There has been much research done on Banaba leaves and their ability to reduce blood sugar. In the Philippines, Banaba is a popular medicinal plant and is used in the treatment of diabetes mellitus. It is a natural plant product, can be taken orally, and has no side effects according to Japanese research. Its active constituent is corosolic acid which is known for its "insulin-like activity".
Banaba leaf extract contains a triterpenoid compound known as corosolic acid - which has actions in stimulating glucose transport into cells. As such, banaba plays a role in regulating levels of blood sugar and insulin in the blood. For some people, fluctuations in blood sugar and insulin are related to hunger and various food cravings, particularly cravings for carbohydrates such as bread and sweets. By keeping blood sugar and insulin levels in check, banaba may be an effective supplement for promoting weight loss in certain individuals.
As a dietary supplement to help reduce elevated levels of sugar and insulin in the blood, banaba extract has been shown to be safe and effective. As a weight loss aid, a handful of small (unpublished) studies in humans have suggested that tighter control of blood glucose and insulin can help promote moderate weight loss.
Adults: 1,600 mg 3 times daily after meals. Ages 7 and under: 800 mg 3 times daily after meals
Latin Name - Lagerstroemia speciosa (L.) Pers. (syn. L. flos-reginae Retz.)
Common Names - Banaba (Tagalog/Philippines), Queen's crape- myrtle (English)
Traditional Uses - Leaves and flowers: diabetes, bark: abdominal pain, blood in urine, diarrhea root: jaundice.
Pharmacology - Leaf extract: Hypoglycemia, glucose transport stimulating, anti-obesity, antioxidant. Corosolic acid: Hypoglycemia, glucose transport stimulating
Clinical Application - Diabetes, Syndrome X
Toxicity - No adverse effects found in clinical studies
The effect of plant extracts on D-glucose uptake by Ehrlich ascites tumour cells was examined. Among the 23 extracts of medicinal plants, five samples inhibited, and six samples activated the uptake significantly. From one of the active plants, Lagerstroemia speciosa, two triterpenoids, corosolic acid and maslinic acid were isolated. Corosolic acid was shown to be a glucose transport activator. Since this compound was known to have hypoglycemic activity, our simple in vitro bioassay method can at least be used as a first screening for anti-diabetic activity. 
Banaba in the Tagalog name, Lagerstroemia speciosa L., has been used as a folk medicine for a long time among diabetics in the Philippines. Extracts from banaba leaves have been reported to reduce diabetic symptoms in genetically diabetic mice (Type II, KK-Ay). In the present study, female mice of the same strain showing remarkable body weight gain were used to examine the antiobesity effect of dietary banaba extract. Five-week-old female KK-Ay mice were fed a control diet or test diet containing 5% of a hot-water extract from banaba leaves instead of cellulose for 12 wks. Neither group showed any changes in diet intake during the experimental period. Body weight gain and parametrial adipose tissue weight were lowered significantly in the banaba diet group. Blood glucose levels were not suppressed in the banaba diet group, but hemoglobin A1C was found to be suppressed at the end of the experiment. No effects on the serum lipids were observed, but the mice fed banaba extract showed a significant decrease, to 65% of the control level in total hepatic lipid contents. This decrease was due to a reduction in the accumulation of triglyceride. These results suggest that banaba had a beneficial effect on obese female KK-Ay mice. 
The hypoglycemic effects of Lagerstroemia speciosa L., known by the Tagalog name
of banaba in the Phillipines, were studied using hereditary diabetic mice (Type
II, KK-AY/Ta Jcl). The mice were fed a test diet containing 5% of the hot-water
extract (HWE) from banaba leaves, 3% of the water eluent of the partial fraction
unadsorbed onto HP-20 resin of HWE (HPWE), and 2% of the methanol eluent of the
partial fraction adsorbed onto HP-20 resin of it (HPME) for a feeding period of
5 weeks. The elevation of blood plasma glucose level in non-insulin dependent
diabetic mice fed the cellulose as control (CEL) diet were almost entirely
suppressed by addition of either HWE or HPME in place of cellulose in the CEL
diet. Water intakes were inclined to increase gradually in the group fed either
CEL or HPWE, but lower in the mice fed either HWE or HPME than in the animals
given either CEL or HPME. The level of serum insulin and the amount of urinary
excreted glucose were also lowered in mice fed HWE. Plasma total cholesterol
level was also lowered in mice fed either HWE or HPME. It is suggested that HWE,
especially HPME, obtained from banaba leaves have beneficial effects on control
of the level of plasma glucose in non- insulin dependent diabetes mellitus.