Green Tea Catechin Plus Inulin Improves Insulin Resistance Without Reducing Visceral Fat and Shows Exploratory Gut Microbiota Signals in Adults with Visceral Obesity: A Double-Blind Randomized Controlled Trial

Background: Green tea catechins and inulin may improve metabolic health and modulate the gut microbiota; however, double-blind trials evaluating visceral fat, insulin resistance, and microbiota concurrently are limited. Methods: We conducted a double-blind, parallel-group, randomized, placebo-controlled trial in Japanese adults aged 20–75 years with visceral fat area (VFA) ≥ 80 cm2 and BMI ≥ 23 kg/m2 (trial registration: Japan Registry of Clinical Trials (jRCT), jRCTs021230004 (registered 16 May 2023)). Participants were randomized to a catechin + inulin beverage (catechins 400 mg/day; inulin 2.3 g/day) or placebo for 12 weeks. The primary outcome was the change in VFA (bioelectrical impedance). Secondary outcomes included HOMA-IR, metabolic markers, liver fat assessed by the controlled attenuation parameter (CAP), and genus-level gut microbiota. Results: Ninety-six participants were randomized (catechin + inulin, n = 49; placebo, n = 47); 47 and 44, respectively, were included in the full analysis set for the primary outcome. The 12-week change in VFA did not differ between groups (mean change: +0.91 vs. +4.61 cm2; p = 0.243). HOMA-IR decreased in the catechin + inulin group and increased in the placebo group, yielding a significant between-group difference (mean change: −0.32 vs. +0.18; p = 0.020). No other secondary outcomes showed significant between-group differences. In exploratory microbiota analyses, no genus remained significant after false discovery rate correction; however, Coprococcus and Bifidobacterium showed nominal between-group differences (unadjusted p < 0.05). Changes in Coprococcus were inversely correlated with changes in HOMA-IR (r = −0.28; p = 0.010). Conclusions: Catechin plus inulin for 12 weeks did not reduce visceral fat but was associated with improved insulin resistance. Exploratory analyses suggest a potential association between increased Coprococcus and improved HOMA-IR, warranting further investigation.