Synopsis

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

L-cysteine is an amino acid that plays a critical role in the synthesis of keratin, the primary structural protein in hair. Scientific research supports its involvement in hair health, primarily due to its function as a building block for keratin and its contribution to the strength and structure of hair fibers. Several studies have indicated that cysteine, including its stable form N-acetyl-L-cysteine (NAC), can enhance hair growth and reduce hair loss, especially in individuals with nutritional deficiencies or oxidative stress-related hair disorders. Clinical trials examining oral supplementation with cysteine—often in combination with other ingredients such as B vitamins and trace minerals—have demonstrated improvements in hair density and quality, although the effects can be modest and are not universally observed.

Despite the promising evidence, it is important to note that most research involves multi-ingredient supplements, making it difficult to isolate the effects of L-cysteine alone. Nonetheless, its direct role in keratin synthesis and antioxidant properties provide a plausible biochemical rationale for its use in supporting hair health. L-cysteine is commonly included in nutraceuticals for hair, and its use is scientifically grounded, though more robust, large-scale studies are needed to definitively quantify its efficacy.

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