Corosolic acid has high inhibitory activity against α-glucosidase with an IC50 of 3.53 μg/mL [1].
Corosolic acid (CRA) is an extract of banana leaves. It has been reported that corosolic acid had many beneficial effects like decreasing blood sugar level, antihyperlipidemic, antiviral, anti-inflammatory, and antifungal activities [2].
When tested with Chinese hamster ovary cell, treated with corocolic acid showed high hypoglycemic effects via stimulating glucose uptake and inhibiting several diabetes-related nonreceptor protein tyrosine phosphatase enzymes [3].
In mouse model with genetically diabetic disease, feeding the mice with banana leaf extraction (containing highly purified corosolic) significantly suppressed the blood glucose level [4]. When tested with SHR/NDmcr-cp (cp/cp) rat (animal model of metabolic syndrome), compared with control group, rats were fed a high fat diet containing 0.072% corosolic acid for 14 weeks decreased the blood pressured with significant difference[5]
In a human clinical study, type 2 diabetic subjects were given daily 0.32 and 0.48 mg corosolic acid respectively for 2 weeks and a 30% decreasement of blood glucose level was observed [6].
References:
[1].    Hou, W., et al., Triterpene acids isolated from Lagerstroemia speciosa leaves as alpha-glucosidase inhibitors. Phytother Res, 2009. 23(5): p. 614-8.
[2].    Stohs, S.J., H. Miller, and G.R. Kaats, A review of the efficacy and safety of banaba (Lagerstroemia speciosa L.) and corosolic acid. Phytother Res, 2012. 26(3): p. 317-24.
[3].    Shi, L., et al., Corosolic acid stimulates glucose uptake via enhancing insulin receptor phosphorylation. Eur J Pharmacol, 2008. 584(1): p. 21-9.
[4].    Kakuda, T., et al., Hypoglycemic effect of extracts from Lagerstroemia speciosa L. leaves in genetically diabetic KK-AY mice. Biosci Biotechnol Biochem, 1996. 60(2): p. 204-8.
[5].    Yamaguchi, Y., et al., Corosolic acid prevents oxidative stress, inflammation and hypertension in SHR/NDmcr-cp rats, a model of metabolic syndrome. Life Sci, 2006. 79(26): p. 2474-9.
[6].    Judy, W.V., et al., Antidiabetic activity of a standardized extract (Glucosol) from Lagerstroemia speciosa leaves in Type II diabetics. A dose-dependence study. J Ethnopharmacol, 2003. 87(1): p. 115-7.