Effect of black bean tannins on in vitro carbohydrate digestion and absorption

Legume seeds are an important source of protein and carbohydrate for both human and animal consumption. Nevertheless the bioavailability of these nutrients is affected by several antinutritional factors present in edible pulses. Most of them, except tannins, are destroyed, at least partially, during cooking. Tannins impair macronutrient utilization by forming tannin-protein complexes with various digestive enzymes precluding the formation of products absorbable by the small intestine. There is ample experimental evidence on the effect of these compounds on protein utilization, but little attention has been paid to the impairment of starch and disaccharide assimilation. In this study, it is shown that alpha-amylase, maltase, sucrase, and lactase were strongly inhibited in vitro by condensed tannins isolated from black beans. These pigments also affected in vitro glucose uptake by rat-everted intestinal sleeves, although the transport system was less susceptible to tannin inhibition than the hydrolases mentioned above. The inhibition of alpha-amylase and sucrase by bean tannins appeared to be of the mixed type with changes in both Vmax and Km. The effect of these compounds on enzyme activity was decreased by treating the tannin solution with polyvinyl-polypyrrolidone but not by heating. Tannin interference with the digestive machinery may explain the decrease in carbohydrate bioavailability observed in animals fed diets of high tannin content.