[subungual]

Maybe I wasn't specific enough here. When I said "smaller peptides," I should have instead written "free amino acids, dipeptides and tripeptides." The two papers you cited refer to topical application, but your statement in your first comment said that it is easier to get collagen from dietary sources. What I'm asking for is what size of peptide you're calling a "collagen peptide" and a source supporting a mechanism of absorption. Most references I find to "collagen peptides" are for unvetted dietary supplements.

There is not good evidence that peptides larger than 2-3 amino acids can be absorbed into circulation via the gut in more than trace quantities (1). Digested collagen will be rich in certain amino acids (glycine and proline in particular), but calling something as small as a di or tripeptide a "collagen peptide" seems like an oddly arbitrary choice. I am a medical student with significant background in biochemistry, so please don't feel the need to dumb it down for me when you explain.

1. https://www.cambridge.org/core/journals/nutrition-research-r...

[gamblor956]

Collagen peptides are some of the bioactive peptides referred to in your citation that are known to be capable of passing beyond the gut in more than de minimis quantities... (Though in this context, it's still a small amount of the collagen originally consumed, between 1-10%.)

See https://academic.oup.com/fqs/article/1/1/29/4791729 for a discussion of bioactive peptides.