A high glycaemic response to food is problematic because chronic raised levels of blood sugar are implicated as a causative factor in disease. In particular, high glycaemic responses to carbohydrates are thought to lead to weight gain, metabolic syndrome, type 2 diabetes and cardiovascular disease. The glycaemic index (GI) is a measure of the ability of a carbohydrate food to raise blood sugar levels and this index can be converted to the glycaemic load (GL) by adding in the amount (in grams) of the carbohydrate food consumed. Recommendations are to eat more foods with a low glycaemic index in order to favourably moderate levels of blood sugar and subsequently cause improvements in health. Of the factors affecting the glycaemic index, much has been written about the beneficial effects of fibre. Soluble fibre for example is thought to form a viscous gel in the gut and this can act as a physical barrier to glucose absorption, thus lowering the glycaemic response. This may explain some of the health benefits of high fibre diets.
However, the fibre content of carbohydrates cannot explain all of the variation in glycaemic response. Some foods with lower levels of dietary fibre have lower glycaemic indices than those with more fibre. In fact, many refined starches have lower glycaemic indices than whole grain starches. Therefore some other mechanisms must be responsible for variation in glycaemia following consumption of carbohydrates. The amylose to amylopectin ratio is a measure of the ratio of straight chain to branched chain starches within a carbohydrate food and this may be an important determinant of postprandial glycaemia. Because branched chain starches have a larger surface area, amylopectin should be digested at a faster rate, and carbohydrates with a high concentration of their starch as amylopectin should therefore be absorbed at a faster rate resulting in a greater glycaemic response. A number of studies have assessed the physiological responses to ingestion of foods with varying amylose to amylopectin ratios.
For example, in one study, researchers fed three types of rice to healthy volunteers to assess their glucose and insulin responses1. The rice contained either 0 % (low), 14 to 17 % (medium) or 23 to 25 % (high) amylose concentrations. The results showed that the serum glucose response to the high amylose rice was significantly lower at 30 minutes compared to the low amylose rice. In addition, the high amylose rice produced a more gradual decline in glucose levels from 30 to 180 minutes, compared to the low amylose rice. The lower glucose levels of the high amylose rice was reflected in lower 30 and 60 minute insulin levels in the high amylose group, as well as a lower total secretion over the 3 hour test. The fibre content and the surface area of the rice grains were identical between the different rices, and as such this suggests that the amylose to amylopectin ratio has a profound effect on the glycaemic response to rice in healthy individuals. The amylose content of starches may therefore be as important as fibre in determining glycaemic response.
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