nowledge of the differing digestion and absorption characteristics of carbohydrates has lead to the construction of the glycaemic index. The glycaemic index is a value given to carbohydrate foods that is a measure of the rate of blood sugar rise following consumption of that carbohydrate food in isolation. The value is given in comparison to white bread or glucose (depending on the method employed), both of which are given a value of 100. The lower the number on the glycaemic index the lower the blood sugar response following ingestion. Generally, evidence suggests that carbohydrate foods that cause rapid and large rises in blood sugar are detrimental to the health if consumed frequently and in large amounts in the diet. Refined carbohydrates tend to be graded higher on the glycaemic index that whole grains and legumes, which may explain the detrimental health effects of refined carbohydrates.
However, foods are not eaten in isolation, and a number of factors can affect the glycaemic index of a carbohydrate when included in a meal. In particular, factors that can influence the gastric emptying rate of the carbohydrate foods might alter the glycaemic response. For example, a study published in the American Journal of Clinical Nutrition1 in 1988 presented data to show that gastric emptying rate accounts for around 36 % of the variation in postprandial glycaemic response to the carbohydrate foods tested (spaghetti, bread, mashed potato and rice). While the area under the curve for the insulin and glucose responses to the foods was similar, the maximum insulin and glucose response was significantly inversely associated with the rate of gastric emptying. Therefore gastric emptying rate is one of the variables that determines the glycaemic index of foods.
Because both fibre and protein can influence the rate of gastric emptying, they might have a strong influence on the glycaemic index of carbohydrates. However, some studies that have attempted to alter the protein and fibre content of the meal have not been able to change the glycaemic responses of carbohydrate foods, suggesting that the relationship is more complex than might be considered. Digestible food components only exit the stomach after they have been reduced in size to less that 2mm in diameter. Therefore the initial composition of the foods may have an influence on the gastric emptying rates, based on the initial particle sizes of the foods. This may explain the high glycaemic index values attributed to processed foods such as mashed potato, which have their particle sizes decreased considerably prior to ingestion. It may also explain the low glycaemic index value of some whole grains and legumes.
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