They're a good source of ergothioneine Ergothioneine is an antioxidant found in plants and animal tissues. It works with other antioxidants to control oxidative stress in mitochondria, cells that help convert food into energy. Mitochondria are also critical in signaling, cell death, and other processes. Eating foods high in antioxidants may help prevent or delay many diseases, including cancer and cardiovascular disease. Mushrooms are one good source of ergothioneine. Many types of mushrooms contain ergothioneine, including portabellas, criminis, and white button mushrooms. A three-ounce serving of these mushrooms contains about five milligrams (mg) of ERG. Exotic mushrooms are even better, containing as much as thirteen milligrams per ounce. Moreover, cooking mushrooms does not affect their ergothioneine content.