Amino acids and proteins are fundamental components for the structure and function of hair. Hair is primarily composed of a protein called keratin, which is synthesized in hair follicles from various amino acids. Scientific research supports the role of adequate protein and specific amino acids, such as cysteine, methionine, and lysine, in promoting healthy hair growth and preventing hair loss associated with protein deficiency. For example, clinical studies have shown that individuals with low protein intake or specific amino acid deficiencies may experience thinning hair or increased shedding, which can often be reversed with nutritional supplementation. Additionally, some studies suggest that oral supplementation with certain amino acids can modestly improve parameters like hair strength and rate of growth, particularly in cases of suboptimal dietary intake or increased physiological demand (e.g., during stress or rapid growth periods).

However, evidence for the efficacy of generalized "amino acid blends" or protein supplementation in individuals without deficiency is less robust. Most benefits are observed in the context of correcting a deficiency rather than providing superphysiological amounts. As such, while there is a solid scientific rationale for the use of amino acids/protein to support hair health—especially in cases of inadequate intake—the overall evidence for supplementation in otherwise healthy individuals is moderate.