That's very good for you. Someone did an experiment on healthy, slim
prison inmates, by asking them to eat as much as they could and to try
to gain weight.

`` Sims was interested in whether the metabolic differences observed
between fat and thin people were the result or the cause of their body
type. Put simply, he wanted to know whether people are born fat or
made fat. He decided that the best way to sort this out was to
convince a group of slim volunteers to eat themselves fat and to
observe what happened to them when they reduced to their original
weight.

Sims was fortunate to have nearby a ready source of experimental
subjects: the inmates at Vermont state prison, sufficient numbers of
whom were willing to gorge themselves for science. At first the
prisoners proved enthusiastic trenchermen, as much as doubling their
usual daily intake of food. But as they fattened, they became
increasingly reluctant to overeat. Most found it extremely difficult
to gain weight, and eventually some started to drop out of the
study. Only 20 made it through the requisite 200 days, achieving an
average weight gain of 20-25lbs. Relieved of the high-calorie,
low-exercise regimen, all but two of the inmates quickly dropped the
newly acquired ballast. The pair of inmates who found it most
difficult to lose weight were those who had experienced the least
difficulty gaining weight in the first place. It was later discovered
that both these men had a family history of obesity.

From this experiment Sims concluded that the body was remarkably well
equipped to balance energy intake and output, and to reach an energy
equilibrium, or "homeostasis", at which it felt naturally
comfortable. What was particularly interesting was that body weight
seemed somehow fixed, and was in most subjects resistant to change
over the short term. The prisoners with obesity in their backgrounds
were, it seemed, genetically inclined to reach homeostasis at a higher
weight than were others; the high-calorie diet only helped manifest
their genetic proclivity.''