Hydroxyproline isn't essential though, humans can produce their need from collagen, which they also produce as well as most other animals.

> You need to eat a ton of quinoa

Most don't: at 4.4% protein, a 65kg man like me needs 1.5kg of cooked quinoa per day and it's not a big deal:

- You'll digest it like a king: quinoa is full of soluble and insoluble fibers and you won't feel puffy for eating too much. Easy in, easy out.

- Like milk or wheat, there's many transformations possible like flour, flakes, marinades, beverages, soups... alway a joy to cook and eat, no boredom with that grain.

Can't speak for the bodybuilders though but I'm sure most manage they nutrition. I think soy/pea is more popular.

By the way, very few eats only quinoa or any other single aliment. They also get amino acids from grains, pulses and seeds... even fruits like tomatoes but it's obviously negligible.

Quinoa is also fulled with minerals, vitamins and it's proteins have the same biological value (BV) as beef - or more depending on the source.

> its high-quality protein, complete set of amino-acids, and high content of minerals and vitamins. [0]

> exceptional balance between oil, protein and fat [1]

> Quinoa has a high biological value (73%), similar to that of beef (74%) [2]

0 https://www.tandfonline.com/doi/full/10.1080/15528014.2022.2...

1 https://scijournals.onlinelibrary.wiley.com/doi/10.1002/jsfa...

2 https://www.researchgate.net/publication/303845280_Quinoa_Ch...

There are plenty of studies showing that collagen supplementation helps athletes. Which means there are cases where the body doesn’t produce enough collagen for itself. And as you age, your body produces less collagen. Reasons enough to supplement with collagen.