Ergothioneine is an amino acid that is found mainly in mushrooms, but also in king crab, meat from animals that have grazed on grasses containing ergothioneine, and other foods. Amino acids are chemicals that are the building blocks for proteins.Ergothioneine is used as medicine.People take ergothioneine for liver damage, cataracts, Alzheimer's disease, diabetes, and heart disease. Ergothioneine is sometimes applied directly to the skin to prevent wrinkles, reduce signs of aging skin, and reduce sun damage.