Across 3 studies, all reported beneficial effects on insulin levels, with effect sizes ranging from moderate to large. The evidence is drawn from clinical populations with non-alcoholic fatty liver disease (NAFLD) or metabolic syndrome, using grape seed extract at doses of 100–520 mg/day over a median of 58 days. All three studies reached statistical significance.
Caveats: Evidence base is small (only 3 studies) — conclusions should be considered preliminary. 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). All studies used grape seed extract, not whole red grape or other forms — findings may not generalize to other red grape preparations. Effects were observed in clinical populations with metabolic disturbances; relevance to healthy individuals is unclear.
Flavonoids intervention significantly decreased ... insulin levels
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)
The MD of insulin between the GSE group (-7.05 ± 2.11) and the placebo group (-1.71 ± 2.12), (p = 0.024), were significant.