Consumption of legumes is associated with weight loss, and this might result from the ability of legumes to improve glycaemic function. A large part of this effect may derive from the fibre content of legumes, which is thought to produce favourable glycaemic changes by reductions in the rate of carbohydrate absorption. However, legumes also contain enzyme inhibitors which may cause weight loss through inhibition of nutrient digestion. Such substances are often called anti-nutrients because they reduce the nutritional value of food. For example, trypsin and chymotrypsin inhibitors (protease inhibitors) have been identified in beans and been found to be highly resistant to heat treatment. Lectins (phytohaemagglutinins) have also been characterised from a large number of varieties of beans and some studies have suggested that lectins may exert toxicity through interference with digestion, the production of digestive lesions and their subsequent ability to allow gut bacteria through the intestinal barrier to the circulation.
As well as lectins and protease inhibitors legumes also contain amylase inhibitors. Amylase is one of the enzymes responsible for the digestion of starch, and amylase inhibitors have also been suggested to be a contributory factor in the beneficial glycaemic effects of legumes. Extracts of protein concentrate from legumes such as Phaseolus vulgaris have been used to create commercial starch blockers aimed at controlling weight gain and obesity. Analysis of the amylase inhibitors in such commercially starch blockers has been performed in order to assess their chemical composition1. The results of this analysis showed that the commercial starch blockers had highly variable concentrations of amylase inhibitors within the tablets. This likely reflects the fact that the starch blockers contain crude extracts of beans, and the variability in the tablets was due to the natural variability of amylase inhibitors. Analysis of amylase and protease inhibitors and lectins from kidney beans demonstrated this highly variable protein contents.
The lectins, amylase inhibitors and protease inhibitor have been suggested to contribute to the poor nutritive qualities of beans. This may to some extent explains the weight loss effect of beans if consumed regularly. In fact certain varieties of kidney beans appear to show toxicity to animals including humans if eaten raw. However, the toxicity of raw beans is thought to be related to the lectin content of the beans, and therefore the protease and amylase inhibitors are not contributory factors in this respect. Further, it is not known if animal experiments are completely applicable to humans eating free living diets because animal experiments, often performed in vitro, may not be representative. In addition, the lectin content of beans varies considerably between varieties. Beans are not all the same and their nutrient content differs considerably. So while raw kidney beans may show toxicity to animals and humans because of their high lectin content, other varieties with lower concentrations of lectin may not be toxic.
The toxicity of raw beans, based on their lectin content and protein utilisation by rats have been investigated2. kidney (Phaseolus vulgaris), runner (Phaseolus coccineus) and tepary (Phaseolus acutifolius) showed high reactivity with all cell types and were therefore classified as toxic. Butter beans (Phaseolus lunatus) and winged beans (Psophocarpus tetragonolobus) agglutinated only pronase-treated erythrocytes and were therefore less toxic. Lentils (Lens culinaris), peas (Pisum sativum), chick peas (Cicer arietinum), black eyed peas (Vigna sinensis), pigeon peas (Cajanus cajan), mung beans (Phaseolus aureus), broad beans (Vicia faba) and adzuki beans (Phaseolus angularis) had low reactivity with all cell types and were essentially non-toxic. Soya (Glycine max) and pinto (Phaseolus vulgaris) has low reactivity with all cell types but did inhibit grows somewhat. The authors classified this last group as essentially non-toxic, and suggested the inhibition of growth was likely due to anti-nutrient factors other than lectins within the beans.
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