Glycemic Index

Glycemic Index
by Sandra Woodruff, MS, RD, LD/N

What is the Glycemic Index?
The glycemic index (GI) is a numerical ranking of carbohydrate-containing foods based on their potential to raise blood sugar levels. High-GI foods are quickly digested and absorbed, producing a rapid rise in blood sugar and insulin levels. Low-GI foods, on the other hand, are slowly digested and absorbed, producing a smaller, more gradual rise in blood sugar and insulin levels. Why is this important? High levels of blood sugar and insulin have been linked to many of the health problems that are so common today.

History of the Glycemic Index
The glycemic index has been the subject of scientific research for over 20 years. It was originally developed as a dietary strategy to help people with diabetes gain better control over their blood sugar levels. Today the GI is an accepted part of medical nutrition therapy in Canada, Australia, and much of Europe and its use has expanded to include roles in treating obesity, cardiovascular disease, and various other health problems.

What Determines the GI of a Food?
Many people assume that simple carbohydrates, like sugars, have a higher GI than complex carbohydrates like starches. However, it’s not that simple. The GI of a food is influenced by a variety of factors including the degree to which a food is processed; how long the food is cooked; the kind of starch, sugar, or fiber the food contains; and the foods acidity. In general, anything that speeds the rate at which a food is digested and absorbed will raise its glycemic index.

Which Foods Have a High Glycemic Index?
Generally speaking, foods that rank high on the glycemic index include products made from finely ground flours like bread and baked goods; processed breakfast cereals; snack foods like chips and pretzels; baked, mashed, and French fried potatoes; and short-grain (sticky) rice. Foods that rank lower on the glycemic index include most vegetables and fruits; sweet potatoes; legumes; minimally processed whole grains such as thick-cut oatmeal, oat bran, long-grain brown rice, barley, and bulgur wheat; pasta; and dairy products.

How do Researchers Determine the Glycemic Index of a Food?
Researchers measure out a portion of food that contains 50 grams of available (digestible) carbohydrate. For instance, 4 1/2 slices of bread, 1 1/4 cups of rice, 1 1/2 pounds of carrots, and 3 medium apples each contains about 50 grams of available carbohydrate. A food is fed to a group of test subjects and their blood sugar responses are measured. The test subjects’ blood sugar response to the food is then compared with their response to eating 50 grams (about 3 tablespoons) of pure glucose. For instance, if the food is brown rice, and the rice raises blood sugar only 55 percent as much as glucose does, the rice is assigned a GI of 55.

Glycemic Index Versus Glycemic Load
The glycemic index tells you how quickly a carbohydrate-containing food turns into sugar, but it doesn’t tell you how much carbohydrate is in a serving of a food. To assess the full impact of a food on blood sugar levels you should have an idea of both. This is where glycemic load comes in.

Glycemic load is a relatively new term that considers both the glycemic index and the amount of carbohydrate in a food. The carbohydrate in carrots, for example, has a high GI. But carrots are pretty low in carbohydrate compared to other foods like potatoes, bread, and sweets, so carrots glycemic load is relatively low. The bottom line is that if a food has a high GI but very little carbohydrate, it will not have much impact on blood sugar and insulin levels. On the other hand, if a food has both a high GI and a high carbohydrate content, it should be limited. The following table illustrates this principle.

Food	Glycemic Index	Carbohydrates (g)	Glycemic Load
1 medium carrot	71	8	6
1 cup watermelon	72	11	8
1 cup mashed potatoes	83	31	26
1 cup brown rice	55	44	24
1 cup pasta	41	39	16
1 cup lentils	29	39	11
12-ounce soda	68	37	25

Who can Benefit from Eating Low on the Glycemic Index?
By helping to maintain lower blood sugar and insulin levels, a low-GI diet may be useful in preventing and treating a variety of the health problems. Here are some examples of how eating low on the glycemic index can help promote excellent health:

Diabetes - Substituting low-GI carbohydrates (like thick-cut oats, pasta, and legumes) for high-GI carbohydrates (like processed cereals, white bread, and potatoes) can help lower blood glucose levels in people with diabetes. This is why the GI has been an integral part of medical nutrition therapy for diabetes in Australia, New Zealand, Canada, and Europe for many years.

A low-GI diet may also help prevent diabetes from ever developing in the first place. Harvard University researchers who tracked the eating habits of over 100,000 men and women found that people whose diets are low in fiber and high in refined and high-GI carbohydrates are more than twice as likely to develop type 2 diabetes, as are people who eat a fiber-rich diet with a low glycemic load.

Cancer - Insulin is a cellular growth factor. Many studies have shown an association between high insulin levels and a variety of cancers including breast, colorectal, prostate, and pancreas. Other studies have shown links between diets high in sugar, refined carbohydrates, glycemic load, and cancer. This suggests that lifestyle changes like maintaining a healthy body weight, exercising, and eating a healthy low-GI diet may help protect against cancer at least partly by lowering insulin levels.

Cardiovascular disease - As with type 2 diabetes, researchers have found that a diet high in refined and high-GI carbohydrates may substantially raise the risk for heart disease. These foods increase blood insulin levels, which in turn contribute to a higher blood pressure, higher levels of blood fats (triglycerides), lower levels of HDL (good) cholesterol, and an increased tendency for dangerous clots to form and linger in the blood.

Hypoglycemia - People who have meal-related reactive hypoglycemia secrete too much insulin after eating. This causes the cells to remove so much sugar from the blood that they feel weak, shaky, irritable, headachy, unable to concentrate, and very hungry with a few hours of eating. Choosing low-GI carbohydrates can help prevent this type of hypoglycemia because eating foods that promote a gradual rise in blood sugar and a lower insulin response reduces the likelihood that blood sugar levels will drop too low.

Obesity - Since low-GI foods are slowly digested, they provide a gradual and sustained rise in blood sugar. This keeps you feeling full and satisfied and delays the return of hunger between meals. Conversely, high-GI carbohydrates provide short bursts of energy that satisfy you in the short term but soon leave you hungry. Many of the fat-free and low-fat foods that have become popular over the last decade—such as bagels, processed cereals, rice cakes, crackers, snack chips, and cookies—tend to rank high on the glycemic index and may actually contribute to a pattern of overeating in some people.

Polycystic Ovary Syndrome (PCOS) - This disorder affects about 7 percent of reproductive age women. Symptoms include altered hormone levels, disturbances in the menstrual cycle, infertility, and acne. Because PCOS is often associated with insulin resistance, women with this disorder are at increased risk for developing diabetes, high blood pressure, and heart disease. Measures that improve insulin sensitivity, such as weight loss and exercise can help reverse PCOS symptoms. A low-GI diet, which minimizes insulin secretion, may also confer benefits.

Athletic Performance - Low glycemic index carbohydrates should be chosen for precompetition meals and snacks. These slowly digested carbs provide a steady stream of glucose to working muscles during exercise and enhance performance by making glycogen stores last longer.

When high-GI foods are eaten just prior to competition they can actually impair performance because they stimulate more insulin, which speeds removal of glucose from the blood and can lead to hypoglycemia. On the other hand, high-GI carbs are the preferred choice after the event because the insulin surge helps replenish muscle glycogen stores.

Some Cautions about Using the Glycemic Index
While the GI is an important dietary strategy, the total amount of calories and carbohydrate must also be considered when planning a healthful diet. Furthermore, not all low-GI foods are healthful choices. For instance, many candy bars and sweets have a low to moderate GI. However, these foods are also high in calories and low in nutrients and should be eaten with your overall nutrition goals in mind.

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