How Do Herbivores Get Protein?

Factual Questions
1

We’ve always been told that humans require protein in order to build strong muscles. My question is: how do herbivores get the protein they need to build strong muscles (think horses, gorillas, etc.)? Or, if they’re not ingesting protein, is their body manufacturing it somehow?

2

Of course they’re ingesting protein! You think an elephant can get that big without protein?

Plants have plenty of protein.

3

Grass and other plants contain protein. It’s just not as concentrated or easy to extract as it is in meat. That’s why grazing animals have such complicated digestive systems and devote so much time to eating.

4

Eh… Plants have plenty of protein, as Qagdop said. The difference is that the digestive system of herbivores is much more efficient at digesting and absorbing those proteins from the plants than a normal carnivore (or even an omnivore like the humans).

The body manufactures the proteins it needs, but:

  1. It needs the basic ingredients, amino acids, to form the proteins. Those are either formed by the body or obtained by the diet. In the case of herbivores, that means plant protein.

  2. As with amino acids, sometimes there are proteins that the humans may need, but cannot synthesize them. Therefore those must come from the diet.

5

Protein is a long strand of amino acids stuck together, sorta like lego blocks. For the most part, proteins themselves cannot cross the stomach lining (iirc, I could be wrong…), but are broken down into amino acids in the stomach, then rebuilt into the right kind of protein when needed.

There are some 20 different types of amino acid; our cells can produce some from even smaller building blocks (sugars, other amino acids, etc), whereas some, we need to get from the diet.

It is possible to get all the amino acids we need from plants, which is why all those vegetarians are still alive. However, this requires a bit of thought, as most plants are missing some (but different) amino acids that we need.

Pure herbivores have three options:

  1. eat a varied diet so that they get all the amino acids they can’t syntesise from some of the plants they eat.
  2. evolve cellular mechanisms for synthesising all 20-odd amino acids, so they don’t need any specific ones in the diet. (or, at least synthesise the ones that are not common in what they eat)
  3. develop a symbiosis with bacteria in the rumen that synthesise amino acids for them.
6

You are correct about complete proteins, they cannot cross the stomach lining. But then, protein digestion occurs mostly in the small intestine, not stomach. :wink: Even so, most proteins cannot cross all in one. The notable exception are the colostrum antibodies that can cross the barrier when the animals are just a few days old. Dipeptides and tripeptides can be absorbed by the intestinal lining cells.