Legumes have beneficial glycaemic effects. The exact reason for this effect is not known, but evidence suggests that this may result from a combination of a high fibre and protein content, tough cell walls that inhibit digestive enzymes, the presence of enzyme inhibitors, as well as the presence of polyphenolic phytochemicals. Eating legumes with other foods slows the digestion of the starch contained within, slows the absorption of the subsequently produced glucose, and thereby flattens the glucose and insulin curves that result from the consumption of these foods. This explains the beneficial weight loss effects of legumes, that may results solely from this beneficial glycaemic effect. Lentils are one member of the legume family that shows these beneficial glycaemic effects. In fact, the benefits of a legume meal may go beyond providing benefits in the present, as evidence suggests that subsequent meals may have their glycaemic responses altered by an intake of lentils.
For example in one study1, researchers fed healthy subjects a high carbohydrate breakfast. One meal contained 226 grams of lentils (dry weight) with 11 grams of butter and 150 grams of tomatoes. Another meal contained 280 grams of whole meal bread with 213 grams of low fat cottage cheese and 150 grams of tomatoes. These meals were identical in their protein, carbohydrate and fat contents. Another breakfast was identical to the previously mentioned bread meal, but was taken in divided doses over 4 hours. Another meal contained just 70 grams of wholemeal bread but was identical to the previous two bread meals in all other respects. The results showed that the lentil, divided dose wholemeal bread and reduced quantity wholemeal bread meals had significantly flattened blood glucose responses (29 %, 54 % and 46 %, respectively) compared to the full wholemeal bread meal. The total area of the blood glucose response from the lentils was reduced by 71 % compared to the full wholemeal bread meal.
Therefore lentils produce a blood sugar response that is lower even when compared to that of a lesser amount of wholemeal bread. In addition, the lentil meal showed a flattened insulin response and reductions in the release of gastric inhibitory peptide (GIP; also called glucose dependent inhibitory peptide), a hormone known to stimulate the release of insulin. However, when a lunchtime meal was given (containing 150 grams of wholemeal bread, 60 grams of cottage cheese, 100 grams of tomatoes, 15 grams of marmite and a 150 gram banana), those subjects consuming the lentils for breakfast displayed a significantly flattened blood glucose response (38 % of the wholemeal bread response). However, those consuming the reduced quantity of wholemeal bread for breakfast actually showed an increased blood glucose response to the subsequent lunch. That lentils are superior even to wholemeal bread at flattening blood glucose responses shows how good the glycaemic effect of legumes really is.
Dr Robert Barrington’s Nutritional Recommendation: Lentils, like all legumes therefore display beneficial glycaemic effects in humans. This effect may spill over into subsequent meals. The authors suspected that the reduced blood glucose responses may be due to malabsorption of the lentils. However, subsequent hydrogen breath tests (a measure of the amount of carbohydrate reaching the colon and therefore not being absorbed) was not significantly different between the groups, suggesting that this was not the reason for the beneficial glycaemic effects. It therefore appears that the beneficial glycaemic effects of lentils are simply due to the slower digestion of the starch and subsequent absorption of the glucose within the cells. That this effect is able to delay the absorption of carbohydrate from subsequent meals suggests that lentils do not have to be eaten at every meal to exert their effects.
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