Effect of corosolic acid on postchallenge plasma glucose levels

Corosolic acid (CRA) is a substance extracted from Lagerstroemia speciosa L. (Banaba) and has been reported to have biological activities in in vitro and experimental animal studies, In a study using KK-Ay mice (a model of type 2 diabetes), elevation of plasma glucose levels in the group fed with a diet containing the extract from the leaf of L. speciosa was significantly suppressed compared to the control group In studies using Ehrlich ascites tumour cells and 3T3-L1 cells, glucose uptake was stimulated by the extract from the leaf of L. speciosa Recently, Judy et al. described the anti-diabetic activity of the leaf extract (including 1% corosolic acid) in humans in a dosedependent manner.It is known that the main content of the plant leaf extract is polyphenol, which has a blood glucose lowering effect. Screening of the compounds with blood glucose lowering activities.from the crude extract of L. speciosa leaf elucidated lagerstroemin (a kind of polyphenol) as the fraction with glucose lowering activity, but it was not enough to explain the glucose lowering effect of the total leaf extract in vivo . CRA is contained in the leaf of L. speciosa, but it is still unclear whether CRA per se has an effect on a glucose challenge in humans. In this study, we have clarified the effect of CRA on postchallenge plasma glucose levels in vivo in humans.

In the present study, the CRA treatment subjects had lower postchallenge plasma glucose levels at 90 min than the placebo treatment subjects. Sixty minutes after the challenge, plasma glucose levels began to show a difference, and reached statistical significance at 90 min. At 180 min, plasma glucose levels in CRA treatment subjects returned to the levels of the control. The mechanism by which CRA reduces postchallenge plasma glucose levels is not known at present. The main regulators of plasma glucose levels in humans are insulin, glucagon, and somatostatin .Gut hormones such as GIP and GLP-1 also modulate plasma glucose levels in response to glucose challenge. It is well described that intake of polyphenols shows glucose lowering effects by the inhibition of carbohydrate absorption. In a previous report, a negative correlation was observed between glycemic index and the concentration or total intake of polyphenols in both normal and diabetic individuals . Using human intestinal cells, polyphenols were shown to decrease glucose uptake . We have examined the content of an extract from the leaf of L. speciosa and detected a large amount of polyphenols (water: 5.3%, protein:1.2%, lipid: 6.3%, carbohydrate: 73.1%, dietary fiber: 2.1%, and polyphenols: 10.3%). Thus, it was considered that the hypoglycemic effect of the crude extract from the leaf was an additive result of polyphenols and other factors. In this study, we have focused on the effect of CRA per se on postchallenge glucose levels.The structure of CRA is very different from polyphenols and other hypoglycemic agents as shown in Further studies are necessary to elucidate the mechanism of the hypoglycemic activities of CRA.

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