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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.