Cardiovascular and mental health benefits of soy consumption: role of soy isoflavones.

Regular soy consumption has been shown to reduce cardiovascular (CV) risk through plasma cholesterol reduction. According to the current health claim, this benefit is attributed to soy protein (SP). Dietary intervention trials indicate that isoflavones (ISO), weak phytoestrogens in soy, may also contribute by offering additional vascular and metabolic protection. Equol, a metabolite of the ISO daidzein (DAZ) with greater estrogenic potency, may be an important mediator of such effects. This thesis examines effects of soy, in particular, ISO consumption on CV risk factors and the potential for ISOs to enhance cognition, possibly through improvements of circulatory function. Two crossover design intervention trials were undertaken: a food-based intervention, investigating differential effects of SP and ISO on plasma lipids and other risk factors for CVD, and an ISO supplementation trial, examining effects on cognition and vascular function. Both addressed whether benefits were dependent on equol production. In the first trial, 91 subjects with untreated mild hypercholesterolemia were randomised to consume each of the following three diets in random order for sequential 6 week periods: (S) soy foods containing 24 g of SP and 75-90 mg ISO per day, (SD) soy/dairy foods containing 12 g SP, 12 g dairy protein (DP) and 75-90 mg ISO per day or (D) dairy foods containing 24 g DP only per day. At the end of each diet period, blood lipids, flow-mediated dilatation (FMD) of the brachial artery, blood pressure, arterial compliance and anthropometric measures were assessed. Compared with the control diet (D), there was a small but significant reduction in total cholesterol on the S diet only (2.8 + 1.1%, P<0.05), which could be accounted for by a decrease in saturated fat intake. FMD was found to be significantly improved when SD and S diet data were nested (P=0.03). Plasma triglycerides (TG) improved on both the SD and S diets compared with D (P<0.01). Other lipid, metabolic and vascular parameters did not differ between diets. There were no differences in outcomes between equol (n=30) and non equol producers (n=61). In a subsequent 12 week double-blind supplementation trial, 34 healthy males were randomised to take 4 capsules providing 120mg ISO per day or a matching placebo for 6 weeks, after which they crossed over to the alternate supplement. FMD and cognitive assessments relating to measures of memory and executive function were performed at the beginning and end of each treatment phase. Spatial working memory, a test in which females consistently perform better than males, was significantly improved by ISO supplementation (P