Legumes are plants belonging to the fabaceae family and include lentil, bean and pea plants. The dried seeds of such plants are pulses, and consumption of these seeds is associated with health and longevity. Evidence shows that those who regularly consume legumes are not only thinner, but also healthier. The chemistry of legumes is highly complex, and in this regard it is not surprising that their biochemical effects are not fully characterised. One aspect of legumes that makes them nutritionally significant is their ability to regulate the glycaemic effects of foods, due to their ability to produce very slow and controlled rises in blood sugar. Further, their ability to modulate the blood glucose rises seen from other foods is evident and in this way consuming legumes with a mixed meal favourable effects the postprandial insulin and glucose profiles. These lower more controlled postprandial rises in insulin and glucose are associated with protection from Western lifestyle disease and are known to be pivotal in fat loss.
Both the fibre and protein content of legumes have been theorised to contribute to the beneficial postprandial glucose and insulin profiles seen with pulse consumption. In this respect the soluble fibre contained within pulses may provide a physical barrier that delays the arrival of glucose to the intestinal mucosa and thus hinders glucose absorption. Protein is known to reduce the gastric emptying of food because the presence of undigested protein in the duodenum results in closure of the pyloric sphincter and therefore the reasonably high protein content of beans and peas may contribute to their beneficial postprandial glycaemic effect. In addition to these mechanisms, pulses also contain α-amylase inhibitors that include the protein phaseolamin. These inhibitors are known to bind to the digestive enzyme α-amylase and inhibit its action. Because α-amylase is required to digest starch to glucose, legume seeds containing α-amylase inhibitors may prevent this process and thus delay or prevent the absorption of some dietary carbohydrates.
The research into the effects of α-amylase inhibitors is not extensive, but some studies have investigated their effects on body weight. For example, in one study1, researchers used fasted rats to investigate the effects of an α-amylase inhibitor called acarbose. Acarbose is similar to the α-amylase inhibitors found in pulses. Following a period of fasting that caused weight loss, the rats were refed at a normal calorie intake. However, one group was concomitantly fed 500 mg per kg body weight of the α-amylase inhibitor acarbose. The results showed that acarbose reduced the weight regain seen in the rats when compared to the control group. In addition, The acarbose also reduced the mid jejunal mass and the rise of some digestive enzymes seen in the control group (including disaccharidases and intracellular glycolytic enzymes). However, when the researchers compared the acarbose fed rats to non-fasted control rats, the acarbose rats still gained more weight, suggesting that the acarbose did not completely inhibit weight regain.
These results therefore suggest that α-amylases are responsible for preventing weight regain in rats following fasting. This suggests that some of the weight loss effects of pulse may be derived from their high concentrations of α-amylase inhibitors. One question that arises from results such as these is ‘what happens to the undigested carbohydrate?’ However, this is not an easy question to answer. One possibility is that it is eventually digested and the α-amylase inhibitors only slow rather than prevent the absorption of the glucose. Another possibility is that the undigested carbohydrates pass to the colon where they are fermented by bacteria, perhaps to short chain fatty acids. These would then be absorbed and used as a source of energy. This implies that the carbohydrate present in the gut would eventually be absorbed through one route or another. The beneficial effects of α-amylase inhibitors are not therefore that they inhibit energy intake, but that they delay this energy and thus produce beneficial glycaemic effects.
Dr Robert Barrington’s Nutritional Recommendation: Pulses are high quality foods and are known to produce fat loss when included as part of a healthy diet. Try to use pulses as the main form of dietary carbohydrate by incorporating as many different forms of legume plants into the diet as possible. Frozen baby broad beans and garden peas are convenient and easy to prepare and can therefore be considered if preparation time is limited.
RdB
