Cystine is the most important amino acid for hair health. Not enough of it can lead to fraying, brittle, dull hair. Methionine is a sulfur-rich amino acid, encouraging the strength and elasticity of hair. Lysine is one of the body’s nine essential amino acids needed for growth and cellular repair, and studies have shown a correlation between lysine deficiency and hair loss. Arginine is another essential amino acid found in hair follicles, and is vital to the body’s detox process. Taurine, a beta-amino acid, helps with hair follicle development.