The thermic effect of food and obesity: a critical review

A psychobioecological model to understand the income-food insecurity-obesity relationship

Food insecurity, defined by unpredictable access to food that may not meet a person's nutritional needs, is associated with higher BMI (kg/m2) and obesity. People with food insecurity often have less access to food, miss meals and go hungry, which can lead to psychological and metabolic changes that favor energy conservation and weight gain. We describe a conceptual model that includes psychological (food reinforcement and delay discounting) and physiological (thermic effect of food and substrate oxidation) factors to understand how resource scarcity associated with food insecurity evolves into the food insecurity-obesity paradox. We present both animal and human translational research to describe how behavioral and metabolic adaptations to resource scarcity based on behavioral ecology theory may occur for people with food insecurity. We conclude with ideas for interventions to prevent or modify the behaviors and underlying physiology that characterize the income-food insecurity-obesity relationship.

The role of postprandial thermogenesis in the development of impaired glucose tolerance and type II diabetes

Accounting for 5%-15% of total daily energy expenditure, postprandial thermogenesis (PPT) refers to an acute increase in resting metabolic rate (RMR) in the hours after eating. This is largely explained by the energy costs of processing the macronutrients of a meal. Most individuals spend the majority of the day in the postprandial state, thus over one's lifetime even minor differences in PPT may possess true clinical significance. In contrast to RMR, research indicates that PPT may be reduced in the development of both prediabetes and type II diabetes (T2D). The present analysis of existing literature has found that this impairment may be exaggerated in hyperinsulinemic-euglycemic clamp studies compared with food and beverage consumption studies. Nonetheless, it is estimated that daily PPT following carbohydrate consumption alone is approximately 150 kJ lower among individuals with T2D. This estimate fails to consider protein intake, which is notably more thermogenic than carbohydrate intake (20%-30% vs. 5%-8%, respectively). Putatively, dysglycemic individuals may lack the insulin sensitivity required to divert glucose toward storage-a more energy-taxing pathway. Accordingly, the majority of findings has associated an impaired PPT with a reduced "obligatory" energy output (i.e., the energy costs associated with nutrient processing). More recently, it has been reported that "facultative" thermogenesis [e.g., the energy costs associated with sympathetic nervous system (SNS) stimulation] may also contribute to any impairment in PPT among individuals with prediabetes and T2D. Further longitudinal research is required to truly ascertain whether meaningful changes in PPT manifest in the prediabetic state, before the development of T2D.

The Role and Regulatory Mechanism of Brown Adipose Tissue Activation in Diet-Induced Thermogenesis in Health and Diseases

Brown adipose tissue (BAT) has been considered a vital organ in response to non-shivering adaptive thermogenesis, which could be activated during cold exposure through the sympathetic nervous system (SNS) or under postprandial conditions contributing to diet-induced thermogenesis (DIT). Humans prefer to live within their thermal comfort or neutral zone with minimal energy expenditure created by wearing clothing, making shelters, or using an air conditioner to regulate their ambient temperature; thereby, DIT would become an important mechanism to counter-regulate energy intake and lipid accumulation. In addition, there has been a long interest in the intriguing possibility that a defect in DIT predisposes one to obesity and other metabolic diseases. Due to the recent advances in methodology to evaluate the functional activity of BAT and DIT, this updated review will focus on the role and regulatory mechanism of BAT biology in DIT in health and diseases and whether these mechanisms are applicable to humans.

Differential fuel utilization in liver transplant recipients and its relationship with non-alcoholic fatty liver disease

Metabolic flexibility is the ability to match biofuel availability to utilization. Reduced metabolic flexibility, or lower fatty acid (FA) oxidation in the fasted state, is associated with obesity. The present study evaluated metabolic flexibility after liver transplantation (LT).

METHODS

Patients receiving LT for non-alcoholic steatohepatitis (NASH) (n = 35) and non-NASH (n = 10) were enrolled. NASH was chosen as these patients are at the highest risk of metabolic complications. Metabolic flexibility was measured using whole-body calorimetry and expressed as respiratory quotient (RQ), which ranges from 0.7 (pure FA oxidation) to 1.0 is (carbohydrate oxidation).

RESULTS

The two cohorts were similar except for a higher prevalence of obesity and diabetes in the NASH cohort. Post-prandially, RQ increased in both cohorts (i.e. greater carbohydrate utilization) but peak RQ and time at peak RQ was higher in the NASH cohort. Fasting RQ in NASH was significantly higher (0.845 vs. 0.772, p < .001), indicative of impaired FA utilization. In subgroup analysis of the NASH cohort, body mass index but not liver fat content (MRI-PDFF) was an independent predictor of fasting RQ. In NASH, fasting RQ inversely correlated with fat-free muscle volume and directly with visceral adipose tissue.

CONCLUSION

Reduced metabolic flexibility in patients transplanted for NASH cirrhosis may precede the development of non-alcoholic fatty liver disease after LT.

Ketogenic Diet and Weight Loss: Is There an Effect on Energy Expenditure?

A dysregulation between energy intake (EI) and energy expenditure (EE), the two components of the energy balance equation, is one of the mechanisms responsible for the development of obesity. Conservation of energy equilibrium is deemed a dynamic process and alterations of one component (energy intake or energy expenditure) lead to biological and/or behavioral compensatory changes in the counterpart. The interplay between energy demand and caloric intake appears designed to guarantee an adequate fuel supply in variable life contexts. In the past decades, researchers focused their attention on finding efficient strategies to fight the obesity pandemic. The ketogenic or “keto” diet (KD) gained substantial consideration as a potential weight-loss strategy, whereby the concentration of blood ketones (acetoacetate, 3-β-hydroxybutyrate, and acetone) increases as a result of increased fatty acid breakdown and the activity of ketogenic enzymes. It has been hypothesized that during the first phase of KDs when glucose utilization is still prevalent, an increase in EE may occur, due to increased hepatic oxygen consumption for gluconeogenesis and for triglyceride-fatty acid recycling. Later, a decrease in 24-h EE may ensue due to the slowing of gluconeogenesis and increase in fatty acid oxidation, with a reduction of the respiratory quotient and possibly the direct action of additional hormonal signals.

The counterbalancing effects of energy expenditure on body weight regulation: Orexigenic versus energy-consuming mechanisms

OBJECTIVE

Weight change is a dynamic function of whole-body energy balance resulting from the interplay between energy intake and energy expenditure (EE). Recent reports have provided evidence for the existence of a causal effect of EE on energy intake, suggesting that increased EE may drive overeating, thereby promoting future weight gain. This study investigated the relationships between ad libitum energy intake and 24-hour EE (24-h EE) in sedentary conditions versus long-term, free-living weight change using a mediation analysis framework.

METHODS

Native American individuals (n = 61, body fat by dual-energy x-ray absorptiometry: 39.7% [SD 9.5%]) were admitted to the clinical inpatient unit and had baseline measurements as follows: 1) 24-h EE accurately measured in a whole-room indirect calorimeter during energy balance and weight stability; and 2) ad libitum energy intake objectively assessed for 3 days using computerized vending machines. Free-living weight change was assessed after a median follow-up time of 1.7 years (interquartile range: 1.2-2.9).

RESULTS

The total effect of 24-h EE on weight change (-0.23 kg per 100-kcal/d difference in EE at baseline) could be partitioned into the following two independent and counterbalanced effects: higher EE protective against weight gain (-0.46 kg per 100-kcal/d difference in EE at baseline) and an orexigenic effect promoting overeating, thereby favoring weight gain (+0.23 kg per 100-kcal/d difference in EE at baseline).

CONCLUSIONS

The overall impact of EE on body weight regulation should be evaluated by also considering its collateral effect on energy intake. Any weight loss intervention aimed to induce energy deficits by increasing EE should take into account any potential orexigenic effects that promote compensatory overeating, thereby limiting the efficacy of these obesity therapies.

Ubiquitin-like processing of TUG proteins as a mechanism to regulate glucose uptake and energy metabolism in fat and muscle

In response to insulin stimulation, fat and muscle cells mobilize GLUT4 glucose transporters to the cell surface to enhance glucose uptake. Ubiquitin-like processing of TUG (Aspscr1, UBXD9) proteins is a central mechanism to regulate this process. Here, recent advances in this area are reviewed. The data support a model in which intact TUG traps insulin-responsive "GLUT4 storage vesicles" at the Golgi matrix by binding vesicle cargoes with its N-terminus and matrix proteins with its C-terminus. Insulin stimulation liberates these vesicles by triggering endoproteolytic cleavage of TUG, mediated by the Usp25m protease. Cleavage occurs in fat and muscle cells, but not in fibroblasts or other cell types. Proteolytic processing of intact TUG generates TUGUL, a ubiquitin-like protein modifier, as the N-terminal cleavage product. In adipocytes, TUGUL modifies a single protein, the KIF5B kinesin motor, which carries GLUT4 and other vesicle cargoes to the cell surface. In muscle, this or another motor may be modified. After cleavage of intact TUG, the TUG C-terminal product is extracted from the Golgi matrix by the p97 (VCP) ATPase. In both muscle and fat, this cleavage product enters the nucleus, binds PPARγ and PGC-1α, and regulates gene expression to promote fatty acid oxidation and thermogenesis. The stability of the TUG C-terminal product is regulated by an Ate1 arginyltransferase-dependent N-degron pathway, which may create a feedback mechanism to control oxidative metabolism. Although it is now clear that TUG processing coordinates glucose uptake with other aspects of physiology and metabolism, many questions remain about how this pathway is regulated and how it is altered in metabolic disease in humans.

Acute carbohydrate overfeeding: a redox model of insulin action and its impact on metabolic dysfunction in humans

A role for fat overfeeding in metabolic dysfunction in humans is commonly implied in the literature. Comparatively less is known about acute carbohydrate overfeeding (COF). We tested the hypothesis that COF predisposes to oxidative stress by channeling electrons away from antioxidants to support energy storage. In a study of 24 healthy human subjects with and without obesity, COF was simulated by oral administration of excess carbohydrates; a two-step hyperinsulinemic clamp was used to evaluate insulin action. The distribution of electrons between oxidative and reductive pathways was evaluated by the changes in the reduction potentials (Eh) of cytoplasmic (lactate, pyruvate) and mitochondrial (β-hydroxybutyrate, acetoacetate) redox couples. Antioxidant redox was measured by the ratio of reduced to oxidized glutathione. We used cross-correlation analysis to evaluate the relationships between the trajectories of Eh, insulin, glucose, and respiratory exchange during COF. DDIT3 and XBP1s/u mRNA were measured as markers of endoplasmic reticulum stress (ER stress) in adipose tissue before and after COF. Here, we show that acute COF is characterized by net transfer of electrons from mitochondria to cytoplasm. Circulating glutathione is oxidized in a manner that significantly cross-correlates with increasing insulin levels and precedes the decrease in cytoplasmic Eh. This effect is more pronounced in overweight individuals (OW). Markers of ER stress in subcutaneous fat are detectable in OW within 4 h. We conclude that acute COF contributes to metabolic dysfunction through insulin-dependent pathways that promote electron transfer to the cytoplasm and decrease antioxidant capacity. Characterization of redox during overfeeding is important for understanding the pathophysiology of obesity and type 2 diabetes.NEW & NOTEWORTHY Current principles assume that conversion of thermic energy to metabolically useful energy follows fixed rules. These principles ignore the possibility of variable proton uncoupling in mitochondria. Our study shows that the net balance of electron distribution between mitochondria and cytoplasm is influenced by insulin in a manner that reduces proton leakage during overfeeding. Characterization of the effects of insulin on redox balance is important for understanding obesity and insulin resistance.

Dynamic changes in energy expenditure in response to underfeeding: a review

The observation that 64% of English adults are overweight or obese despite a rising prevalence in weight-loss attempts suggests our understanding of energy balance is fundamentally flawed. Weight-loss is induced through a negative energy balance; however, we typically view weight change as a static function, in that energy intake and energy expenditure are independent variables, resulting in a fixed rate of weight-loss assuming a constant energy deficit. Such static modelling provides the basis for the clinical assumption that a 14644 kJ (3500 kcal) deficit translates to a 1 lb weight-loss. However, this '3500 kcal (14644 kJ) rule' is consistently shown to significantly overestimate weight-loss. Static modelling disregards obligatory changes in energy expenditure associated with the loss of metabolically active tissue, i.e. skeletal muscle. Additionally, it disregards the presence of adaptive thermogenesis, the underfeeding-associated fall in resting energy expenditure beyond that caused by loss of fat-free mass. This metabolic manipulation of energy expenditure is observed from the onset of energy restriction to maintain weight at a genetically pre-determined set point. As a result, the observed magnitude of weight-loss is disproportionally less, followed by earlier weight plateau, despite strict compliance to a dietary intervention. By simulating dynamic changes in energy expenditure associated with underfeeding, mathematical modelling may provide a more accurate method of weight-loss prediction. However, accuracy at an individual level is limited due to difficulty estimating energy requirements, physical activity and dietary intake in free-living individuals. In the present paper, we aim to outline the contribution of dynamic changes in energy expenditure to weight-loss resistance and weight plateau.

Energy Balance and Control of Body Weight: Possible Effects of Meal Timing and Circadian Rhythm Dysregulation

Conservation of the energy equilibrium can be considered a dynamic process and variations of one component (energy intake or energy expenditure) cause biological and/or behavioral compensatory changes in the other part of the system. The interplay between energy demand and caloric intake appears designed to guarantee an adequate food supply in variable life contexts. The circadian rhythm plays a major role in systemic homeostasis by acting as “timekeeper” of the human body, under the control of central and peripheral clocks that regulate many physiological functions such as sleep, hunger and body temperature. Clock-associated biological processes anticipate the daily demands imposed by the environment, being synchronized under ideal physiologic conditions. Factors that interfere with the expected demand, including daily distribution of macronutrients, physical activity and light exposure, may disrupt the physiologic harmony between predicted and actual behavior. Such a desynchronization may favor the development of a wide range of disease-related processes, including obesity and its comorbidities. Evidence has been provided that the main components of 24-h EE may be affected by disruption of the circadian rhythm. The sleep pattern, meal timing and meal composition could mediate these effects. An increased understanding of the crosstalk between disruption of the circadian rhythm and energy balance may shed light on the pathophysiologic mechanisms underlying weight gain, which may eventually lead to design effective strategies to fight the obesity pandemic.

An update on brown adipose tissue biology: a discussion of recent findings

Brown adipose tissue (BAT) has been encouraged as a potential treatment for obesity and comorbidities due to its thermogenic activity capacity and contribution to energy expenditure. Some interventions such as cold and β-adrenergic drugs are able to activate BAT thermogenesis as well as promote differentiation of white adipocytes into brown-like cells (browning), enhancing the thermogenic activity of these cells. In this mini-review, we discuss new mechanisms related to BAT and energy expenditure. In this regard, we will also discuss recent studies that have revealed the existence of important secretory molecules from BAT "batokines" that act in autocrine, paracrine, and endocrine mechanisms, which in turn may explain some of the beneficial roles of BAT on whole body glucose and fat metabolism. Finally, we will discuss new insights related to BAT thermogenesis with an additional focus on the distinct features of BAT metabolism between rodents and humans.

Oral Processing, Satiation and Obesity: Overview and Hypotheses

Increasing the speed of eating or decreasing the amount of chewing of a test meal significantly decreases its satiation, increases concomitant caloric intake, and influences entero-endocrine secretion. Speed of eating is a strong risk factor for obesity and longitudinal studies suggest an etiological relationship. Individuals with obesity have an increase in bite size, less chewing per bite, decreased satiation, and greater food intake. Oral processing in terms of bite size and amount of chewing per gram of food is influenced by food texture and textural complexity. Soft foods increase bite size and decrease chewing per gram of food and meal duration compared to hard foods. An ultra-processed diet can lead to greater weight gain than a non-processed diet and a significant increase in eating rate. Many children with obesity are noted by their parents to have persistent hunger on a questionnaire and this is often extreme. Results of attempts to change eating behavior have been mixed in terms of producing long-term changes in eating behavior and body weight. It is hypothesized that there may be a unidirectional relationship between changes in oral processing, satiation and weight gain. However, the presence of persistent hunger can produce a vicious cycle that may exacerbate obesity and make treatment difficult. The increased energy density of foods as found particularly in ultra-processed foods also influences energy intake and obesity.

Adrenergic-Independent Signaling via CHRNA2 Regulates Beige Fat Activation

Maintaining energy homeostasis upon environmental challenges, such as cold or excess calorie intake, is essential to the fitness and survival of mammals. Drug discovery efforts targeting β-adrenergic signaling have not been fruitful after decades of intensive research. We recently identified a new beige fat regulatory pathway mediated via the nicotinic acetylcholine receptor subunit CHRNA2. Here, we generated fat-specific Chrna2 KO mice and observed thermogenic defects in cold and metabolic dysfunction upon dietary challenges caused by adipocyte-autonomous regulation in vivo. We found that CHRNA2 signaling is activated after acute high fat diet feeding and this effect is manifested through both UCP1- and creatine-mediated mechanisms. Furthermore, our data suggested that CHRNA2 signaling may activate glycolytic beige fat, a subpopulation of beige adipocytes mediated by GABPα emerging in the absence of β-adrenergic signaling. These findings reveal the biological significance of the CHRNA2 pathway in beige fat biogenesis and energy homeostasis.

Dietary Factors and Eating Behaviors Affecting Diet-Induced Thermogenesis in Obese Individuals: A Systematic Review

Obese individuals are considered to have lower energy expenditure (EE) rates than non-obese individuals. We aimed to investigate the effects of various factors related to food intake on diet-induced thermogenesis (DIT) in the EE of obese individuals. Using Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, we reviewed relevant literature from PubMed, Embase, and Medline databases from study inception till the end of July 2019. Studies on dietary factors affecting DIT in obese individuals were included. Fifteen studies were included; these studies assessed macronutrient, single-nutrient, or supplement meal compositions, as well as dietary patterns and behaviors. The effect of obesity on DIT was not constant in each study. Differences in DIT pertained to the protein ratio being higher than the fat ratio or the carbohydrate ratio being higher than the fat ratio. High intake of calcium and vitamin D as well as high-oleic peanut supplements increased DIT in obese people. In addition, ascorbic acid intake, fatty acid saturation, and the chain length of various fatty acids had no effects on DIT. In conclusion, the findings suggest that in obese individuals, DIT is associated with various factors related to meal, nutrient, and dietary habits. However, because of the complexity of the relationship between DIT and obesity, it is difficult to determine the critical element underlying this association.

Energy Metabolism and Intermittent Fasting: The Ramadan Perspective

Intermittent fasting (IF) has been gaining popularity as a means of losing weight. The Ramadan fast (RF) is a form of IF practiced by millions of adult Muslims globally for a whole lunar month every year. It entails a major shift from normal eating patterns to exclusive nocturnal eating. RF is a state of intermittent liver glycogen depletion and repletion. The earlier (morning) part of the fasting day is marked by dominance of carbohydrate as the main fuel, but lipid becomes more important towards the afternoon and as the time for breaking the fast at sunset (iftar) gets closer. The practice of observing Ramadan fasting is accompanied by changes in sleeping and activity patterns, as well as circadian rhythms of hormones including cortisol, insulin, leptin, ghrelin, growth hormone, prolactin, sex hormones, and adiponectin. Few studies have investigated energy expenditure in the context of RF including resting metabolic rate (RMR) and total energy expenditure (TEE) and found no significant changes with RF. Changes in activity and sleeping patterns however do occur and are different from non-Ramadan days. Weight changes in the context of Ramadan fast are variable and typically modest with wise inter-individual variation. As well as its direct relevance to many religious observers, understanding intermittent fasting may have implications on weight loss strategies with even broader potential implications. This review examines current knowledge on different aspects of energy balance in RF, as a common model to learn from and also map out strategies for healthier outcomes in such settings.

Metabolic Determinants of Weight Gain in Humans

One of the fundamental challenges in obesity research is to identify subjects prone to weight gain so that obesity and its comorbidities can be promptly prevented or treated. The principles of thermodynamics as applied to human body energetics demonstrate that susceptibility to weight gain varies among individuals as a result of interindividual differences in energy expenditure and energy intake, two factors that counterbalance one another and determine daily energy balance and, ultimately, body weight change. This review focuses on the variability among individuals in human metabolism that determines weight change. Conflicting results have been reported about the role of interindividual differences in energy metabolism during energy balance in relation to future weight change. However, recent studies have shown that metabolic responses to acute, short-term dietary interventions that create energy imbalance, such as low-protein overfeeding or fasting for 24 hours, may reveal the underlying metabolic phenotype that determines the degree of resistance to diet-induced weight loss or the propensity to spontaneous weight gain over time. Metabolically "thrifty" individuals, characterized by a predilection for saving energy in settings of undernutrition and dietary protein restriction, display a minimal increase in plasma fibroblast growth factor 21 concentrations in response to a low-protein overfeeding diet and tend to gain more weight over time compared with metabolically "spendthrift" individuals. Similarly, interindividual variability in the causal relationship between energy expenditure and energy intake ("energy sensing") and in the metabolic response to cold exposure (e.g., brown adipose tissue activation) seems, to some extent, to be indicative of individual propensity to weight gain. Thus, an increased understanding and the clinical characterization of phenotypic differences in energy metabolism among individuals (metabolic profile) may lead to new strategies to prevent weight gain or improve weight-loss interventions by targeted therapies on the basis of metabolic phenotype and susceptibility to obesity in individual persons.

Core body temperature, energy expenditure, and epinephrine during fasting, eucaloric feeding, and overfeeding in healthy adult men: evidence for a ceiling effect for human thermogenic response to diet

BACKGROUND

In homeothermic animals, approximately 50% of daily energy expenditure (EE) is spent to maintain a consistent core body temperature (CBT). In humans, little is known about CBT responses to feeding and overfeeding and their relationship to diet-related changes in EE.

OBJECTIVE

To study the effects of feeding and overfeeding on CBT and its association with diet-induced thermogenesis (DIT).

DESIGN

Fifty-three healthy men with normal glucose regulation and a wide range of body composition (mean ± SD, body fat: 25 ± 8%, range: 7-43%) had 24-h EE assessed during fasting in a whole-room indirect calorimeter with concomitant CBT measurement by ingestible capsules and 24-h urinary collection for catecholamine measurements. Changes in 24-h EE (DIT) and CBT compared to fasting were assessed during three normal-protein (20%) diets using a cross-over design: one eucaloric diet (EBL, 50% carbohydrate, n = 37) and two overfeeding diets with 200% energy requirements: a high-fat (FNP, 60% fat, n = 25) and a high-carbohydrate (CNP; 75% carbohydrate, n = 24) diet.

RESULTS

The average 24-h CBT (avgCBT) during fasting was 36.81 ± 0.14 °C (inter-individual CV = 0.4%) and positively correlated with 24-h urinary epinephrine (r = 0.61, p < 0.001), but not with body composition measures (p > 0.05). AvgCBT increased during EBL (Δ = 0.06 ± 0.11 °C, p = 0.002), FNP (Δ = 0.13 ± 0.14 °C, p < 0.001), and CNP (Δ = 0.19 ± 0.13 °C, p < 0.001) and associated with increased DIT during EBL (r = 0.43, p = 0.01, β = 31 kcal/day/0.1 °C) and FNP (r = 0.60, p = 0.002, β = 43 kcal/day/0.1 °C), but not CNP (p = 0.47). A ceiling effect for the increase in CBT, but not in DIT, was observed during feeding and, particularly, overfeeding.

CONCLUSIONS

CBT increases with feeding and is moderately associated with DIT to a different degree depending on the macronutrient composition of the overfeeding diet. There is a ceiling effect such that individuals with a higher CBT during fasting have limited capacity to increase CBT with feeding. Because of body thermoregulatory mechanisms that maintain a constant CBT, these results indicate that CBT has a limited role in the inter-individual variability in DIT.

Effects of Polyphenols on Thermogenesis and Mitochondrial Biogenesis

Obesity is a health problem worldwide, and energy imbalance has been pointed out as one of the main factors responsible for its development. As mitochondria are a key element in energy homeostasis, the development of obesity has been strongly associated with mitochondrial imbalance. Polyphenols are the largest group of phytochemicals, widely distributed in the plant kingdom, abundant in fruits and vegetables, and have been classically described as antioxidants owing to their well-established ability to eliminate free radicals and reactive oxygen species (ROS). During the last decade, however, growing evidence reports the ability of polyphenols to perform several important biological activities in addition to their antioxidant activity. Special attention has been given to the ability of polyphenols to modulate mitochondrial processes. Thus, some polyphenols are now recognized as molecules capable of modulating pathways that regulate mitochondrial biogenesis, ATP synthesis, and thermogenesis, among others. The present review reports the main benefits of polyphenols in modulating mitochondrial processes that favor the regulation of energy expenditure and offer benefits in the management of obesity, especially thermogenesis and mitochondrial biogenesis.

Diet-induced thermogenesis: fake friend or foe?

Diet-induced thermogenesis (DIT) is energy dissipated as heat after a meal, contributing 5-15% to total daily energy expenditure (EE). There has been a long interest in the intriguing possibility that a defect in DIT predisposes to obesity. However, the evidence is conflicting; DIT is usually quantified by indirect calorimetry, which does not measure heat. Using gas exchange, indirect calorimetry measures total post-prandial EE, which comprises heat energy produced from brown adipose tissue (BAT) and energy required for processing and storing nutrients. We questioned whether DIT is reliably quantified by indirect calorimetry by employing infrared thermography to independently assess thermogenesis. Thermogenic activity of BAT was stimulated by cold and by a meal that induced a parallel increase in energy production. These stimulatory effects on BAT thermogenesis were inhibited by glucocorticoids. However, glucocorticoids enhanced postprandial EE in the face of reduced BAT thermogenesis and stimulated lipid synthesis. The increase in EE correlated significantly with the increase in lipogenesis. As energy cannot be destroyed (first law of thermodynamics), the energy that would have been dissipated as heat after a meal is channeled into storage. Post-prandial EE is the sum of heat energy that is lost (true DIT) and chemical energy that is stored. Indirect calorimetry does not reliably quantify DIT. When estimated by indirect calorimetry, assumed DIT can be a friend or foe of energy balance. That gas exchange-derived DIT reflects solely energy dissipation as heat is a false assumption likely to explain the conflicting results on the role of DIT in obesity.

Increased thermogenesis by a noncanonical pathway in ANGPTL3/8-deficient mice

Dietary triglyceride (TG) is the most efficient energy substrate. It is processed and stored at substantially lower metabolic cost than is protein or carbohydrate. In fed animals, circulating TGs are preferentially routed for storage to white adipose tissue (WAT) by angiopoietin-like proteins 3 (A3) and 8 (A8). Here, we show that mice lacking A3 and A8 (A3^-/-A8^-/- mice) have decreased fat mass and a striking increase in temperature (+1 °C) in the fed (but not fasted) state, without alterations in food intake or physical activity. Subcutaneous WAT (WAT-SQ) from these animals had morphologic and metabolic changes characteristic of beiging. O₂ consumption rates (OCRs) and expression of genes involved in both fatty acid synthesis and fatty acid oxidation were increased in WAT-SQ of A3^-/-A8^-/- mice, but not in their epididymal or brown adipose tissue (BAT). The hyperthermic response to feeding was blocked by maintaining A3^-/-A8^-/- mice at thermoneutrality or by treating with a β3-adrenergic receptor (AR) antagonist. To determine if sympathetic stimulation was sufficient to increase body temperature in A3^-/-A8^-/- mice, WT and A3^-/-A8^-/- animals were maintained at thermoneutrality and then treated with a β3-AR agonist; treatment induced hyperthermia in A3^-/-A8^-/- , but not WT, mice. Antibody-mediated inactivation of both circulating A3 and A8 induced hyperthermia in WT mice. Together, these data indicate that A3 and A8 are essential for efficient storage of dietary TG and that disruption of these genes increases feeding-induced thermogenesis and energy utilization.

Convergence between biological, behavioural and genetic determinants of obesity

Multiple biological, behavioural and genetic determinants or correlates of obesity have been identified to date. Genome-wide association studies (GWAS) have contributed to the identification of more than 100 obesity-associated genetic variants, but their roles in causal processes leading to obesity remain largely unknown. Most variants are likely to have tissue-specific regulatory roles through joint contributions to biological pathways and networks, through changes in gene expression that influence quantitative traits, or through the regulation of the epigenome. The recent availability of large-scale functional genomics resources provides an opportunity to re-examine obesity GWAS data to begin elucidating the function of genetic variants. Interrogation of knockout mouse phenotype resources provides a further avenue to test for evidence of convergence between genetic variation and biological or behavioural determinants of obesity.

Effects of the light-dark cycle on diurnal rhythms of diet-induced thermogenesis in humans

This study aimed to clarify the effect of light exposure during the daytime and nighttime on diet-induced thermogenesis (DIT), which is one kind of energy expenditure, and the contribution of autonomic nervous activities (ANA) to the mechanism behind such effects. We found that the light-dark cycle significantly induced a diurnal rhythm of DIT, with afternoon levels tending to be higher than nighttime levels. By contrast, no such rhythms were observed under constant light or dark conditions. There were also no significant differences in ANA between the light conditions. These findings demonstrate that a diminished light-dark cycle leads to disruption of the diurnal rhythm of metabolism and so the retention of ordinary light-dark cycles may be recommended for health maintenance.

Gut-Brain Cross-Talk in Metabolic Control

Because human energy metabolism evolved to favor adiposity over leanness, the availability of palatable, easily attainable, and calorically dense foods has led to unprecedented levels of obesity and its associated metabolic co-morbidities that appear resistant to traditional lifestyle interventions. However, recent progress identifying the molecular signaling pathways through which the brain and the gastrointestinal system communicate to govern energy homeostasis, combined with emerging insights on the molecular mechanisms underlying successful bariatric surgery, gives reason to be optimistic that novel precision medicines that mimic, enhance, and/or modulate gut-brain signaling can have unprecedented potential for stopping the obesity and type 2 diabetes pandemics.

Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition

Weight changes are accompanied by imbalances between calorie intake and expenditure. This fact is often misinterpreted to suggest that obesity is caused by gluttony and sloth and can be treated by simply advising people to eat less and move more. Rather various components of energy balance are dynamically interrelated and weight loss is resisted by counterbalancing physiological processes. While low-carbohydrate diets have been suggested to partially subvert these processes by increasing energy expenditure and promoting fat loss, our meta-analysis of 32 controlled feeding studies with isocaloric substitution of carbohydrate for fat found that both energy expenditure (26 kcal/d; P <.0001) and fat loss (16 g/d; P <.0001) were greater with lower fat diets. We review the components of energy balance and the mechanisms acting to resist weight loss in the context of static, settling point, and set-point models of body weight regulation, with the set-point model being most commensurate with current data.

Reliability of low-power cycling efficiency in energy expenditure phenotyping of inactive men and women

Standardized approaches to assess human energy expenditure (EE) are well defined at rest and at moderate to high-intensity exercise, but not at light intensity physical activities energetically comparable with those of daily life (i.e., 1.5-4 times the resting EE, i.e., 1.5-4 METs). Our aim was to validate a graded exercise test for assessing the energy cost of low-intensity dynamic work in physically inactive humans, that is, those who habitually do not meet the guidelines for moderate-to-vigorous aerobic physical activity levels. In healthy and inactive young men and women (n = 55; aged 18-32 years), EE was assessed in the overnight-fasted state by indirect calorimetry at rest and during graded cycling between 5 and 50W for 5 min at each power output on a bicycle ergometer. Repeatability was investigated on three separate days, and the effect of cadence was investigated in the range of 40-90 rpm. Within the low power range of cycling, all subjects perceived the exercise test as "light" on the Borg scale, the preferred cadence being 60 rpm. A strong linearity of the EE-power relationship was observed between 10 and 50 W for each individual (r > 0.98), and the calculation of delta efficiency (DE) from the regression slope indicated that DE was similar in men and women (~29%). DE showed modest inter-individual variability with a coefficient of variation (CV) of 11%, and a low intra-individual variability with a CV of ~ 5%. No habituation or learning effect was observed in DE across days. In conclusion, the assessment of the efficiency of low power cycling by linear regression - and conducted within the range of EE observed for low-intensity movements of everyday life (1.5-4 METs) - extends the capacity for metabolic phenotyping in the inactive population.

Postprandial Increase in Energy Expenditure Correlates with Body Weight Reduction in Patients with Type 2 Diabetes Receiving Diet Therapy

AIM

The clinical significance of energy expenditure (EE) in the treatment of type 2 diabetes has not been fully elucidated. Here we analyzed the relationships between EE and clinical measurements in patients with type 2 diabetes receiving diet therapy.

METHODS

A total of 100 patients (34 women and 66 men) with type 2 diabetes admitted to our hospital for glycemic control were enrolled. The participants received an energy-restricted diet during their hospitalization (median, 15 days). EE was measured in the fasted (FEE) and postprandial (PPEE) states using indirect calorimetry. The postprandial increment of EE (ΔEE) was calculated from the FEE and PPEE (ΔEE=PPEE-FEE).

RESULTS

FEE, PPEE, and ΔEE were 0.997±0.203, 1.104±0.213, and 0.107±0.134 kcal/min, respectively. Body weight decreased from 68.7±16.6 to 66.8±16.0 kg (p＜0.0001) during hospitalization. FEE and PPEE showed positive correlations with height, body weight, body mass index, and abdominal circumference at admission, but ΔEE was not correlated with these anthropometric measurements. On the other hand, ΔEE was inversely correlated with the body weight change. The association between ΔEE and the body weight change was independent of age, sex, and HbA1c.

CONCLUSIONS

Postprandial increase in energy expenditure may be a determinant of individual differences in weight reduction in patients with type 2 diabetes on diet therapy. As a simple surrogate for diet-induced thermogenesis, ΔEE may serve as a useful predictive marker for the efficacy of diet therapy.

The effect of exercise on non-exercise physical activity and sedentary behavior in adults

It is widely assumed that structured exercise causes an additive increase in physical activity energy expenditure (PAEE) and total daily energy expenditure (TDEE). However, the common observation that exercise often leads to a less than expected decrease in body weight, without changes in energy intake, suggests that some compensatory behavioral adaptations occur. A small number of human studies have shown that adoption of structured exercise can lead to decreases in PAEE, which is often interpreted as a decrease in physical activity (PA) behavior. An even smaller number of studies have objectively measured PA, and with inconsistent results. In animals, high levels of imposed PA induce compensatory changes in some components of TDEE. Recent human cohort studies also provide evidence that in those at the highest levels of PA, TDEE is similar when compared to less physically active groups. The objective of this review is to summarize the effects of structured exercise training on PA, sedentary behavior, PAEE and TDEE. Using models from ecological studies in animals and observational data in humans, an alternative model of TDEE in humans is proposed. This model may serve as a framework to investigate the complex and dynamic regulation of human energy budgets.

Effects of prednisolone on energy and fat metabolism in patients with rheumatoid arthritis: tissue-specific insulin resistance with commonly used prednisolone doses

OBJECTIVE

Glucocorticoids can cause postprandial hyperglycaemia, but the effects on postprandial energy and fat metabolism are uncertain. We investigated the effects of acute and chronic low-dose prednisolone on fasting and postprandial energy expenditure and substrate metabolism.

DESIGN

An open interventional and cross-sectional study was undertaken.

PATIENTS AND MEASUREMENTS

Eighteen patients who had not taken oral glucocorticoids for ≥6 months were studied before and after 7 days prednisolone (6 mg/day) to assess the acute effects of prednisolone. Baseline data from patients, not on glucocorticoids, were compared with 18 patients on long-term prednisolone (6·5 ± 1·8 mg/day for >6 months) to assess the chronic effects. Energy expenditure and substrate oxidation were measured using indirect calorimetry before and after a mixed meal. Adipocyte insulin resistance index and insulin-mediated suppression of NEFA were calculated from fasting and postprandial insulin and NEFA concentrations.

RESULTS

There were no significant differences in resting energy expenditure or diet-induced thermogenesis with prednisolone. Acute (-2·1 ± 6·2 vs -16·3 ± 4·8 mg/min, P = 0·01) and chronic (-1·4 ± 2·8 vs -16·3 ± 4·8 mg/min, P = 0·01) prednisolone attenuated postprandial suppression of fat oxidation. Chronic (31·6 ± 3·8 vs 17·0 ± 3·3, P = 0·007), but not acute, prednisolone increased adipocyte insulin resistance index. However, insulin-mediated suppression of NEFA was not significantly different after acute or chronic prednisolone.

CONCLUSIONS

Prednisolone does not alter energy expenditure. However, even at low doses, prednisolone exerts adverse effects on fat metabolism, which could exacerbate insulin resistance and increase cardiovascular risk. Attenuated postprandial suppression of fat oxidation, but not lipolysis, suggests that prednisolone causes greater insulin resistance in skeletal muscle than in adipocytes.

Analysis of energy metabolism in humans: A review of methodologies

Background

Obesity is a consequence of chronic energy imbalance. We need accurate and precise measurements of energy intake and expenditure, as well as the related behaviors, to fully understand how energy homeostasis is regulated in order to develop interventions and evaluate their effectiveness to combat the global obesity epidemic.

Scope of review

We provide an in-depth review of the methodologies currently used to measure energy intake and expenditure in humans, including their principles, advantages, and limitations in the clinical research setting. The aim is to provide researchers with a comprehensive guide to conduct obesity research of the highest possible quality.

Major conclusions

An array of methodologies is available to measure various aspects of energy metabolism and none is perfect under all circumstances. The choice of methods should be specific to particular research questions with practicality and quality of data the priorities for consideration. A combination of complementary measurements may be preferable. There is an imperative need to develop new methodologies to improve the accuracy and precision of energy intake assessments.

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Image-based technology is a significant step to improve energy intake measurement.

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Physical activity informs patterns but not absolute energy expenditure.

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Combining complementary measurements overcomes shortfalls of individual methods.

A novel approach to calculating the thermic effect of food in a metabolic chamber

The thermic effect of food (TEF) is the well‐known concept in spite of its difficulty for measuring. The gold standard for evaluating the TEF is the difference in energy expenditure between fed and fasting states (ΔEE). Alternatively, energy expenditure at 0 activity (EE₀) is estimated from the intercept of the linear relationship between energy expenditure and physical activity to eliminate activity thermogenesis from the measurement, and the TEF is calculated as the difference between EE₀ and postabsorptive resting metabolic rate (RMR) or sleeping metabolic rate (SMR). However, the accuracy of the alternative methods has been questioned. To improve TEF estimation, we propose a novel method as our original TEF calculation method to calculate EE₀ using integrated physical activity over a specific time interval. We aimed to identify which alternative methods of TEF calculation returns reasonable estimates, that is, positive value as well as estimates close to ΔEE. Seven men participated in two sessions (with and without breakfast) of whole‐body indirect calorimetry, and physical activity was monitored with a triaxial accelerometer. Estimates of TEF by three simplified methods were compared to ΔEE. ΔEE, EE₀ above SMR, and our original method returned positive values for the TEF after breakfast in all measurements. TEF estimates of our original method was indistinguishable from those based on the ΔEE, whereas those as EE₀ above RMR and EE₀ above SMR were slightly lower and higher, respectively. Our original method was the best among the three simplified TEF methods as it provided positive estimates in all the measurements that were close to the value derived from gold standard for all measurements.

MECHANISMS IN ENDOCRINOLOGY: Brown adipose tissue in humans: regulation and metabolic significance

The recent discovery that functional brown adipose tissue (BAT) persists in adult humans has enkindled a renaissance in metabolic research, with a view of harnessing its thermogenic capacity to combat obesity. This review focuses on the advances in the regulation and the metabolic significance of BAT in humans. BAT activity in humans is stimulated by cold exposure and by several factors such as diet and metabolic hormones. BAT function is regulated at two levels: an acute process involving the stimulation of the intrinsic thermogenic activity of brown adipocytes and a chronic process of growth involving the proliferation of pre-existing brown adipocytes or differentiation to brown adipocytes of adipocytes from specific white adipose tissue depots. BAT activity is reduced in the obese, and its stimulation by cold exposure increases insulin sensitivity and reduces body fat. These observations provide strong evidence that BAT plays a significant role in energy balance in humans and has the potential to be harnessed as a therapeutic target for the management of obesity.

The thermic response to food intake in persons with thoracic spinal cord injury

[Purpose] To investigate the influence of the level of spinal cord injury on the thermic effect of food intake (TEF) in persons with thoracic spinal cord injury. [Subjects and Methods] Seven male subjects with spinal cord injury (SCI; age, 40 ± 6 years) and six able-bodied subjects (AB; age, 37 ± 8 years) volunteered to participate in the present study. The subjects consumed an identical test meal consisting of 7.9 kcal/kg of body weight. Energy expenditure and plasma norepinephrine concentrations were measured over a 3-hour period. [Results] The adjusted TEF at 60 min was almost the same among the three groups [AB, SCI with high thoracic cord (T5–6) injury (HSCI), and SCI with low thoracic cord (T9–12) injury (LSCI)]. Although the LSCI group had almost the same adjusted TEF at 120 min as the AB group, the adjusted TEF at 120 min of the HSCI group was significantly lower than that of the AB group. The changes in plasma norepinephrine concentration and heart rate in response to food intake were similar among the three groups. [Conclusion] SCI at the T5–6 level results in a lower TEF due to sympathetic decentralization.

Determinants of body weight regulation in humans

Body weight is regulated by the ability of hypothalamic neurons to orchestrate behavioral, endocrine and autonomic responses via afferent and efferent pathways to the brainstem and the periphery. Weight maintenance requires a balance between energy intake and energy expenditure. Although several components that participate in energy homeostasis have been identified, there is a need to know in more detail their actions as well as their interactions with environmental and psychosocial factors in the development of human obesity. In this review, we examine the role of systemic mediators such as leptin, ghrelin and insulin, which act in the central nervous system by activating or inhibiting neuropeptide Y, Agouti-related peptide protein, melanocortin, transcript related to cocaine and amphetamine, and others. As a result, modifications in energy homeostasis occur through regulation of appetite and energy expenditure. We also examine compensatory changes in the circulating levels of several peripheral hormones after diet-induced weight loss.

Metabolically healthy obese individuals: Key protective factors

OBJECTIVES

Obesity is a significant quality of life-impairing health problem affecting industrialized nations. However, despite carrying a large fat mass, some very obese individuals exhibit normal metabolic profiles (metabolically healthy obesity). The physiological factors underlying their protective and favorable metabolic profiles remain poorly defined.

METHODS

A search of the National Library of Medicine PubMed database was performed using the following keywords: Metabolically healthy obese, metabolically normal obese, insulin resistance, metabolically unhealthy normal weight, and uncomplicated obesity.

RESULTS

This article reviewed factors associated with severe obesity that lacks complications, and suggests putative activities by which these obese individuals avoid developing the clinical features of metabolic syndrome, or the metabolic complications associated with severe obesity.

CONCLUSIONS

Despite the knowledge that visceral fat deposition is the seminal factor that ultimately causes insulin resistance (IR) and the detrimental inflammatory and hormonal profile that contributes to increase risk for cardiovascular disease, it remains unknown whether metabolically healthy obesity (MHO) has genetic predisposing factors, and whether MHO ultimately succumbs to IR and the metabolic syndrome, indicating a need for prophylatic bariatric surgery.

Postprandial metabolism in adults with Prader-Willi syndrome

OBJECTIVE

Individuals with Prader-Willi syndrome (PWS) are commonly restricted to 60-75% of height-appropriate calorie intake because they rapidly become obese on a normal diet. This study measured changes in energy expenditure, glucose and lipid homeostasis, and metabolic flexibility in response to a meal in PWS adults.

METHODS

11 adults with PWS were compared with 12 adiposity-matched and 10 lean subjects. Indirect calorimetry was conducted at baseline and 210 min after a standardized 600 kCal breakfast to assess energy expenditure and substrate utilization. Circulating glucose, insulin, C-peptide, glucagon, nonesterified fatty acids, and triglycerides were measured up to 240 min. Insulin sensitivity and insulin secretion rate were assessed by HOMA-IR and C-peptide deconvolution, respectively. Body composition was determined by dual-energy X-ray absorptiometry.

RESULTS

The PWS group had lower lean mass than the obesity control group. Corrected for lean mass, there were no differences between the PWS and obesity groups in resting metabolic rate or metabolic flexibility. Total and abdominal fat mass, insulin sensitivity, and insulin secretion rate were also similar between these groups.

CONCLUSIONS

This study did not detect an intrinsic metabolic defect in individuals with PWS. Rather, lower lean mass, combined with lower physical activity, may contribute to weight gain on an apparent weight-maintenance diet.

Obesity and cardiovascular diseases: implications regarding fitness, fatness, and severity in the obesity paradox

Obesity has been increasing in epidemic proportions, with a disproportionately higher increase in morbid or class III obesity, and obesity adversely affects cardiovascular (CV) hemodynamics, structure, and function, as well as increases the prevalence of most CV diseases. Progressive declines in physical activity over 5 decades have occurred and have primarily caused the obesity epidemic. Despite the potential adverse impact of overweight and obesity, recent epidemiological data have demonstrated an association of mild obesity and, particularly, overweight on improved survival. We review in detail the obesity paradox in CV diseases where overweight and at least mildly obese patients with most CV diseases seem to have a better prognosis than do their leaner counterparts. The implications of cardiorespiratory fitness with prognosis are discussed, along with the joint impact of fitness and adiposity on the obesity paradox. Finally, in light of the obesity paradox, the potential value of purposeful weight loss and increased physical activity to affect levels of fitness is reviewed.

Acute effect on satiety, resting energy expenditure, respiratory quotient, glucagon-like peptide-1, free fatty acids, and glycerol following consumption of a combination of bioactive food ingredients in overweight subjects

OBJECTIVE

A combination of bioactive food ingredients (capsaicinoids, epigallocatechin gallate, piperin, and l-carnitine, CBFI) may promote satiety and thermogenesis. The study was conducted in order to assess whether there is any effect on satiety, resting energy expenditure (REE), respiratory quotient, glucagon-like peptide-1 (GLP-1), free fatty acids (FFA) and glycerol release, following a standardized mixed meal with or without single consumption of a CBFI.

DESIGN

An 8-week randomized double-blind placebo-controlled trial.

SETTING

Dietetic and Metabolic Unit, Azienda di Servizi alla Persona, University of Pavia and "Villa delle Querce" Clinical Rehabilitation Institute, Rome, Italy.

PARTICIPANTS

Thirty-seven overweight adults (body mass index [BMI]: 25-35).

INTERVENTION

Nineteen overweight subjects were included in the supplemented group (14 women, 5 men; age 46.4 ± 6.4; BMI: 30.5 ± 3.3) and 18 in the placebo group (13 women, 5 men; age 40.8 ± 11.5; BMI: 30.1 ± 2.6). Satiety was assessed using 100-mm visual analogue scales (VAS) and the area under the curve was calculated.

RESULTS

All measured parameters increased significantly in comparison with baseline in response to meal, both with CBFI and with placebo. However, throughout the study day, the supplemented group experienced a significantly greater increase than the placebo group in their sensation of satiety following acute administration of the supplement.

CONCLUSION

CBFI may therefore be of great value in the treatment of overweight patients by increasing satiety and stimulating thermogenesis.

The carbohydrate sensitive rat as a model of obesity

BACKGROUND

Sensitivity to obesity is highly variable in humans, and rats fed a high fat diet (HFD) are used as a model of this inhomogeneity. Energy expenditure components (basal metabolism, thermic effect of feeding, activity) and variations in substrate partitioning are possible factors underlying the variability. Unfortunately, in rats as in humans, results have often been inconclusive and measurements usually made after obesity onset, obscuring if metabolism was a cause or consequence. Additionally, the role of high carbohydrate diet (HCD) has seldom been studied.

METHODOLOGY/FINDINGS

Rats (n=24) were fed for 3 weeks on HCD and then 3 weeks on HFD. Body composition was tracked by MRI and compared to energy expenditure components measured prior to obesity.

RESULTS

1) under HFD, as expected, by adiposity rats were variable enough to be separable into relatively fat resistant (FR) and sensitive (FS) groups, 2) under HCD, and again by adiposity, rats were also variable enough to be separable into carbohydrate resistant (CR) and sensitive (CS) groups, the normal body weight of CS rats hiding viscerally-biased fat accumulation, 3) HCD adiposity sensitivity was not related to that under HFD, and both HCD and HFD adiposity sensitivities were not related to energy expenditure components (BMR, TEF, activity cost), and 4) only carbohydrate to fat partitioning in response to an HCD test meal was related to HCD-induced adiposity.

CONCLUSIONS/SIGNIFICANCE

The rat model of human obesity is based on substantial variance in adiposity gains under HFD (FR/FS model). Here, since we also found this phenomenon under HCD, where it was also linked to an identifiable metabolic difference, we should consider the existence of another model: the carbohydrate resistant (CR) or sensitive (CS) rat. This new model is potentially complementary to the FR/FS model due to relatively greater visceral fat accumulation on a low fat high carbohydrate diet.

Extent and determinants of thermogenic responses to 24 hours of fasting, energy balance, and five different overfeeding diets in humans

CONTEXT

Individual variation in the ability to convert excess calories to heat and the effects of dietary macronutrient composition are unclear.

OBJECTIVE

Stability and determinants of the energy expenditure (EE) response to overconsumption were assessed.

DESIGN, SETTING, AND PARTICIPANTS

Twenty subjects (75% male) with normal glucose regulation were evaluated during 24 hours each of energy balance, fasting, and 5 different diets with 200% energy requirements in a clinical research unit.

INTERVENTIONS

Five 1-day overfeeding diets were given in random order: high carbohydrate (75%) and low protein (3%); high carbohydrate and normal protein (20%); high fat (46%) and low protein; high fat (60%) and normal protein; and balanced (50% carbohydrates, 20% protein).

MAIN OUTCOME MEASURES

The 24-hour EE, sleeping EE, and thermic effect of food (TEF) during each diet were measured with a metabolic chamber. Appetitive hormones were measured before and after the diets.

RESULTS

The EE response to overfeeding exhibited good intraindividual reproducibility. Similar increases above eucaloric feeding in 24-hour EE (mean 10.7 ± 5.7%, P < .001; range 2.9-18.8%) and sleeping EE (14.4 ± 11.3%, P < .001; range 1.0-45.1%) occurred when overfeeding diets containing 20% protein, despite differences in fat and carbohydrate content, but the EE response during overfeeding diets containing 3% protein was attenuated. The percent body fat negatively correlated with TEF during normal protein overfeeding (r = -0.53, P < .01). Fasting peptide YY negatively correlated with TEF (r = -0.56, P < .01) and the increase in sleeping EE (r = -0.54, P < .01) during overfeeding.

CONCLUSIONS

There is an intrinsic EE response to overfeeding that negatively associates with adiposity, although it represents a small percentage of consumed calories.

Brown adipose tissue in adult humans: a metabolic renaissance

Brown adipose tissue (BAT) plays a key role in energy homeostasis and thermogenesis in animals, conferring protection against diet-induced obesity and hypothermia through the action of uncoupling protein 1 (UCP1). Recent metabolic imaging studies using positron emission tomography computerized tomography (PET-CT) scanning have serendipitously revealed significant depots of BAT in the cervical-supraclavicular regions, demonstrating persistence of BAT beyond infancy. Subsequent cold-stimulated PET-CT studies and direct histological examination of adipose tissues have demonstrated that BAT is highly prevalent in adult humans. BAT activity correlates positively with increment of energy expenditure during cold exposure and negatively with age, body mass index, and fasting glycemia, suggesting regulatory links between BAT, cold-induced thermogenesis, and energy metabolism. Human BAT tissue biopsies express UCP1 and harbor inducible precursors that differentiate into UCP1-expressing adipocytes in vitro. These recent discoveries represent a metabolic renaissance for human adipose biology, overturning previous belief that BAT had no relevance in adult humans. They also have implications for the understanding of the pathogenesis and treatment of obesity and its metabolic sequelae.

The carbon dioxide production rate assumption biases gastric emptying parameters in healthy adults

RATIONALE

An altered gastric emptying (GE) rate has been implicated in the aetiology of obesity. The (13)C-octanoic acid breath test (OBT) is frequently used to measure GE, and the cumulative percentage of (13)C recovered (cPDR) is a common outcome measure. However, true cPDR in breath is dependent on accurate measurement of carbon dioxide production rate (VCO(2)). The current study aimed to quantify differences in the (13)C OBT results obtained using directly measured VCO(2) (VCO(2DM)) compared with (i) predicted from resting VCO(2) (VCO(2PR)) and (ii) predicted from body surface area VCO(2) (VCO(2BSA)).

METHODS

The GE rate of a high-fat test meal was assessed in 27 lean subjects using the OBT. Breath samples were gathered during the fasted state and at regular intervals throughout the 6-h postprandial period for determination of (13)C-isotopic enrichment by continuous-flow isotope-ratio mass spectrometry. The VCO(2) was measured directly from exhaled air samples and the PDR calculated by three methods. The bias and the limits of agreement were calculated using Bland-Altman plots.

RESULTS

Compared with the VCO(2DM), the cPDR was underestimated by VCO(2PR) (4.8%; p = 0.0001) and VCO(2BSA) (2.7%; p = 0.02). The GE T(half) was underestimated by VCO(2PR) (13 min; p = 0.0001) and VCO(2BSA) (10 min; p = 0.01), compared with VCO(2DM).

CONCLUSIONS

The findings highlight the importance of directly measuring VCO(2)production rates throughout the (13)C OBT and could partly explain the conflicting evidence regarding the effect of obesity on GE rates.

Metabolic adaptations to over--and underfeeding--still a matter of debate?

Weight changes in response to a change in energy intake are smaller than calculated from the excess or deficit of energy intake. Digestion efficiency is not affected by intake level when consuming the same diet. Over- or underfeeding induces an increase or decrease in energy expenditure. Intake-induced expenditure changes are largely explained by proportional changes in diet-induced energy expenditure, in activity-induced energy expenditure and in maintenance expenditure as a function of changes in body weight and body composition. Additionally, underfeeding causes a metabolic adaptation as reflected in a reduction of maintenance expenditure below predicted values and defined as adaptive thermogenesis. Thus, alternating overfeeding and underfeeding with an iso-energetic amount results in a positive energy balance. The latter might be one of the explanations for the increasing incidence of obesity in our current society with an ample food supply.

Adaptive thermogenesis in human body weight regulation: more of a concept than a measurable entity?

According to Lavoisier, 'Life is combustion'. But to what extent humans adapt to changes in food intake through adaptive thermogenesis--by turning down the rate of heat production during energy deficit (so as to conserve energy) or turning it up during overnutrition (so as to dissipate excess calories)--has been one of the most controversial issues in nutritional sciences over the past 100 years. The debate nowadays is not whether adaptive thermogenesis exists or not, but rather about its quantitative importance in weight homoeostasis and its clinical relevance to the pathogenesis and management of obesity. Such uncertainties are likely to persist in the foreseeable future primarily because of limitations to unobtrusively measure changes in energy expenditure and body composition with high enough accuracy and precision, particularly when even small inter-individual variations in thermogenesis can, in dynamic systems and over the long term, be important in the determining weight maintenance in some and obesity and weight regain in others. This paper reviews the considerable body of evidence, albeit fragmentary, suggesting the existence of quantitatively important adaptive thermogenesis in several compartments of energy expenditure in response to altered food intake. It then discusses the various limitations that lead to over- or underestimations in its assessment, including definitional and semantics, technical and methodological, analytical and statistical. While the role of adaptive thermogenesis in human weight regulation is likely to remain more a concept than a strictly 'quantifiable' entity in the foreseeable future, the evolution of this concept continues to fuel exciting hypothesis-driven mechanistic research which contributes to advance knowledge in human metabolism and which is bound to result in improved strategies for the management of a healthy body weight.

Increase in the Thermic Effect of Food in Women by Adrenergic Amines Extracted from Citrus Aurantium

OBJECTIVE

To compare the thermic response to a meal between men and women of varied body composition and to determine whether adrenergic amines extracted from citrus aurantium (CA) induce an increase in metabolic rate and enhance the thermic response to the meal.

RESEARCH METHODS AND PROCEDURES

In 30 healthy weight-stable subjects (17 women, 13 men; BMI: 20 to 42 kg/m2), body composition was determined by bioimpedance analysis followed by resting energy expenditure for 20 minutes, and the thermic effect of food (TEF) of a 1.7-MJ, 30-gram protein meal was determined intermittently for 300 minutes by indirect calorimetry. In a subset of 22 subjects, the TEFs of CA alone and when added to the same 1.7-MJ meal were determined. Blood pressure and pulse before and throughout the studies and catecholamine excretion were determined.

RESULTS

TEF was significantly lower in women than men (152 +/- 7 vs. 190 +/- 12 kJ and 8.8 +/- 0.4% vs. 11.0 +/- 0.7% of meal), independently of age and magnitude of adiposity. The thermic response to CA alone was higher in men, but, when added to the meal, CA increased TEF only in women and to values no longer different from men. CA had no effect on blood pressure and pulse rate but increased epinephrine excretion by 2.4-fold.

DISCUSSION

A 20% lower TEF in women suggests a diminished sympathetic nervous system response to meals, because with CA, TEF increased by 29% only in women. However, this acute response may not translate into a chronic effect or a clinically significant weight loss over time.

Inconspicuous assessment of diet-induced thermogenesis using whole-body indirect calorimetry

We report a novel technique for computing diet-induced thermogenesis using data from 24-h respiration chamber measurements of 76 subjects. Physical activity (PA) was determined using a radar system to assess its duration and an accelerometer to evaluate its intensity. The regression line relating PA and energy expenditure facilitated calculation of the integrated thermogenic response to the total energy ingested (11.4% ± 3.8%), which is consistent with the values classically reported in the literature (10%) at the group level.

Effect of dental status on changes in mastication in patients with obesity following bariatric surgery

Background

Patients scheduled for bariatric surgery (BS) are encouraged to chew slowly in order to optimise the digestion process. The influence of dental status on patients' ability to comply with advice on chewing behaviour is poorly documented. This study aims to compare modifications of chewing function before and after BS in three groups of obese patients differing in dental status.

Method and Findings

A cohort of 46 obese women provided three groups: FD group: fully dentate (7–10 functional dental units [FU]); PD group: partially dentate (4–6 FU) without partial dentures; DW group: partial and complete denture wearers. Chewing time (CT), number of chewing cycles (CC), and chewing frequency (CF) were measured before and after surgery during mastication of standardised samples of raw carrot, peanuts, banana, apple and jelly. The median particle-size distribution (D50) of the pre-swallowed bolus was also evaluated for peanut and carrot. Before surgery, the PD and DW groups exhibited greater mean CCs and CTs than the FD group (SNK p<0.05) and produced a bolus with higher granulometry (SNK, p<0.05) than the FD group. After surgery, CT and CC increased for all groups and for all foods, but not statistically significant for jelly. The resulting changes in bolus granulometry observed depended on both food and dental status. The granulometry of carrot bolus remained as fine or as coarse in FD and DW groups respectively as it was before surgery while it was significantly decreased in the PD group (Student's test, p<0.001).

Conclusions

After bariatric surgery, all the obese patients, regardless of dental status modified their chewing kinematics. The effects of this chewing behaviour on bolus granulometry depended on dental status and type of food. Further studies are needed to understand better the impact of dental status on feeding behaviour and nutrition in patients with obesity.