L-methionine is an essential amino acid that serves as a building block for proteins, including keratin, which is a major structural component of nails. The rationale for using L-methionine to support nail health is based on its biological role in protein synthesis and in providing sulfur, a key element in keratin structure. Some laboratory and animal studies have shown that methionine deficiency can lead to impaired keratin production, which could theoretically impact nail strength and appearance. However, direct human studies evaluating L-methionine supplementation for improving nail health are limited and often lack rigorous design or large sample sizes.

Most evidence comes from the understanding of amino acid metabolism and the known composition of nails, rather than from clinical trials. While some dietary supplements include L-methionine for nail support, the claims are generally extrapolated from its role in protein and sulfur metabolism rather than from robust clinical research. Therefore, scientific validation exists at a basic biochemical level, but direct clinical evidence for efficacy in improving nail health through supplementation is weak. The overall evidence rating is modest (2 out of 5).