Corosolic acid improves glucose and insulin responses in middle-aged men with impaired fasting glucose: A randomized, double-blinded, placebo-controlled crossover trial

• Corosolic acid (CA) improved glucose tolerance in healthy, hyperglycemic individuals. • CA intake (1 mg/d) for 2 weeks improved postprandial glucose and insulin sensitivity. • CA improved the liver insulin sensitivity index after glucose loading. • CA intake did not alter gastrointestinal hormone levels or appetite measures. Corosolic acid (CA) improves glucose metabolism in diabetics and prediabetics, but its effects in healthy subjects with borderline hyperglycemia are unclear. Non-diabetic middle-aged men (n = 14) with impaired fasting glucose tolerance (mean ± SD; age, 51.7 ± 9.0 years; fasting blood glucose, 6.0 ± 0.4 mmol/L) underwent an oral glucose tolerance test (OGTT) after taking 1 mg/d CA or placebo for 2 weeks in a randomized double-blind crossover trial. In the 13 subjects who completed the study, the incremental area under the curve (iAUC) of plasma glucose after the OGTT was significantly lower in the CA condition than in the placebo condition (p = 0.028). The AUC and iAUC of the serum insulin and C-peptide concentrations, and liver insulin sensitivity values were also significantly lower in the CA condition. Glucagon, glucagon-like peptide-1, gastric inhibitory peptide, and non-esterified fatty acid levels did not differ significantly between conditions. CA may improve glucose tolerance in both healthy and prediabetic subjects. Trial registration: UMIN-CTR; http://www.umin.ac.jp/ ; Registration No. UMIN000034130.