Glutathione is a tripeptide of three amino acids, cysteine, glutamic acid and glycine, produced naturally in every cell of the body. It is often described as the body's principal endogenous antioxidant.
How it works as an antioxidant
Glutathione cycles between a reduced form, which donates electrons to neutralise free radicals, and an oxidised form, which is then regenerated by enzymes. That recycling is what allows a relatively small pool to do a large amount of work.
It also participates in regenerating vitamins C and E after they have been oxidised, which is why the three are frequently formulated together.
The pigmentation pathway
Glutathione's cosmetic interest is chiefly in skin tone rather than antioxidant activity. Melanin is produced in two broad forms: eumelanin, which is brown to black, and pheomelanin, which is red to yellow. Glutathione appears to shift production toward pheomelanin, and separately to inhibit tyrosinase, the enzyme that initiates melanin synthesis.
What is and is not established
This needs stating plainly, because glutathione is heavily marketed.
Most human research on glutathione and skin tone has used oral or intravenous administration, not topical. Intravenous glutathione for skin lightening is a significant public health concern in several countries, and regulators including the Philippine FDA have issued warnings about unapproved use.
Topical glutathione faces a real delivery problem. It is a water-soluble tripeptide, and the skin barrier is specifically structured to exclude molecules of that description. Some studies of topical glutathione and its oxidised form report measurable effects, but the evidence base is thinner than the marketing implies, and considerably thinner than for vitamin C or niacinamide.
Stability
Glutathione oxidises readily in aqueous formulations. As with green tea catechins, formulation and packaging matter more than the percentage stated on the label.
