An Overview of the
Etiology of Obesity
JAMES O. HILL
The current global epidemic of obesity is fueled by an environment that “overpowers” the body’s energy balance regulatory system. The impressive stability of body weight seen in most individuals over time implies that the body does have a system to regulate energy balance. But the worldwide increase in body weights suggests that the capacity of this system to maintain energy balance at a “healthy” body weight in response to environmental challenges is limited. The current environment is exerting constant, unidirectional effects on energy balance that exceed the adaptive capacity of the system (see Chapters 74 and 78). Under these conditions, an increase in body weight and body fat mass is necessary for the achievement of energy balance.
We do not fully understand the physiological mechanisms in the body by which energy intake and energy expenditure are linked, but the existence of such a system is suggested by results of under- and overfeeding studies. In response to underfeeding, decreased resting energy expenditure tends to limit negative energy balance and minimize changes in body weight. Similarly, with overfeeding, resting energy expenditure increases in a fashion that opposes positive energy balance and weight gain. The adaptive response appears to be somewhat stronger to oppose weight loss than to oppose weight gain (see Chapter 8). Furthermore, body weights of these subjects return to baseline levels relatively quickly after the under- or overfeeding stops, suggesting an ability to restore the “usual” state of energy balance after the challenge to the system is withdrawn.
While there are clearly adaptive responses to under- and overfeeding, this physiological system has a limited capacity to maintain energy balance in the face of environmental challenges (e.g., under- or overfeeding). If such challenges are sufficiently large, energy