Benefits
Joint comfort and stiffness
In one randomized, placebo-controlled trial of 50 adults with knee osteoarthritis, 500 mg per day for 60 days reduced pain scores about twice as much as placebo and reduced stiffness. Flexibility and range of motion were not measured.
Sulfur amino acids (unproven for joints)
Keratin is a sulfur-rich structural protein of hair, nails and skin, not of cartilage, which is built from collagen and proteoglycans. Any contribution of its sulfur amino acids to joint or cartilage structure is an unproven mechanism and was not measured in the trial.
Antioxidant sulfur support (unproven)
Whether the sulfur in keratin affects antioxidant systems or joint oxidative stress is an unproven mechanism. No antioxidant or oxidative-stress outcome was measured in the trial.
Mechanism of action
Sulfur amino acid delivery
Solubilized keratin provides bioavailable cysteine and other sulfur amino acids used to build and maintain connective tissue and antioxidant molecules.
Proposed anti-inflammatory action (not measured)
The trial did not measure inflammatory markers, so any anti-inflammatory action is a proposed mechanism rather than a demonstrated effect.
Clinical trials
Randomized, double-blind, placebo-controlled trial of solubilized keratin (Cynatine FLX) for symptoms of knee osteoarthritis. (Beer et al. 2013, Journal of Dietary Supplements)
50 adults with knee osteoarthritis (25 active, 25 placebo).
Over 60 days at 500 mg per day, pain scores fell about twice as much as placebo and stiffness was reduced. Flexibility and range of motion were not reported. This is the only trial and it was industry supported.