Berries show beneficial effects on blood sugar. Specifically, berries appear to lower the glucose response to a test meal. The reason for this effect is not fully understood but is thought to relate mainly to the fibre and flavonoid content of the berries. Berries are a good source of the soluble fibre pectin, and soluble fibre absorbs water within the gut and this creates a viscous gel-like fluid. The gel-like properties of soluble fibre can act as a physical barrier along the endothelial lining of the gut inhibiting the interaction between the glucose and the transporters located on the enterocytes. In addition, the gel-like matrix created in the gut by the presence of soluble fibre may entrap glucose and prevents its diffusion to the brush border of the enterocytes. Further, soluble fibre may physically separate starch and sugars from their enzymes and thus inhibit digestion rates significantly. These mechanisms likely work together to slow the absorption of glucose from the gut to the circulation, lowering the subsequent rise in blood sugar.
Another mechanisms that may explain the glycaemic benefits of berries centres on the presence of high concentrations of flavonoids in the skins of the fruit. Quantitatively the most important flavonoid group in berries are the anthocyanins. Anthocyanins are present mainly in the skins of berries and they give the fruits their distinctive red, purple and blue colours. Plants likely use flavonoids as protection from environmental stressors such as sunlight, pollution and predators. In humans, flavonoids may interfere with starch digesting enzymes. For example, animal experiments have shown that grape skin extracts are able to inhibit the intestinal α-glucosidases (maltase and sucrase) but not the structurally related pancreatic α-glucosidases in diabetic rats1. The inhibition seen in such experiments was competitive at a half inhibitory concentration (IC50) of 0.384 mg/mL. The same authors used 400 mg per kg body weight grape skin extract with mice to lower postprandial blood glucose by around 31 % following starch consumption.
The amount of grape skin extract given to the mice was a large quantity. In fact 400 mg per kg body weight would equate to around 30 grams of grape skin extract in a typical human being weighing 75 kg. However, the fact that the grape skin was effective at inhibiting postprandial glycaemia supports other studies showing that berries have beneficial effects in humans. Interestingly, flavonoids such as the anthocyanins in berries may also inhibit the enzyme aldose reductase. This enzyme is involved in the conversion of glucose to sorbitol in humans, and high concentrations of sorbitol the high plasma blood glucose concentrations in diabetics are a cause of many of the complications of the disorder. Therefore the anthocyanins in berry skins may be beneficial in the treatment of hyperglycaemia because they lower postprandial blood glucose levels and because they inhibit sorbitol production. In addition the antioxidant effects of anthocyanins may improve vascular tone, which is often dysfunctional in hyperglycaemics.
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