The glycemic index (GI) was developed as a research tool to examine how a person’s glucose (blood sugar) level 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 level 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. Foods with a low GI score are converted to sugar more slowly and do not cause a rapid spike in blood glucose. 

A scale of 0 to 100 is used to rank the foods. A GI of less than 55 is considered low, a GI over 70 is considered high, and everything in between is considered intermediate or moderate.

In general, refined grains and sugars have a high GI, legumes and unprocessed or whole grains have a moderate GI, and fruits and vegetables have a low GI. As with fats, scientists categorized foods as “good” and “bad” carbohydrates. In theory, some people, especially those with diabetes, should choose more of the low-glycemic foods at meals to keep glucose levels stable.

However, using the glycemic index for meal planning is a complicated process, and may not adequately control glucose. GI values have many variables. First, a GI score is not necessarily connected with how nutritious a particular food is. The score is ranked solely for the rate at which it raises glucose levels. In addition, GI scores of a food vary depending on how it is prepared and its interaction with other foods. The GI of a specific food may also vary from individual to individual. Many foods do not have a known GI, which can make it difficult to plan a meal using the glycemic index as a guide.

Although some scientists believe that a low-glycemic diet may help control glucose and fat (lipids), the results are inconclusive. There is not enough evidence of long-term benefits to recommend use of the glycemic index for dietary control. The glycemic index can be used in meal planning but should not be the only consideration when making food choices. Foods may have a different GI from day to day, based on the individual’s glucose reading and any insulin resistance (e.g., in people with diabetes).

The glycemic index (GI) is a classification system used to rank carbohydrate-containing foods to reflect their effect on the body’s blood glucose levels. Scientists developed the glycemic index about 20 years ago as a nutritional research method. This research indicates that the way carbohydrate foods are converted into sugar is more complicated than previously thought.

When a person eats a piece of food, it is broken down and digested in the body. A person’s glycemic response refers to the speed at which glucose is absorbed when a person eats, the level to which blood glucose rises and the rate at which blood glucose returns to a normal level. Slow absorption, modest rises in blood glucose levels and a smooth return to normal are desired during the glycemic response.

Different foods impact blood glucose levels in different ways. It is particularly important for people with diabetes to identify the rate of glucose absorption resulting from the consumption of various foods. Researchers have studied the glycemic response of foods and developed the glycemic index, which can be used to rank certain 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 a set amount of a food is eaten, compared to the reference amount of white bread, which is ranked 100. In the second method, scientists used glucose as a reference, which has a rank of 143. Foods that are ranked with a GI of less than 100 are converted more slowly than white bread or glucose. Foods with a GI greater than 100 turn into sugar more quickly. In general, high-GI foods appear to cause a rapid jump in blood glucose followed by a sharp plunge, whereas low-GI foods raise blood glucose at a slow, steady pace. GI also may have an effect on appetite. Foods with a low GI tend to keep a person feeling full for longer periods of time. In contrast, a person is more likely to feel hungry again sooner after eating foods with a high GI.

The benefits of using the glycemic index to regulate the diet are not entirely clear. People do not eat meals containing one single food. Meals that contain a variety of carbohydrates, fats and proteins have effects on blood sugar that cannot be measured using the glycemic index.

Some researchers believe that eating low-GI foods will help keep blood glucose levels steady, lower blood fats (lipids) and reduce the risk for obesity and heart disease. 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-meal testing of blood glucose levels is necessary to determine whether some foods raise levels more than others.

Recent research on the glycemic index has produced mixed results. For example, some studies suggest that eating high-GI potatoes might raise the risk of insulin resistance and type 2 diabetes and that low-GI high-fiber foods may reduce the risk of diabetic heart disease. Other studies have found no link between high-GI foods and hyperglycemia.

According to the American Dietetic Association:

• Science does not show that a low-GI diet reduces appetite or results in significant weight loss. In addition, this type of diet may deprive the body of some phytonutrients.
  
  
• Patients with diabetes should track their grams of carbohydrates.
  
  
• The GI used in conjunction with glucose monitoring may help some people with diabetes modestly improve after-meal (postprandial) glucose levels.

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. The GI of foods is not necessarily consistent with nutritional value. For example, ice cream has a lower GI than baked potatoes and honey has a lower GI than mashed potatoes. There are also several important aspects beyond the GI number that influence the effect of foods on glucose. These factors include:

• How it combines with other foods. Foods are not usually eaten alone. Instead, people eat a variety of foods in the same meal. These other foods (e.g., protein and fat-containing foods in a meal) have an effect on the GI of the carbohydrate-containing foods.
  
  
• The type of nutrients in the food. Not all carbohydrates are alike. Different types have a different GI. The fiber, fat and protein content of the food are important. These other nutrients affect the GI of the food. For example, higher levels of fiber or fat help to slow down the glycemic response, thus reducing the GI.
  
  
• The way the food is prepared. The GI of a food will vary depending on how it is prepared (e.g., cooked, chopped, mashed). The amount of processing the food item has undergone is also important. Highly processed foods tend to have a higher GI than their minimally processed counterparts.
  
  
• Region where the food is grown and processed. For example, foods are processed differently in different parts of the United States, and an item such as rice can be grown in many different ways from one part of the world to another.
  
  
• The ripeness of the fruit or vegetable. The GI of a food varies depending on how ripe the particular food happens to be. Riper foods frequently have a higher GI than their unripe counterparts.
  
  
• Fat and acid content of the food. Foods with fats and acids influence the conversion of carbohydrates to blood sugar. The higher the fat and acid content, the slower the carbohydrate is converted to glucose.
  
  
• Individual differences. These include the individual’s age and activity level, glucose level before eating and individual differences in digestion. Different people digest foods at different rates and speeds.

Knowing the general glycemic index of foods is therefore not enough information to plan a safe weight-loss or diabetic diet. Current guidelines already suggest a number of healthy foods that have a low GI, including whole grains, legumes, vegetables, fruits, and dairy products. In addition, people who eat smaller and more frequent meals may be able to spread out their absorption of glucose in a fashion that mimics the benefits of eating foods with low glycemic response.

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 a particular individual.

Despite continuing debate over the value of the glycemic index, organizations such as the American Diabetes Association and the Canadian Diabetes Association have stated that this tool can be helpful when planning a sound diet.

The body converts all carbohydrates into glucose in the bloodstream, which is burned for energy or stored in the body. In the glycemic index theory, the faster the carbohydrates are broken down by the digestive system, the faster they travel to the blood and the quicker the blood glucose level rises. This causes greater insulin production. The rapid rise in glucose may result in a burst of energy quickly followed by hunger. This cycle is not considered healthy for some people, including those with diabetes.

Examples of foods with a GI over 70 (high-glycemic) include:

• White bread
• Mashed potatoes
• Some cereals
• Snacks such as corn chips, potato chips and pretzels
• Honey and jams

Although certain high-glycemic foods may be considered the “bad” carbohydrates, some of the foods are valuable in the diet. People should not totally avoid high-GI foods. Instead, they are advised to look for high-GI foods that are more nutritious and lower in calories. For example, a high-fiber cereal may have a GI of 80. However, it provides more nutrients than corn chips, which have a GI of 72.

Some carbohydrates cause a rise in moderate rise in glucose levels. These foods are categorized as having an intermediate or moderate glycemic index. The GI for this category is 55 to 70 and includes:

• Some cereals
• Crackers and whole grain breads
• Canned fruits
• Table sugar

Along with the high-GI foods, it is important to realize that the foods in this category are not all the same. Some foods should be eaten sparingly because they have the same GI but little nutritional value. For example, table sugar and beets both have a GI of 65, but beets contain far more nutrients and fiber.

Foods that produce the most gradual rise in glucose levels is placed in the low-glycemic group. These foods 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. However, there is still food in this category that contains 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.

Glycemic index of some common foods:

The glycemic index (GI) can measure only how quickly one particular carbohydrate is converted into glucose in the body. It does not describe how much carbohydrate is in a certain food, or the influence on blood glucose levels of a variety of foods consumed in the typical single meal. Some foods have high-GI carbohydrate but do not have a great deal of it. It is important to know both how much carbohydrate is in a food and 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 carbohydrates minus fiber).
  
  
• Multiply that number by the food’s GI.
  
  
• Divide by 100 for glycemic load.

For example, the GL of a half-cup serving of carrots would be calculated as:

8 (grams of carbohydrates) X 101 (GI) = 808 / 100 = 8.08, rounded to a GL of 8.

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 have a lower GL. This comparison demonstrates that using glycemic load rather than just the glycemic index in choosing food provides more information. A lower glycemic load typically means less carbohydrates and more nutrients and fiber. In general, lower glycemic load foods provide a better diet.

Glycemic Index vs. Glycemic Load
Table of Common Single Foods

   * lower than 131 according to some calculations

Ranges for GI and GL

Foods with a low GL include high-fiber fruits and vegetables (other than potatoes), bran cereals, and many legumes (e.g., chick peas, kidney beans, black beans, pinto beans). Foods with a medium GL include pearled barley, brown rice, bulgur, rice cakes, whole grain pasta and fruit juices (unsweetened). Other high GL foods include French fries, refined cereal products, sugar-sweetened beverages, jelly beans, candy bars and white rice and pasta.

The glycemic index (GI) that is assigned to a particular food is an average of the glycemic responses of many individuals. It is possible for people to measure their own personal response to a food to determine how it affects their glucose level. By testing individually, people 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 (e.g., for diabetes).

The first step in determining how food affects the glucose level requires a glucose test immediately before and two hours after eating. This reading will give a general idea of the way the food raises glucose in the individual. 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.

Testing more frequently after eating a specific food can help. By testing at numerous times, a personal glucose curve can be produced. 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 the 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

Research continues on using the glycemic index to regulate diets for losing weight or the treatment of diabetes. Scientists differ in opinions regarding its effect on controlling glucose (blood sugar), losing weight and preventing obesity. Some do not find the glycemic index a helpful method for losing weight or managing diabetes, but other researchers have advocated having the glycemic index placed on food labels.

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 weight control or 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.

Preparing questions in advance can help patients have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to glycemic index:

1. Do you recommend the glycemic index for me?
   
   
2. If the glycemic index cannot be my primary dietary guideline, can it be a helpful supplement for me?
   
   
3. Will the glycemic index help me to lose weight?
   
   
4. Can using the glycemic index reduce my frequency of checking my glucose, or will I need to maintain my schedule of glucose monitoring?
   
   
5. What do I need to know about factors that affect the glycemic index of food?
   
   
6. 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?
   
   
7. What do I need to know about the difference between glycemic index and glycemic load?
   
   
8. How can I estimate my personal glycemic index?
   
   
9. If I use carbohydrate counting as a measure to control my glucose, can I still use glycemic index to measure food values as well?
   
   
10. Does the latest research on the glycemic index reveal anything that is relevant for me?