Glutathione is a tripeptide protein composed of the amino acids cysteine, glycine, and glutamic acid. One of nature's premier antioxidants and free-radical scavengers, glutathione is found in a wide variety of plant and animal tissues, where it serves as an oxygen carrier in respiration and as a coenzyme in certain metabolic processes. While there are many food sources of glutathione, including fruits, vegetables, meat, and fish, most circulating glutathione in humans is manufactured in the body from amino acid components rather than obtained directly through the diet. Glutathione is marketed as a dietary supplement, usually in tablet or capsule form.
Although glutathione supplements are often promoted as natural antioxidants and immune-system boosters, there is little scientific evidence to support such claims; as research indicates that oral glutathione is poorly absorbed in humans. While studies conducted on mice and rats found oral glutathione supplements to be well-absorbed and effective in raising plasma glutathione levels, these animals lack an enzyme found in the human digestive tract (gamma-glutamyltransferase) that breaks down glutathione, preventing it from being absorbed intact. Researchers in one study concluded that dietary glutathione has little influence on circulating glutathione levels in humans, after three grams of glutathione failed to significantly increase plasma glutathione in seven healthy human subjects. Nevertheless, supplemental glutathione may serve as a significant source of amino acids.
There is no known therapeutic dosage for supplemental glutathione. Label recommendations range from 50 to 250 mg per day. There are no known side effects associated with supplemental glutathione at this dosage range. Glutathione is a common nutritional component of many foods and has no known toxicity.