Evidence supporting the use of: L-cysteine
For the body system: Nails

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Synopsis

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

L-cysteine is an amino acid that plays a key role in the synthesis of keratin, the primary structural protein found in nails, hair, and skin. Its inclusion in dietary supplements marketed for nail health is based on the biological necessity of sulfur-containing amino acids for keratin production. Several in vitro and animal studies have demonstrated that cysteine is a limiting amino acid in keratin synthesis, and deficiencies can result in weaker or slower-growing nails. However, direct clinical evidence in humans linking L-cysteine supplementation to improved nail strength or growth is limited. Some small studies and anecdotal reports suggest potential benefits, but these are not robust or widely replicated. The rationale for its use is grounded in biochemistry, but the translation to meaningful improvements in nail health for individuals who are not deficient in cysteine remains uncertain. As such, the scientific evidence supporting L-cysteine as a nail health supplement is limited (rating: 2/5), and more high-quality human trials are needed to confirm its efficacy in this context.

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