The term glycemic index was coined by researchers more than 25 years ago, and it offered a new way of classifying foods containing carbohydrate according to their effect on blood glucose level following a meal. Up until that point, for the purposes of diabetes meal planning, carbohydrates were divided into complex carbohydrates (or starches) and simple carbohydrates (or sugars). It was assumed that sugars had a much more pronounced effect on blood glucose level than starches, and people with diabetes were routinely told to avoid sugar. But nutrition studies in which people ate different foods and then had their blood glucose level measured showed that this assumption was incorrect. Today it is known, for example, that a bagel can raise blood glucose more than a serving of table sugar containing the same amount of carbohydrate. In other words, the bagel has a higher glycemic index.
Like any new research that shakes up long-held beliefs, information on the glycemic index of foods sparked both excitement and controversy (as well as a whole new category of popular weight-loss books and programs). Twenty-five years later, that controversy is still alive. So how useful is the glycemic index in improving blood glucose control, helping with weight loss or maintenance, and improving long-term health? This article takes a look at what’s been learned about the glycemic index in the years since the term was introduced.
How a food’s glycemic index is determined
Determining a food’s calorie and nutrient content involves only a laboratory technician and a machine. Determining a food’s glycemic index, on the other hand, involves significantly more people and more work. Normally the way it works is that a group of 5–10 volunteers first eats an amount of either glucose or white bread containing 50 grams of carbohydrate. (White bread has a glycemic index similar to that of pure glucose.) The volunteers’ blood glucose level is then measured at intervals over the next two hours, and the numbers are plotted on a graph. The points on the graph form a curved line, and the area under the curve is the food’s glycemic index.
The glycemic index of the glucose or bread is given a value of 100, and all other foods tested are compared to it and given a relative value. For example, the volunteers might next eat a bowl of cornflakes containing 50 grams of carbohydrate and have their blood glucose level similarly measured and plotted on a graph. The area under that curve is calculated for each volunteer, and based on the average of all the volunteers’ scores, cornflakes are assigned a glycemic index of 80, indicating that cornflakes cause somewhat less of a rise in blood glucose than glucose (or bread) causes.
A glycemic index of 80 is still high, however. Generally, a glycemic index of 55 or lower is considered low, a glycemic index from 56 to 69 is considered intermediate, and a glycemic index of 70 or higher is considered high.
While determining a food’s glycemic index is somewhat labor-intensive, it would seem that if enough foods were tested, it would be easy enough to construct a meal plan incorporating mostly foods with a low glycemic index, resulting in lower blood glucose levels after eating. In practice, however, it’s not that simple.
In the laboratory, study subjects eat only one test food at a time, but in real life, people usually eat meals or snacks containing several different foods. The presence of protein or fat in the accompanying foods can alter a food’s effect on blood glucose level. Some other factors that can affect a food’s glycemic index include the following:
- The botanical variety of the food. Different types of rice or potatoes, for example, have different glycemic index values.
- How the food has been processed. The more finely ground a grain, for example, the higher its glycemic index, so instant oats have a higher glycemic index than steel-cut oats. Conversely, parboiling rice lowers its glycemic index, so parboiled, or “converted,” rice products generally have a lower glycemic index than regular rice.
- How a food is prepared. For example, pasta cooked al dente has a lower glycemic index than pasta that is cooked until fully soft.
- Acidity. Acidic foods, such as sourdough bread, tend to have a lower glycemic index than similar, less-acidic foods. In addition, adding lemon juice or vinegar to a dish, such as a salad, will lower its glycemic index.
- Presence of sucrose. Table sugar (sucrose) is half glucose, which has a high glycemic index, and half fructose, which has a low glycemic index. As a result, the glycemic index of table sugar is intermediate, and foods that are high in sugar may similarly have an intermediate glycemic index.
- Fiber. Some types of fiber lower a food’s glycemic index, but some do not. Insoluble fiber, the type found in wheat, has little effect on glycemic index, so whole wheat bread has a glycemic index similar to that of white bread. Soluble fiber, on the other hand, lowers glycemic index. Foods high in soluble fiber such as oats, barley, and legumes (dried beans and lentils) have low glycemic index values.
To make matters more complicated, external factors can also alter how much a given food raises blood glucose, such as stress, time of day, prior food intake, and blood glucose level at the time of the meal; level of physical fitness; and having diabetes. (People with diabetes tend to have higher blood glucose levels in response to foods compared with people who don’t have diabetes.) All of this potential for variability has led some people to doubt the usefulness of the glycemic index in meal planning. On the other hand, studies have shown that the combination of the carbohydrate content of a meal and the source of carbohydrate in the meal accounts for 90% of a person’s blood glucose response to the meal. Paying attention to amount and type of carbohydrate, therefore, is clearly important.
What about portion size? The glycemic index of a food remains the same no matter how much is eaten. But eating a large amount of a carbohydrate-containing food will raise your blood glucose level more than eating a small amount of that food. That’s where glycemic load comes in.
Glycemic load is calculated by multiplying a food’s glycemic index by the number of grams of carbohydrate in a serving of food and dividing the result by 100. Here’s how the equation looks:
Glycemic load = (glycemic index × grams of carbohydrate) ÷ 100.
To assess the glycemic load of a small, 4-ounce apple, therefore, the calculation would be: (40 × 15 grams of carbohydrate) ÷ 100 = 6.
Similarly, for a 10-ounce slice of watermelon, the calculation would be: (76 × 15 grams of carbohydrate) ÷ 100 = 11.
A glycemic load of 10 or lower is considered low, 11–19 intermediate, and over 20 is considered high. Since both amount of carbohydrate and glycemic index are used to determine glycemic load, there are two ways to lower the glycemic load of a snack or meal: Eat a smaller portion, or choose a food with a lower glycemic index.
Glycemic index and diabetes management
With all of the variables that can affect the glycemic index of a meal, the question remains whether glycemic index values are useful in helping people with diabetes maintain blood glucose control. Two recently published reports address this question.
In 2003, one of the best-known researchers of the glycemic index and author of several books on the topic, Jennie Brand-Miller, Ph.D., reviewed 14 experiments that studied the effects of following a low-GI diet on people with diabetes. (People with both Type 1 and Type 2 diabetes were represented in the studies.) Her results showed a 0.43 percentage-point average decrease in glycosylated hemoglobin (HbA1c) level, which is a measure of blood glucose control over the preceding 2–3 months. Less than a percentage point of improvement of HbA1c may sound paltry, but in fact, other research has shown that as little as a 1 percentage-point decrease in HbA1c is associated with a reduced risk of death, heart attack, and so-called microvascular complications such as diabetic neuropathy. What’s more, the improvement seen with the use of a low-glycemic-index diet is comparable to that seen with some prescription medicines that target high blood glucose after meals.
In 2004, James W. Anderson, M.D., Professor of Internal Medicine at the University of Kentucky Graduate Center for Nutritional Sciences, and his colleagues reviewed nine randomized control trials (considered the strongest experimental research) on low-glycemic-index diets. They found that the consensus among the nine trials is that low-glycemic-index diets are associated with lower blood glucose levels, lower LDL (“bad”) cholesterol levels, lower triglycerides, and higher HDL (“good”) cholesterol levels. As a result, they recommended that information about the glycemic index be included in diabetes education and in the Exchange Lists for Meal Planning.
Currently the diabetes associations of Australia, Britain, Canada, Germany, Japan, and South Africa endorse the use of the glycemic index in meal planning. Diabetes Australia recommends eating at least three foods with a low glycemic index daily, preferably one at each meal. The World Health Organization also backs the use of the glycemic index in diabetes management. The American Diabetes Association clinical recommendations for diabetes care acknowledge the potential benefits of following a low-glycemic diet, particularly for people who currently have a high-glycemic diet.
Chronic disease and weight control
Use of glycemic index values and the concept of glycemic load may have benefits that go beyond blood glucose control. Some research—though not all—has found associations between high-glycemic-index and/or high-glycemic-load diets and cancer (breast, ovarian, endometrial, and colorectal), coronary heart disease and heart attacks, macular degeneration, high cholesterol and triglycerides, and high C-reactive protein (a marker of general inflammation in the body).
In 2006, The New England Journal of Medicine published a study that found a 30% reduction in risk of coronary heart disease among women on low-carbohydrate (and therefore low-glycemic-load) diets in which the sources of protein and fat were predominantly vegetarian. This study also found a strong positive association between high-glycemic-load diets and coronary heart disease. Since no data on lipids (cholesterol and triglycerides) were obtained, the study did not establish any association between a low-glycemic-load diet and blood lipid levels, but other research has found a beneficial effect on them.
Since many chronic diseases are linked with overweight and obesity, some researchers have examined whether low-glycemic-index, low-glycemic-load diets can affect appetite and encourage weight loss. While the previously mentioned study showed no association between low-carbohydrate diets and weight over the long term, two studies have shown that low-glycemic-index diets may be especially effective in helping women reduce waist circumference, body fat, and body weight. (In the study reported in The New England Journal of Medicine, the subjects were not following the low-carbohydrate diet to lose weight, and that may have affected results.) The direct effect of the glycemic index on body weight and body fat is still widely debated.
Yet another study using a group of both men and women showed an increased risk of Type 2 diabetes in subjects who consumed high-glycemic-index diets and did not have abdominal obesity, particularly those whose waist size increased during the study. However, the researchers did not find the same increase in risk based on dietary glycemic load or carbohydrate intake.
Appetite control is one of the main tenets of popular diet programs that advocate low-glycemic-index, low-glycemic-load diets for weight loss, but scientific research on appetite control has had conflicting results. For example, a study published in the September 2005 issue of the journal Diabetes Care in which some subjects consumed only high-glycemic-index foods and some consumed only low-glycemic-index foods reported no differences in perceived appetite or in food intake after eight days of these diets. However, this study has been criticized by other researchers who say that the glycemic index values of the foods used in the study were miscalculated, so the results are faulty.
Using glycemic index in meal planning
One of the main drawbacks to relying on the glycemic index is that the methods used to determine the glycemic index of foods are not standardized. Some research uses blood samples taken from a vein, while other research uses blood samples taken from capillaries; blood glucose levels in venous samples can vary more than in capillary samples. Some research uses white bread as the reference food, while other research uses glucose. These different methods can produce different results.
Among the general population, the glycemic response to even similar foods can vary immensely, and currently only a relatively small number of foods have been tested. Furthermore, one brand of whole wheat bread may very well have a substantially different glycemic index than its competitor, and a shopper would have no way of knowing it unless both were tested. Currently, some Australian companies are testing their food products for glycemic index and adding that information to labels of packaged goods. This may prove to be a future trend in food labeling.
Until this trend crosses the globe, however, how can one use this potentially powerful tool in meal planning? First, remember that the basics of good nutrition still apply. Foods that are mostly fat or protein have an undetectable glycemic index and may even slightly reduce the glycemic index of carbohydrate-containing meals. However, this doesn’t mean they have no potentially negative health effects. Fat is a very dense source of calories, and even healthy fats (and protein) can contribute to weight gain when consumed in excess. In addition, saturated and trans fats have negative effects on heart health and should be eaten only in moderation. Second, according to Brand-Miller, “If you can cook something more quickly, you can digest it more quickly.” This means that choosing the least processed foods will help reduce the glycemic index of your overall diet.
Consult the sidebar “High, Low, and Intermediate” for a list of low-glycemic-index foods and incorporate them into as many meals as possible, especially breakfast, which tends to raise blood glucose higher than other meals. At the same time, go easy on the highest-glycemic-index foods like most breakfast cereals (even many whole-grain varieties have high glycemic index values) and potatoes. (Check out “Glycemic Index and Glycemic Load of Select Foods” to learn more about which foods to eat in moderation.) Limiting all sources of carbohydrate to about 45% of your daily calories (but not less than 45%) will decrease the glycemic load of your diet while still providing adequate energy and nutrients. Also, since the vast majority of fruits and vegetables have a low glycemic index, this is yet another reason to eat your fruits and vegetables every day. (For more tips on planning meals using the glycemic index, see “Tips For Meal Planning.”) You can use your daily blood glucose monitoring results to determine your individual glycemic response to foods and adjust your diet accordingly. Last but not least, get moving. Physically fit people have a lower glycemic response to foods than sedentary people.
Using the glycemic index and other meal-planning tools may not be enough for everyone with diabetes to stay within target blood glucose ranges after meals. If you are particularly prone to post-meal high blood glucose, you may want to consult your physician about medicines that specifically target after-meal blood glucose levels.
Carbohydrate counting and exchanges still hold their rightful place as the first priority of meal planning for diabetes, but by incorporating low-glycemic-index foods into the equation, you may be able to reach the full potential of diabetes control through dietary intervention.