Vitamin A2 is a form of vitamin A — chemically dehydroretinol — found principally in the livers of freshwater fish. It is closely related to the standard vitamin A (retinol, or A1) but has an extra double bond in its ring structure, making it slightly less potent in supporting vision and growth. Its nutritional importance is minor for most people, who obtain vitamin A as retinol from animal foods or as beta-carotene from plants. It appears in nutrition science mainly in the analysis of fish liver oils, where distinguishing the A vitamers matters for accurate potency labelling. Like all retinoids, excessive intake is toxic, so fish liver products are not foods for unlimited consumption.