Collagen peptides do enter the bloodstream at significant levels:
http://www.ncbi.nlm....pubmed/16076145
In the present study, we identified several food-derived collagen peptides in human blood after oral ingestion of some gelatin hydrolysates. Healthy human volunteers ingested the gelatin hydrolysates (9.4-23 g) from porcine skin, chicken feet, and cartilage after 12 h of fasting. Negligible amounts of the peptide form of hydroxyproline (Hyp) were observed in human blood before the ingestion. After the oral ingestion, the peptide form of Hyp significantly increased and reached a maximum level (20-60 nmol/mL of plasma) after 1-2 h and then decreased to half of the maximum level at 4 h after the ingestion. Major constituents of food-derived collagen peptides in human serum and plasma were identified as Pro-Hyp. In addition, small but significant amounts of Ala-Hyp, Ala-Hyp-Gly, Pro-Hyp-Gly, Leu-Hyp, Ile-Hyp, and Phe-Hyp were contained.
http://www.ncbi.nlm....pubmed/10498764
...this study investigated the time course of gelatin hydrolysate absorption and its subsequent distribution in various tissues in mice (C57/BL). Absorption of (14)C labeled gelatin hydrolysate was compared to control mice administered (14)C labeled proline following intragastric application. ... In cartilage, measured radioactivity was more than twice as high following gelatin administration compared to the control group. The absorption of gelatin hydrolysate in its high molecular form, with peptides of 2.5-15kD, was detected following intestinal passage. These results demonstrate intestinal absorption and cartilage tissue accumulation of gelatin hydrolysate and suggest a potential mechanism for previously observed clinical benefits of orally administered gelatin.
Collagen peptides are not merely passive building blocks of collagen proteins, they also act as triggers for biosythesis of collagen, osteoblasts, fibroblasts, and more:
http://www.ncbi.nlm....es/PMC4057461/
http://www.bloodjour...so-checked=true
http://www.ncbi.nlm....pubmed/20618556
https://www.jstage.j..._3_211/_article
http://pubs.acs.org/....1021/mp300549d
However, as of 2011 "a cause and effect relationship has not been established between the consumption of collagen hydrolysate and maintenance of joints" according to the European Food Safety Authority's evaluation of one product's claims: http://www.efsa.euro...al/doc/2291.pdf
And oral supplementation with type II collagen may even trigger flare-ups in rheumatoid arthritis: http://www.ncbi.nlm....pubmed/11072596
I'm not advocating for or against supplementing with collagen, but just wanted to clear up the misconception about all peptides being broken down to constituent amino acids before entering the bloodstream. This does happen with most peptides, but some remain intact and enter the bloodstream in significant amounts, an important process which leads to the stimulation of many other processes.
I hope I didn't give the impression that peptides were fully hydrolyzed before absorption. I meant that they would have to be fully hydrolyzed prior to being used in the synthesis of new protein. Short peptides are known to be absorbed, in fact, even better than individual amino acids. I thought that the 14C label experiment was going to lay all the arguments to rest, but I don't understand it. The group that got labeled proline also got gelatin. Why did they do that? It sounds like fairly long oligopeptides of collagen are getting absorbed and are getting incorporated into collagen without passing through the collagen synthesis apparatus, but the mice in this experiment were dosed with 1% of their body weight in collagen. That's a hell of a lot of collagen, more than anyone is going to use. The question is, do the effects they see also happen with sane levels of supplementation, or is the massive dose inducing some unusual behavior?
Thanks for all that research, deeptrance. That's a nice collection of papers.