The Glycemic Index: How Useful Is It?

Several popular diets, such as the South Beach Diet and Sugar Busters, are based on the glycemic index; however, many patients are confused about its significance. Here I discuss how to interpret the glycemic index and the glycemic load, which is derived from the index. I also show how the index can be used to provide nutritional guidance to patients.

THE GLYCEMIC INDEX: A QUICK OVERVIEW

Simply stated, the glycemic index is a numeric ranking of carbohydrates based on the increase they induce in blood glucose levels. Carbohydrates have typically been classified as simple or complex. However, some nutritionists believed that this classification could be misleading. Because carbohydrates include fiber as well as sugar, some complex carbohydrates actually raise blood glucose levels faster than do simple carbohydrates. Therefore, about 25 years ago, the glycemic index was developed as an alternative to the "simple/complex" classification.

INTERPRETATION OF THE GLYCEMIC INDEX

What the numbers mean. The glycemic index of a food is calculated by measuring the change in blood glucose level that occurs after a person who has been fasting consumes enough of that food to supply 50 g of carbohydrate. The magnitude of the change is then compared with the blood glucose response associated with a standard high-carbohydrate food (typically glucose or white bread), which is given a ranking of 100.

Carbohydrates that break down quickly during digestion cause a rapid rise in blood glucose levels and, consequently, have a high glycemic index. Carbohydrates that break down slowly, releasing glucose gradually into the bloodstream, result in a slow rise in blood glucose level and thus have a low glycemic index.

Low: ≤ 10
Moderate: 11 - 19
High: ≥ 20

In general, glycemic index values of 70 or more are considered high, values between 56 and 69 are considered moderate, and values of 55 or less are considered low when glucose has a glycemic index of 100 (Table 1). Examples of foods with a low glycemic index include beans, yogurt, many fruits, and whole grains. Processed grains and potatoes have some of the highest glycemic indices.

Clinical implications. The rapid increase in blood glucose levels caused by foods with a high glycemic index leads to insulin secretion; this can ultimately result in fat deposition. In addition, the rapid rise in blood glucose and the subsequent sudden increase in insulin sometimes lead to an abrupt decrease in blood glucose. This rapid drop in blood glucose can produce a craving for more carbohydrates, which often leads to ingestion of additional calories and subsequent weight gain.

In contrast, foods with a low glycemic index may improve the body's sensitivity to insulin, resulting in reduced insulin demands. A reduction in insulin demand may improve glucose control in patients with diabetes. Foods with a low glycemic index may also promote satiety, thereby decreasing caloric intake and subsequent weight gain.

LIMITATIONS OF THE GLYCEMIC INDEX

A number of factors undercut the utility of the glycemic index. First, the index is not listed on food labels, so patients must search for the information online or in reference books. Often, they will find significant variation among the values assigned to the same food by different researchers (a result, in part, of the need to rely on measurement of reactions in live volunteers). Moreover, the glycemic index of many foods has not been determined. In addition, the index pertains only to single foods eaten individually, which is not typically how people eat. Most people eat several types of food (eg, meat, vegetables, noodles) at one time; the glycemic index of a meal such as beef stew can be difficult to calculate. The way in which foods are prepared also affects their glycemic index. Baking, frying, grilling, and steaming all result in slightly different indices for the same food.

Finally, the glycemic index is not based on portion-sized quantities. As noted previously, it is based on whatever amount is required to supply 50 g of carbohydrate; for many foods, this does not equal a realistic serving size. Consequently, use of the glycemic index can be misleading to both patients and physicians and can result in misguided food choices. For foods that have a high glycemic index but contain only a small amount of carbohydrate, the high index overstates their effect on blood glucose levels. Conversely, the glycemic index of foods that contain a high percentage of carbohydrate may underestimate their glycemic effect when consumed in typical quantities.

A number of fruits are good examples of foods whose glycemic index poorly represents their appropriateness for a healthful diet. Even though some fruits have a moderately high glycemic index, typical serving sizes of most of them do not cause a surge in blood glucose levels. For example, pineapple has a glycemic index of 66, but you would have to eat 1 3/4 lb of pineapple at a sitting to affect your blood glucose level to the degree predicted by the index.

GLYCEMIC LOAD

The concept of "glycemic load" was developed to correct the serving-size problem that detracted from the practical value of the glycemic index. Glycemic load values provide a measure of the actual glycemic response to typical servings of foods. The glycemic index is multiplied by the amount of carbohydrate in a typical serving of food to produce the glycemic load; this value indicates how likely the serving is to cause a rapid rise in blood glucose. The lower a food's glycemic load, the less likely a serving of it is to cause blood glucose levels to rise quickly. The following formula is used to calculate glycemic load:

Glycemic load = glycemic index (expressed as a percentage) × grams of carbohydrate per serving

Let us look at how this formula applies in the case of popcorn. The glycemic index ranking of popcorn is 72, which is high. However, a typical serving of popcorn is 20 g, which contains only 11.1 g of carbohydrate. Thus, the glycemic load of popcorn is .72 × 11.1, or 8; this is low.

Glycemic loads of 20 or more are considered high; those between 11 and 19 are considered moderate; and those of 10 or less are considered low (see Table 1). The glycemic index and glycemic load of some common foods are listed in Table 2.

RESULTS OF RECENT TRIALS

Use of the glycemic index and glycemic load values may be particularly helpful to patients who have diabetes or who are at risk for diabetes. Several studies have suggested an association between the glycemic index or glycemic load of foods eaten and a patient's glucose tolerance. In the Nurses' Health Study, women whose diets provided the highest glycemic loads were more likely to develop type 2 diabetes than women whose diets provided the lowest glycemic loads.¹

A recent study published in Diabetes Care showed that a low­glycemic index diet improved glycemic control and some lipid profiles, as well as fibrinolysis, in men with type 2 diabetes.² Twelve men with type 2 diabetes were randomized to two 4-week periods of either a low­glycemic index diet or a high­glycemic index diet; a 4-week washout interval separated the 2 periods during which they ate a specialized diet. At the end of the study, the low­ glycemic index diet resulted in reductions in postprandial glucose and insulin levels, fasting plasma glucose concentrations, and hemoglobin A_1c levels. The low­glycemic index diet was also associated with a decrease in fasting plasma total cholesterol and low-density lipoprotein cholesterol levels, apolipoprotein B levels, and plasminogen activator inhibitor 1 activity.

Although low­glycemic index diets are often recommended as an effective way to lose weight, it is notable that neither diet used in the Diabetes Care study was associated with significant weight loss.

In a study of the effects of a low­glycemic load diet on heart disease risk factors and resting energy expenditures during weight loss, researchers found that low­glycemic load eating patterns are less likely to slow down metabolism and are more likely to improve cardiac risk factor profiles than a conventional low-fat diet. Pereira and colleagues³ studied 39 overweight patients; 22 followed a low­glycemic load diet while 17 followed a low-fat diet. After all participants had lost 10% of their body weight, patients in the low­glycemic load group demonstrated a greater decrease in triglyceride levels, blood pressure, insulin resistance, and C-reactive protein levels than those in the low-fat group. However, there was no significant difference between the rate of weight loss in those who followed a low­glycemic load diet and those who followed a low-fat diet.

These studies are encouraging. Still, much more evidence is needed before definitive conclusions can be made about the proper role of a low­ glycemic index diet in the prevention or management of diabetes, heart disease, or obesity.

HOW TO ADVISE PATIENTS

Diets that focus on low glycemic index rankings are more moderate than some other diets; for example, they are less "anti-carbohydrate" than Atkins-type diets and not as fat-restrictive as some popular low-fat diets. However, even when the more practical glycemic load values are used, this approach may be too complex for many patients. Many of the principles associated with the glycemic index and glycemic load can be distilled into a few salient points (points that are also included in the recent US Department of Agriculture Dietary Guidelines⁴). Instead of recommending a particular diet, advise patients to focus on moderation and to follow the basic principles listed in Table 3 as an aid to making healthful choices.

References:

REFERENCES:

1.

Wu T, Giovannucci E, Pischon T, et al. Fructose, glycemic load, and quantity and quality of carbohydrate in relation to plasma C-peptide concentrations in US women.

Am J Clin Nutr.

2004;80:1043-1049.

2.

Rizkalla SW, Taghrid L, Laromiguiere M, et al. Improved plasma glucose control, whole-body glucose utilization, and lipid profile on a low-glycemic index diet in type 2 diabetic men: a randomized controlled trial.

Diabetes Care.

2004;27:1866-1872.

3.

Pereira MA, Swain J, Goldfine AB, et al. Effects of a low-glycemic load diet on resting energy expenditure and heart disease risk factors during weight loss.

JAMA.

2004;292:2482-2490.

4.

Dietary guidelines for Americans 2005. Available at: http://www. healthierus.gov/dietaryguidelines. Accessed March 4, 2005.

5.

Foster-Powell K, Holt SH, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002.

Am J Clin Nutr.

2002; 76:5-56.