Thats kind of his point. if being overweight doesn't cause heart disease, if eating fatty foods doesn't make you gain weight, or raise bad cholesterol, then whats the real problem? it basically comes down to nature. what IS natural for humans? meats, fruits, leafy stuff, natural fats. what isn't? grains, agricultural goods. especially wheat. basically, the food pyramid thats been shoved down kids throats is wholey wrong. the general rule of thumb is, if its something that an animal could get on its own, its good, if its something that is human grown, the product of agriculture i mean, its bad.
a modified Atkins diet is really what he's doing. eat the burger, not the bun, cut carbs. that kind of stuff. and don't OVER-EAT. its been awhile since i watched the film proper so i can't give exact numbers and such, but basically he kept on a strict diet, setting a certain amount of calories and carbs a day. its just that all of it is McDonalds, KFC and wendy's foods.