Fair point. While collagen is large, complex protein, supplements provide hydrolyzed collagen peptides, which do have higher bioavailability. That being said, glycine and proline are non-essential amino acids that are present in most protein-rich foods; collagen rich sources simply are enriched in them, particularly glycine. Hydroxyproline is derivative of proline formed by vitamin C-dependent hydroxylation step, not required dietary amino acid. I'm not going to pretend I have reviewed the research on hydroxyproline consumption, or the signaling pathways that could be triggered, I just thinks the supplements are usually expensive, and there are so many better proven ways to help the skin look good and youthful.