Are sweets that good?

I love Death Note a LOT but I've always wondered this, how can L just eat sweets and tea and not get sick, fat, or diabetic? It scares me! Even though he claims that he uses brain power to burn the fat, how can he?!?!?! Plus he never exercises. He played tennis once but that isn't enough!!! I think he needs to borrow some of ryuk's apples ;_: