what

(edit) Okay sorry that was rude but no, that's still not how it works. Really not. You're trying to align Island "fat"/food mechanics with real-world food mechanics, but you're not accepting that fat is, in the real world, a long-term storage system. You use it up when your particular stretch of savanna goes mysteriously empty of antelope and you can't find food for weeks. It's meant to keep you alive, not give you short-term energy. (Plus, having fat and being fat are two different things. People generally have a store of fat, but they don't have to be overweight. The Island is saying that you are overweight, which should not give you stamina if you are trying to make real-world sense of the way the Island uses fat.)

You cannot make real-world sense of how the Island uses the word 'fat'. In the real world the body can only process so much. No matter how much you overeat, no matter how poorly you choose your diet, one day simply will not ever take you from being slim and trim to being grossly overweight! It takes years.

Moreover, if you are using up all the calories you take in each day with vigorous hard labour, and flopping into bed each night totally drained, it doesn't make any difference how much fat is in your food, it isn't going to be stored as fat. It isn't going to get past your digestive system.

This, however, is not the real world. Food works the peculiar way it does because CMJ wanted to give us a complicated puzzle to work out. I'm with FML that some of the components could really use re-naming, but, whatever. It's a game. Wave a hand, chalk it up to Improbability.