Utilization of the Glycemic Index

Several food characteristics that influence GI are summarized in Table 6-2. Broadly speaking, the two main factors that influence GI are carbohydrate type and physical determinants of the rate of digestion, such as whether grains are intact or ground into flour, food firmness resulting from cooking, ripeness, and soluble fiber content (Wolever, 1990). Intrinsic factors such as amylose:amylopectin ratio, particle size and degree of gelatinization, as well as extrinsic factors such as enzyme inhibitors and food preparation and processing, affect GI in their ability to interact with digestive enzymes and the consequent production of monosaccharides. With progressive ripeness of foods, there is a decrease in starch and an increase in free sugar content. The ingestion of fat and protein has been shown to decrease the GI of foods by increasing plasma glucose disposal through the increased secretion of insulin and possibly other hormones (Gannon et al., 1993; Nuttall et al., 1984). Significantly high correlations between GI and protein, fat, and total caloric content were observed and

TABLE 6-2 Factors That Reduce the Rate of Starch Digestibility and the Glycemic Index

Intrinsic

High amylose:amylopectin ratio

Intact grain/large particle size Intact starch granules Raw, ungelatinized or unhydrated starch Physical interaction with fat or protein

Extrinsic

Protective insoluble fiber seed coat as in whole intact grains Viscous fibers Enzyme inhibitors Raw foods (vs. cooked foods) Minimal food processing Reduced ripeness in fruit Minimal (compared to extended) storage

270 DIETARY REFERENCE INTAKES

explained 87 percent of the variation in glycemic response among foods (Hollenbeck et al., 1986). In addition to these factors, the GI of a meal can affect the glycemic response of the subsequent meal (Ercan et al., 1994; Wolever et al., 1988). Examples of published values for the GI of pure carbohydrates and other food items are shown in Table 6-3.

A number of research groups have reported a significant relationship between mixed-meal GI predicted from individual food items and either the GI measured directly (Chew et al., 1988; Collier et al., 1986; Gulliford et al., 1989; Indar-Brown et al., 1992; Järvi et al., 1995; Wolever and Jenkins, 1986; Wolever et al., 1985, 1990) or metabolic parameters such as high

TABLE 6-3 Glycemic Index (GI) of Common Foods

Food Item (White Bread = 100)

Rice, white, low-amylose 126

Baked potato 121

Corn flakes 119

Rice cakes 117

Jelly beans 114

Cheerios 106

Carrots 101

White bread 101

Wheat bread 99

Soft drink 97

Angel food cake 95

Sucrose 92

Cheese pizza 86

Spaghetti (boiled) 83

Popcorn 79

Sweet corn 78

Banana 76

Orange juice 74

Rice, Uncle Ben's converted long-grain 72

Green peas 68

Oat bran bread 68

Orange 62

All-Bran cereal 60

Apple juice 58

Pumpernickel bread 58

Apple 52

Chickpeas 47

Skim milk 46

Kidney beans 42

Fructose 32

SOURCE: Foster-Powell and Brand Miller (1995).

DIETARY CARBOHYDRATES: SUGARS AND STARCHES 271

density lipoprotein cholesterol concentration that are known to be influenced by GI (Liu et al., 2001). Although the glycemic response of diabetics is distinctly higher than that of healthy individuals, the relative response to different types of mixed meals is similar (Indar-Brown et al., 1992; Wolever et al., 1985). The prediction of GI in mixed meals by Wolever and Jenkins (1986) is shown in Figure 6-1. In contrast, some studies reported no such relationship between the calculated and measured GI of mixed meals (Coulston et al., 1984; Hollenbeck et al., 1986; Laine et al., 1987).

There are a number of reasons why different groups have reported different findings on the calculation of GI in mixed meals. As previously discussed, there are a number of intrinsic (e.g., particle size) and extrinsic (e.g., ingestion of fat and protein, degree of food preparation) factors that can affect the glycemic response of a meal (Table 6-2), some of which are known to also affect the absorption of other nutrients such as vitamins and minerals. For instance, coingestion of dietary fat and protein can sometimes have a significant influence on the glucose response of a carbohydrate-containing food, with a reduction in the glucose response generally seen with increases in fat or protein content (Gulliford et al., 1989; Holt et al.,

0 100 200 300

Incremental Plasma Glucose Area (mg/dl-h)

FIGURE 6-1 Correlation between calculated glycemic index (GI) of four test meals (•) and incremental blood glucose response areas. Based on data from Coulston et al. (1984). Reproduced, with permission, from Wolever and Jenkins (1986). Copyright 1986 by the American Society for Clinical Nutrition.

272 DIETARY REFERENCE INTAKES

1997). Palatability can have an influence on GI, independent of food type and composition (Sawaya et al., 2001). Furthermore, there are expected inherent biological variations in glucose control and carbohydrate tolerance that are unrelated to the GI of a meal. Finally, varied experimental design and methods for calculating the area under the blood glucose curve can result in a different glycemic response to meals of a similar predicted GI (Coulston et al., 1984; Wolever and Jenkins, 1986). For instance, it is important that the incremental area, rather than the absolute area, under the blood glucose curve be measured (Wolever and Jenkins, 1986). Taken together, the results from these different studies indicate that the GI of mixed meals can usually be predicted from the GI of individual food components.

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