system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do most animals only have a diet containing foods from 1 or 2 food groups while humans need a balanced diet from all different food groups?

Humans need grain, vegetables, fruits, meat, and dairy. Most animals eat from just one food group and that's considered healthy for them. Does it have anything to do with evolution?
gpt
It's not about the groups, it's about what our body can and cannot do. In herbivores the bacteria in stomach/intestine can digest fibers and synthetize proteins and precursors to fats and sugars, but humans can't and so need some fundamental proteins from other sources (be them vegetable or animals). Herbivores can digest meat products too (meat meal is added in nutrition and improves the qualities of the feed), but they are not adapted for hunting ( yet some can still put up quite a fight!). Pure carnivores can also eat food of plant origin (kibble is also made from grains) but lack the ability to digest pure fibers (like cats eating grass to vomit) and is adapted to hunt.