People can't break down tough plant fiber like cows or rabbits can, so we get almost no energy from it — their stomachs are built for that job, but ours aren't.

Humans cannot digest cellulose or most dietary fibers using their own enzymes. Instead, bacteria in the gut partially break down these fibers through fermentation, which suggests that human evolution has favored energy extraction from animal-based foods rather than plant materials.

Humans cannot produce enzymes that break down most plant fibers on their own, unlike herbivores. Instead, bacteria in the human gut partially break down these fibers to produce short-chain fatty acids.

Humans have a digestive system with a shorter small intestine and smaller cecum than herbivores, which allows for more efficient digestion of animal fats and proteins and less reliance on bacteria to break down plant fiber.