Synopsis

Source of validity: Scientific
Rating (out of 5): 3

L-cystine is a dimeric amino acid formed by the oxidation of two cysteine molecules and is a key structural component of keratin, the main protein in hair. Its use for supporting the hair system is primarily based on its biochemical role: cystine provides the disulfide bonds essential for giving hair its strength, resilience, and structure. There is scientific evidence supporting the role of L-cystine in hair health. Several clinical studies have investigated the effects of L-cystine supplementation (often in combination with other nutrients such as B vitamins and millet extract) on hair growth and quality. For example, randomized controlled trials have found that oral supplementation with L-cystine can improve hair density, reduce hair shedding, and enhance hair tensile strength in individuals with diffuse hair loss or telogen effluvium. Mechanistically, L-cystine supports keratin synthesis and may help in conditions where dietary cysteine availability limits hair growth. However, while these findings are promising, most studies use L-cystine in combination with other nutrients, making it difficult to isolate its individual effect. Overall, the scientific evidence is moderate (rated 3 out of 5), supporting the use of L-cystine for hair health, particularly in cases of deficiency or increased need, but more high-quality research is needed to confirm its efficacy as a standalone supplement.

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