Evidence exists indicating that substitution of soy for animal protein reduces both total ahd LDL-cholesterol concentrations in humans. There are a number of biologically active compounds associated with soy protein; however, the precise mechanism and the component(s) of soy responsible have not been fully established. Some studies suggest that, when soy protein is fed, cholesterol absorption or bile acid reabsorption, or both, is impaired. This is observed in some animal species such as rabbits and rats but not in humans, nor when amino acids replace intact soy protein. Other workers have proposed that changes in endocrine status are responsible, however, this again has not been observed in humans. Increases in LDL receptor activity in both animals and humans have been reported after ingestion of soy protein or various extracts of soy, or both. Furthermore, the soybean isoflavone genistein may inhibit lesion and thrombus formation via inhibition of second messengers.