Obesity: an evolutionary context

The vices and virtues of medical models of obesity

Despite numerous public health organizations supporting the pathologization of obesity and considering recent obesity rates a health crisis, many researchers in the humanities, social sciences, and even in the health sciences remain unconvinced. In this paper, we address a set of arguments coming from these academic fields that criticize medical models of obesity for their supposedly flawed diagnostic categories that shift focus onto individuals and support moralizing judgements. Clarifying some key claims in these models and explicating the view of obesity in terms of energy dysregulation, we aim to tease apart misunderstandings and argue that not only do these models not say what they are often accused of saying, but their apparent vices may actually be virtues in helping to combat stigma. Building on the social psychology of stigma and disease labeling, we then suggest that current medical models are largely supportive of many moral and political aims promoted by critics of these models.

Enhanced Intrusion of Exogenous Airborne Fine Particles toward Eyes in Humans and Animals: Where Damaged Blood-Ocular Barrier Plays a Crucial Role

Emerging data suggest a close correlation between ambient fine particle (AFP) exposure and eye disorders and pinpoint potential threats of AFPs to eye health in humans. However, the possible passage (including direct intrusion) and the interactions of AFPs with the eye microenvironment in addition to morphological and physiological injuries remain elusive. To this end, the likely transport of AFPs into the eyes via blood-ocular barrier (BOB) in humans and animals was investigated herein. Exogenous particles were recognized inside human eyes with detailed structural and chemical fingerprints. Importantly, comparable AFPs were found in sera with constant structural and chemical fingerprints, hinting at the translocation pathway from blood circulation into the eye. Furthermore, we found that the particle concentrations in human eyes from patients with diabetic retinopathy were much higher than those from patients with no fundus pathological changes (i.e., myopia), indicating that the damaged BOB increased the possibility of particle entrance. Our diseased animal model further corroborated these findings. Collectively, our results offer a new piece of evidence on the intrusion of exogenous particles into human eyes and provide an explanation for AFP-induced eye disorders, with substantially increased risk in susceptible individuals with BOB injuries.

Novel Pharmaceuticals in Appetite Regulation: Exploring emerging gut peptides and their pharmacological prospects

Obesity, a global health challenge, necessitates innovative approaches for effective management. Targeting gut peptides in the development of anti-obesity pharmaceuticals has already demonstrated significant efficacy. Ghrelin, peptide YY (PYY), cholecystokinin (CCK), and amylin are crucial in appetite regulation offering promising targets for pharmacological interventions in obesity treatment using both peptide-based and small molecule-based pharmaceuticals. Ghrelin, a sole orexigenic gut peptide, has a potential for anti-obesity therapies through various approaches, including endogenous ghrelin neutralization, ghrelin receptor antagonists, ghrelin O-acyltransferase, and functional inhibitors. Anorexigenic gut peptides, peptide YY, cholecystokinin, and amylin, have exhibited appetite-reducing effects in animal models and humans. Overcoming substantial obstacles is imperative for translating these findings into clinically effective pharmaceuticals. Peptide YY and cholecystokinin analogues, characterized by prolonged half-life and resistance to proteolytic enzymes, present viable options. Positive allosteric modulators emerge as a novel approach for modulating the cholecystokinin pathway. Amylin is currently the most promising, with both amylin analogues and dual amylin and calcitonin receptor agonists (DACRAs) progressing to advanced stages of clinical trials. Despite persistent challenges, innovative pharmaceutical strategies provide a glimpse into the future of anti-obesity therapies.

Energy Expenditure in Humans: Principles, Methods, and Changes Throughout the Life Course

Humans require energy to sustain their daily activities throughout their lives. This narrative review aims to (a) summarize principles and methods for studying human energy expenditure, (b) discuss the main determinants of energy expenditure, and (c) discuss the changes in energy expenditure throughout the human life course. Total daily energy expenditure is mainly composed of resting energy expenditure, physical activity energy expenditure, and the thermic effect of food. Total daily energy expenditure and its components are estimated using variations of the indirect calorimetry method. The relative contributions of organs and tissues determine the energy expenditure under different physiological conditions. Evidence shows that energy expenditure varies along the human life course, at least in part due to changes in body composition, the mass and specific metabolic rates of organs and tissues, and levels of physical activity. This information is crucial to estimate human energy requirements for maintaining health throughout the life course.

Obesity and cardiovascular health

Obesity has risen to epidemic levels worldwide over the past few decades and has become a huge global health burden owing to its direct contribution to the development of some of the most prevalent chronic diseases including diabetes, hypertension, hyperlipidaemia, and other cardiovascular diseases. Obesity is a disease of positive energy balance resulting from complex interactions between abnormal neurohumoral responses and an individual's socioeconomic, environmental, behavioural, and genetic factors leading to a state of chronic inflammation. Understanding the complex nature of the disease is crucial in determining the best approach to combat its rising numbers. Despite recent advancements in pharmacological therapy for the treatment of obesity, reversing weight gain and maintaining weight loss is challenging due to the relapsing nature of the disease. Prevention, therefore, remains the key which needs to start in utero and continued throughout life. This review summarizes the role obesity plays in the pathophysiology of various cardiovascular diseases both by directly affecting endothelial and myocyte function and indirectly by enhancing major cardiovascular risk factors like diabetes, hypertension, and hyperlipidaemia. We highlight the importance of a holistic approach needed to prevent and treat this debilitating disease. Particularly, we analyse the effects of plant-based diet, regular exercise, and non-exercise activity thermogenesis on obesity and overall cardiorespiratory fitness. Moreover, we discuss the significance of individualizing obesity management with a multimodal approach including lifestyle modifications, pharmacotherapy, and bariatric surgery to tackle this chronic disease.

Changes in weight status among "Ju/'hoansi" San hunter-gatherers between 1968/69 and 1987-The effects of nutritional transition, sex and age

BACKGROUND

Changes in lifestyle and dietary habits that hunter-gatherer populations have undergone in recent decades have often led to rising obesity rates with disastrous consequences for their health.

OBJECTIVES

The associations between dietary habits and weight status were studied in 238 "Ju/'hoansi" San (93 women and 145 men) aged between 18 and 65 years in northern Namibia in 1987. Weight status was estimated based on the World Health Organization body mass index (BMI) categories, and dietary habits were recorded using food recall methods. Anthropometrics and weight status were compared with those of a sex- and age-matched sample of "Ju/'hoansi" San people collected by Nancy Howell in 1968/69.

RESULTS

Body weight had increased significantly among "Ju/'hoansi" San people from 1968/69 to 1987. The number of underweight people decreased from 1968/69 to 1987. In 1987, most participants (60.9%) were of normal weight. Overweight was found in 1.3% of the women, but not among men. No participants were obese. Less than 4% of the women and less than 2% of the men consumed exclusively traditional hunter-gatherer food. Westernized food products were significantly (p < 0.001) more common among men and younger people. Dietary patterns were significantly associated with weight status. The less traditional the diet, the higher the BMI (p < 0.001).

CONCLUSION

The transition to domestic agricultural and westernized foods was positively associated with increasing BMI. Overweight, however, was still an extremely rare condition in this population in 1987.

Reduction of body weight by increased loading is associated with activation of norepinephrine neurones in the medial nucleus of the solitary tract

We previously provided evidence supporting the existence of a novel leptin-independent body weight homeostat ("the gravitostat") that senses body weight and then initiates a homeostatic feed-back regulation of body weight. We, herein, hypothesize that this feed-back regulation involves a CNS mechanism. To identify populations of neurones of importance for the putative feed-back signal induced by increased loading, high-fat diet-fed rats or mice were implanted intraperitoneally or subcutaneously with capsules weighing ∼15% (Load) or ∼2.5% (Control) of body weight. At 3-5 days after implantation, neuronal activation was assessed in different parts of the brain/brainstem by immunohistochemical detection of FosB. Implantation of weighted capsules, both subcutaneous and intraperitoneal, induced FosB in specific neurones in the medial nucleus of the solitary tract (mNTS), known to integrate information about the metabolic status of the body. These neurones also expressed tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DbH), a pattern typical of norepinephrine neurones. In functional studies, we specifically ablated norepinephrine neurones in mNTS, which attenuated the feed-back regulation of increased load on body weight and food intake. In conclusion, increased load appears to reduce body weight and food intake via activation of norepinephrine neurones in the mNTS.

Models of body weight and fatness regulation

Body weight and fatness appear to be regulated phenomena. Several different theoretical models are available to capture the essence of this idea. These include the set-point, dynamic equilibrium, adiposity force, control theory-settling point, Hall-Guo, operation point and dual intervention point (DIP) models. The set-point model posits a single reference point around which levels of fat are regulated. The dynamic equilibrium model suggests that the apparent regulation of body fat around a reference point is an illusion owing to the necessary impacts of weight change on energy expenditure. Control theory focuses on the importance of feedback gain and suggests set-point and dynamic equilibrium are ends of a continuum of feedback gain. Control theory models have also been called 'settling point' models. The Hall-Guo, operation point and DIP models also bring together the set-point and dynamic equilibrium ideas into a single framework. The DIP proposes a zone of indifference where dynamic equilibrium 'regulation' predominates, bounded by upper and lower intervention points beyond which physiological mechanisms are activated. The drifty gene hypothesis is an idea explaining where this individual variation in the upper intervention point might come from. We conclude that further experiments to test between the models are sorely required. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

The dual hypothesis of homeostatic body weight regulation, including gravity-dependent and leptin-dependent actions

Body weight is tightly regulated when outside the normal range. It has been proposed that there are individual-specific lower and upper intervention points for when the homeostatic regulation of body weight is initiated. The nature of the homeostatic mechanisms regulating body weight at the lower and upper ends of the body weight spectrum might differ. Previous studies demonstrate that leptin is the main regulator of body weight at the lower end of the body weight spectrum. We have proposed that land-living animals use gravity to regulate their body weight. We named this homeostatic system the gravitostat and proposed that there are two components of the gravitostat. First, an obvious mechanism involves increased energy consumption in relation to body weight when working against gravity on land. In addition, we propose that there exists a component, involving sensing of the body weight by osteocytes in the weight-bearing bones, resulting in a feedback regulation of energy metabolism and body weight. The gravity-dependent homeostatic regulation is mainly active in obese mice. We, herein, propose the dual hypothesis of body weight regulation, including gravity-dependent actions (= gravitostat) at the upper end and leptin-dependent actions at the lower end of the body weight spectrum. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

Promoting metabolic inefficiency for metabolic disease

Recent advances in pharmacotherapies that promote appetite suppression have shown remarkable weight loss. Therapies targeting energy expenditure lag behind, and as such none have yet been identified to be safe and efficacious for sustaining negative energy balance toward weight loss. Multiple energy dissipating pathways have been identified in adipose tissue and muscle. The molecular effectors of some of these pathways have been identified, but much is still left to be learned about their regulation. Understanding the molecular underpinnings of metabolic inefficiency in adipose tissue and muscle is required if these pathways are to be therapeutically targeted in the context of obesity and obesity-accelerated diseases.

An adiposity force induces obesity in humans independently of a normal energy balance system-a thought experiment

Obesity in humans represents a cumulative retention of a tiny fraction of total energy intake as fat, which is accompanied by growth of the metabolically active, energy-demanding, lean body mass. Since the energy balance regulation operates irrespective of the excess fat storage, availability of the required energy supplies is a permissive condition for obesity development. It occurs predominantly among people genetically predisposed and/or living with social or mental challenges. I propose a theory in which the body responds to social disruptions as threats of a future lack of food by an adiposity force building a reserve of energy independent of the regulation of the energy balance. It is based on the assumption that our evolutionary development required collaboration in gathering and sharing of food, combined with precautionary measures against anticipated failing food supplies. Social challenges are perceived as such threats, which activate the adiposity force through the brain to instigate the growth of fat and lean mass by neuro-hormonal signalling. If both perceived social threats and food abundance continue, the adiposity force pushes the fat accretion process to continue without inhibition by feedback signals from the fat mass, eventually leading to more obesity, and more so among the genetically predisposed. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part I)'.

Energy homeostasis from Lavoisier to control theory

The intellectual history of energy homeostasis, focusing on food intake and energy storage, is briefly reviewed. Physiological energetics was founded by Lavoisier, who in the late eighteenth century invented direct and indirect calorimetry and discovered the role of oxygen in combustion and respiration. Energy was understood well enough by the mid-nineteenth century to realize the physiological energy-balance equation, that energy intake - energy expenditure = energy storage, but this did not greatly influence physiological research for another century. Homeostasis, the concept that many vital physiological variables are actively regulated in narrow envelopes, was developed by Bernard and Cannon between approximately 1870-1940 and remains a central principle of physiology. Kennedy coined the term lipostasis in 1953 to refer to the constancy of fat mass, which Mayer argued was the mechanism regulating body weight. A parameterized control-theory model suggests that a proportional negative-feedback control system incompletely compensates weight loss during persistent negative energy balance, suggesting that Cannon's idea of constancy within a narrow envelope may not fit body-weight regulation well. This modelling encourages further application of control theory to issues in energy homeostasis, including to the development of obesity. It also sets the stage for understanding the underlying neuroendocrine mechanisms. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part I)'.

Physiological protection against weight gain: evidence from overfeeding studies and future directions

Body weight is under physiological regulation. When body fat mass decreases, a series of responses are triggered to promote weight regain by increasing food intake and decreasing energy expenditure. Analogous, in response to experimental overfeeding, excessive weight gain is counteracted by a reduction in food intake and possibly by an increase in energy expenditure. While low blood leptin and other hormones defend against weight loss, the signals that oppose overfeeding-induced fat mass expansion are still unknown. In this article, we discuss insights gained from overfeeding interventions in humans and intragastric overfeeding studies in rodents. We summarize the knowledge on the relative contributions of energy intake, energy expenditure and energy excretion to the physiological defence against overfeeding-induced weight gain. Furthermore, we explore literature supporting the existence of unidentified endocrine and non-endocrine pathways that defend against weight gain. Finally, we discuss the physiological drivers of constitutional thinness and suggest that overfeeding of individuals with constitutional thinness represents a gateway to understand the physiology of weight gain resistance in humans. Experimental overfeeding, combined with modern multi-omics techniques, has the potential to unveil the long-sought signalling pathways that protect against weight gain. Discovering these mechanisms could give rise to new treatments for obesity. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part I)'.

A critical update on the leptin-melanocortin system

The discovery of leptin in 1994 was an "eureka moment" in the field of neurometabolism that provided new opportunities to better understand the central control of energy balance and glucose metabolism. Rapidly, a prevalent model in the field emerged that pro-opiomelanocortin (POMC) neurons were key in promoting leptin's anorexigenic effects and that the arcuate nucleus of the hypothalamus (ARC) was a key region for the regulation of energy homeostasis. While this model inspired many important discoveries, a growing body of literature indicates that this model is now outdated. In this review, we re-evaluate the hypothalamic leptin-melanocortin model in light of recent advances that directly tackle previous assumptions, with a particular focus on the ARC. We discuss how segregated and heterogeneous these neurons are, and examine how the development of modern approaches allowing spatiotemporal, intersectional, and chemogenetic manipulations of melanocortin neurons has allowed a better definition of the complexity of the leptin-melanocortin system. We review the importance of leptin in regulating glucose homeostasis, but not food intake, through direct actions on ARC POMC neurons. We further highlight how non-POMC, GABAergic neurons mediate leptin's direct effects on energy balance and influence POMC neurons.

The physiology of experimental overfeeding in animals

BACKGROUND

Body weight is defended by strong homeostatic forces. Several of the key biological mechanisms that counteract weight loss have been unraveled over the last decades. In contrast, the mechanisms that protect body weight and fat mass from becoming too high remain largely unknown. Understanding this aspect of energy balance regulation holds great promise for curbing the obesity epidemic. Decoding the physiological and molecular pathways that defend against weight gain can be achieved by an intervention referred to as 'experimental overfeeding'.

SCOPE OF THE REVIEW

In this review, we define experimental overfeeding and summarize the studies that have been conducted on animals. This field of research shows that experimental overfeeding induces a potent and prolonged hypophagic response that seems to be conserved across species and mediated by unidentified endocrine factors. In addition, the literature shows that experimental overfeeding can be used to model the development of non-alcoholic steatohepatitis and that forced intragastric infusion of surplus calories lowers survival from infections. Finally, we highlight studies indicating that experimental overfeeding can be employed to study the transgenerational effects of a positive energy balance and how dietary composition and macronutrient content might impact energy homeostasis and obesity development in animals.

MAJOR CONCLUSIONS

Experimental overfeeding of animals is a powerful yet underappreciated method to investigate the defense mechanisms against weight gain. This intervention also represents an alternative approach for studying the pathophysiology of metabolic liver diseases and the links between energy balance and infection biology. Future research in this field could help uncover why humans respond differently to an obesogenic environment and reveal novel pathways with therapeutic potential against obesity and cardiometabolic disorders.