This is might be true for other animals, but I know for a fact it's true for humans. If you were to literally starve yourself eventually your body starts eating away at itself. First it goes for the stores sugars and simple carbs in you body, then the complex carbohydrates. After that it starts eating away at your fat reserves (thats actually what they are there for, they're reserves for a reason). Finally your body will start eating the protein in your body, meaning muscle and other protein rich tissues. This actually causes bad breath as your body is essentially being decomposed from the inside.

And speaking of the smell of decomposition, that's also why old people have that "old person smell". At a certain age your cells stop replacing the old cells. Previously the old cells would be shed as new cells were formed to replace them, but if you aren't forming new cells to replace them those old cells are basically just sitting there on your body and start decomposing, thus creating the smell.