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Question: A few years ago you and some colleagues wrote a book called "How to Lower Your Fat Thermostat." In that book, you said that the body had a thermostat or setpoint that controlled body weight. You wrote that if you went on a low-calorie diet, the body would decrease the amount of energy burned, a "starvation reflex," to protect the fat stores and that it would be more difficult to lose weight. Is this idea still correct? Does the body really adjust metabolism in an effort to maintain a certain body weight? If so, how do you lose excess weight?

Answer: In an elegant experiment just published in the "New England Journal of Medicine" (March 1995), Dr. Rudolph Leibel and his associates at Rockefeller University showed that the body does indeed change its metabolism to resist either weight loss or weight gain. Using 18 obese and 23 volunteer subjects who had never been overweight, these researchers established a baseline metabolic rate using a series of metabolic measurements balanced with a liquid diet, which was adjusted until weight was maintained exactly for 14 days. Body composition was also measured carefully, using several exacting techniques at each stage of the experiment.After the initial metabolic rate was carefully established, 11 of the obese and 13 of the non-obese subjects were fed 5,000 to 8,000 calories a day until they had gained 10 percent of their initial body weight (this took from four to six weeks). The metabolic rate was again measured and a liquid formula fed to maintain the weight gain. These subjects were then returned to their initial weight using an 800 calorie diet, and the metabolic studies were repeated.

After the weight-gain studies were done, nine obese and 11 non-obese subjects were fed 800 calories a day until their body weight was reduced by 10 percent of their initial weight. They were again studied for metabolic rate and body composition and a liquid formula was given to maintain this new weight. Ten of the obese subjects were then given an 800 calorie diet until they had lost 20 percent of their initial weight, and all metabolic studies were repeated.

The rate of energy expenditure changed in both the obese and the non-obese subjects after changes in body weight. When subjects increased their weight by 10 percent, they used 16 percent more calories than would be expected. When they lost 10 percent of their body weight, their bodies compensated by burning up 15 percent fewer calories. These results substantiate the idea that the body tries to resist changes in body weight and explains why it is so difficult to lose weight by simply decreasing caloric intake.

It was interesting that the changes occurred equally in both fat people and thin. It was also interesting that weight change even changed the number of calories used after eating a meal. Whenever we eat, the metabolic rate goes up for a while (the thermic effect of feeding). When at a reduced body weight, the thermic effect of meals was lower; at a higher body weight, the thermic effect was higher.

However, there were no differences between the group losing 10 percent or 20 percent of their initial weight in terms of the change in metabolic rate. This suggests that the maximal adaptation in metabolic rate occurs at the 10 percent level, and no more change occurs, even if more than 10 percent of body weight is lost.

I will tie this research to the question of a "fat thermostat" next week.