Carbohydrates are an important nutrient and a significant part of a balanced diet. Experts generally encourage people to build their diets around nutrient-dense carbohydrates, incorporating a moderate amount of protein and some fat. Exercise and weight control are key elements of a healthful lifestyle to help reduce risk for heart disease, diabetes, and high blood pressure.
About 20 years ago a group of researchers developed the glycemic index as a tool to measure how much a carbohydrate-containing food is likely to raise blood sugar. Since that time “glycemic index” has become a popular nutritional catchphrase and the basis for some popular “magic bullet” diets that advocate strict limitations on carbohydrate intake. The National Academies’ Dietary Reference Intake for Energy, Carbohydrates, and Fiber states, “Research is needed to determine the effect of low glycemic index foods and low glycemic load diets on serum lipids and other risk factors for chronic disease and complications, especially in high risk groups.” Is it really necessary to “eat by the numbers” to lose or maintain weight? Here are some answers to questions about the glycemic index, glycemic load, and their relationship to diet and health.
What is the Glycemic Index?
Almost all carbohydrates (sugars and starches), regardless of the form in which they are consumed, are digested to glucose which enters the bloodstream, causing a temporary rise in blood glucose levels. This glycemic response is influenced by many factors, including how much food is eaten, the amount and type of carbohydrate in the food, how the food is processed, or how it is prepared. Under laboratory conditions, glycemic index (GI) is assessed by having a group of people eat a specific amount of a single food (usually 50 grams of digestible carbohydrate, i.e., total carbohydrate minus fiber) and then measuring the change in blood sugar levels compared with the levels achieved after they have eaten a control food (either glucose or white bread) containing the same amount of digestible carbohydrate. The International Table of Glycemic Index and Load lists the values of both control foods. The average change in blood sugar levels over a set period of time, relative to the levels after consumption of the control food, is the food’s glycemic index.
The index assigns foods a number on a scale of one to 100, in comparison with pure glucose, which has a reference score of 100. Regular pasta has a glycemic index of 41, meaning that it raises blood sugar levels by less than half (41 percent) of what an equal amount of glucose does.
What Is Glycemic Load?
Glycemic load (GL) is a related concept that considers both the glycemic index and the amount of carbohydrate in a typical serving of food, giving a somewhat more accurate picture of a food’s effect on blood sugar. For example, carrots have a relatively high glycemic index because the carbohydrates in carrots are mostly sugars. But a serving of carrots contains a low amount of carbohydrate compared to high GI foods such as bread and potatoes. To calculate glycemic load, you multiply the grams of carbohydrate in a serving of food by that food’s glycemic index. Carrots, which have a glycemic index of 71, have 8 grams of carbohydrates per serving and are assigned a glycemic load of 6 (8 x 0.71).
It’s Not That Simple
On the surface GI and GL sound like straightforward concepts for enabling meal and diet planning, but experts say it’s not that simple. Here’s why.
The GI value of a food varies depending on how ripe it is, its variety and how it is cooked and processed. Also, GI is determined using standard test portions of foods, which are not the usual portion sizes that people consume.
Many variables affect GI calculations, including the fact that the GI of a food depends on whether it is eaten alone or consumed with another food. “In the real world, people eat meals and diets, not single foods,” says Susan Borra, a registered dietitian and president of the International Food Information Council Foundation. A high GI food combined with a low GI food produces a moderate response. Think of spreading some strawberry jam on whole wheat bread with peanut butter. The protein, fat, and fiber in the sandwich temper the relatively high GI of the jam.
What about using glycemic load instead of GI? Again, it’s not that simple, since the GL calculation begins with a food’s glycemic index and thus includes all the inherent uncertainties that pertain to GI.
GI’s rising popularity has been fueled by claims that low GI foods can help control appetite, weight, and may be useful to people with diabetes. These claims are based on the theory that high GI foods raise blood sugar levels, cause excess insulin to be secreted, and lead to the storage of fat. However, the American Dietetic Association (ADA) says: “At this time, research does not support the claim that a low GI diet causes significant weight loss or helps control appetite.” For people with diabetes, monitoring total grams of carbohydrate continues to be the key strategy, ADA says, although the GI concept may supplement blood glucose monitoring to “fine-tune” food choices and modestly improve blood glucose levels.
A recent recommendation for people with diabetes from the American Diabetes Association, published in the September 2006 issue of Diabetes Care says, “The use of glycemic index and load may provide a modest additional benefit over that observed when total carbohydrate is considered alone.”
Think Nutrient Density
GI and GL calculations depend on too many factors to enable science-based meal planning. However, most nutrition scientists concur that a food’s nutrient density (a measure of the proportion of nutrients a food supplies to the calories it contains) is more important than the glycemic index. The good news is that consumers can plan healthful meals and diets without complicated calculations for each item on their dinner plates.
It is helpful to remember that many low glycemic index foods are naturally high in fiber. They take longer to digest, and may, therefore, be more satisfying. Often they are less energy dense than their higher GI counterparts. These are the very foods—whole grains, fruits, vegetables, legumes—that nutrition experts have always recommended and continue to emphasize as the cornerstones of a healthful diet.
The Bottom Line
Over its 20-year history the glycemic index has generated much interest and controversy. A number of hypotheses relate high GI foods to increased blood sugar and insulin levels, suggesting the potential for increased disease risk. While worthy of further investigation, these theories remain unproven. Also, concerns about the measurement and reliability of GI have yet to be resolved. Glycemic index may have the potential to be a valuable clinical tool, but it is clear that more research, including long-term clinical studies, is needed to prove its worth in reducing risk of disease.
Prevailing nutrition wisdom is represented by the 2005 Report of the Dietary Guidelines Advisory Committee which states, “Current evidence suggests that the glycemic index and/or glycemic load are of little utility for providing dietary guidance for Americans.”
