Beneficial effects of blackcurrant and apple polyphenols on glucose homeostasis

Epidemiological evidence and randomised controlled trials suggests that dietary patterns with high intakes of fruit and vegetables are associated with reduced risk of cardiovascular disease (CVD) and type 2 diabetes (T2D). Frequent elevated excursions in postprandial glucose concentrations are thought to increase risk of T2D, therefore dietary strategies to control chronic postprandial hyperglycaemia would strengthen efforts to reduce the risk for T2D. Fruit polyphenols are dietary constituents that might help to delay glucose absorption following a carbohydrate-containing meal or beverage and epidemiological studies have shown a negative relationship between ingestion of polyphenols and T2D. Polyphenol dietary intakes and urinary excretions were analysed in a free-living population following the UK dietary guidelines and compared with a population following the average UK diet. In the intervention group, increased consumption of fruit and vegetables (> 5 portions/day) and wholegrain cereals led to an increased intake in polyphenols associated with health benefits, such as anthocyanins, isoflavones and lignans (P<0.05). Randomised clinical trials were conducted using highly purified anthocyanin-rich blackcurrant extract (BE) and polyphenol-rich apple extracts (AE) consumed immediately before a high-carbohydrate (starch and sucrose) test meal. BE (600 mg of blackcurrant anthocyanins), AE (1200 mg of apple polyphenols) and BE+AE (600 mg of blackcurrant anthocyanins + 600 mg of apple polyphenol) reduced postprandial glycaemia relative to control by 57, 63 and 73 %, respectively (P<0.005), but lower doses of an apple extract had no effect on postprandial responses to an oral glucose load. Effective doses of the fruit extracts are equivalent to 100 g of fresh blackcurrant and 600 and 300 g of raw apple. In vitro studies testing physiological concentrations of the blackcurrant and apple extracts used in the randomised clinical trials showed that the polyphenols contained in blackcurrant and apple extracts inhibited total and GLUT-mediated glucose uptake in Caco-2/TC7 cells (a well-known model of the small intestine) and inhibited the sodium-dependent glucose transporter SGLT1 expressed in oocytes. Our results suggest blackcurrant and apple polyphenols may reduce postprandial glycaemia in vivo at least partly by inhibiting glucose uptake in the small intestine. Altogether the findings presented show that advice to consume more fruits and vegetables can effectively increase intakes of specific polyphenols that have been associated with reduced risk of cardio-metabolic diseases, and provides evidence for mechanisms where fruit polyphenols might regulate glucose homeostasis