system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can the body of some animals like Cows, Horses, etc. that only eat grass make those huge muscles?

They only eat vegetables, yet they have huge muscles. Their bodies are full of protein. How is that possible?

**In bodybuilding they say you need to eat high protein foods like beef and chicken to make muscles. It doesn't make any sense!**
gpt
First of all you need to consider that all plant matter, or every living being to be exact, contains all essential amino acids that are needed to assemble the proteins needed. The common claim that only some foods contain "complete protein" is utter bullshit - even apples and potatoes theoretically could supply us with sufficient amino acids to get by, we only need eat insane quantities to do so. The efficiency at which the pure protein of food can be used also varies by a large margin.

Animals that can digest grass need to eat a lot of it, but by doing so they can get plenty of amino acids and most other essential nutrients to thrive.

Now the reason bodybuilders usually recommend eating meat is that they aim for very low body fat and optimum muscle growth. One way to achieve that is to eat way more proteins than the body can use to grow (the amino acids can be used as source of energy otherwise) while avoiding fat and sugar, so lean meat and eggs are pretty good foods for that.

If you merely want to get ripped you can be a vegan and grow muscles just fine. Legumes like beans and peas, and grains contain a significant amount of protein, and combined they can be used very efficiently (grain lacks one amino acid legumes have plenty of)