Wild Strawberry, Blackberry, and Blueberry Leaf Extracts Alleviate Starch-Induced Hyperglycemia in Prediabetic and Diabetic Mice

Intestinal alpha-glucosidase and alpha-amylase break down nutritional poly- and oligosaccharides to monosaccharides and their activity significantly contributes to postprandial hyperglycemia. Competitive inhibitors of these enzymes, such as acarbose, are effective antidiabetic drugs, but have unpleasant side effects. In our ethnopharmacology inspired investigations, we found that wild strawberry (Fragaria vesca), blackberry (Rubus fruticosus), and European blueberry (Vaccinium myrtillus) leaf extracts inhibit alpha-glucosidase and alpha-amylase enzyme activity in vitro and are effective in preventing postprandial hyperglycemiain vivo. Toxicology tests on H9c2 rat embryonic cardiac muscle cells demonstrated that berry leaf extracts have no cytotoxic effects. Oral administration of these leaf extracts alone or as a mixture to normal (control), obese, prediabetic, and streptozotocin-induced diabetic mice attenuated the starch-induced rise of blood glucose levels. The efficiency was similar to that of acarbose on blood glucose. These results highlight berry leaf extracts as candidates for testing in clinical trials in order to assess the clinical significance of their effects on glycemic control.