Across 3 studies, 2 reported beneficial effects (small to moderate magnitude) on reducing ALT levels, with statistically significant findings. The most-studied population was patients with non-alcoholic fatty liver disease (NAFLD). Effects were typically observed at 60 days (median study duration). The only reported effective dose was 520 mg/day of grape seed extract.
Caveats: Evidence base is small (only 3 studies) — conclusions should be considered preliminary. The beneficial effects were observed with grape seed extract and mixed flavonoids, not necessarily whole red grape; the neutral study used quercetin in a different population (thalassemia), suggesting effects may be population- or form-specific. Effect sizes vary from small to moderate, and clinical significance remains unclear.
Flavonoids intervention significantly decreased ... alanine aminotransferase (ALT)
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)
In contrast, quercetin and curcumin supplementation did not show a statistically significant difference compared to control groups in these outcomes.