A study published in the British Journal of Nutrition suggests that soy protein also protects against atherosclerosis by increasing blood levels of nitric oxide, a small molecule known to improve blood vessel dilation and to inhibit oxidative (free radical) damage of cholesterol and the adhesion of white cells to the vascular wall (two important steps in the development of atherosclerotic plaques).
In this study, when researchers gave laboratory animals bred to be apoliprotein-E deficient a purified diet containing either casein, the principal protein in dairy products, soy protein or rice protein, the mice given casein developed the largest atherosclerotic lesions. (In humans as well as animals, apolipoprotein E plays an important role in cholesterol transport, so a deficiency of this protein increases risk for the development of atherosclerosis.) Animals given soy or rice protein fared much better.
In trying to understand why, the researchers evaluated blood levels of nitric oxide. Mice fed either soy or rice protein diets were found to have increased blood levels of L-arginine (the amino acid that the body uses to produce nitric oxide) and nitric oxide metabolites when compared to those given casein-based feed. However, the L-arginine content of the soy and rice diets was not high enough to explain the amount of protective benefit they conferred, so the researchers concluded that these foods must also contain other protective compounds.