High blood pressure, or hypertension, is often symptomless and can lead to heart attack or failure.1 Diet is commonly investigated as a therapeutic method for treating hypertension, necessitating the research of food and compounds therein for the condition. Some research suggests that isoflavones from soy (Glycine max, Fabaceae) may benefit arterial vasodilatation, endothelial function, and blood pressure, but studies examining the effect of soy on blood pressure have produced inconsistent results. The authors of this paper conducted a meta-analysis of all randomized, double-blind, placebo-controlled trials examining the effect of soy protein and isoflavoneson blood pressure.
Databases used included PubMed from 1950-2009, Embase from 1966-October 2009, and The Cochrane Library. The search terms used were “soy” or “soy protein” or “soya;” “isoflavones” or “isoflavone;” and “blood pressure” or “hypertension.” References in the articles were also searched. Studies were included if they were published in English as full-length articles, had at least 10 adult subjects or patients that ingested soy isoflavones for 1 to 12 months, and were published, randomized, double-blind, placebo-controlled trials. Trials also had to have means or differences between means for systolic blood pressure (SBP) and diastolic blood pressure (DBP) reported at baseline and the endpoint of the study, as well as standard deviation, standard error, 95% confidence interval (CI), or probability values. Reporting of dose and duration of study was also necessary.
The authors found 1,080 articles, but excluded 1,045 due to study design or incomplete reporting. Of the remaining 35, 24 were excluded due to inappropriate source material, having fewer than 10 participants, incomplete data, or insufficient duration. A total of 11 trials were included in the meta-analysis. The sample size ranged from 18 to 302 participants with average ages from 48.5 to 66.7 years and a body mass index (BMI) of 25.5 to 32.2 kg/m2. Soy protein consumption ranged from 20 to 50 g/day with isoflavone intake ranging from 65-153 mg/day. Duration of treatment ranged from 1 to 12 months. Two trials included diabetic patients, and 4 trials had patients with hypercholesterolemia. Quality scores ranged from 3-5.
Four trials reported reduced SBP and DBP, one reported an increase in SBP and DBP, and the remaining trials showed no change in blood pressure. When all data were pooled and analyzed, soy isoflavones were shown to significantly decrease SBP by 2.5 mmHg (95% CI, -5.35 to 0.34 mmHg, P=0.08) and DBP by 1.5 mmHg (95% CI, -3.09 to 0.17 mmHg, P=0.08). Since there was heterogeneity between the studies, a meta-regression was performed. Age—but not dose, duration, or BMI—was found to be a significant predictor of heterogeneity.
When data were divided into hypertensive and normotensive subgroups, the reduction in both SBP and DBP compared to placebo was statistically significant in the hypertensive group but not in the normotensive group (for hypertensive group, 5 of 11 trials, SBP: -5.94, 95% CI, [-10.55, -1.34] mmHg, P=0.01; DBP: -3.35, 95% CI, [-6.52, -0.19] mmHg, P=0.04). Blood pressure reductions were found to be greater in younger patients. According to the authors, the effectiveness of soy isoflavones appeared to be “significantly disturbed” by the presence of diabetes or hypercholesterolemia in some of the subjects, though due to low statistical power, these findings should be interpreted with caution. “The real effects of both diabetes and hypercholesterolemia on BP need to be investigated further in the studies aimed at hypertensive subjects without other comorbidities,” wrote the authors. There were no significant effects of duration, dosage, or sex. No publication bias was detected for either SBP or DBP.
The reductions in SBP seen in hypertensive patients with soy treatment are comparable to those seen with antihypertensive medications (-5 mmHg). Therefore, the authors suggested that ingestion of 65-153 mg/day of soy protein-containing isoflavones for 1-12 months can make a significant impact on cardiovascular disease risk in the hypertensive population. The mechanism for this effect is not well understood, but may involve improvement of endothelial function. It is also unclear whether the causal compound is soy protein or soy isoflavones, or whether the 2 act synergistically. As there was considerable heterogeneity in the trials analyzed, additional studies in homogeneous populations and at a wide range of doses are needed to confirm the results.
—Amy C. Keller, PhD
1. High blood pressure. Centers for Disease Control and Prevention. 2011. Available at: http://www.cdc.gov/bloodpressure/diastolic_systolic.htm. Accessed April 15, 2011.