Carbohydrates play a role in the development of obesity and diabetes. In particular, high intakes of refined carbohydrates, particularly disaccharides and monosaccharides, are associated with increased glycaemia, insulin release, and the development of blood sugar disorders than can contribute to insulin resistance and increase the risk of metabolic syndrome. Low carbohydrate diets can be effective in controlling glycaemia, which makes them useful tools in the treatment and prevention of blood sugar disorders such as type 2 diabetes. However, some diets that do not restrict carbohydrates are also effective in the treatment of blood sugar disorders, because they control glycaemia via other mechanisms. While reducing the glycaemic load therefore appears to be the necessary step to controlling certain blood sugar disorders, there are a number of way in which this can be effectively achieved other than through the limitation of carbohydrate intake.
Absorption of starches from the gastrointestinal tract relies on the effective hydrolysis of glucose from the original polysaccharide molecule via the action of amylase and alpha-glucosidase. Amylase is secreted from the saliva and the pancreas, whereas alpha-glucosidase is located in the brush boarder of the small intestine. Only 5% of starch breakdown occurs through the action of salivary amylase, with the majority of digestive action occurring in the small intestine. The sequential removal of glucose units from the starch molecule takes time, and the duration of breakdown can change depending on a number of factors. Starch can comprise of amylose or amylopectin, which represent branched or straight chains of glucose, respectively. Because branch points increase the surface area, enzymatic hydrolysis of amylopectin is faster, and so digestion and absorption proceeds at an accelerated rate. The amylose to amylopectin ratio therefore influences glycaemia.
The non-digestible or resistant carbohydrate component of plants is often universally referred to as fibre, although this is a simplification of a large diverse group of polysaccharides. When plant material is ingested and the content of fibre is high, hydrolysis of the digestible component of the plant material, the starch, is slowed. This is particularly true for soluble fibre, which absorbs water and interferes with the interaction between the starch molecules and the brush boarder of the enterocytes in the unstirred layer. High soluble fibre intake has been shown to lower the risk of cardiovascular disease, possibly because improved glycaemic control lowers the risk of metabolic syndrome. This explains the benefits of whole grain carbohydrates that are rich in fibre, increased intakes of which have been shown to improve type 2 diabetes. A shift to refined low carbohydrate diets in Western nations has been a disaster for health.
Beans may be beneficial to the control of glycaemia because of their high fibre content. However, beans are also thought to contain substances that can inhibit the absorption of carbohydrates, through enzymatic inhibition. In rats studies, area under the curve (AUC) for glucose elevations from rice were 40% of their control values following administration of a bean extract. In pigs a bean extract significantly deceased both sucrase and starch absorption. A bean extract was administered to humans with white bread and resulted in a 66% reduction in carbohydrate absorption as measured by glucose AUC. With a full meal, 0.75g of bean extract resulted in a 41% decrease in carbohydrate absorption. A white kidney bean extract resulted in a 2.3% and 1.3kg/m2 decrease in body fat and body mass index, respectively, when administered with meals for 12 weeks to obese subjects, supporting epidemiological evidence suggesting weight benefits with bean consumption.
Bean extracts may be effective at inhibiting carbohydrate absorption because they contain the 5-carbon sugar L-arabinose which can inhibit sucrase, and may also contain amylase inhibitors. Some evidence also suggests that green tea extract may inhibit carbohydrate absorption from the small intestine. Both bean and green tea consumption is associated with inhibition of weight gain, and weight loss in humans. Decreasing carbohydrate absorption may explain the mechanism of action of these foods. Amylose inhibitors have also been reportedly isolated from wheat. Other potential carbohydrate blockers include phloridzin from apple extract and Gymnema sylvestra, an Indian herb in traditional medicine. One possible caveat is that L-arabinose will not prevent the absorption of high fructose corn syrup (HFCS), due to the fact that sucrase is not needed for its absorption. However, avoidance of HFCS should be a priority if disease prevention is a concern.
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