Botanical Aspects of Phaseolus vulgaris
Common names: Common bean, White Kidney bean
Phaseolus vulgaris belongs to the botanical family of Leguminosae. It is a sub-erect or twining annual herb, native of tropical America and now grown extensively throughout the warm regions of the world. The kidney beans leaves are trifoliate; flowers white to violet-purple; pods slender, 10-26 cm long, straight or slightly curved, the surface glabrous or faintly pubescent, the beak prominent. Seeds are more or less kidney shaped, elongated or nearly globular, or somewhat compressed; white, red, purple blackish or mottled in appearance.
Varieties of the white common bean have been classified variously; some classifications are based on morphological characters of the plant, pod and seeds, while others are based on their mode of use.
The seeds of Phaseolus vulgaris contain an inhibitor of alpha-amylase. This alpha amylase inhibitor is identified in the protein fraction of the bean. This protein binds with the active sites of alpha- amylase and prevents the starch metabolizing activity; hence it is also called “Starch Blocker” or “Carbohydrate blocker”.
The alpha-amylase inhibitor from Phaseolus vulgaris is a protein composed of two glycosylated protomers of 30 kDa, which are cleaved into two polypeptides of 16 and 14 kDa, respectively.
Mechanism of “carbohydrate blocking” action
The alpha-amylase inhibitors are also called “Carbo blockers” and these compounds are not directly involved in the weight loss process. However, the carbo blockers are indirectly helpful in weight loss due to inhibition of sugar assimilation, through inhibiting starch breakdown. With reduced amount of amylase available for break down, the complex carbohydrate has a better chance of travelling through the body without being assimilated, and is eventually excreted from the body instead of being converted into storage fat. An animal model study revealed that amylase inhibitors alter the amount and pattern of food intake and reduce weight gain probably through inducing satiety and increasing carbohydrate delivery to the distal (farthest) part of the small intestine in rats.
Health benefits of carbohydrate blockers
An alpha-amylase inhibitor inhibits the digestion of starch and carbohydrates thereby potentially improving postprandial carbohydrate tolerance in people with low glucose tolerance. As excess dietary carbohydrate is metabolized to fat; inhibition of carbohydrate digestion may help in weight management as well.
A partially purified white bean amylase inhibitor was found to reduce starch digestion and inactivate intraduodenal amylase in humans by 94 to 99.9%.16 The authors of this study reported that perfusion of the partially purified inhibitor (2.0, 3.5 or 5 mg/ml at the rate of 5 ml/min) into the duodenum of humans rapidly inhibited alpha-amylase activity in a dose-dependent manner. The inhibitor also inactivated intralieal and salivary amylase in vitro studies. Additionally, the specific activity of the inhibitor was not affected by exposure to gastric juice and was only minimally affected by exposure to duodenal juice.
In studies in normal subjects and in patients with diabetes mellitus, a purified amylase inhibitor from beans was found to exert a “starch blockade” effect. In comparison with the placebo, ingestion of this inhibitor with 50 g starch substantially reduced post-prandial increases in plasma concentrations of glucose and insulin in both normal subjects and those with diabetes.