Corosolic acid is a substance extracted from the herb Lagerstroemia speciosa and has been reported to have biological activities, as discovered in in vitro and experimental animal studies, particularly due to its influence on blood sugar. Thus, it may have an influence on diabetes. It is found in many plants, particularly banaba, but also in almond hulls, Weigela subsessilis, Perilla frutescens, Campsis grandiflora and other herbs. It is a pentacyclic triterpene and inhibits glycogen phosphorylases.

Corosolic acid research - Blood sugar and diabetes
Effect of corosolic acid on postchallenge plasma glucose levels.
Diabetes Research Clinical Practice. 2006. Department of Diabetes and Clinical Nutrition, Kyoto University, Kyoto, Japan.
In this study, 31 subjects were orally administered 10mg corosolic acid or a placebo, on different occasions, in a capsule 5 min before the 75-g oral glucose tolerance test (OGTT). Nineteen subjects had diabetes, seven had impaired glucose tolerance, one had impaired fasting glucose, and four had normal glucose tolerance. There were no significant differences in plasma glucose levels before and 30min after the administration. Corosolic acid treatment subjects showed lower glucose levels from 60min until 120min and reached statistical significance at 90min. In this study, we have shown for the first time that corosolic acid has a lowering effect on postchallenge plasma glucose levels in vivo in humans.

Antidiabetic effects of corosolic acid in KK-Ay diabetic mice.
Biol Pharm Bull. 2006. Miura T, Ueda N, Yamada K, Fukushima M, Ishida T. Department of Clinical Nutrition, Suzuka University of Medical Science, Mie, Japan.
The antidiabetic effects of corosolic acid were investigated in KK-Ay mice, an animal model of type 2 diabetes. Corosolic acid (2 mg/kg body weight) reduced the blood glucose levels of KK-Ay mice 4 h after a single oral dose. Corosolic acid (2 mg/kg) reduced the blood glucose levels in KK-Ay mice 2 weeks after a single oral dose and also significantly lowered plasma insulin levels were in KK-Ay mice under similar conditions. Corosolic acid -treated KK-Ay mouse blood glucose significantly decreased in an insulin tolerance test. These results support the hypothesis that corosolic acid improves glucose metabolism by reducing insulin resistance. Therefore corosolic acid may be useful for the treatment of type 2 diabetes.

Corosolic acid induces GLUT4 translocation in genetically type 2 diabetic mice.
Biol Pharm Bull. 2004. Department of Clinical Nutrition, Suzuka University Medical Science, Mie, Japan.
The effect of corosolic acid on blood glucose was studied in KK-Ay mice, an animal model of type 2 diabetes. Corosolic acid (10 mg/kg) reduced the blood glucose (p<0.05) of KK-Ay mice 4 h after single oral administration when compared with the control group. However, corosolic acid did not change the plasma insulin. The muscle facilitative glucose transporter isoform 4 (GLUT4) translocation from low-density microsomal membrane to plasma membrane was significantly increased in the orally Corosolic acid -treated mice when compared with that of the controls. These results suggest that the hypoglycemic effect of corosolic acid is derived, at least in part, from an increase in GLUT4 translocation in muscle. Therefore, it may be that corosolic acid has beneficial effects on hyperglycemia in type 2 diabetes.

Antidiabetic activity of a standardized extract (Glucosol) from Lagerstroemia speciosa leaves in Type II diabetics. A dose-dependence study.
J Ethnopharmacol. 2003. Judy WV, Hari SP, Stogsdill WW, Judy JS, Passwater R. SIBR, Bradenton, FL
The antidiabetic activity of an extract from the leaves of Lagerstroemia speciosa standardized to 1% corosolic acid (Glucosol) has been demonstrated in a randomized clinical trial involving Type II diabetics. Subjects received a daily oral dose of Glucosol and blood glucose levels were measured. Glucosol at daily dosages of 32 and 48mg for 2 weeks showed a significant reduction in the blood glucose levels. Glucosol in a soft gel capsule formulation showed a 30% decrease in blood glucose levels compared to a 20% drop seen with dry-powder filled hard gelatin capsule formulation suggesting that the soft gel formulation has a better bioavailability than a dry-powder formulation.

What's the right dosage of corosolic acid to treat diabetes or blood sugar problems?
    Since so little research has been done with banaba, corosolic acid, and diabetes, that it is very difficult to say at this time what the ideal dosages should be, or how often to take these supplements, and how effective they are in the long run.

Cancer
Corosolic acid isolated from the fruit of Crataegus pinnatifida is a protein kinase C inhibitor as well as a cytotoxic agent.
Planta Med. 1998.
Corosolic acid isolated from the fruit of Cratoegus pinnatifida var. psilosa was tested for anticancer activity. It displayed about the same potent cytotoxic activity as ursolic acid against several human cancer cell lines. In addition, the compound displayed antagonistic activity against the phorbol ester-induced morphological modification of K-562 leukemic cells, indicating the suppression of protein kinase C (PKC) activity by the cytotoxic compound. Corosolic acid showed PKC inhibition with dose-dependent pattern in an in vitro PKC assay.

Ursolic acid and its natural derivative corosolic acid suppress the proliferation of APC-mutated colon cancer cells through promotion of β-catenin degradation. S.Food Chem Toxicol. 2014.

Side effects and safety
Thus far no major side effects have been reported in medical literature studies but we need more time to find out. There is one anecdote of concern.

Am J Kidney Dis. 2010. Corosolic acid-induced acute kidney injury and lactic acidosis in a patient with impaired kidney function.

Phytother Res. 2012. A review of the efficacy and safety of banaba (Lagerstroemia speciosa) and corosolic acid. Banaba extracts have been used for many years in folk medicine to treat diabetes, with the first published research study being reported in 1940. This review summarizes the current literature regarding banaba and its constituents. The hypoglycemic effects of banaba have been attributed to both corosolic acid as well as ellagitannins. Pure corosolic acid has been reported to decrease blood sugar levels within 60 min in human subjects. Corosolic acid also exhibits antihyperlipidemic, antioxidant, antiinflammatory, antifungal, antiviral, antineoplastic and osteoblastic activities. The beneficial effects of banaba and corosolic acid with respect to various aspects of glucose and lipid metabolism appear to involve multiple mechanisms, including enhanced cellular uptake of glucose, impaired hydrolysis of sucrose and starches, decreased gluconeogenesis and the regulation of lipid metabolism. No adverse effects have been observed or reported in animal studies or controlled human clinical trials. Banaba extract, corosolic acid and other constituents may be beneficial in addressing the symptoms associated with metabolic syndrome, as well as offering other health benefits.

Questions
I have routinely seen 1 to 1.5 % corosolic acid extracts typically sold in 24 to 48 mg capsules or softgels (the glucosol or glucotrim products) but have only seen 3% extract in combination formulas. A banaba corosolic product states that it is a 1.5% extract or a total 250 mg dose. The information on the web site states this to be 3 mg of corosolic acid, however I figure that 1.5% of 250 mg is 3.75 milligrams of corosolic acid. I just wanted to check with you because if I can buy a 3 or 3.75 mg dose of corosolic acid per capsule I'm going to jump for joy, lol. Your info on banaba states that no clinical trials have been performed, however your site references a clinical trial using Lagerstroemia speciosa which is banaba. I just wanted to bring this to your attention.
   The research is not very clear. We're not sure if the research with Glucotrol or GlucoTrim refers to 24 or 48 mg of corosolic acid, or 24 - 48 mg of banaba extract with 1% corosolic acid extract.

According to a magazine article, to reduce blood sugar, it is best to use a supplement with 24 mg of corosolic acid.
   There's very little human research with banaba, so not much can be said with confidence at this time regarding the ideal corosolic acid dosage and potential benefits and long term side effects.

Q. I saw an ad for a NOW Foods product called GlucoTrim. It said, GlucoTrim is derived from the Crepe Myrtle (Lagerstroemia speciosa), a tree native to Southern Asia. It is standardized to contain min. 1% Coroscolic Acid, the active ingredient that supports natural glucose metabolism. Each softgel contains 24 mg of  Gluco Trim leaf (1% corosolic acid." Does this mean GlucoTrim has 0.24 mg of corosolic acid, or 24 mg of corosolic acid?
   A. We really are not sure, but it appears the way it is explained that it may be 0.24 mg of corosolic acid.
      Q. I checked with NOW foods on their GlucoFit product. Here's their reply: 18% of 1.334mg. This is 0.24mg or 240mcg: GlucoFit Banaba (Lagerstroemia speciosa L.) extract (leaf) [std. to 18% Corosolic Acid (.24 mg)].

Lagerstroemia speciosa (banaba) is thought to help with controlling blood sugar. Does lagerstroemia indica (crepe myrtle commonly grow as an ornamental tree inthe southern states) also contain corosolic acid? Both trees look very similar in bloom and foliage based on pictures online.
   A search on Medline corosolic acid lagerstroemia indica did not indicate any such studies so I don't know.