1 · Collagen · incomplete protein
As a supplement, collagen lacks tryptophan: it is an incomplete protein that cannot stand in for dietary protein when building muscle.Collagen is the most abundant protein in the body (about 25-30% of total protein), found in skin, bone, tendons, ligaments, cartilage. But as a supplement, it's an incomplete protein — it contains no tryptophan, and very little methionine.Protein scoring works on the shortest plank: with tryptophan at zero, the whole PDCAAS is pinned to 0. So collagen cannot replace dietary protein for MPS — it can be part of your daily protein without wrecking your indispensable amino acid balance, but it cannot stand in for the rest.
Its amino acid profile is unusual: ~33% glycine, ~12% proline, ~10% hydroxyproline — exactly what the collagen triple helix requires, though none of these amino acids is scarce.
One more point marketing skips: hydroxyproline you eat is not built directly into new collagen. There is no codon for hydroxyproline, so cells put in proline first and add the hydroxyl group on the finished chain — a step that needs vitamin C (step 3).