Across 3 studies, all reported beneficial effects of red grape (primarily grape seed extract) on insulin sensitivity, with effect sizes ranging from small to large. The most studied population is patients with non-alcoholic fatty liver disease (NAFLD), and the median study duration is 58 days, indicating effects typically observed at 8–12 weeks. Doses of 100–520 mg/day of grape seed extract showed improvements in insulin resistance markers such as HOMA-IR and QUICKI.
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, but the form was not consistently specified; findings may not generalize to other red grape preparations (e.g., juice, whole fruit).
Flavonoids intervention ... augmented the quantitative insulin sensitivity check index (QUICKI) 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 (mean difference ±SEM) of HOMA-IR between the GSE group (-1.46 ± 0.45) and the placebo group (-0.48 ± 0.47), (p = 0.020)...