Why do some people never seem to get fat?

"The UK is a country obsessed by the threat of obesity. As the average person's weight has grown, so has coverage of the subject. The chief medical officer for England, Sir Liam Donaldson, has said we are facing an "obesity timebomb". Culinary celebrities like Jamie Oliver have launched campaigns, in homes and school kitchens, to fight the fat war. Yet the science of weight gain is less straightforward than the headlines sometimes suggest. Why, for example, do some people seem to eat what they like and not put on weight, while others limit their diet yet struggle to shed their bulk?" - BBC