Anyone who has ever been on a diet knows it is extremely difficult to keep weight off once it is lost. Sure, she might shed a few pounds in the first weeks or so but, try as she might, that weight almost invariably returns. When it does, the person usually blames the diet or, more typically, herself. “It’s my fault,” she wails. “I have no willpower. I’m a failure-a washout.”

When I hear such remarks from a patient, I tell her as emphatically as I possibly can: “It is not your fault.” And I proceed to tell her about a scientific discovery that has revolutionized the way we think about body weight.

Simply put, that discovery-known as the set point model-has revealed that each person is biologically programmed to reach and maintain a body weight that falls within a certain, relatively narrow range. Within that range, body weight is strongly guarded against either increasing or decreasing substantially.

For the individual, the actual weight range is not a matter of personal choice or aesthetic preference, nor is it a response to cultural pressure. It is part of our genetic, physiological destiny (with perhaps some influence from the way we are fed in infancy). Just as we have no control over whether our eyes will be brown or blue, we have no choice over the body weight that our DNA says will be right for us.

The human body is an organic system that takes in and expends energy. Like the coal that stokes the furnace, food in the form of calories provides energy. Metabolism determines the rate at which our bodies convert the food into energy.

Normally, the metabolic rate represents a balance between intake and expenditure. If we consume roughly two thousan calories a day, we will usually burn off roughly two thousand calories a day. Such burning occurs actively, through work or exercise, or passively, through sweating or regulation of body temperature. The very act of thinking burns calories. Even when we are sitting or sleeping, we are burning energy at a certain rate. Our metabolic balance is controlled by interaction among a number of different factors, including hormones and their receptors in the cells, neurotransmitters, diet, amount of exercise, our genetic inheritance-even the temperature of the air around us.

The set point model, supported by much scientific evidence, shows that metabolism tries to reach and hold a certain weight level. Some people are naturally skinnier and others are naturally heavier. The set point model implies that different body weights are appropriate and healthy for different people.

A person biologically programmed to weigh 160 pounds is healthy and normal at that weight. If she suddenly dropped thirty pounds, her metabolism would react as though the body were in a state of siege-which, in a sense, is what starvation is. Her metabolism would slow down tremendously-by as much as a whopping 30 percent-to help conserve energy. Her body would take the few calories it gets and use them more efficiently until it returned to its higher weight. Conversely, someone who should weigh 100 pounds and who balloons up to 140 will experience an acceleration of metabolism, a cranking up of the physiological furnace in an effort to burn off the unwanted weight.

You can see the vicious cycle. A person decides to diet-say, by cutting down her food intake by 20 percent. She loses some weight. Her body senses the change and reacts as though it is in danger of starvation. Her metabolism then drops as the efficiency by which her body converts food to body weight increases.

Thus, although the dieter eats less food, her metabolism also decreases. A recent study on a group of women found that after nine weeks of dieting they had lost an average of 3 percent of their original weight. Yet their consumption of oxygen-which the body uses to burn energy and which thus fuels metabolism- dropped by a substantial 17 percent.

When weight rises beyond the set point range, the body burns off energy-by raising body temperature, for example. Conversely, loss of too much weight triggers a kind of biological blackout. The person will feel fatigued and may begin to sleep more. Body temperature may drop to conserve calories. As we have seen, anorexics often complain of feeling cold.

Menstruation also stops. There is genetic wisdom-I’m tempted to call it “bio-logic”-behind this development. A starving woman can’t spare the energy needed to build another human being. In the interest of survival of both the mother and her future children, the reproductive system shuts down.

By undergoing this radical energy-conservation program, the body struggles to push weight back up to the right level for this particular person. But when her weight starts to rise again-as it inevitably will-the dieter begins to panic. She may take even more drastic steps to lose weight, triggering further metabolic disruption. And so on.

*47/35/5*