The Typical Western diet is increasingly being seen as a causative factor in the development of Western lifestyle disease such as cardiovascular disease, obesity and type 2 diabetes. The typical Western diet contains high amounts of processed cereal grains and these can cause disease because they are devoid of meaningful levels of vitamins and minerals, which are stripped from the grain in the bran and germ layers during processing. The removal of the bran layer from the grain also removes most of the fibre and this can also cause detrimental health effects. In particular the fibre may be necessary to inhibit the digestion rate of the starch and prevent large increases in postprandial plasma insulin and glucose levels. Excessive postprandial glycaemic effects from foods may be a cause of the liver overload syndrome, a condition characterised by oversupply of energy to the liver. This may increase flux through the de novo lipogenesis pathway, resulting in fatty acid production which may be a driver of insulin resistance.
Decreasing postprandial glycaemia is therefore beneficial because it may contribute to improved insulin sensitivity. In turn the improved insulin sensitivity may cause weight loss and its associated health benefits such as improved lipoprotein profiles, lower blood pressure and decreased inflammation and oxidative stress. Berries have been shown to possess beneficial glycaemic effects, although the exact reason for this is not fully understood. It has been suggested that the fibre content of the berries may be the reason for their ability to decrease postprandial glycaemia as they are rich source of the soluble fibre pectin. However, a number of studies have shown that the polyphenol content of the berries may be the reason for their beneficial glycaemic effects. Berries are a rich source of anthocyanins which belong to the flavonoid group of polyphenols. Anthocyanins may be potent inhibitors of the α-glucosidase enzymes that are involved in the digestion of starch and disaccharides.
The inhibitory effects of polyphenol extracts of black currants and rowanberries have been investigated in comparison to the pharmaceutical α-glucosidase inhibitor acarbose1. The polyphenol extract inhibited the activity of α-glucosidase with IC50 values of 20 and 30 μg GAE/mL (gallic acid equivalents per mL) for black currants and rowanberries, respectively. This is in comparison to acarbose which inhibited the activity of α-glucosidase with an IC50 value of 40 μg GAE/mL. Both berry extracts potentiated the inhibitory effects of acarbose on α-glucosidase activity and as doses of acarbose were lowered, the polyphenols in the berries could replace the activity of the acarbose to maintain α-glucosidase inhibition. However, no synergistic effects were noted with black currant and rowanberry extracts in combination. Black currant and rowanberry extracts are therefore at least as effective as the pharmaceutical acarbose at inhibiting intestinal α-glucosidase activity.
In this study, the black currant extract contained around 70 % anthocyanins, whereas the rowanberry extract had low levels of anthocyanins but contained around 65 % chlorogenic acid. In addition, black currents also contain approciable concentrations of flavonols and hydroxycinnamic acid and their derivatives. The high chlorogenic acid content of rowanberries may be responsible for its α-glucosidase inhibitory activity, as chlorogenic acid has been suggested to be responsible for the beneficial glycaemic effects of coffee. It has been suggested that anthocyanin glycosides may act as substrates for the α-glucosidase enzymes and therefore inhibit the enzymes through competitive inhibition. For example cyanidin-3-O-galactoside showed a three-fold greater inhibitory effect for sucrase (an α-glucosidase) compared to the aglycone cyanidin in rats. The more effective whole berry extracts compared to isolated polyphenols suggest that other components in the berries may also inhibit α-glucosidase activity.
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