Cystine

Cystine is a sulfur-containing amino acid naturally present in the body, formed by the union of two cysteine molecules joined by a disulfide bond. It is found in significant amounts in animal proteins such as eggs, meat, and dairy products, as well as in certain legumes. It belongs to the family of so-called semi-essential amino acids, as the body can synthesise it, although this capacity may be limited under certain physiological conditions. Cystine plays an important structural role in keratin, the protein that makes up hair, nails, and skin. It contributes to the stability of this fibrous protein through the disulfide bonds it forms. Furthermore, as a precursor of cysteine, it indirectly participates in the synthesis of glutathione, a major endogenous antioxidant involved in protecting cells against oxidative stress. Supplementation with cystine may be of interest when dietary intake of sulfur amino acids is insufficient, or in contexts where keratin requirements are increased. It is often combined with other nutrients involved in the health of hair and nails to optimise its effectiveness.