L-cysteine is a conditionally essential amino acid, one of only three sulfur-containing amino acids, the others being taurine (which can be produced from L-cysteine) and L-methionine from which L-cysteine can be produced in the body by a multi-step process. Cysteine plays a role in the sulfation cycle, acting as a sulfur donor in phase II detoxification and as a methyl donor in the conversion of homocysteine to methionine. Cysteine also helps synthesize glutathione, the body’s most important intracellular antioxidant and a vital detoxifier.Glutathione (gamma-L-glutamyl-L-cysteinyl-glycine) is a peptide-like (short-protein) molecule synthesized in the body from the three amino acids L-glutamic acid, L-cysteine and glycine. Glutathione is one of the body’s most important and powerful antioxidants,116 helping to detoxify xenobiotics.117-120 A major function of vitamin C is to keep glutathione in its reduced form so that it can continue to provide free radical-quenching effects.

Even though many published studies show that garlic, selenium, alpha-lipoic acid, L-cysteine, and N-acetyl-cysteine can boost cellular glutathione levels, people with health problems may benefit from taking glutathione.