he growing intakes of refined carbohydrates is worrying because they are though to contribute to the development of disease through detrimental blood sugar effects. For this reason it is recommended that refined carbohydrates are replaced in the diet with whole grain alternatives. Whole grains are associated with protection from Western lifestyle disease, although the exact reason for this is not known. It is often assumed that whole grains are protective because they have low glycaemic indexes, but this is not necessarily true. Table sugar for example has a lower glycaemic index than many whole grain foods. In addition, although tables showing the glycaemic index of particular foods can give a broad approximation of the effects of that food on blood sugar rises, there are many factors than can affect the final glycaemic index of a food following harvest.
For example, researchers1 have investigated the factors that affect the glycaemic index of nine barley cultivars in 10 healthy volunteers. The researchers put the barley whole grains through various levels of processing to create a whole grain (hull removal), a processed grain (bran and germ removal) and a pearled barley (removal of bran). The results showed that the glycaemic index of the barley was correlated with the fibre content, but not the starch characteristics (such as the amylose to amylopectin ratio). There were differences in the glycaemic index of the various barley cultivars, but this glycaemic index ranking changed when the barley was processed into pasta. When whole grain barley was processed into whole grain pasta causing destruction of the cellular matrix, the glycaemic index increased, despite the same bran, endosperm and germ content being present in both products.
These results demonstrate just how difficult it is to predict the glycaemic index of a product made from cereal grains. In this case barley was investigated, but there is no reason to suspect the same is not true for wheat. Cultivars of barley may have different glycaemic indexes not because of variation in the amylose to amylopectin ratio, but because of variation in the total fibre content. The cultivars were selected by the researchers because of their differing β-glucan (soluble fibre) content, and it is likely this influenced the glycaemic index. That fibre was influential in the glycaemic index was also suggested by the increase in glycaemic index of the barley with bran removal. That ground whole grains pasta had a higher glycaemic index than non-processed whole grain barley suggests that the matrix of the grain when intact, lowers the glycaemic index.
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