Does the Body Have a Set Point for Weight?

The set point theory of weight maintenance espouses the notion that weight is closely regulated by the body. It proposes that humans have a genetically predetermined body weight or body fat content, which the body attempts to defend. Some research suggests that the hypothalamus monitors the amount of body fat in humans and tries to keep that amount constant over time. This regulation of body fat content is referred to as a “set point.” You have already seen in this chapter that the hormone leptin forms one communication link between adipose cells and the brain that allows for some weight regulation.

In the major studies of humans cited to support the set-point theory, volunteers who lost weight through starvation later ate in a way to regain their original weight or a little more. In addition, studies in the 1960s using prisoners with no history of obesity found it was hard for some men to gain weight. This was supported by later studies (see the previous section on thermo genesis). Also, after an illness is resolved, a person generally gains lost weight.

Sound physiological evidence also suggests that body weight tends to be regulated. If energy intake is reduced, the blood concentration of the thyroid hormones fall, and the metabolic rate slows. In addition, lower body weight decreases the energy cost of each future weight-bearing activity, and the total energy used by lean tissue falls because some of these tissues are also lost. Furthermore, the enzyme used by adipose and muscle cells to take up fat from the bloodstream (lipoprotein lipase) often increases its activity. Through these changes, the body resists further weight loss.

If a person overeats, I the short run the metabolic rate tend to increase. This causes some resistance to weight gain. People often recognize the body’s resistance to weight loss when dieting but do not think much about the resistance to weight gain after eating a big holiday meal. However, in the long run, resistance to weight gain is much less than resistance to weight loss. When a person gains weight and stays at that weight for a while, the body tends to defend the new weight.

Arguments against the set-point theory cite the fact that, during pregnancy, women slowly increase body weight and fat. Also an average person’s weight does not remain constant throughout adulthood; it usually increases slowly, at least until old age. This means that a person must be able to shift his or her set point. It is also argued that, if an individual is placed in a different social, emotional, or physical environment, weight can become markedly higher or lower and is maintained. These arguments suggest that humans, rather than having a set point determined by genetics or number of adipose cells, actually settle into a particular stable weight based on an interaction between nature and nurture influences.

In the final analysis, we must bear much of the responsibility for weight maintenance ourselves since set point is weaker in preventing weight gain than in preventing weight loss. The odds are against the likelihood that, even with a set point helping us, we can avoid creeping weight gain in adulthood without attention to this tendency.

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