TIn the United States population, 30% of adults are obese and another 35% are overweight. (Obesity is defined in terms of body mass index (BMI): BMI = weight in kg/(height in m)2. A BMI below 25 is considered normal; 25 to 30 is overweight, and greater than 30, obese.) Obesity is life-threatening. It significantly increases the chances of developing type II diabetes as well as heart attack, stroke, and cancers of the colon, breast, prostate, and endometrium. Consequently, there is great interest in understanding how body mass and the storage of fats in adipose tissue are regulated. ■
To a first approximation, obesity is the result of taking in more calories in the diet than are expended by the body's energy-consuming activities. The body can deal with an excess of dietary calories in three ways: (1) convert excess fuel to fat and store it in adipose tissue, (2) burn excess fuel by extra exercise, and (3) "waste" fuel by diverting it to heat production (ther-mogenesis) in uncoupled mitochondria. In mammals, a complex set of hormonal and neuronal signals act to keep fuel intake and energy expenditure in balance, so as to hold the amount of adipose tissue at a suitable level. Dealing effectively with obesity requires understanding these various checks and balances under normal conditions, and how these homeostatic mechanisms fail in obesity.
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