/6 DIO mice.

Effects of experimental weight perturbation on skeletal muscle work efficiency, fuel utilization, and biochemistry in human subjects

Maintenance of a body weight 10% above or below that "customary" for lean or obese individuals results in respective increases or decreases in the energy expended in low levels of physical activity (nonresting energy expenditure, NREE). These changes are greater than can be accounted for by the altered body weight or composition and are due mainly to altered skeletal muscle work efficiency at low levels of power generation. We performed biochemical analysis of vastus lateralis muscle needle biopsy samples to determine whether maintenance of an altered body weight was associated with changes in skeletal muscle histomorphology. We found that the maintenance of a 10% reduced body weight was associated with significant declines in glycolytic (phosphofructokinase, PFK) enzyme activity and, in particular, in the ratio of glycolytic to oxidative (cytochrome c oxidase, COX) enzyme activity without significant changes in the activities of enzymes relevant to mitochondrial density, respiratory chain activity, or fuel transport; or in skeletal muscle fiber type or glycogen stores. The fractional change in the ratio of PFK/COX activity in subjects following weight loss was significantly correlated with changes in the systemic respiratory exchange ratio (RER) and measures of mechanical efficiency of skeletal muscle at low workloads (pedaling a bicycle to generate 10 or 25 W of power). Thus, predictable changes in systemic skeletal muscle biochemistry accompany the maintenance of an altered body weight and account for a significant portion of the variance in skeletal muscle work efficiency and fuel utilization at reduced body weight.

Energy metabolism, fuel selection and body weight regulation

Energy homeostasis is critical for the survival of species. Therefore, multiple and complex mechanisms have evolved to regulate energy intake and expenditure to maintain body weight. For weight maintenance, not only does energy intake have to match energy expenditure, but also macronutrient intake must balance macronutrient oxidation. However, this equilibrium seems to be particularly difficult to achieve in individuals with low fat oxidation, low energy expenditure, low sympathetic activity or low levels of spontaneous physical activity, as in addition to excess energy intake, all of these factors explain the tendency of some people to gain weight. Additionally, large variability in weight change is observed when energy surplus is imposed experimentally or spontaneously. Clearly, the data suggest a strong genetic influence on body weight regulation implying a normal physiology in an 'obesogenic' environment. In this study, we also review evidence that carbohydrate balance may represent the potential signal that regulates energy homeostasis by impacting energy intake and body weight. Because of the small storage capacity for carbohydrate and its importance for metabolism in many tissues and organs, carbohydrate balance must be maintained at a given level. This drive for balance may in turn cause increased energy intake when consuming a diet high in fat and low in carbohydrate. If sustained over time, such an increase in energy intake cannot be detected by available methods, but may cause meaningful increases in body weight. The concept of metabolic flexibility and its impact on body weight regulation is also presented.

Enhanced weight loss with pramlintide/metreleptin: an integrated neurohormonal approach to obesity pharmacotherapy

The neurohormonal control of body weight involves a complex interplay between long-term adiposity signals (e.g., leptin), and short-term satiation signals (e.g., amylin). In diet-induced obese (DIO) rodents, amylin/leptin combination treatment led to marked, synergistic, fat-specific weight loss. To evaluate the weight-lowering effect of combined amylin/leptin agonism (with pramlintide/metreleptin) in human obesity, a 24-week, randomized, double-blind, active-drug-controlled, proof-of-concept study was conducted in obese or overweight subjects (N = 177; 63% female; 39 +/- 8 years; BMI 32.0 +/- 2.1 kg/m(2); 93.3 +/- 13.2 kg; mean +/- s.d.). After a 4-week lead-in period with pramlintide (180 microg b.i.d. for 2 weeks, 360 microg b.i.d. thereafter) and diet (40% calorie deficit), subjects achieving 2-8% weight loss were randomized 1:2:2 to 20 weeks of treatment with metreleptin (5 mg b.i.d.), pramlintide (360 microg b.i.d.), or pramlintide/metreleptin (360 microg/5 mg b.i.d.). Combination treatment with pramlintide/metreleptin led to significantly greater weight loss from enrollment to week 20 (-12.7 +/- 0.9%; least squares mean +/- s.e.) than treatment with pramlintide (-8.4 +/- 0.9%; P < 0.001) or metreleptin (-8.2 +/- 1.3%; P < 0.01) alone (evaluable, N = 93). The greater reduction in body weight was significant as early as week 4, and weight loss continued throughout the study, without evidence of a plateau. The most common adverse events with pramlintide/metreleptin were injection site events and nausea, which were mostly mild to moderate and decreased over time. These results support further development of pramlintide/metreleptin as a novel, integrated neurohormonal approach to obesity pharmacotherapy.

Obesity does not increase external mechanical work per kilogram body mass during walking

Walking is the most common type of physical activity prescribed for the treatment of obesity. The net metabolic rate during level walking (W/kg) is approximately 10% greater in obese vs. normal weight adults. External mechanical work (W(ext)) is one of the primary determinants of the metabolic cost of walking, but the effects of obesity on W(ext) have not been clearly established. The purpose of this study was to compare W(ext) between obese and normal weight adults across a range of walking speeds. We hypothesized that W(ext) (J/step) would be greater in obese adults but W(ext) normalized to body mass would be similar in obese and normal weight adults. We collected right leg three-dimensional ground reaction forces (GRF) while twenty adults (10 obese, BMI=35.6 kg/m(2) and 10 normal weight, BMI=22.1 kg/m(2)) walked on a level, dual-belt force measuring treadmill at six speeds (0.50-1.75 m/s). We used the individual limb method (ILM) to calculate external work done on the center of mass. Absolute W(ext) (J/step) was greater in obese vs. normal weight adults at each walking speed, but relative W(ext) (J/step/kg) was similar between the groups. Step frequencies were not different. These results suggest that W(ext) is not responsible for the greater metabolic cost of walking (W/kg) in moderately obese adults.

Metabolic and behavioral compensations in response to caloric restriction: implications for the maintenance of weight loss

Background

Metabolic and behavioral adaptations to caloric restriction (CR) in free-living conditions have not yet been objectively measured.

Methodology and Principal Findings

Forty-eight (36.8±1.0 y), overweight (BMI 27.8±0.7 kg/m²) participants were randomized to four groups for 6-months; Control: energy intake at 100% of energy requirements; CR: 25% calorie restriction; CR+EX: 12.5% CR plus 12.5% increase in energy expenditure by structured exercise; LCD: low calorie diet (890 kcal/d) until 15% weight reduction followed by weight maintenance. Body composition (DXA) and total daily energy expenditure (TDEE) over 14-days by doubly labeled water (DLW) and activity related energy activity (AREE) were measured after 3 (M3) and 6 (M6) months of intervention. Weight changes at M6 were −1.0±1.1% (Control), −10.4±0.9% (CR), −10.0±0.8% (CR+EX) and −13.9±0.8% (LCD). At M3, absolute TDEE was significantly reduced in CR (−454±76 kcal/d) and LCD (−633±66 kcal/d) but not in CR+EX or controls. At M6 the reduction in TDEE remained lower than baseline in CR (−316±118 kcal/d) and LCD (−389±124 kcal/d) but reached significance only when CR and LCD were combined (−351±83 kcal/d). In response to caloric restriction (CR/LCD combined), TDEE adjusted for body composition, was significantly lower by −431±51 and −240±83 kcal/d at M3 and M6, respectively, indicating a metabolic adaptation. Likewise, physical activity (TDEE adjusted for sleeping metabolic rate) was significantly reduced from baseline at both time points. For control and CR+EX, adjusted TDEE (body composition or sleeping metabolic rate) was not changed at either M3 or M6.

Conclusions

For the first time we show that in free-living conditions, CR results in a metabolic adaptation and a behavioral adaptation with decreased physical activity levels. These data also suggest potential mechanisms by which CR causes large inter-individual variability in the rates of weight loss and how exercise may influence weight loss and weight loss maintenance.

Trial Registration

ClinicalTrials.gov NCT00099151

Molecular physiology of weight regulation in mice and humans

Evolutionary considerations relating to efficiency in reproduction, and survival in hostile environments, suggest that body energy stores are sensed and actively regulated, with stronger physiological and behavioral responses to loss than gain of stored energy. Many physiological studies support this inference, and suggest that a critical axis runs between body fat and the hypothalamus. The molecular cloning of leptin and its receptor-projects based explicitly on the search for elements in this axis-confirmed the existence of this axis and provided important tools with which to understand its molecular physiology. Demonstration of the importance of this soma-brain reciprocal connection in body weight regulation in humans has been pursued using both classical genetic approaches and studies of physiological responses to experimental weight perturbation. This paper reviews the history of the rationale and methodology of the cloning of leptin (Lep) and the leptin receptor (Lepr), and describes some of the clinical investigation characterizing this axis.