Glutathione (γ-L-Glutamyl-L-cysteinylglycine) is a small amino acid containing molecule (peptide) comprised of one molecule of L-glutamic acid, L-cysteine, and Glycine each. The molecule exists normally in the food supply and in the the human body where it acts as a central antioxidant.
The actions of glutathione rely heavily on the entirety of the 'glutathione system', which comprises the enzymes that synthesize glutathione within a cell as well as dedicated enzymes that use glutathione as the means to exert antioxidant effects. Supplementation of glutathione is thought to support this pool of glutathione in a cell and thus maintain the efficacy of the entire glutathione system.
Despite the above theory, glutathione per se does not have a niche role in nutritional supplementation and when it does have efficacy it is perhaps the most expensive and inefficient means to the desired end. This is ultimately due to a few pharmacokinetic aspects which render supplementation of glutathione per se mostly useless:
(1) Glutathione is a tripeptide formed of three amino acids, and while this particular tripeptide is somewhat resistant to hydrolysis it is still mostly digested in the intestines.
(2) There may be some absorption of glutathione intact from the intestines, but this glutathione cannot actually enter cells intact; it must be metabolized to form L-cystine (two molecules of L-cysteine bound together) before being taken up.
(3) Provision of L-cysteine to within the cell is all that is needed to increase glutathione synthesis, andN-Acetylcysteine does this more reliably and more potently at a lower financial cost than glutathione.
In effect, glutathione is an indirect and expensive way to provide dietary L-cysteine which is ultimately all that is needed for the benefits of glutathione. Glutathione will be synthesized in the cell once the rate limiting step, provision of L-cysteine, is bypassed by any means. Dietary protein itself, including L-cysteine rich sources such as Whey Protein, are effective but inefficient ways to increase L-cysteine intake in the diet and N-Acetylcysteine is both more efficient and cheaper than glutathione.