Defining the molecular mechanisms linking obesity with insulin resistance is important for developing new therapies against the rising incidence of type 2 diabetes in industrialized nations. Maintaining a balance between calorie intake and energy expenditure is critical for preventing insulin resistance, the precursor for type 2 diabetes (1). Mouse genetics has made enormous contributions to theoretical models explaining how organisms balance energy intake and energy expenditure. A seminal event was the positional cloning of the obese gene (now called the Leptin gene) by Friedman and colleagues in the early 1990s (2). Leptin deficiency causes severe obesity in mice and humans (3), and leptin was proposed to regulate energy homeostasis by suppressing appetite and increasing energy expenditure (4). However, it has clearly been forgotten that the interpretation of energy expenditure data from mice homozygous for the Lepob mutation was challenged shortly after the initial publication (5). Increasingly sophisticated technologies for manipulating the mouse genome are now used routinely to analyze new genes linked to energy homeostasis, resulting in many new mouse models with obese or lean phenotypes. Altered energy expenditure is frequently cited as the primary mechanism underlying the obese or lean phenotype. However, in many cases the same issues with interpretation of energy expenditure data are evident. Here, we discuss what has developed into a recurring problem in the literature with the analysis of energy balance. Specifically, we shall discuss the practice of using body weight as a denominator in analyzing energy balance to overestimate the role of energy expenditure.
The growing number of individuals with chronic metabolic diseases like type 2 diabetes provides a powerful incentive for investigating mechanisms linking obesity with insulin resistance. That a balance between food intake and energy expenditure (thermogenesis) is maintained through homeostatic mechanisms is a central tenet of obesity research. A major goal is to discover mechanisms to avoid a positive energy balance, a pathway to weight gain, increased susceptibility to insulin resistance, and diseases of the metabolic syndrome (6,7).
There has been significant progress in understanding how organisms regulate caloric intake and adiposity. Identification of factors secreted from peripheral organs, including leptin and adiponectin from adipocytes, insulin from the pancreas, ghrelin from the stomach, and fibroblast-growth factor-21 from the liver and examining how they regulate adiposity and insulin sensitivity has been essential for the evolving concepts of energy homeostasis (8–10). These factors have been found to interact with networks of specific neurons in the hypothalamus and brain stem, modulating behaviors relevant to energy homeostasis including satiety, reward, and motivation (11–13). Through regulating autonomic and neuroendocrine output, and by acting directly on peripheral tissues, these factors can regulate glucose and lipid homeostasis. Importantly, aberrant regulation and action of these molecules has been linked to the development of insulin resistance and diabetes in the obese state. These findings have given hope that therapies designed to restore or replace the normal function of these peptides will provide effective treatment for obesity and the associated metabolic disorders.
Neuroendocrine factors that regulate food intake have been reported to have reciprocal effects on thermogenesis that should, in principle, facilitate attainment of energy balance. A frequently cited example is the ability of leptin to inhibit food intake and stimulate thermogenesis (4), the latter occurring through stimulating the activity of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) (14). The central nervous melanocortin system, primarily acting through melanocortin-4 receptors (Mc4r), has also been suggested to function similarly. Mc4r-deficient mice do not exhibit diet-induced thermogenesis (15) linked to impaired regulation of BAT UCP1 expression (16). Collectively, these and other observations form the basis of a neuromolecular model linking calorie intake and diet-induced thermogenesis, a system for maintaining energy homeostasis proposed by Rothwell and Stock nearly 30 years ago (17).
While such a model linking food intake to thermogenesis is theoretically satisfying, it falls short for several reasons. The clinical relevance is unclear, as it has long been debated whether nonshivering thermogenesis is the basis for diet-induced thermogenesis in humans (18). However, recent reports on the presence of deposits of brown adipocytes in adult humans (19–21), and the discovery of molecular mechanism for transforming myocytes into brown adipocytes (22), will likely further stimulate this debate. Several studies have also failed to demonstrate that leptin stimulates thermogenesis, as determined by measuring oxygen consumption, even when administered to Lepob/Lepob mice (23). That the marked decline in leptin associated with food restriction instigates a neuroendocrine response affecting a compensatory reduction in energy expenditure is not disputed (24). Also not disputed is the experimental evidence for systems residing in the hypothalamus responding to hormones and metabolites that, when disrupted, lead to rapid weight gain and insulin resistance (25). However, the ability of leptin per se to stimulate a compensatory thermogenic effect in the well-fed state is negligible (23), while lean animals fail to respond to pharmacological doses of leptin (4). A positive energy balance is also rapidly associated with a state of leptin resistance and deterioration in the hypothalamic control of energy homeostasis (26).
Another key point is that we have little documented knowledge for a thermogenic mechanism that has the prevention of positive energy balance as its primary function. An alternative hypothesis is that the physical effort to feed life and limb and escape predators precluded the evolution of a thermogenic system to protect those few with the luxury of having a positive energy balance (27). However, complementation of this perceived evolutionary deficiency in energy homeostasis through pharmacological stimulation of thermogenesis remains an important goal for developing drugs against obesity and diabetes.
Finally, recent data from studies investigating how calorie restriction affects mitochondrial activity also question our assumptions of a correlation between calorie intake and thermogenesis. Calorie restriction extends lifespan in rodents and is associated with reduced energy expenditure. However, calorie restriction stimulates mitochondrial biogenesis in skeletal muscle, a response involving nuclear factors that regulate mitochondrial biogenesis, such as Pgc1a (28). In other words, while a positive correlation between calorie intake and physical energy expenditure exists, it is not necessarily associated with stimulating mitochondrial respiratory activity at a molecular level.