Promote healthy aging at the cellular level with Essential Youth L-Ergothioneine from Life Extension. What is ergothioneine? This amino acid found primarily in mushrooms is a potent antioxidant with a variety of health applications. The most common source of ergothioneine is reishi (also called lingzhi, Ganoderma lucidum) mushroom, popular in Eastern health practices for supporting longevity, vigor and vitality. Emerging research suggests benefits for liver and brain health as well.