Eating Disorders and Obesity: A Comprehensive Handbook

By Christopher G. Fairburn; Kelly D. Brownell | Go to book overview

10
Energy Expenditure and Body Weight

ERIC RAVUSSIN

There has been considerable speculation concerning the reasons the human genome could harbor genes predisposing to positive energy balance and obesity in our “obesegenic” environment (see Chapters 3, 5, and 12). The most frequently stated theory is that of the “thrifty genotype.” During human history, food was not very abundant and required much physical work to obtain. Survival mechanisms evolved to confer protection against periods of food scarcity. It is therefore not surprising that highly industrialized populations now struggle with the problem of obesity due to rapid environmental changes (see Chapter 78). This has led us to propose another hypothesis: that obesity in our present environment is an “essential” condition and only those with fewer obesity susceptibility genes (the former nonsurvivors) can resist our “obesigenic” environment and maintain a normal weight without conscious effort (see Chapter 4). The common thread between these two hypotheses is that what was an asset to early humans is now rapidly becoming a liability. Although the pathogenesis of obesity is not completely understood, recent studies of energy intake and expenditure in humans have shown that obesity is not simply the result of bad behavior or so-called “sloth and gluttony.”


RELEVANCE OF ENERGY EXPENDITURE COMPONENTS TO OBESITY

Obesity results from a chronic positive energy imbalance between energy intake and energy expenditure. Since food intake assessment is precise only under laboratory conditions that do not reflect everyday life, and measurement in free-living conditions is inaccurate, scientists have concentrated on the energy expenditure side of the energy–balance equation (see Chapter 24). Technological advances have made possible the use of indirect calorimetry for measures of metabolic rates over periods of hours using a ventilated hood system, or over 1 day or more using a respiratory chamber. In such chambers, all components of sedentary energy expenditure can be measured (i.e., sleeping metabolic rate, the energy cost of arousal, the thermic effect of food, and the energy cost of spontaneous

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