Each person's unique genetic makeup, these experts think, may affect what he craves, how much he craves and how his body uses fat and burns calories.
"We are hard-wired to be a bit more hungry than we need to, because until very recently — in evolutionary terms — the vast majority of our fellow humans had no idea whether the next meal would be available or not," said Francesco S. Celi, a clinical investigator at the NIH research unit.
Yet for some people, there is a profound imbalance between what they eat and the amount of energy they expend. Most of these people become obese as a result, but some, like Michael, don't.
"Some are more sensitive" to that imbalance, said Rudolph Leibel, a diabetes researcher at New York's Columbia University who has been studying the biochemistry and genetics of obesity for 25 years. "That's the genetics."
"There are people in the population who are skinnier or more slender with a different genetic response to the environment," he said. That is why "just yelling at people and telling them it is sinful or gluttony is not a particular fruitful way to deal with the problem. It's not very effective to insinuate that someone has moral failings when a behavior is involved."
To try to unravel the complexity of all this, researchers at an NIH diabetes and obesity lab in Phoenix have begun to incorporate thin people into their studies. Why "some [people] tend to overeat more than they need more consistently and why this occurs is clearly complex and involves levels of behavior that we are just beginning to understand," said Jonathan Krakoff, an endocrinologist at the lab.
Krakoff and his colleagues are recruiting for a study in which thin people will consume about 4,000 calories in a 24-hour period, about twice the amount an average healthy person needs in a day.