Biological Factors in Weight Control
How does heredity affect our weight? Part of the answer seems to be described in the fixed point theory, which proposes that each person’s body has a certain or “set” weight that it strives to maintain. The body tries to keep your weight close to the set point by means of a physiological mechanism similar to a thermostat. When a person’s weight starts from the set point, the body takes corrective action, such as increasing or decreasing metabolism. According to the theory, people whose caloric intake is drastically reduced or increased over a few months should show rapid corresponding changes in weight initially, but then the weight should show slower changes and reach a limit. Studies have found that these predictions are correct and that people quickly return to their original weight when they can eat what they want again. But the set-point theory is incomplete: it doesn’t explain, for example, why some people who lose a lot of weight manage to keep it off.
The mechanism that controls the set point appears to involve the hypothalamus. Animal research has shown that damage to specific parts of the hypothalamus causes weight to shift and eventually plateau, suggesting that a new benchmark has been established. If the damage is in the lateral region of the hypothalamus, the new set-point has less weight; damage to the ventromedial region leads to obesity. One way the hypothalamus might regulate body weight is by monitoring some aspect of fat cells. One study found, for example, that after obese people lost weight, they began to produce large amounts of an enzyme that facilitates fat storage in cells and weight gain. Also, the more obese people were before losing weight, the more of this enzyme they produced. It may be that the loss of fat in the cells causes the hypothalamus to start producing enzymes to maintain the set point.
Another way the hypothalamus can affect the weight management process is by regulating the level of insulin in a person’s blood. Insulin is a hormone produced by the pancreas, it accelerates the conversion of sugar (glucose) into fat and promotes the storage of fat in adipose tissue. Obese people tend to have high serum levels of insulin, which is called hyperinsulinemia. Elevations in serum insulin levels increase a person’s feelings of hunger, perceived pleasure from sweet tastes, and food consumption. Taken together, these findings indicate that weight gain is the result of a biopsychosocial process in which physiological factors interact with psychological and environmental factors.
It seems likely that the setting and function of the set point in regulating a person’s weight depends on the number and size of fat cells in the body. Psychologist Kelly Brownell has suggested that people whose weight is above the set point can reduce their weight quite easily until fat cells reach their lower size limit. The body weight at which this level is reached would depend on the number of fat cells in the body. Since the number of fat cells increases primarily in childhood and adolescence, it is likely that people’s diets during this period of life are very important. Obese children between the ages of 2 and 10 have fat cells the same size as those of adults. As these children gain weight, they do so primarily by adding fat cells. The size of fat cells in normal-weight children does not reach adult levels until age 12, and the number of fat cells does not increase much between 2 and 10 years of age.