A few months ago, I ran across a BBC documentary on Youtube called "Why Are Thin People Not Fat?". You can watch Part One here. Just follow the links on the right-hand side of the page to watch the other episodes (total of 7).
This self-described "controversial" program documents an experiment done with ten young, decidedly slim people who went on a 5,000 calorie a day diet with no exercise allowed for four weeks. The seven episodes intersperse the regular "fat epidemic!!!!!!! we've got to do something or we're all damned!!!" fear mongering with discussion by medical professionals and, of course, interviews with the subjects themselves.
The goal of the experiment, as the Swedish scientist who led it states, is to "learn from those who can't get obese". Most studies are done on those who are already overweight and the prevailing attitude in society is that people are fat because the eat too much and exercise too little. Generally (and certainly in the weight loss universe), overweight is seen as a personal failing and "normal" or slim people are seen to be morally superior. This program turns that premise on its head and looks at the people who frustrate us beyond belief: the naturally slim.
Do different bodies cope differently with an excess of food? According to this experiment, the answer is a resounding "yes!"
This is a fascinating program and the results are, for the most part, not surprising. While the subjects all gained some weight during the experiment, they also lost it effortlessly once the experiment had ended. I was particularly struck by the young Asian man whose body at a certain point went into metabolic "overdrive" and literally prevented him from continuing to gain weight, despite the large quantity of food he ate every day and his total lack of exercise.
I highly recommend you watch this documentary. Despite the film maker's need to repeat all the old saws about the dangers of overweight (because, as we all know [eye rolling allowed], you can only be healthy if your BMI is under 25), the documentary still makes an important point about how different people's weights vary due to factors far beyond how much they may eat or exercise.