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The Glycemic Index/Load Sarah K. Khan, RD, MPH, PhD At present, the American Diabetes Association (ADA)1 does not recognize a role for the glycemic index in the prevention of disease. On the basis of its review, diets with a low glycemic index may reduce postprandial glycemia but may be difficult to maintain long term. The ADA holds that additional evidence to support the glycemic and lipid benefits of low– glycemic index diets is needed.1,2 Data from a later review of the literature, however, suggest that glycemic index information should be incorporated into exchanges and teaching materials3 specifically for disease prevention of diabetes, ­cardiovascular disease, inflammation, and, possibly, some cancers. In this chapter, the definitions of glycemic index and glycemic load are reviewed, and studies demonstrating their possible use in disease prevention are provided.

Glycemic Index The glycemic index (GI) has proved to be a useful nutritional concept—the chemical classification of carbohydrate (simple or complex, sugars or starches, available or unavailable)— that fosters new insights into the relationship between the physiologic effects of carbohydrate-rich foods and health.4 The GI measures how quickly a consumed carbohydrate affects postprandial serum glucose levels in a specified time. By definition, the GI compares equal quantities of available carbohydrate in foods and provides a measure of carbohydrate quality. Available carbohydrates can be calculated by summing the quantity of available sugars, starch, oligosaccharides, and maltodextrins.5 In effect, the GI is an indicator of the relative glycemic response to dietary carbohydrates. Glucose and white bread are often used as the “gold standard” because they cause the fastest and most dramatic rise in glucose levels. In the evaluation of individual foods, either glucose or white bread is assigned a value of 100, the highest index possible. All other foods are then assigned proportionately lower values on the basis of how they affect serum glucose levels in comparison with glucose or white bread.6 The GI is now widely recognized as a reliable, physiologically based classification of foods according to their postprandial

glycemic effects.4 For example, in healthy individuals, stepwise increases in GI have been shown to predict stepwise elevations in postprandial blood glucose and insulin levels (Fig. 85-1).5,7 The GI measures how quickly a consumed carbo­ hydrate affects postprandial serum glucose levels in a specified time.

Glycemic Load To provide a more accurate description of the quantity and quality of carbohydrate in a meal simultaneously, researchers developed the concept of the glycemic load (GL), which takes the concept of GI a step further, accounting not only for how rapidly a food's carbohydrates are converted to glucose but also the relative amounts of carbohydrate the food contains in an average serving. GL is generally held to be a more accurate measure of a food's overall effect on pancreatic insulin release and serum glucose levels. The GL of a food is calculated by multiplying the GI value by the amount of carbohydrates in grams provided by a serving of food and dividing the total by 100.5 Therefore, the GL provides a summary measure of the relative glycemic impact of a “typical” serving of the food. In general, items with a low GI tend to have a correspondingly low GL. However, foods with a high GI may vary as to whether their GL is low or high. For example, the carbohydrates in watermelon are rapidly converted to glucose, so watermelon's GI is high at 72. However, because watermelon is made up primarily of water and contains little absolute carbohydrate content, its GL is relatively low at a value of 4.4 Several prospective observational studies have documented an independent association between a long-term consumption of a diet with a high GL and a higher risk for development of type 2 diabetes mellitus, cardiovascular disease, and certain cancers.4 In another study, implementation of a low-GL diet was associated with substantial and ­sustained improvements in abdominal obesity, cholesterol 789

790   Part Three  Tools for Your Practice: Nutrition

c­ oncentration, and glycemic control.8 A low-GL diet has been associated with more weight loss in young adults who are insulin resistant.9 Use of GL to guide dietary choices has been found to have several benefits. High-GL foods can often lead to rapid release of large amounts of insulin, which can ultimately cause blood glucose levels to fall below fasting levels a few hours after eating. This rebound hypoglycemia can be characterized by fatigue, which decreases substantially when high-GL foods are removed from the diet. Interestingly, a low-GI meal leads to a lower glycemic response to subsequent meals as well.10 On average, people who eat low-GL diets tend to eat smaller meals; their food cravings diminish. Diets of predominantly high-GI foods have been associated with a higher risk of insulin resistance syndrome and type 2 diabetes mellitus (Tables 85-1 and 85-2).11 Figure 85-1

Mean incremental blood glucose responses in healthy subjects 65 to 70 years of age. (Modified from Bjorck I, Granfeldt Y, Liljeberg H, et al. Food properties affecting the digestion and absorption of ­carbohydrates. Am J Clin Nutr. 59[suppl]:699 S–705 S, 1994.)

Glucose (mmol/l Blood)

3.0

Consumption of white bread Spaghetti 1.8 mm Thin linguine 2.2 x 1.2 mm Thin linguine w/egg Thick linguine 2.2 x 3.3 mm

2.5 2.0 1.5 1.0 0.5 0.0

0

20

40

60

80

100 120 140 160 180

Time (Minutes)

Table 85-1. Food Rating Values for Glycemic

Index and Glycemic Load

VALUE

GLYCEMIC INDEX

GLYCEMIC LOAD

High

>70

>20

Medium

56-69

11-19

Low