A sterol is any of a group a steroid-based secondary alcohols with a fused ring structure of three six-membered carbon rings and one five-membered carbon ring, all of which are hydrogenated and contain in total one or more double bonds. Sterols are generally colorless crystalline nonsaponifiable compounds. They exist either as free sterols or as esters of fatty acids. Animals sterols, called zoosterols, include cholesterol and lanesterol. The major plant sterol, or phytosterol, is beta-sitosterol.