How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?

Question

How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?

In:

Answer 1

The most basic life forms only consume one thing and their bodies produce the rest of what they need. As diet diversifies, we get more of what we need. If your diet contains a lot of Vitamin C, for example, you won’t be affected by genetic defects that stop your body producing Vitamin C.

So, over the millennia, organisms lose the ability to produce vital nutrients and if their diets compensate, then all is well and the genes are passed on. If not, they die just like every other unsuccessful genetic variant.