Legumes are of interest to nutritional scientists because they have beneficial effects on blood sugar. This is evident from their low rating on the glycaemic index (GI). The glycaemic rating of carbohydrate foods is widely believed to be due to the fibre content of the foods. However, this is provably false, as some low fibre refined carbohydrates have much lower glycaemic indices compared to high fibre whole grain alternatives. Studies investigating the glycaemic effects of legumes have reported that the beneficial effects are not due to the fibre content (here). Indeed, the beneficial blood sugar effects of legumes may be due to the rigid walls of the parenchymal cells that surround the starch. These act as a physical barrier to digestion, slowing the hydrolysis of starch to glucose and thus delaying and extending the postprandial rise in blood sugar. This ability of legumes to beneficially affect postprandial glycaemia may explain epidemiological evidence showing inverse associations between consumption of legumes and Western lifestyle diseases.
A number of studies have investigated the beneficial glycaemic effects of legumes. In one study published in the American Journal of Clinical Nutrition1, researchers compared the glycaemic response of subjects to white kidney bean flakes or equivalent flakes made from potato. The two test meals were standardised for the fibre and protein content of the beans, because some evidence suggests that beneficial glycaemic effects may be attributed to protein or fibre. The results showed that the plasma glucose and insulin rise was significantly lower following the bean flake meal, compared to the potato flake meal. In addition, the glucose oxidation rate, as measured by indirect calorimetry, was significantly lower in the subjects following consumption of the beans flakes. Further evidence that legumes have beneficial glycaemic effects was provided when the researchers fed the legume flakes to subjects diagnosed with type 2 diabetes. The meal produced only moderate rises in blood sugar postprandially, suggesting that legumes are effective at controlling glycaemia in subjects with severe insulin resistance.
These results suggest that the fibre and protein content of legumes may not be responsible for their beneficial glycaemic effects. This is interesting because other research suggests that both fibre and protein may play a role in favourably modulating postprandial glycaemia. It is worthy of note however that the authors of this study did not control for the presence of enzyme inhibitors in the legumes, which could account for the blood sugar effects reported. Such inhibitors are thought to decrease the activity of amylase and other enzymes involved in the digestion of starch. Therefore some of the beneficial effects seen in this study could have been due to the presence of such inhibitors. Another interesting finding from this study was the fact that after around 100 min, the bean flake meal produced higher glucose and insulin responses compared to the potato flake meal, despite lower initial rises. This suggests that legumes offer the advantage of a prolonged rise in blood sugar, which may also explain their appetite satiating effects.
