Soy protein and isoflavones: Effects on body weight, oxidative stress and sex hormone activity

Few studies have assessed the effect of soy on weight reduction. The vast majority of studies have endeavored to keep body weight constant. Nevertheless there is a tendency in many soy studies to see a trivial greater weight loss on soy than on the control treatment. Specific weight loss studies are therefore required to test this facet of soy's effects. The evidence is strong for an antioxidant effect of soy. Soy isoflavones have demonstrated antioxidant activity in vitro. However it is possible that other components of soy also share these properties. Our own studies have consistently shown reduced conjugated dienes in the LDL fraction as evidence of antioxidant activity after soy feeding. However these effects were seen even when cholesterol reduction was not demonstrated and when isoflavone intakes were low (< 10 mg/d). The effect of soy on sex hormone activity in humans might provide a way to reduce the risk of hormone dependent cancers. Deleterious effects appear less likely. At present the effect of soy in prostate cancer prevention appears the most promising. Studies in this area are at an early stage. Our own assessment of total sex hormone activity in urine and blood from those consuming soy indicates no clearly significant effect, although in one study a trend for reductions in both estrogenic and androgenic activity was observed. Soy therefore appears to have potential use in CHD and possibly cancer risk reduction. The extent to which isoflavones account for all these effects is still an issue.