With diabetes, a controlled diet is one of the most important factors to maintain normal glucose (blood sugar). The glycemic index (GI) was developed as a research tool to examine how blood glucose is affected by foods that contain carbohydrates.
The glycemic index ranks food with a value based on how quickly that food is converted to sugar in the blood and how fast the glucose returns to normal.
Foods with a high GI score are digested and absorbed by the body more quickly, causing a rapid rise in blood glucose. Some research has found diets with a high glycemic load to be a risk factor for type 2 diabetes. Foods with a low GI score are converted to sugar more slowly and do not cause a rapid spike in blood glucose.
GI scores should not be confused with how nutritious a particular food is. The score is ranked solely for the rate at which it raises glucose levels.
In general, refined starches and sugars have a high GI, legumes and whole grains have a moderate GI, and many fruits and vegetables have a low GI. As with fats, scientists categorized foods as “good” and “bad” carbohydrates. In theory, people with diabetes should choose more of the low-glycemic foods at meals to keep glucose levels stable. Using the glycemic index for meal planning, however, is complicated, and it may not adequately control glucose.
Certain factors may change the final GI score of a food depending on how it is prepared and its reaction with other foods. In addition, the GI of a specific food may vary from individual to individual, and many foods do not have a GI, which can make it difficult to plan a meal using the glycemic index as a guide. It is important to note the GI of a food eaten alone may change when added to a meal.
Although some scientists believe that a low-glycemic diet may help control glucose and fat (lipids) in people with diabetes, the results are inconclusive. There is not enough evidence of long-term benefits to recommend use of the glycemic index for diabetic care. The glycemic index can be used in meal planning but should not be the only consideration when making food choices. According to the American Diabetes Association, for people with diabetes, foods may have a different GI than stated, from day to day, based on the individual’s glucose reading and insulin resistance.
The use of low-GI diets remains controversial in the treatment of diabetes. If a person with diabetes is interested in using the glycemic index for dietetic reasons, this should be done with the assistance of a physician or registered dietitian.
About glycemic index
Up until several years ago, scientists believed that carbohydrates converted to sugar in the body according to their type. Complex carbohydrates, such as potatoes or whole grain bread, were thought to break down into sugar more slowly. Simple carbohydrates, such as an orange or apple, were believed to turn into sugar more quickly. With research, however, it appears that the way food is converted into sugar is more complicated.
When a person eats a piece of food, it is broken down and digested in the body. The speed at which the food raises glucose (blood sugar) is called the glycemic response. Researchers studied the glycemic response of foods and developed the glycemic index (GI), which is now used to rank more than 1,000 foods.
To determine the glycemic index of a specific food, scientists used two methods. In one, they measured the time it takes for blood glucose levels to be affected after the food is eaten compared to the glycemic index of a slice of white bread, which is ranked 100. In the second way, scientists used glucose, which is given a rank of 143. Foods that are given a GI of less than 100 are converted into sugar more slowly than white bread or glucose. Foods with a GI greater than 100 turn into sugar more quickly. In general, the high-GI foods cause a rapid jump in blood glucose, whereas the low-GI foods raise blood glucose at a slow, steady pace.
|Less than 55||Low|
|55 to 70||Intermediate/moderate|
|More than 70||High|
The benefits of using the glycemic index for preventing or controlling diabetes are not entirely clear. Some researchers believe that eating low-GI foods will help keep levels of blood glucose steady, lower blood fats (lipids) and reduce the risk for obesity. Others believe that the GI of foods is not accurate enough to determine the effect on glucose because too many factors not reflected in the GI may cause different reactions in individuals. Also, consistent pre– and post–testing of meals is necessary to determine whether some foods raise blood glucose levels more than others.
Recent research has produced some support for the glycemic index. For example, the long-term, large-scale Nurses’ Health Study, Nurses’ Health Study II and Health Professionals Follow-up Study indicated a lower risk of insulin resistance and type 2 diabetes in women and men who followed a low-GI, high-fiber diet than those who ate a lot of high-GI foods such as potatoes, white bread, white rice and soft drinks. Some research has also suggested that a low-GI diet may reduce the risk of diabetic heart disease. And some scientists have blamed increased consumption of high-GI foods in the increasing rates of childhood obesity, adult obesity and type 2 diabetes.
Yet some other studies have found no link between high-GI foods and hyperglycemia, and the Insulin Resistance Atherosclerosis Study found no relationship between the glycemic index and diabetic risk factors such as insulin sensitivity and obesity.
According to the American Dietetic Association:
- Science does not show that a low-GI diet reduces appetite or results in significant weight loss.
- The primary method diabetic patients should use is tracking grams of carbohydrates.
- Using the GI in conjunction with glucose monitoring may help some people modestly improve after-meal (postprandial) glycemic control.
In its updated dietary guidelines issued in 2006, the American Diabetes Association stated that there was insufficient evidence that low-GI diets prevented type 2 diabetes but that low-GI foods rich in fiber and nutrients were encouraged. It also said that switching from a high-GI to a low-GI diet could yield a modest reduction in a person’s postprandial hyperglycemia.
Patients should seek advice from a physician or registered dietitian before choosing to follow a GI diet. Decisions on foods must be made on the basis of overall nutrition, as well as the effect on glucose. Only knowledgeable professionals can determine if the use of the glycemic index is beneficial for an individual with diabetes. Furthermore, there are times when diabetic patients need to consume high-GI foods, such as eating an amount of sugar specified by their physician to treat an episode of hypoglycemia.
Glycemic index in foods
The body converts all carbohydrates into glucose (blood sugar), which is burned for energy or stored in the body. In the theory of the glycemic index, the faster the carbohydrates are broken down by the digestive system, the faster they travel to the blood and the quicker the glucose rises.
The increased glucose in the bloodstream causes greater production of insulin, which may over time stress the pancreas, and a person with diabetes must handle the extra glucose in the body. The rapid rise in glucose (hyperglycemia) may result in a burst of energy quickly followed by hypoglycemia and hunger. This cycle is not considered healthy.
Examples of high-glycemic foods with a GI over 70 include:
- White bread
- White potatoes
- Some cereals
- Snacks such as corn chips, potato chips and pretzels
- Honey and jams
Although high-glycemic foods may be considered the “bad” carbohydrates, some of the foods are valuable in the diet. People with diabetes need not abstain from high-GI foods. Instead, they are advised to consider high-GI foods that are more nutritious and lower in calories. For example, a high-fiber cereal may have a GI index of 80. However, it provides more nutrients than corn chips, which have a GI of 72.
Some carbohydrates cause a rise in glucose that is not as dramatic as with the high-glycemic foods. These foods are categorized as having an intermediate/moderate glycemic index. The GI for this category is 55 to 70 and includes:
- Some cereals
- White and brown rice
- Crackers and whole-grain breads
- Canned fruits
- Table sugar
Along with the high-GI foods, it is important for people with diabetes to realize that the foods in this category are not all the same. Some foods should be eaten sparingly as they have the same GI but little nutritional value. Table sugar and beets both have a GI of 65, but beets contain far more nutrients.
The food that produces the most gradual rise in glucose is placed in the low-glycemic group. These foods are considered the easiest on the body and may control hunger for a longer period of time. Foods with a GI lower than 55 include:
- Green vegetables
- Most fresh fruit
- Whole grains and beans
- Some fruit juices
In general, the low-glycemic carbohydrate foods are lower in fat and higher in fiber. There are still some foods in this category that contain empty calories. Peanut candies and frozen lima beans have the same GI of 32, but the lima beans are more beneficial in nutrients and fiber.
Using the glycemic index can be complicated and confusing. In addition to checking the GI value, the individual must consider the type of food, its nutrients and the overall diet. There are several factors beyond just the GI number that influence the effect of foods on glucose. These factors include:
- Region where an individual lives. For example, foods are processed differently in different parts of the United States, and an item such as rice can be grown in many ways depending on which part of the world the rice is coming from.
- The way the food is prepared (e.g., cooked, chopped, mashed).
- How it combines with other food (e.g., the protein and fat–containing food in a meal).
- The type of carbohydrate in the food.
- The amount of processing the food item has undergone.
- The ripeness of the fruit or vegetable.
- Fiber, fat and protein content of the food.
- The individual’s glucose level before eating.
- How quickly the body digests food (this varies from person to person).
- The individual’s activity level.
Knowing the general glycemic index of foods is therefore not enough information to plan a safe diabetic diet.
Glycemic index and glycemic load
The glycemic index (GI) measures only how quickly a particular carbohydrate turns into glucose (blood sugar) in the body. It does not show how much of the carbohydrate is in a certain food, or the influence on glucose of a variety of foods consumed in a single meal. Some foods have high-GI carbohydrate but do not have a great deal of it. It is important to know both of these factors:
- How much carbohydrate is in a food
- How quickly it converts into glucose
The glycemic load (GL) calculates this information to more clearly determine the effect of food on blood glucose. The glycemic load of food is calculated using the following formula:
- Take the available amount of carbohydrates in a serving of food (total carbs minus fiber).
- Multiply that number by the food’s GI.
- Divide by 100 for glycemic load.
For a half-cup serving of carrots, the GL would be calculated as:
8 (grams of carbohydrates) X 131 (GI) = 1048 / 100 = 10.4.
For a half-cup serving of white rice, the GL would be calculated as:
35 (grams of carbohydrates) X 81 (GI) = 2835 / 100 = 28.3.
Based on these calculations, if people chose foods using only the glycemic index, they might avoid some nutritious items. The carrots have a higher GI than rice but are actually better for the individual. This comparison demonstrates the importance of using the glycemic load rather than just the glycemic index in choosing food. A lower glycemic load means fewer carbohydrates and more nutrients and fiber. In general, foods with a lower glycemic load provide a better diet.
Glycemic Index vs. Glycemic Load
Table of Common Foods
|Food (one serving)||Carbohydrate content (grams)||Glycemic index||Glycemic load|
* lower than 131 according to some calculations
Ranges for GI and GL
|High||70 or more||20 or more|
|Medium||56 to 69||11 to 19|
|Low||55 or less||10 or less|
Calculating the GL is time-consuming and not always easy. It may be difficult to determine how many carbohydrates are in a serving of food or if that number changes in preparation and in combination with other foods, such as protein. This is another reason use of the glycemic index is viewed with caution when dealing with people with diabetes and why people with diabetes should consult a registered dietitian for nutrition counseling.
Estimating personal glycemic index
The glycemic index (GI) that is assigned to foods is an average of the glycemic responses of many individuals. It is possible for people with diabetes to measure their own personal response to a food to determine how it affects the glucose (blood sugar) level. By testing individually, people with diabetes are able to get their own glycemic response for specific foods. The process requires consistent, timed blood testing that is usually more involved than what would be done for normal glucose monitoring.
The first step in determining how food affects the glucose level requires testing glucose immediately before and two hours after eating. This reading will give the person with diabetes a general idea of the way the food raises glucose. However, because different foods raise the glucose level at different speeds, this testing may not provide enough information. Eating an apple may peak the blood glucose at 60 minutes, whereas a piece of bread may peak at 90 minutes.
The way to overcome this problem is to test more often after eating a specific food. By testing at numerous times, the person with diabetes will produce a glucose curve. This curve will show the baseline, when the blood glucose peaks and when it returns back to the baseline.
To be most accurate, the testing of foods should be done at different times of the day. A person may respond differently to a food in the morning or after activity. If a person chooses to do glycemic testing, it should be done at the time when the food is usually eaten.
The difficulties in calculating personal glycemic responses for food include:
- Multiple finger sticks and testing (always use same blood source – either capillary or vein)
- Time consuming
- Must follow strict schedule for accurate results
- Responses may vary for many reasons
- Questionable accuracy due to variables with testing
Glycemic testing can be a difficult process with less-than-accurate results due to the variables. If a person with diabetes wishes to calculate the GI for foods, this should be done after consulting medical professionals for specific recommendations and guidelines.
Future of glycemic index
Research continues on using the glycemic index in the treatment of diabetes. Scientists differ in opinions regarding its effect on controlling glucose (blood sugar), losing weight and preventing obesity. Some researchers have pushed to have the glycemic index placed on food labels. Others would like the carbohydrate information on labels replaced with information about glycemic index and load.
Because food testing for the glycemic index is not universal, this information is not placed on labels. There are contrasting views about the glycemic index, and the use of a low-GI diet for diabetes remains controversial. More carefully designed research studies are necessary to develop an understanding of the role of the glycemic index in a healthy diet.
Questions for your doctor about glycemic index
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about the glycemic index:
- Do you recommend the glycemic index for me?
- If the glycemic index cannot be my primary dietary guideline, can it be a helpful supplement for controlling my diabetes?
- Can using the glycemic index reduce my frequency of checking my glucose, or will I need to maintain my schedule of glucose monitoring?
- What do I need to know about factors that affect the glycemic index of food?
- If I monitor the glycemic index of the food I eat, do I need to continue following the prescribed diet given to me, or can it be adjusted?
- What do I need to know about the difference between glycemic index and glycemic load?
- How do I calculate glycemic load?
- How can I estimate my personal glycemic index?
- If I use carb counting as a measure to control my glucose, can I still use glycemic index to measure food values as well?
- If I have prediabetes, insulin resistance, a family history of diabetes or other risk factors, can the glycemic index help me prevent diabetes?
- Does the latest research on the glycemic index reveal anything that pertains to me?