Ergothioneine is a sulfur-containing amino acid derivative produced by fungi and certain bacteria. Humans cannot make it, yet the body has evolved a dedicated transporter to take it up and distribute it. That fact is what makes it interesting.
The transporter
In 2005 researchers identified that a protein previously catalogued as an organic cation transporter, OCTN1, is highly selective for ergothioneine. The body builds a specific mechanism to absorb and retain a compound it does not produce.
Ergothioneine accumulates in tissues under high oxidative stress: liver, kidney, bone marrow, red blood cells, the lens of the eye, and skin. Some researchers have proposed it should be considered a vitamin on that basis, though it has not been formally classified as one.
How it works
A potent scavenger of free radicals, notably hydroxyl radicals and singlet oxygen, and a chelator of metal ions that would otherwise catalyse oxidation reactions.
Two properties distinguish it from most antioxidants. It is unusually stable, resisting oxidation itself under conditions that degrade vitamin C or green tea catechins. And it appears to concentrate where oxidative stress is highest rather than distributing evenly.
In skin
Skin is continuously exposed to UV and pollution, both generators of oxidative stress, and it does contain ergothioneine. Whether topical application meaningfully supplements what is there is less established than the underlying biology, which is the usual gap.
Source
Commercially it is obtained from fungal fermentation or from mushrooms, with oyster and king oyster mushrooms among the richest natural sources.
Honestly on the evidence
The biology is genuinely compelling: a dedicated human transporter is not a small finding. The topical skincare evidence is thinner than the mechanistic research, and considerably thinner than for vitamin C or niacinamide. It is a well-founded ingredient rather than a proven one.
