If Body Fatness is Under Physiological Regulation, Then How Come We Have an Obesity Epidemic?

Obesity: an evolutionary context

People completely lacking body fat (lipodystrophy/lipoatrophy) and those with severe obesity both show profound metabolic and other health issues. Regulating levels of body fat somewhere between these limits would, therefore, appear to be adaptive. Two different models might be contemplated. More traditional is a set point (SP) where the levels are regulated around a fixed level. Alternatively, dual-intervention point (DIP) is a system that tolerates fairly wide variation but is activated when critically high or low levels are breached. The DIP system seems to fit our experience much better than an SP, and models suggest that it is more likely to have evolved. A DIP system may have evolved because of two contrasting selection pressures. At the lower end, we may have been selected to avoid low levels of fat as a buffer against starvation, to avoid disease-induced anorexia, and to support reproduction. At the upper end, we may have been selected to avoid excess storage because of the elevated risks of predation. This upper limit of control seems to have malfunctioned because some of us deposit large fat stores, with important negative health effects. Why has evolution not protected us against this problem? One possibility is that the protective system slowly fell apart due to random mutations after we dramatically reduced the risk of being predated during our evolutionary history. By chance, it fell apart more in some people than others, and these people are now unable to effectively manage their weight in the face of the modern food glut. To understand the evolutionary context of obesity, it is important to separate the adaptive reason for storing some fat (i.e. the lower intervention point), from the nonadaptive reason for storing lots of fat (a broken upper intervention point). The DIP model has several consequences, showing how we understand the obesity problem and what happens when we attempt to treat it.

Effects of temperature on feeding and digestive processes in fish

As most fish are ectotherms, their physiology is strongly affected by temperature. Temperature affects their metabolic rate and thus their energy balance and behavior, including locomotor and feeding behavior. Temperature influences the ability/desire of the fish to obtain food, and how they process food through digestion, absorb nutrients within the gastrointestinal tract, and store excess energy. As fish display a large variability in habitats, feeding habits, and anatomical and physiological features, the effects of temperature are complex and species-specific. The effects of temperature depend on the timing, intensity, and duration of exposure as well as the speed at which temperature changes occur. Whereas acute short-term variations of temperature might have drastic, often detrimental, effects on fish physiology, long-term gradual variations might lead to acclimation, e.g. variations in metabolic and digestive enzyme profiles. The goal of this review is to summarize our current knowledge on the effects of temperature on energy homeostasis, with specific focus on metabolism, feeding, digestion, and how fish are often able to "adapt" to changing environments through phenotypic and physiological changes.

The drive to eat in homo sapiens: Energy expenditure drives energy intake

The drive to eat is a component of appetite control, independent of the omnivorous habit of humans, and separate from food choice, satiety and food reward. The drive forms part of the tonic component of appetite and arises from biological needs; it is distinct from episodic aspects of appetite which are heavily influenced by culture and the environment (and which reflect the omnivorous habit). It is proposed that the tonic drive to eat reflects a need state generated by metabolic energy expenditure (EE) required to maintain the functioning and integrity of vital organs. Specifically, the tonic drive is quantitatively associated with fat-free mass (FFM) and resting metabolic rate (RMR). A rational proposition is that high metabolic rate organs (such as heart, liver, kidneys, brain) together with skeletal muscle generate a metabolic need which drives energy intake (EI). The basic phenomenon of a relationship between FFM, RMR and EI, first published in 2011, has been substantially replicated and there are at least 14 concordant published studies carried out in 9 different countries (and 4 continents) with various ethnic groups of lean and obese humans. These studies demonstrate that FFM and RMR represent major determinants of the drive to eat, and this is rational from an evolutionary perspective. The EE of bodily movements through skeletal muscle activity (namely physical activity and exercise) represents another driver which is clearly but more weakly associated with an increase in EI. This account of appetite control, developed within an energy balance framework, is consistent with the apparent inexorable escalation of fatness in individual humans, and for the progressive increase in the prevalence of obesity which, among other factors, reflects the difficulty of managing the biological drive to eat.

The unidentified hormonal defense against weight gain

Human biology has evolved to keep body fat within a range that supports survival. During the last 25 years, obesity biologists have uncovered key aspects of physiology that prevent fat mass from becoming too low. In contrast, the mechanisms that counteract excessive adipose expansion are largely unknown. Evidence dating back to the 1950s suggests the existence of a blood-borne molecule that defends against weight gain. In this article, we discuss the research supporting an "unidentified factor of overfeeding" and models that explain its role in body weight control. If it exists, revealing the identity of this factor could end a long-lasting enigma of energy balance regulation and facilitate a much-needed breakthrough in the pharmacological treatment of obesity.

Increasing Energy Flux to Maintain Diet-Induced Weight Loss

Long-term maintenance of weight loss requires sustained energy balance at the reduced body weight. This could be attained by coupling low total daily energy intake (TDEI) with low total daily energy expenditure (TDEE; low energy flux), or by pairing high TDEI with high TDEE (high energy flux). Within an environment characterized by high energy dense food and a lack of need for movement, it may be particularly difficult for weight-reduced individuals to maintain energy balance in a low flux state. Most of these individuals will increase body mass due to an inability to sustain the necessary level of food restriction. This increase in TDEI may lead to the re-establishment of high energy flux at or near the original body weight. We propose that following weight loss, increasing physical activity can effectively re-establish a state of high energy flux without significant weight regain. Although the effect of extremely high levels of physical activity on TDEE may be constrained by compensatory reductions in non-activity energy expenditure, moderate increases following weight loss may elevate energy flux and encourage physiological adaptations favorable to weight loss maintenance, including better appetite regulation. It may be time to recognize that few individuals are able to re-establish energy balance at a lower body weight without permanent increases in physical activity. Accordingly, there is an urgent need for more research to better understand the role of energy flux in long-term weight maintenance.

Keeping Slim When Food Is Abundant: What Energy Mechanisms Could Be at Play?

The obesity epidemic in humans is juxtaposed by observations of passerine birds exhibiting fine-scale body mass regulation. The ecology literature is replete with research into why these animals regulate body weight, citing tradeoffs between competing pressures such as emaciation and predation. Yet studies on the underlying mechanisms of mass regulation in these animals are scarce. Maintaining or decreasing weight could obviously be achieved by limiting food intake. However, there are numerous reasons why an animal may not control ingestion, at least precisely. This Opinion article investigates the plausibility of possible behavioural and physiological mechanisms to adaptively maintain or decrease body mass in birds and other animals. Candidate behavioural mechanisms include exercising and fidgeting, while physiological mechanisms could include reducing digestive efficiency or mitochondrial efficiency.

Quantitative approaches to energy and glucose homeostasis: machine learning and modelling for precision understanding and prediction

Obesity is a major global public health problem. Understanding how energy homeostasis is regulated, and can become dysregulated, is crucial for developing new treatments for obesity. Detailed recording of individual behaviour and new imaging modalities offer the prospect of medically relevant models of energy homeostasis that are both understandable and individually predictive. The profusion of data from these sources has led to an interest in applying machine learning techniques to gain insight from these large, relatively unstructured datasets. We review both physiological models and machine learning results across a diverse range of applications in energy homeostasis, and highlight how modelling and machine learning can work together to improve predictive ability. We collect quantitative details in a comprehensive mathematical supplement. We also discuss the prospects of forecasting homeostatic behaviour and stress the importance of characterizing stochasticity within and between individuals in order to provide practical, tailored forecasts and guidance to combat the spread of obesity.

Mechanisms responsible for homeostatic appetite control: theoretical advances and practical implications

Homeostatic appetite control is part of a psychobiological system that has evolved to maintain an adequate supply of nutrients for growth and maintenance. The system links the physiological needs for energy with the behaviour that satisfies these needs (feeding), and is shaped by excitatory and inhibitory signals. Owing to rapid shifts in the food environment, homeostatic appetite control is not well adapted for modern-day human functioning. Areas covered: Homeostatic appetite control has two divisions. Tonic processes exert stable and enduring influences, with signals arising from bodily tissues and metabolism. Episodic processes fluctuate rapidly and are related to nutrient ingestion and the composition of foods consumed. Research in these areas incorporates potent endocrine signals that can influence behaviour. Expert commentary: The regulation of adipose tissue, and its impact on appetite (energy) homeostasis, has been heavily researched. More recently however, it has been demonstrated that fat-free mass has the potential to act as a tonic driver of food intake. A challenging issue is to determine how the post-prandial action of episodic satiety hormones and gastrointestinal mechanisms can effectively brake the metabolic drive to eat, in order to keep food intake under control and prevent a positive energy balance and fat accumulation.

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.

Attenuating the Biologic Drive for Weight Regain Following Weight Loss: Must What Goes Down Always Go Back Up?

Metabolic adaptations occur with weight loss that result in increased hunger with discordant simultaneous reductions in energy requirements—producing the so-called energy gap in which more energy is desired than is required. The increased hunger is associated with elevation of the orexigenic hormone ghrelin and decrements in anorexigenic hormones. The lower total daily energy expenditure with diet-induced weight loss results from (1) a disproportionately greater decrease in circulating leptin and resting metabolic rate (RMR) than would be predicted based on the decline in body mass, (2) decreased thermic effect of food (TEF), and (3) increased energy efficiency at work intensities characteristic of activities of daily living. These metabolic adaptations can readily promote weight regain. While more experimental research is needed to identify effective strategies to narrow the energy gap and attenuate weight regain, some factors contributing to long-term weight loss maintenance have been identified. Less hunger and greater satiation have been associated with higher intakes of protein and dietary fiber, and lower glycemic load diets. High levels of physical activity are characteristic of most successful weight maintainers. A high energy flux state characterized by high daily energy expenditure and matching energy intake may attenuate the declines in RMR and TEF, and may also result in more accurate regulation of energy intake to match daily energy expenditure.

Food intake and appetite following school-based high-intensity interval training in 9-11-year-old children

Using a randomised cross-over design, free-living lunch intake and subjective appetite were examined in 10 children (9.8 ± 0.6 years) following high-intensity interval training (HIIT) versus a control sedentary (SED) period, within a school setting. The 22-min HIIT took place during a regular PE lesson and consisted of two rounds of 4 × 30 s sprints. Foods were offered at a regular school lunch immediately following HIIT and SED and were matched between conditions. All food was covertly weighed before and after the meal. Hunger, fullness and prospective consumption were reported immediately before and after HIIT/SED, using visual analogue scales. Heart rate was higher during HIIT than SED (159.3 ± 23.1 vs. 76.9 ± 2.2 bpm, P < 0.05). Lunch energy intake was not different (P = 0.52) following HIIT, compared to SED (2.06 ± 0.35 vs. 2.09 ± 0.29 MJ, respectively). There were no significant differences in macronutrient intake or subjective appetite (P > 0.05). Results suggest that HIIT can be implemented in a PE lesson immediately before lunch, without causing a compensatory increase in food consumption.

Changes in Energy Expenditure with Weight Gain and Weight Loss in Humans

Metabolic adaptation to weight changes relates to body weight control, obesity and malnutrition. Adaptive thermogenesis (AT) refers to changes in resting and non-resting energy expenditure (REE and nREE) which are independent from changes in fat-free mass (FFM) and FFM composition. AT differs in response to changes in energy balance. With negative energy balance, AT is directed towards energy sparing. It relates to a reset of biological defence of body weight and mainly refers to REE. After weight loss, AT of nREE adds to weight maintenance. During overfeeding, energy dissipation is explained by AT of the nREE component only. As to body weight regulation during weight loss, AT relates to two different set points with a settling between them. During early weight loss, the first set is related to depleted glycogen stores associated with the fall in insulin secretion where AT adds to meet brain's energy needs. During maintenance of reduced weight, the second set is related to low leptin levels keeping energy expenditure low to prevent triglyceride stores getting too low which is a risk for some basic biological functions (e.g., reproduction). Innovative topics of AT in humans are on its definition and assessment, its dynamics related to weight loss and its constitutional and neuro-endocrine determinants.

A neuroendocrine role for chemerin in hypothalamic remodelling and photoperiodic control of energy balance

Long-term and reversible changes in body weight are typical of seasonal animals. Thyroid hormone (TH) and retinoic acid (RA) within the tanycytes and ependymal cells of the hypothalamus have been implicated in the photoperiodic response. We investigated signalling downstream of RA and how this links to the control of body weight and food intake in photoperiodic F344 rats. Chemerin, an inflammatory chemokine, with a known role in energy metabolism, was identified as a target of RA. Gene expression of chemerin (Rarres2) and its receptors were localised within the tanycytes and ependymal cells, with higher expression under long (LD) versus short (SD) photoperiod, pointing to a physiological role. The SD to LD transition (increased food intake) was mimicked by 2 weeks of ICV infusion of chemerin into rats. Chemerin also increased expression of the cytoskeletal protein vimentin, implicating hypothalamic remodelling in this response. By contrast, acute ICV bolus injection of chemerin on a 12 h:12 h photoperiod inhibited food intake and decreased body weight with associated changes in hypothalamic neuropeptides involved in growth and feeding after 24 hr. We describe the hypothalamic ventricular zone as a key site of neuroendocrine regulation, where the inflammatory signal, chemerin, links TH and RA signaling to hypothalamic remodeling.

Body composition and energy intake - skeletal muscle mass is the strongest predictor of food intake in obese adolescents: The HEARTY trial

There has been renewed interest in examining the relationship between specific components of energy expenditure and the overall influence on energy intake (EI). The purpose of this cross-sectional analysis was to determine the strongest metabolic and anthropometric predictors of EI. It was hypothesized that resting metabolic rate (RMR) and skeletal muscle mass would be the strongest predictors of EI in a sample of overweight and obese adolescents. 304 post-pubertal adolescents (91 boys, 213 girls) aged 16.1 (±1.4) years with body mass index at or above the 95th percentile for age and sex OR at or above the 85th percentile plus an additional diabetes risk factor were measured for body weight, RMR (kcal/day) by indirect calorimetry, body composition by magnetic resonance imaging (fat free mass (FFM), skeletal muscle mass, fat mass (FM), and percentage body fat), and EI (kcal/day) using 3 day food records. Body weight, RMR, FFM, skeletal muscle mass, and FM were all significantly correlated with EI (p < 0.005). After adjusting the model for age, sex, height, and physical activity, only FFM (β = 21.9, p = 0.007) and skeletal muscle mass (β = 25.8, p = 0.02) remained as significant predictors of EI. FFM and skeletal muscle mass also predicted dietary protein and fat intake (p < 0.05), but not carbohydrate intake. In conclusion, with skeletal muscle mass being the best predictor of EI, our results support the hypothesis that the magnitude of the body's lean tissue is related to absolute levels of EI in a sample of inactive adolescents with obesity.

Predation risk modulates diet-induced obesity in male C57BL/6 mice

OBJECTIVE

In this study, the behavioral and physiological changes induced by experimentally varying the risk of predation in male mice fed a high-fat diet were examined. In particular, the study aimed to assess whether the risk of being predated modulates the body weight gain, providing an ecological context for the obesity resistance observed in many species of small mammals.

METHODS

Body weight, food intake, physical activity, and core body temperature of 35 male C57BL/6 mice were monitored for 20 days, while feeding a high-fat diet. A third of the animals were exposed to elevated risk of predation through exposure to the sounds of nocturnal predatory birds, and these were compared to animals exposed to a neutral noise or silence.

RESULTS

Male mice exposed to predation risk had significantly lower weight gain than the neutral or silent groups. Reduced food intake and increased physical activity were the main proximal factors explaining this effect. The risk of predation also induced changes in boldness.

CONCLUSIONS

This study provides evidence supporting the role of predation risk on body weight gain of small mammals.

Food reward. What it is and how to measure it

We investigated the contribution of hunger and food liking to food reward, and the relationship between food reward and food intake. We defined liking as the pleasantness of taste of food in the mouth, and food reward as the momentary value of a food to the individual at the time of ingestion. Liking and food reward were measured, respectively, by ratings of the pleasantness of the taste of a mouthful, and ratings of desire to eat a portion, of the food in question. Hunger, which we view as primarily the absence of fullness, was rated without food being present. Study 1 provided evidence that hunger and liking contribute independently to food reward, with little effect of hunger on liking. Food intake reduced liking and reward value more for the eaten food than uneaten foods. The results were ambiguous as to whether this food-specific decline in reward value ('sensory-specific satiety') involved a decrease in 'wanting' in addition to the decrease in liking. Studies 2 and 3 compared desire to eat ratings with work-for-food and pay-for-food measures of food reward, and found desire to eat to be equal or superior in respect of effects of hunger and liking, and superior in predicting ad libitum food intake. A further general observation was that in making ratings of food liking participants may confuse the pleasantness of the taste of food with the pleasantness of eating it. The latter, which some call 'palatability,' decreases more with eating because it is significantly affected by hunger/fullness. Together, our results demonstrate the validity of ratings of desire to eat a portion of a tasted food as a measure of food reward and as a predictor of food intake.

Motion sickness: more than nausea and vomiting

Motion sickness is a complex syndrome that includes many features besides nausea and vomiting. This review describes some of these factors and points out that under normal circumstances, many cases of motion sickness go unrecognized. Motion sickness can occur during exposure to physical motion, visual motion, and virtual motion, and only those without a functioning vestibular system are fully immune. The range of vulnerability in the normal population varies about 10,000 to 1. Sleep deprivation can also enhance susceptibility. Systematic studies conducted in parabolic flight have identified velocity storage of semicircular canal signals-velocity integration-as being a key factor in both space motion sickness and terrestrial motion sickness. Adaptation procedures that have been developed to increase resistance to motion sickness reduce this time constant. A fully adequate theory of motion sickness is not presently available. Limitations of two popular theories, the evolutionary and the ecological, are described. A sensory conflict theory can explain many but not all aspects of motion sickness elicitation. However, extending the theory to include conflicts related to visceral afferent feedback elicited by voluntary and passive body motion greatly expands its explanatory range. Future goals should include determining why some conflicts are provocative and others are not but instead lead to perceptual reinterpretations of ongoing body motion. The contribution of visceral afferents in relation to vestibular and cerebellar signals in evoking sickness also deserves further exploration. Substantial progress is being made in identifying the physiological mechanisms underlying the evocation of nausea, vomiting, and anxiety, and a comprehensive understanding of motion sickness may soon be attainable. Adequate anti-motion sickness drugs without adverse side effects are not yet available.