ntakes of whole grain foods and legumes have been shown to improve glycaemic response in humans. This provides protection against diabetes, metabolic syndrome, obesity and cardiovascular disease. One reason for this benefit might be the micronutrients contained within the original plant material. However, dietary fibre is known to decrease the absorption speed of carbohydrates and thus is effective at lowering the glycaemic index (GI) of foods. Therefore fibre contained within whole grains and legumes likely provides the largest proportion of the benefit. Fibre can decrease GI because it reduces the absorption speed of glucose by providing a physical barrier that results in a more controlled interaction of polysaccharide molecules with digestive enzymes and subsequently of glucose molecules with the enterocytes absorptive surface. This in turn has beneficial effects on the area under the curve for glucose and insulin in plasma.
However, fibre is a general term for a large number of indigestible carbohydrates and as such care must be taken not to oversimplify their collective effects. Indeed, evidence suggests that different fibres might have varying effects on the GI of food. For example, research1 published in the American Journal of Clinical Nutrition in 1990 investigated the effects of dietary fibre on the GI of foods. The dietary fibre content of 25 foods were assessed in relation to their GI to identify the effects contributed by different type of fibre or components within it. The results showed that total dietary fibre, insoluble fibre and cellulose were associated with GI. Interestingly however, no significant association was found for soluble fibre. The total uronic acid content of both soluble and insoluble fibre, as well the arabinose, total hexose and the other hexose content of insoluble fibre was also associated with the GI.
When the authors performed a multiple-regression analysis, they observed that the uronic acid content of the fibre explained more of the effect on GI than the total fibre content of the food. The association between both uronic acid and cellulose with GI is interesting because legumes have high concentrations of both, compared to cereal grains, and also cause lower glycaemic responses. Uronic acids are found mainly in hemicelluloses, which form the matrix of the cell wall in which cellulose in embedded. Therefore it might be that intact or semi-intact cell walls slow the digestion of glucose by providing a physical barrier to enzymes such as amylase, and that this effect is most pronounced in those food with the strongest cell walls. This would explain the superior effects of legumes on blood sugar disorders when compared to whole grain foods with equal fibre content.
Of particular interest in this study was the non-significant effect of soluble fibre on the GI of the foods tested. In purified form, as tested in many studies, soluble fibre has a large effect on altering the GI of foods. It is thought that soluble fibre forms a physical barrier in the unstirred layer next to the enterocytes and thus limits glucose diffusion. However in this study, when the soluble fibre is part of a food matrix, the glycaemic index of the food is not significantly reduced. This may related to the viscosity of the soluble fibre in consideration. Highly viscous fibres are able to reduce the glycaemic response of food, whereas less viscous fibres are not. Because soluble fibre was assessed in this study as a whole, the ineffective less viscous fibres will have reduced the significance of the effects of the more soluble fibres.
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