Precursor to Glutathione Supports Liver & Lung Function Dietary SupplementN-Acetyl-L-Cysteine (NAC) is a powerful antioxidant amino acid and a precursor in the body to the critical antioxidant glutathione. Glutathione exerts a variety a variety of protective effects, including detoxification and intracellular defense against oxidative stress. Substances known to reduce glutathione status are acetaminophen and alcohol.