Collagen
The main structural protein in skin, tendon and bone, and a popular supplement.
Collagen is the most abundant protein in the body and the main structural material of skin, tendon, ligament, cartilage and bone. It is a rope like triple helix, rich in the amino acids glycine, proline and hydroxyproline, which is what gives connective tissue its tensile strength.
Production declines with age and is damaged by ultraviolet exposure, which is the principal reason photoaged skin loses firmness and elasticity. Smoking and high blood sugar accelerate the same decline.
The central misunderstanding about supplements is one of digestion. Swallowed collagen is broken down into amino acids and short peptides like any other protein; it does not travel intact to your face. Any effect has to work through the amino acids supplied or through signalling by specific peptides, not through delivery of the material itself.
That said, the evidence is not nothing. Trials of collagen peptides report modest improvements in skin elasticity and hydration, and there is early evidence for tendon and joint outcomes, particularly when combined with vitamin C and with loading. The studies are often small and frequently industry funded, which is worth knowing.
Loading is the part with the better evidence for connective tissue generally. Tendons adapt to progressive mechanical stress, and no supplement substitutes for giving the tissue something to respond to.
As a protein source it is incomplete, lacking tryptophan and low in several essential amino acids, so it should not be counted towards your protein target as though it were whey or meat.
In practice, if you want collagen for skin, sunscreen and a retinoid have far stronger evidence. If you want it for tendon, load the tendon, and treat the supplement as a possible small addition rather than the intervention.