Obesity is second only to tooth decay as a major nutritional problem in this country, says a nutrition expert at the University of Rochester's Medical School.

Dr. Gilbert Forbes spent nearly a half-century researching the role food plays in determining whether we grow up lean or fat. He finally grew skeptical of what patients would tell him about their normal eating habits and their ability to gain or lose weight.So he embarked on a massive study on the effects of overeating in women. Subjects were paid $800 each to live at the University's medical center for a month, eating as much as they could.

They were given three hearty meals a day plus three snacks with milkshakes, peanut butter and crackers, chocolate cookies, pound cake and other high-calorie foods. Toward the end of the month, the women were consuming an average of 1,800 extra calories a day - nearly twice what they said was their normal intake.

The result? The women had gained between 71/2 and 111/2 pounds each.

"Even the two thin women who told us that they had been unable to gain weight in the past gained during the study," says Forbes. "It's just what we expected."

What he learned is that overweight patients tend to underestimate what they eat and underweight people tend to overestimate what they eat.

"I think of obesity as a disease of appetite," he says. "All the evidence we have accumulated under controlled conditions suggests that obese people do tend to eat more than lean people.

"I think what obese people may inherit is a big appetite."

He also says that under controlled conditions, patients always lose weight when their calorie intake is below a normal maintenance diet - even those who said they couldn't lose weight before.

He adds, however, that genetics may still play a part in people who are "born" to be fat.

Forbes recently studied 51 pairs of identical and 38 pairs of fraternal twins of the same sex.

"What we found is that the identical twins were much more concordant than the fraternal twins," he says, "in both the amount of body fat they had, and the distribution of the fat. If one identical twin had a small belly and big hips, the other twin did, too."