Across 4 studies, all reported beneficial effects of red grape extract (primarily grape seed extract) on reducing diastolic blood pressure, with predominantly small effect sizes. The median study duration was 34 days (range: 7–60 days), and the most studied dose was 520 mg/day in one trial, though dosing varied across studies. Evidence primarily comes from hypertensive or metabolic syndrome populations.
Caveats: Available evidence is overwhelmingly positive — clinical literature in this area is subject to publication bias (null-result studies are less likely to be published or indexed). Only 4 studies exist, all significant, with small effect sizes; the evidence base is small, and conclusions should be considered preliminary. Most studies used grape seed extract, not other forms of red grape, so the generalizability to whole grape or juice products is unclear.
GSE decreased resting DBP (75 ± 2 vs. 71 ± 3) compared to the placebo (DBP:71 ± 3 vs. 74 ± 3)
insulin, HOMA-IR, TC, TG, LDL-c, ALT, AST, AST/ALT, SBP, DBP and MAP decreased and QUICKi and HDL-c increased significantly (p-value for all < 0.05)
Intervention with grape seed extract may reduce systolic and diastolic blood pressure in subjects with hypertension or metabolic syndrome.
GSE supplementation significantly reduced DBP (WMD: -2.20 mmHg, 95% CI: -3.79 to -0.60, I² = 88.8%)