Banaba (Lagerstroemia speciosa L., crepe myrtle) has been used as a folk medicine to treat diabetes in various parts of the world, primarily Southeast Asia. The hypoglycemic affect of aqueous (hot water) and methanol extractshave been demonstrated inseveral animal models as well as a number of human studies. Most studies have focused on corosolic acid which is isolated with an organic solvent from the leaves of the plant, and corosolic acid is used to standardize banaba extracts .Some studies indicate that ellagitannins in water soluble fractions may be responsible for at least some of the insulin-like activity of banaba, and the antioxidant, antiinflammatory and glucose regulatory properties of tannins in general have European and South American plants.
This review summarizes studies that have been conducted in animals, humans and in vitro systems on the antihyperglycemic, antihyperlipidemic, antioxidant, antiinflammatory, antifungal, antiviral and antineoplastic activities of banaba extracts, corosolic acid-standardized banaba extracts, and isolated and structurally characterized corosolic acid and ellagitannins.
A single report has suggested that corosolic acid may have been involved in nephrotoxicity and lactic acidosis in a diabetic patient with impaired kidney function who was also taking diclofenac for joint pain (Zheng et al., 2010). Diclofenac is a non-steroidal antiinflammatory drug, and this class of drugs is known to cause renal damage and failure. The role of corosolic acid, if any, is not clear. The use of a drug known for its nephrotoxicity in conjunction with impaired kidney function readily explains the resulting kidney failure. The ability of corosolic acid to inhibit gluconeogenesis could favor lactic acid production. If corosolic acid impaired the metabolism of diclofenac, it could theoretically have exacerbated the known nephrotoxicity of the drug. No evidence was provided to specifically demonstrate this possible effect, and no controlled clinical studies have reported nephrotoxicity in diabetic subjects receiving corosolic acid.
The above clinical studies demonstrate that banaba extract, banaba extract standardized to corosolic acid and corosolic acid itself decrease fasting as well as postprandial blood glucose levels in humans. A decrease in blood glucose levels has been observed within 2h of dosing, and the decrease is typically in the range of 10–15%, although a decrease of 30% has been reported. No adverse effects have been observed or reported in any studies involving human subjects receiving banaba, including one study involving 15 subjects who were given banaba extract daily for up to 1year.