Synopsis

Source of validity: Scientific
Rating (out of 5): 2

L-cystine is an amino acid formed from the oxidation of two cysteine molecules and is a major component of keratin, the structural protein found in nails, hair, and skin. Its use to support nail health is based on the biological role cystine and cysteine play in keratin synthesis. Adequate cystine is required to provide the disulfide bonds that give keratin its strength and rigidity, which is directly relevant to the structure and resilience of nails. Some studies and reviews suggest that supplementation with cystine or its precursor, cysteine, may improve nail strength and growth, particularly in people with deficiencies or brittle nails. For example, certain clinical trials have used cystine-containing supplements to treat brittle nail syndrome, reporting improvements in nail hardness and resistance to breakage. However, the overall body of evidence is limited, with relatively few high-quality, large-scale studies available. Most support comes from small trials, case studies, and the biochemical understanding of keratin formation rather than robust clinical research. Therefore, while the rationale for L-cystine supplementation for nail health is scientifically plausible and supported by some preliminary evidence, the strength of clinical validation remains modest.

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