Dr. Katz
Dear Dr. Katz:
My sister and I eat the same amount of food and get pretty much the same amount of exercise. Yet I am much heavier than she is. I can lose weight on a diet, but then I feel ravenous all the time. She can eat a lot without gaining. What's going on?

Response:
It must be frustrating to gain weight easily and struggle to lose it when your sister maintains her figure without as much effort. It seems your body is genetically programmed to be more fuel efficient, and that's not necessarily a good thing.

There are three ways the body burns calories: doing physical activity, generating heat through digestion, and fueling the heart, brain, liver, and other organs. This may come as a shock, but physical activity and heat generation account for only about 30 percent of your total calorie expenditure. The rest of your energy goes to keeping your heart and lungs pumping and your organs functioning. That's known as your resting energy expenditure (REE), and it's determined genetically—there's not a lot you can do to alter it.

If a person's REE is high—as your sister's seems to be—weight loss is easy; she probably has a tough time adding pounds. But many more of us have inherited a slower REE thanks to evolution. The people most likely to survive famine were fuel efficient—they had a low REE. But what helped our ancestors make it through lean times leaves us vulnerable to obesity in this world awash in tasty calories and labor-saving technology.

Another factor that may be thwarting your efforts is your set point—a weight to which your body is inclined. This is a controversial theory, but research supports aspects of it, such as the fact that being heavy in early childhood or puberty will spur the body to make extra fat cells. (In adulthood weight gain is mostly due to an increase in the size of existing fat cells, though being obese can spur the growth of new cells.) Because every fat cell fights to protect its food supply, the more you have, the more intense your efforts will have to be to slim them down.

Sounds grim, but there's no reason to feel hopeless. First, accept that the calories and activity required to control your weight are unique to you; don't drive yourself crazy by making comparisons to your sister. Second, raise your REE by building muscle. Lean body mass will burn more calories than body fat, even at rest. By choosing foods that fill you up on fewer calories—vegetables, fruits, and lean sources of protein like eggs, chicken, fish, and soy—and adjusting your exercise, you should be able to find a weight you're comfortable maintaining permanently. Remember that losing even 10 percent of your total weight can add years to your life. Once you and your metabolism have struck a workable compromise, give yourself a break and love the body you're in.