FAO/WHO expert consultation on carbohydrates in human nutrition

27

TABLE 4

Calculation of the glycemic index of meals

		Grams		Proportion of		Food		Meal Glycemic
Food		Glycemic		total Glycemic		Glycemic		Index*
		Carbohydrate		Carbohydrate		Index
Bread	25		0.342		100		34.2
Cereal	25		0.342		72		24.6
Milk	6		0.082		39		3.2
Sucrose	5		0.068		87		5.9
Orange juice	12		0.164		74		12.1
TOTAL	73						80.0

* Values for each food equals the proportion of total glycemic carbohydrate multiplied by the food GI. The sum of these values is the meal GI.

Using this type of calculation, there is a good correlation between meal GI and the observed glycemic responses of meals of equal nutrient composition. Blood glucose responses are also influenced by the amount of carbohydrate in the meal. To compare the expected glycemic load of meals with different carbohydrate contents, a non-linear adjustment can be applied (86), but this has only been tested in normal subjects.

For detailed application of the GI, a value of the GI for every food in the diet or meal needs to have been assigned (for many foods the value has to be estimated). The accuracy of the calculation depends upon the accuracy of the GI values ascribed to foods, which may vary from place to place due to local factors such as variety, cooking, processing, etc. Foods particularly prone to such variation include rice, potatoes and bananas.

Practical application of the glycemic index

The glycemic index can be used, in conjunction with information about food composition, to guide food choices. For practical application, the glycemic index is useful to rank foods by developing exchange lists of categories of low glycemic index foods, such as legumes, pearled barley, lightly refined grains (e.g. whole grain pumpernickel bread, or breads made from coarse flour), pasta, etc. Specific local foods should be included in such lists where information is available (e.g. green bananas in the Caribbean and specific rice varieties in Southeast Asia).

In choosing carbohydrate foods, both glycemic index and food composition must be considered. Some low GI foods may not always be a good choice because they are high in fat. Conversely, some high GI foods may be a good choice because of convenience or because they have low energy and high nutrient content. It is not necessary or desirable to exclude or avoid all high GI foods.

Physiologic and therapeutic effects of low glycemic index foods

Meals containing low GI foods reduce both postprandial blood glucose and insulin responses. Animal studies suggest that incorporating slowly digested starch into the diet delays the onset

28	Chapter 4 The role of the glycemic index in food choice

of insulin resistance. Some epidemiologic studies suggest that a low GI diet is associated with reduced risk of developing non-insulin diabetes in men (87) and women (88). Clinical trials in normal, diabetic and hyperlipidemic subjects show that low GI diets reduce mean blood glucose concentrations, reduce insulin secretion and reduce serum triglycerides in individuals with hypertriglyceridemia (89,90,91,92). In addition, the digestibility of the carbohydrate in low GI foods is generally less than that of high GI foods. Thus, low GI foods increase the amount of carbohydrate entering the colon and increase colonic fermentation and short chain fatty acid production. This has implications for systemic nitrogen and lipid metabolism, and for local events within the colon.