Question: Do we have a "fat thermostat?" (continued from last week). Last week I reviewed a recent study showing that the body adjusts its metabolic rate to resist weight change. If you are losing weight, the metabolic rate goes down to slow the loss; if you are gaining weight, the rate goes up.
In an editorial in the same issue of the New England Journal of Medicine, Dr. William Bennett, a weight control expert at the Cambridge (Massachusetts) Hospital, commented on the study I reviewed last week. According to him, this helps substantiate other evidence that the body has a complex, highly sophisticated system for regulating its fat stores. "When actual adipose mass deviates from the internal standard, compensatory efforts come into play to minimize the discrepancy and return the body to its previous status quo." In other words, the body appears to have a "set point" mechanism for body fat.This set point (sometimes called an adipostat) is located in the brain and "compares the actual amount (of fat) with the internal ideal and then takes steps to minimize the difference." According to Bennett, this mechanism, in addition to its influence on metabolic rate discussed before, has a powerful influence on both eating and physical activity. And, with only minor adjustments in these three factors, it can strongly affect the body weight of any person over time.
Some scientists have argued with the set theory because no signal molecule has been found to tell the brain how much fat is stored. Bennett thinks the obesity gene that was recently discovered will in fact be related to the signal mechanism for evaluating fat levels in the body. A defective version of the gene probably misleads the body into thinking that it is too thin.
Although the thought of a defective controller is discouraging for those who have a problem controlling weight, the thermostat can probably be reset. Not only can the fat thermostat affect our metabolic rate, activity level and hunger, it is affected by both exercise and diet (and certain drugs).
People who transfer to a sedentary job almost always gain weight, not only because they use fewer calories, but because the fat thermostat gets reset. Bennett used the example of the weight gain that occurs with aging to make this point. A typical 150-pound man acquires about 22 pounds of fat over a 20-year period, an error of only about 10 calories a day. He doesn't think that this gain is related to poor regulation by the fat thermostat but to the sedentary patterns people drift into as they get older, which allow the thermostat to rise.
A second factor is the type of food we eat. It is quite clear that high-fat diets shift the set point to a higher level, and low-fat diets have become an important part of the weight-control arsenal. However, all of you know people who can eat anything they want or who can be inactive and still remain thin; the response of the set point to external factors is probably determined by several genes.
So, what does this mean to people who have a difficult time controlling weight? First, they must exercise and eat properly. Second, they need to be a little philosophical and accept whatever weight results when they exercise and eat right. Hopefully, the research into the genetics of obesity will solve some of the problems with the set point in the near future.