GLYCEMIC INDEX AND GLYCEMIC LOAD

11.4 GLYCEMIC INDEX AND GLYCEMIC LOAD

The GI concept was developed over 20 years ago by Jenkins et al. 29 to classify foods based on their effects on the blood glucose level. The original aim was to aid diabetic patients in controlling their postprandial hyperglycemia. It ranks individual foods according to their postprandial rate of carbohydrate digestion and absorption. The GI is defined as the incremental area under the postprandial blood glucose curve (change in blood glucose level 3 h after a meal) after the consumption of 50 g of (digestible) carbohydrates from a test food, divided by the area under the corre- sponding curve after a meal containing a similar amount of the reference food,

normally white bread or glucose. 30 The reference is given the value 100, and the lower the response, the smaller the GI. Using white wheat bread as a reference gives higher GI values than using glucose. Also, white rice has been suggested as the

reference, especially for Asian populations. 31 Not only the reference carbohydrate, but also the reference group in which the carbohydrate has been measured pose problems in standardizing the values for GI. Values for the glycemic index have frequently been derived from (small) samples of individuals with normal glucose tolerance, impaired glucose tolerance, or type 1 or type 2 diabetes. The glucose tolerance of the individuals tested may affect the GI obtained, even when comparing with a standard reference.

Carbohydrates frequently have been divided into refined and unrefined or simple and complex carbohydrates. With respect to the glycemic index, however, these

divisions are not very useful: unrefined potato products, which are classified as complex carbohydrates, frequently have GIs over 100, whereas pasta, containing

refined carbohydrates, has a GI of around 53. Fruits, which contain simple carbo- hydrates in the form of disaccharides, also have a low GI. Similarly, whole-wheat

(unrefined) bread has a GI similar to that of white (refined) bread. The GIs of starchy foods range from higher than 130 to lower than 30 (with white bread as the refer-

ence). 32 Many potato and bread products have high values, and unprocessed grains, pasta, and legumes have low values. Summary tables of the GIs of over 750 different food items are available. 32,33

Not only the quality, but also the quantity of carbohydrates in a serving influence the postprandial glycemia. Glycemic load (GL) has been proposed to take into

account both. GL is calculated by multiplying the GI of a food with the amount of total dietary carbohydrate per serving. 32 Assessing the total glycemic effect, GL has proved useful in epidemiological studies. 34–36 Table values are currently available for a range of food items. 32 There are also problems with respect to GL; reducing the total intake of carbohydrates or dietary GI decreases GL, but the long-term effects may not be the same.

One of the problems with the GI concept is that the insulin response is not directly proportional to the glucose response. The glycemic index has explained anywhere from about 49 16 to 79% 37 of the variability. This is in line with our experience in which, in healthy subjects with normal glucose tolerance, there was no difference in the glycemic responses between wheat or different rye breads, whereas insulin responses to rye breads where markedly different, so that the same

amount of carbohydrate required less insulin. 38 Therefore, it seems that in healthy

Functional Food Carbohydrates

persons the use of GI to classify foods with respect to their effects on glucose metabolism is hampered with many difficulties, and insulin responses could be

used instead. However, even if the total glucose responses did not differ between the different types of breads, 38 the shapes of the glucose curves showed interesting patterns: 3 h after ingestion of the wheat bread, plasma glucose levels had declined below the fasting level, whereas after rye bread ingestion glucose levels were still above the fasting level. The rapid decrease of blood glucose may increase the feeling of hunger and may cause more frequent snacking, which may be relevant with respect to weight gain. 39