Fourteen double-blinded RCTs (10 parallel designs and four crossover designs, reported as containing 1,416 participants) were included in the review. Jadad scores across the trials ranged from 3 to 5.
Compared with placebo, IPP/VPP demonstrated statistically significant greater reductions in systolic blood pressure (MD -1.28mmHg, 95%CI -2.09 to -0.48; 15 comparisons; no significant heterogeneity) and diastolic blood pressure (MD -0.59mmHg, 95% CI -1.18 to -0.01; 15 comparisons; no significant heterogeneity).
Removal of one study at a time almost always showed similar, statistically significant results favouring IPP/VPP over placebo for reduction of systolic blood pressure. The exception was one crossover trial that had a normal blood pressure subgroup that decreased the estimate of the effect of IPP/VPP so that it was no longer significantly different from placebo (MD -0.89mmHg, 95% CI -1.72 to -0.05).
Meta-regression showed a decrease in treatment effect of IPP/VPP on systolic blood pressure with increasing age. Subgroup analyses showed significant effects with: IPP/VPP on systolic blood pressure in Nordic countries and Italy; crossover and parallel group study designs; and people without overt hypertension. No evidence of publication bias was found. Further results were reported in the paper.