by Nutrient Journal Editorial
Corosolic acid (2 alpha-hydroxy ursolic acid, glucosol) (Figure 1.) is a pentacyclic triterpene acid which is structurally similar to ursolic acid. Corosolic acid is usually extracted from Banaba (Lagerstroemia speciosa) leaf. It is reported to exhibit antihyperlipidemic, antioxidant, antiinflammatory, antifungal, antiviral, antineoplastic, osteoblastic and protein kinase C inhibition activity. However, there is not many clinical reports to support these claims.
Is Corosolic Acid Future Anti-diabetic Drug?
Banaba leaf extract (or corosolic acid) is typically found in supplements that help support healthy blood sugar levels and is claimed to stimulate glucose uptake and heighten insulin sensitivity.
Rats fed with 1% corosolic acid (glucosol™) exhibited significantly reduced blood glucose after 90 minutes . Same formulation also exhibited significant and dose-dependent reduction in blood glucose (30% decrease after 2 weeks) over the range of 16-48 mg per day in 10 subjects with type II diabetes . A study in KK-Ay mice reported dose-dependent reduction in blood glucose levels 4 h after a single oral dose (2 mg/kg) which supported the hypotesis that corosolic acid improves glucose metabolism by reducing insulin resistance . A 12% decrease in fasting blood glucose was reported in 12 non-diabetic subjects who were given 10 mg of corosolic acid (as banaba extract standardized to 18% corosolic acid) for 2 weeks . In an unpublished study by Xu (mentioned in a review by Stohs et al. ) subjects received soft gel caps containing 10 mg of corosolic acid. Both fasting and 2h post-prandial blood glucose levels decreased by 10% compared to placebo group . In Fukushima et al. study  31 subjects were given 10 mg corosolic acid (99% pure) or placebo 5 minutes after 75 g of glucose tolerance test. It has been reported that corosolic acid reduced blood glucose levels from 60 to 120 minutes, with highest difference at 90 minutes.
In patients with type II diabetes mellitus intestinal cholesterol synthesis and cholesterol esterification is increased . Corosolic acid has been show to reduce cholesterol absorption process in the small intestine via inhibition of activity of cholesterol acyltransferase .
Mechanisms of Action
Corosolic acid is suggested to induce GLUT4 translocation [9,10]. Translocation of more GLUT4 glucose transporters to the cell surface means increased insulin action . Disrupted GLUT4 gene has been shown to cause insulin resistance in skeletal muscles as well as diabetes . Corosolic acid also works as “insulin sensitizer” by inhibiting enzymatic activities of several diabetes-related non-receptor protein tyrosine phosphatases (such as PTP1B) which indirectly enhances insulin receptor B phosphorylation . Inhibition of protein tyrosine phosphatase 1B is an attractive target for treatment of diabetes and obesity [13,14]. Corosolic acid also promotes glycolysis  and suppresses gluconeogenesis (via increased production of fructose-2,6-bisphosphate) .
Figure 1. Corosolic aicd.
Advantage of corosolic acid in treatment of diabetes is that unlike insulin treatment it does not increase anti-insulin antibody production .
“Fat Burning” Properties of Corosolic Acid
In twelve subjects who were taking banaba extract for 2 weeks an average three pound weight loss was reported . Corosolic acid has also been shown to retard the absorption of fatty acid via inhibition of pancreatic lipase enzyme (enzyme responsible for lipid absorption) .
Corosolic acid is a potent and selective inhibitor of the enzyme (11-beta hydroxysteroid dehydrogenase type 1) that converts inactive cortisone to active cortisol . Therefore, corosolic acid may prevent excessive cortisol production.
Corosolic Acid Safety and Dosage
No animal studies have been designed to assess toxicity or LD50 (median lethal dose) values for corosolic acid or banaba extracts. No adverse effects have been observed or reported in animal studies or controlled human clinical trials , however information about toxicity in human subjects is lacking.
Various commercially available formulations are available including softgels, tablets, capsules, powders and cosmetics. Banaba (Lagerstroemia speciosa) leaf extracts usually contain from 1 to 18% of corosolic acid. It appears that corosolic acid is better absorbed in oil-based soft gelatin formulation compared to dry powder . Typical recommended dose (by manufacturers) is 1 capsule 20 to 30 minutes with or before morning and evening meals (some times even 3 times daily). Effects of corosolic acid are dose-dependent [2,3], however more human studies assessing proper dose are warranted.
- Hamamoto S, et al. “Glucosol effect on blood glucose in rats. Yakuri to Chiryo”. 27.6 (1999): 1075-1077.
- Judy, William V., et al. “Antidiabetic activity of a standardized extract (Glucosol™) from Lagerstroemia speciosa leaves in Type II diabetics: A dose-dependence study.” Journal of Ethnopharmacology 87.1 (2003): 115-117.
- Miura, Toshihiro, et al. “Antidiabetic effects of corosolic acid in KK-Ay diabetic mice.” Biological and Pharmaceutical Bulletin 29.3 (2006): 585-587.
- Tsuchibe, S., et al. “An inhibitory effect on the increase in the postprandial glucose by banaba extract capsule enriched corosolic acid.” Journal for the Integrated Study of Dietary Habits 17 (2006): 255-259.
- Stohs, Sidney J., Howard Miller, and Gilbert R. Kaats. “A review of the efficacy and safety of banaba (Lagerstroemia speciosa L.) and corosolic acid.” Phytotherapy Research 26.3 (2012): 317-324.
- Fukushima, M., et al. “Effect of corosolic acid on postchallenge plasma glucose levels.” Diabetes research and clinical practice 73.2 (2006): 174-177.
- Jiao, Sheng, et al. “Decreased activity of acyl-CoA: cholesterol acyltransferase by insulin in human intestinal cell line Caco-2.” Diabetes 38.5 (1989): 604-609.
- Takagi, Satoshi, et al. “Effect of corosolic acid on dietary hypercholesterolemia and hepatic steatosis in KK-Ay diabetic mice.” Biomedical Research 31.4 (2010): 213-218.
- Miura, Toshihiro, et al. “Corosolic acid induces GLUT4 translocation in genetically type 2 diabetic mice.” Biological and pharmaceutical bulletin 27.7 (2004): 1103-1105.
- Shi, Lei, et al. “Corosolic acid stimulates glucose uptake via enhancing insulin receptor phosphorylation.” European journal of pharmacology 584.1 (2008): 21-29.
- Holloszy, John O. “Exercise-induced increase in muscle insulin sensitivity.”Journal of Applied Physiology 99.1 (2005): 338-343.
- Stenbit, Antine E., et al. “GLUT4 heterozygous knockout mice develop muscle insulin resistance and diabetes.” Nature medicine 3.10 (1997): 1096-1101.
- Pei, Zhonghua, et al. “Inhibition of protein tyrosine phosphatase 1B as a potential treatment of diabetes and obesity.” Current pharmaceutical design 10.28 (2004): 3481-3504.
- Zhang, Zhong-Yin, and Seung-Yub Lee. “PTP1B inhibitors as potential therapeutics in the treatment of type 2 diabetes and obesity.” Expert opinion on investigational drugs 12.2 (2003): 223-233.
- Yamada, Kotaro, et al. “Effect of corosolic acid on gluconeogenesis in rat liver.” Diabetes research and clinical practice 80.1 (2008): 48-55.
- Sivakumar, Ganapathy, et al. “Plant‐based corosolic acid: Future anti‐diabetic drug?.” Biotechnology journal 4.12 (2009): 1704-1711.
- Jang, Dae Sik, et al. “A new pancreatic lipase inhibitor isolated from the roots of Actinidia arguta.” Archives of pharmacal research 31.5 (2008): 666-670.
- Deanna J. Fall, BA and Sirid-Aimée Kellermann. “Addressing Adrenal Imbalance: The Future of Adrenal health.” NeuroScience, Inc.