The glycaemic index is a measure of the rate at which plasma glucose levels increase following ingestion of carbohydrate foods. Foods are tested in comparison to a reference standard and given a value on a relative scale. The reference standard used to be glucose, but more recently a shift has occurred and white bread is now used because it produces more consistent results on account of it being less affected by water in the gut. Fifty grams of carbohydrate from white bread are consumed and the rise in plasma glucose measured. Fifty grams of carbohydrate fromt the test food are then consumed and its values compared to that of white bread, which is given a value of 100. Foods with lower values cause a slower release of glucose to the blood. Although The glycaemic index is by no means a perfect measure of the quality of a food choice (here), it does provide researchers with useful information. Increasingly, foods higher on the glycaemic index are being associated with disease and it is being recommended to eat lower glycaemic index foods.
A common misconception of the glycaemic index is that whole grain complex carbohydrates have low values while refined and simple sugars have higher values. Fructose for example is a simple refined sugar with a low value of 20, whereas parsnips have a value of 96. The reason for this is that the digestion rate of carbohydrates and the absorption rate of glucose is dependent on a complex set of factors such as degree of processing, amount of fibre, the intactness of the cell walls, the composition of the polysaccharide chains and the type of sugars within the polysaccharide chains. These factors interact to produce unpredictable results on the glycaemic index. One factor that is often overlooked in this respect is the ability of polyphenols, phytochemicals produced by plants, to inhibit carbohydrate absorption and thus alter the glycaemic index. In particular, larger more complex polyphenols such as tannins may interact with digestive enzymes and in this way affect the rate at which carbohydrate is absorbed.
Blueberries are a good source of anthocyanins, polyphenols from the flavonoid class, and the ability of polyphenols to alter the glycaemic index of blueberries has been well reported. However, foods other than fruits can contain polyphenols including beans, cereals and vegetables. In one study researchers tested the ability of polyphenols in thirteen different foods to affect the glycaemic index1. The polyphenol content of all bran, corn flakes, digestive biscuits, porridge oats, rice, white bread, wholemeal bread, wholemeal bread with cottage cheese black eyed peas, chickpeas, kidney beans, red lentils and soybeans was measured and then the glycaemic indices of the foods tested. When the authors assessed the association between the polyphenol content of the foods and the glycaemic index they found a positive correlaton between the glycaemic index and polyphenol content, but in particular between the glycaemic index and the degree of the larger condensed tannin class of polyphenols.
Tannins may bind to digestive enzymes and lower the glycaemic effects of foods. In particular Tannins may bind to proteolytic enzymes such as trypsin and other proteases, and this may indirectly affect the digestibility of starches through the presence of undigested protein slowing the digestive rate within the gut. For example, a tight association is found between the protein and carbohydrate components of some foods, and proteolysis is required in order to allow the starch accessible to attack by carbohydrate digesting enzymes. However, it has also been shown that tannins may bind to amylose and amylopectin and that this can reduce the digestibility of these compounds directly. Fibre has been suggested to be responsible for the reduced digestibility of legumes. However, these results suggest that some of the reason for the lower glycaemic effect of legumes could be due to the presence of tannins. This therefore adds a further levels of complexity to the selection of carbohydrate and further complicates the glycaemic index.
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