What "hydrolyzed" means
Native collagen is a large triple-helix protein (~300 kDa) that the human gut cannot absorb intact. Hydrolyzed collagen has been enzymatically cleaved (typically with pepsin or microbial proteases) into much smaller peptide fragments — 1-5 kilodaltons on average, often as small as di- and tri-peptides. Asserin et al. 2015 demonstrated using radiolabeled tracers that these small peptides absorb across the gut wall intact and circulate in serum. This is the mechanistic basis for why hydrolyzed collagen has measurable systemic effects while unhydrolyzed gelatin does not.
The bioactive sequences
Specific dipeptide sequences appear to act as biological signals. Pro-Hyp (proline-hydroxyproline) and Hyp-Gly are the most-studied: they reach dermal fibroblasts and appear to upregulate procollagen mRNA expression and hyaluronic acid synthesis. This is a signaling pathway distinct from simply providing amino acid raw materials — small amounts of the right peptides matter more than large amounts of any random collagen fragment.
What the RCT evidence supports
Choi et al. 2019 (Journal of Drugs in Dermatology) meta-analyzed 11 placebo-controlled RCTs (n=805) and reported: skin elasticity improved with standardized mean difference +0.36, hydration +0.43, and dermal collagen density +0.41 over 8-12 weeks. Effective doses ranged 2.5-10g/day. Proksch et al. 2014 demonstrated 2.5g/day matched 5g/day for skin endpoints — suggesting 2.5g is a reasonable floor for most users. For joint pain, Clark et al. 2008 showed 10g/day reduced activity-related joint pain in 147 athletes over 24 weeks. For tendon synthesis, Shaw et al. 2017 demonstrated 15g pre-exercise + vitamin C doubled collagen synthesis markers.
Practical protocol
Take 2.5-10g hydrolyzed collagen + 50-100mg vitamin C daily (vitamin C is a required cofactor for the hydroxylation step in endogenous collagen synthesis). Source — marine, bovine, or chicken — produces comparable clinical results when matched for dose; marine has slightly smaller peptide size but the difference is marginal. Continue for 8-12 weeks before judging effect. For specific brand picks, see our collagen supplement rankings. Note: collagen is NOT a complete protein and is suboptimal as a workout protein — use whey or whey+casein for muscle protein synthesis.
Primary sources: Choi FD et al. 2019 (J Drugs Dermatol), Asserin J et al. 2015 (J Cosmet Dermatol), Proksch E et al. 2014 (Skin Pharmacol Physiol), Clark KL et al. 2008 (Curr Med Res Opin), Shaw G et al. 2017 (Am J Clin Nutr).