The glycaemic response to a food, which in turn affects the insulin response, depends on the rate of gastric emptying, as well as on the rate of digestion and absorption of carbohydrates from the small intestine (Jenkins et al., 1987). Traditionally, carbohydrates were classified as 'simple' and 'complex' based on their degree of polymerization. Sugars (which are mono- and disaccharides) were therefore classified as simple, whereas starches (poly-saccharides) were classified as complex. However, carbohydrates might be better classified on the basis of their physiological effects, for example their ability to increase blood glucose. The glycaemic response depends both on the type of sugar (e.g. glucose, fructose, galactose) and the physical form of the carbohydrate (e.g. particle size, degree of polymerization) (Augustin et al., 2002).
In 1981, Jenkins et al. (1981) proposed the concept of the GI to characterize the rate of carbohydrate absorbed after a meal. The GI was meant to supplement information about chemical composition given in food tables, to help understand and better predict the physiological effects of whole diets. Unexpected differences between the GI values of different foods highlighted the importance of food characteristics not provided in food composition tables. These include food form, particle size, the nature of the starch, food processing and interfering factors, all which may have large effects on the physiological properties of foods.
The GI is defined as the area under the glucose response curve after consumption of 50 g available carbohydrate from a test food divided by the area under the curve after consumption of 50 g available carbohydrate from a control food. The control food can be either white bread or glucose (Wolever et al., 1991). Foods with a high GI produce, per gram of available carbohydrate, a higher peak in postprandial blood glucose and a greater overall blood glucose response during the first 2 h after consumption than the peak for foods with a low GI (Foster-Powell et al., 2002). A higher blood glucose response increases insulin demand and insulin secretion by the pancreas. Repeated episodes of hyperinsulinemia may, over the long term, lead to downregulation of insulin receptors and insulin resistance (Virkamaki et al., 1999). This may in turn increase postprandial blood glucose concentra tions and insulin secretion (Fig. 3.1). Insulin resistance is a central characteristic of type 2 diabetes mellitus (Reaven, 1993). Low-GI diets tend to delay glucose absorption and reduce peak insulin concentrations and overall insulin demand. Several studies have found improvements in glycaemic control with low-GI diets in healthy subjects as well as those with coronary heart disease or diabetes (Burke et al., 1982; Jenkins et al., 1987, 1988; Brand et al., 1991; Frost et al., 1996). In addition, low-GI foods are generally associated with greater satiety compared with high-GI foods, delaying hunger and potentially reducing food intake. Examples of the GI values of different foods are given in Table 3.1.
High glycaemic index foods
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