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.

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.