Much is made of the improved health outcomes from consuming whole grain starch compared to refined starch. Whole grains are believed to improve health because they have their original fibre and micronutrient content and thus increase intakes of both when consumed. Because of their fibre content, whole grains are often considered to produce a slower rise in blood sugar compared to refined equivalents, and to some extent this is true. However, the glycaemic index of many whole grain foods can be high, and so fibre cannot explain the differences in blood sugar responses between carbohydrate foods. In fact, it is known that the amylose to amylopectin ratio plays a far more important role that fibre content when it comes to blood sugar response from the starch. This is because amylose and amylopectin have very different structural characteristics that affect the rate of digestion.
Amylose differs from amylopectin is so far as the former is composed of a straight chain of glucose units, and the later is a highly branched arrangement. During digestion amylose can therefore only release glucose units one at a time from each end of the starch molecule when hydrolysed by amylase. In contract, the highly branched structure of amylopectin allows multiple sites for digestion by amylase and this increases digestion speed considerably. This difference in digestion speed was demonstrated in a study published in the American Journal of Clinical Nutrition in 19881. Researchers fed 13 healthy men 1 gram cornstarch per kg bodyweight consisting of 70 % amylopectin or amylose and measured the blood glucose response to the food. The results showed that plasma glucose levels were significantly lower following consumption of amylose compared to amylopectin at 30 minutes post meal.
Food high in amylose therefore has a lower glycaemic index than food high in amylopectin. Controlling blood sugar rises therefore requires a knowledge of the composition of the glucose molecules within the starch. That the blood glucose levels translates into a reduction in the requirement for insulin was also tested by the researchers. At 30 and 60 minutes after consuming the meal, the plasma insulin levels were significantly lower for amylose when compared to amylopectin. Legumes tend to have a high percent of their starch as amylose and this can be around 30 to 40 % of the total carbohydrates. Most other carbohydrate sources have amylose contents of around 15 to 30 %. This explains their low glycaemic indexes and their usefulness as a carbohydrate source in those who wish to limit insulin release. It also explains the benefits of legumes in weight loss where control of insulin is pivotal.
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