Immunocal: Clinically Proven Glutathione Boost I Immunotec Immunotec Immunocal Glutathione Precursor Whey Protein Isolate, Immune Support Sante Medi As I mention in my book, N acetyl cysteine (NAC) is not found in food sources, but in the body, it converts into cysteine. NAC acts as a precursor to glutathione, a powerful Glutathione Precursor, N Acetyl Cysteine, Improves Mismatch Negativity in Schizophrenia Patients Neuropsychopharmacology