Sitostanol is a hardened chemical derivative of
sitosterol, one of several types of plant
sterols. It has been found to lower
cholesterol levels when consumed in conjunction with cholesterol-containing foods.
Sitostanols are similiar in structure to cholesterol, so they are absorbed by the body instead of any co-existing cholesterol in your food. This prevents cholesterol from entering into a phase (micellar) that makes cholesterol absorbable. HDL ("good") cholesterol is not affected.
You are probably most familiar with the brand-name product Benecol which uses sitostanol ester for it's cholesterol blocking effects.
According to an American Heart Association report, sitostanol is tasteless; users cannot tell the difference between margarine containing sitostanol ester and regular margarine. It can also be used in cooking.