Why are thin people not fat?.

Why is it that some people tend to gain weight while others, seemingly, don't? A group of naturally slim people are brought together and asked to eat so much that they gain up to 15% of their current body weight. The aim of the study is to show how it is possible that some people manage to stay slim in an increasingly fat world. Around this central experiment, experts discuss both the pros and the cons of body fat - in times of famine, for instance, those who are fast to gain fat would be likely to be the survivors. However, in today's environment it is the naturally slim who live longer, healthier lives.