Anthocyanins are a group of phytochemicals belonging to the flavonoid group of polyphenols. Anthocyanins are responsible for the red, purple and blue colours in many fruits and vegetables. Evidence suggests that anthocyanins are bioavailable in animals and humans and that they may have specific health effects. In particular, anthocyanins possess antioxidant effects in mammalian tissues, and this relates to their polyphenolic structure. Multiple hydroxyl groups donate hydrogen ions to allow the reduction of oxidised chemicals. The ring structure of the anthocyanins then stabilises the unpaired electrons dissipating the uneven charge throughout the molecule, thus preventing the anthocyanin radical from propagating further free radical generation. In this way, anthocyanins are chain breaking antioxidants, and this may explain some of their beneficial health effect which includes a protective effect against cardiovascular disease, cancer, inflammation, possibly through improvements to insulin sensitivity.
However, anthocyanins may also have biological effects in mammals that occur prior to absorption. In this regard they may inhibit certain digestive enzymes involved in carbohydrate digestion, and this may slow the absorption of glucose postprandially. Berries are rich sources of anthocyanins and many studies have investigated the beneficial effects of berry consumption on the postprandial glycaemic response to carbohydrates such as glucose and starch. The ability of berries to lower blood glucose levels may explain some of their insulin sensitising effects. Rapid rises in blood sugar overload cells with energy, and this causes the generation of free radicals from the electron transport chain as the energy is oxidised. These free radicals induce oxidative stress and this can interfere with the insulin signal cascade. Foods that slow the absorption of nutrients, particularly glucose may prevent this free radical induced damage to the insulin signal cascade and thus have insulin sensitising effects.
Cell culture experiments have demonstrated possible mechanisms by which anthocyanins may inhibit the digestion of sugars. These mechanisms may include both antioxidant and enzyme inhibitory pathways. The results of these experiments are supported by epidemiological evidence that shows associations between anthocyanins and glucose metabolism. For example, in one study researchers1 investigated the associations between subclasses of flavonoids and various measures of glucose metabolism in female subjects aged 18 to 76 years. The results of the study showed that anthocyanin intake was inversely associated with peripheral insulin resistance in the women (as was another group of flavonoids, the flavones). Anthocyanins and anthocyanin rich foods were also inversely associated with plasma insulin levels and a marker for inflammation, C-reactive protein. Those with the highest intakes of anthocyanins also had increased adiponectin concentrations in plasma, suggesting better glucose and fatty acid regulation.