Full Course from Hell Urusei Yatsura : Season 1 Episode 26

Still at the resort, the group meets some maniac who is having an all-you-can-eat contest. Sakura reveals the secret of her figure to be a diet pill called "Beauty Foods". But we really know the real reason is Sakura can just plain eat like a whale, and we see her do so as she beats Ataru and Cherry in the contest and clean the maniac of all his food.