Red flags in study design cast doubt on soy supplementation study results

Soy contains a number of bioactive agents, including isoflavones, which are estrogen-like compounds with effects on the cardiovascular system that help to increase blood flow. As depicted in Figure 1, estrogen regulates blood flow by inducing^[1] the production of nitric oxide (NO), a potent vasodilator made by endothelial cells, a specialized cell type that lines all blood vessels. After production in the endothelium, NO diffuses into the underlying smooth muscle cells that regulate vascular tone, causing them to relax and increasing blood flow, which is called vasodilation.

Improving blood flow to working muscles during exercise has the potential to increase nutrient delivery and provide a performance-boosting edge. Since the rate of NO production has been shown to correlate^[2] with exercise performance, soy supplementation shows some potential as a performance-enhancer. Although a number of studies in animals^[3] and humans^[4] have noted that isoflavones do have an effect on the vascular endothelium, the acute effects of soy supplementation on exercise performance have only recently begun to be explored.

The study under review attempted to explore the question of whether one acute dose of soy product could provide a performance-enhancing boost during exercise.

Isoflavones can increase NO production in blood vessels, causing vasodilation and increasing blood flow. This, in turn, could improve exercise performance. The study under review examined the potential performance-boosting effects of acute isoflavone-containing soy supplementation.