Recently published meta-analyses have found that soy protein modestly lowers both systolic and diastolic blood pressure. However, because blood pressure changes were not the primary focus of many of these studies, more research is needed before definitive conclusions about the hypotensive effects of soy protein can be made. The latest research to evaluate the soy and blood pressure relationship is an acute study that garnered quite a bit of media attention and generated some controversy.
In this intervention study, elderly participants drank 500 milliliters of soymilk provided in either glass bottles or cans. Two hours following consumption of the soymilk in glass bottles, systolic and diastolic blood pressure decreased by 7.9 and 3.5 mmHg, respectively. These differences were statistically significant. However, when soymilk was provided in cans, the drop in blood pressure was much more modest; systolic and diastolic blood pressure decreased by only 2.9 and 2.8 mmHg, respectively. The authors attributed this difference in response to soymilk to a hypertensive effect of bisphenol-A (BPA). BPA is a chemical used in the production of polycarbonate plastic and epoxy resins, which are used on the inner coating of cans.
One obvious limitation of this study is its short duration. One can only speculate as to whether soymilk would lower blood pressure when consumed over a much longer period of time although other studies have found that to be the case. Also, it is somewhat surprising that the drop in blood pressure in participants consuming soymilk in one can and one glass bottle was similar to the drop in blood pressure in those drinking soymilk in bottles only. Nevertheless, this study adds to the evidence that soyfoods are hypotensive.
Bae S and Hong YC. Exposure to bisphenol A from drinking canned beverage increases blood pressure: Randomized crossover trial. Hypertension. (2014).