Obesity and the Regulation of Energy Balance

total fast of approximately 150 days! This impressive energy reserve is due both to the high energy content of triglycerides versus polysaccharides, and the fact that triglycerides are stored in essentially anhydrous Harvard Medical School form; polysaccharides such as glycogen are hydrated Boston, Massachusetts 02115 in storage form, decreasing their efficiency as fuel. † Beth Israel Deaconess Medical Center To understand obesity, one must understand the con-Division of Endocrinology and the Department cept of energy balance (Figure 1). Assuming that an of Medicine individual has no problem with the absorption of nutri-Harvard Medical School ents, stored energy will increase only if energy intake Boston, Massachusetts 02115 exceeds total body energy expenditure. Energy expenditure takes the form of physical activity, basal metabolism , and adaptive thermogenesis. Physical activity re-Obesity is defined medically as a state of increased body fers to all voluntary movement, while basal metabolism weight, more specifically adipose tissue, of sufficient refers to the myriad biochemical processes necessary magnitude to produce adverse health consequences. to sustain life. Adaptive thermogenesis refers to energy There has been an alarming increase recently in the dissipated in the form of heat in response to environ-prevalence of this heterogeneous group of disorders in mental changes, such as exposure to cold and alter-the Western world (Kuczmarski et al., 1994). Fully one-ations in diet. It should be pointed out that the boundary third of the American population is now considered between what is considered basal metabolism versus obese, and the prevalence of obesity in children is esca-adaptive thermogenesis is not always clear-cut. Mam-lating dramatically, presaging even greater medical mals often live in climates with temperatures below body harm in the decades to come (Troiano and Flegal, 1999). temperature, sometimes far below body temperature. What accounts for this epidemic of energy storage? Thus, the determination of energy expenditure in re-Body weight and composition, and the storage of energy sponse to cold versus that which is considered part of as triglyceride in adipose tissue, are determined by the basal metabolism can be quite arbitrary. Traditionally, interaction between genetic, environmental, and psy-the basal metabolism rate is defined as the energy ex-chosocial factors. These influences ultimately act by penditure of a subject relaxed and at rest, at thermoneu-changing the energy balance equation, that is, the long-trality, 8–12 hr after the last food ingestion. Metabolic term balance between energy intake and expenditure. rates of mammals will obviously vary as a continuum …