Heredity

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People whose parents are obese are 25 to 35 percent more likely to be so, too, and other patterns seem to exist in families. One study, for instance, showed that identical twins who ate the same amount of extra calories gained the same amount of weight and in the same places in their bodies.

Modern genetic research is examining "fat genes" that may affect everything from appetite to fat storage. For instance, being closely studied is one gene that controls the production of lipo protein lipase (LPL), an enzyme that helps store calories as fat.

Nonetheless, having "fat genes" doesn't mean you have to be overweight, says Rena White, M.D., director of the National Weight Control Registry, which tracks the success of people who have lost weight and kept it off. "Clearly, there are genetic factors in obesity," Dr. White says.