Some of you may have seen this already but I've just watched a BBC documentary on Youtube with this title.
Here is the link;
It followed a group of naturally thin young people as they ate 5000 cals a day for a month. Interesting differences in results e.g. some people's bodies wouldn't let them eat that much and one turned the weight he gained into muscle rather than fat.
The programme covered pre-birth influences, variations in our reactions to food, our body's regulation of what it sees as our perfect weight and why we still feel hungry even when we have had sufficient cals. It even discussed a chicken virus which makes some people gain weight.... but it doesn't affect all overweight people so we can't all use that excuse..... dammit!
Anyway I think it's worth a watch if you're interested....