Across 6 studies on red grape (grape seed extract) supplementation and reduced systolic blood pressure, 4 reported beneficial effects and 2 found neutral effects. The predominant effect size was small, with one study showing a moderate effect. Most evidence comes from small randomized controlled trials (RCTs) with median study duration of 34 days, and doses were inconsistently reported but included 300 mg and 520 mg/day in clinical populations such as NAFLD patients, prehypertensive men, and obese males.
Caveats: Available evidence is overwhelmingly positive (4 of 6 studies beneficial) — clinical literature in this area is subject to publication bias (null-result studies are less likely to be published or indexed). Additionally, many studies had very small sample sizes (e.g., 9–10 participants), and the meta-analysis (2022) found no significant effect overall (WMD: -3.55 mmHg, 95% CI: -7.59 to 0.49). The effect may be modest and inconsistent, and most studies used grape seed extract (no data on whole red grape forms).
There were no differences on the SBP … between placebo and GSE treatments
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.
Compared with the PL, GSE supplementation significantly decreased SBP (NBW: 103±4 vs. 99±3 mmHg; obese: 118±3 vs. 112±5 mmHg)
GSE supplementation attenuated MAP at both workloads...
no significant effects on ... SBP (WMD: -3.55 mmHg, 95% CI: -7.59 to 0.49, I² = 97.4%)