There has been much research done on Banaba leaves and their ability to reduce blood sugar, and its "insulin-like principle."  In the Philippines, Banaba is a popular medicine plant and is used in treatment of diabetes mellitus. It is high in corosolic acid which is used in many treatments for diabetes. It is a natural plant insulin, can be taken orally, and has no side effects, according to Japanese research. 

Numerous studies have been done on this remarkable herb, much of it in Japan, with researchers such as Dr. Yamazaki, professor of Pharmaceutical Science, Hiroshima University School of Medicine. One study mixed banaba dried leaf powder with chicken feeds, and then analyzed the yolk of the chicken egg. When the banaba enriched egg yolk was fed to diabetic mice, their blood sugar level was normalized. In another study, the alcohol extract of banaba leaves was sprayed into the air of a room at night while the patient was sleeping via a mist generating device. It was found that as the person slept, their lungs received trace amounts of corosolic acid which helped regulate blood sugar levels.

Following the studies done by the Japanese which isolated corosolic acid as the active component, other studies followed. In 1999 a clinical study was conducted on diabetes patients by Dr. William V. Judy at the Southwestern Institute of Biomedical Research, Brandenton, Florida, which found that corosolic acid universally lowered blood sugar levels of all patients. The higher the dosage of corosolic acid, the lower blood sugar levels dropped.

However, more recent studies have shown that the entire herb is useful in lowering blood sugar, and that corosolic acid is probably not the only active ingredient in banaba leaves. A study published in 2001 by Edison Biotechnology Institute, Department of Biomedical Sciences, College of Osteopathic Medicine, in Athens, Ohio compared a whole leaf extract of banaba with insulin in cell cultures. They concluded that the whole herb had a glucose lowering effect and could be used to fight diabetes and obesity. A study reported in the Feb. 2002 volume of Planta Med stated that corosolic acid alone could not account for the glucose transport effect of banaba extracts, and identified at least three active components. (Go here to see details of these studies.)

Because of the way banaba helps the body to handle glucose (glucose transport activator) it is also used effectively in weight-loss products. Supplementwatch.com says: "An interesting 'side-effect' of tighter control of blood sugar and insulin levels is a significant tendency of banaba to promote weight loss (an average of 2-4 lbs. per month) – without significant dietary alterations. It is likely that modulation of glucose and insulin levels reduces total caloric intake somewhat and encourages moderate weight loss." In Japan, Banaba tea has become quite popular. In the US, many weight-loss formulas are now using banaba as a safe, natural component for reducing weight.