The glycemic index (GI) is a measure of how fast carbohydrates are converted into glucose in the body or how fast they raise blood glucose levels. The GI is based on a scale of 0 to 100, in which the lower numbers suggest a slower rise in glucose, and higher numbers indicate a faster rise in glucose levels. The higher reference value of 100 is represented by pure glucose. The glycemic index of a food is determined by measuring the blood glucose level two hours after consumption of a fixed serving size (typically 50 grams) of carbohydrate. The GI is supported by leading international health organizations, such as the American Diabetes Association (ADA).
The human body is healthiest and performs at an optimal level when the blood glucose levels remain constant. When glucose levels are elevated, the body secretes more insulin. Insulin helps stabilize glucose levels in the body by converting excess sugar to fat. When glucose levels drop, the body becomes lethargic and/or experiences increased hunger. Chronic elevated blood glucose levels may promote diseases, such as type 2 diabetes, obesity and heart disease.
A low GI food releases glucose more slowly. Some examples of a low GI food (55 or less) include most fruits and vegetables (with the exception of starchy vegetables, like potatoes), legumes, nuts, milk, yogurt and some cheeses. A food with a high GI index causes a more rapid rise in blood glucose levels. Some examples of foods with a high GI index include some breakfast cereals, potatoes, watermelons, white bread, white rice, white pasta, instant oatmeal, saltine crackers and straight glucose. Some foods also fall into the medium GI index range and include foods made with 100 percent whole wheat, rye bread, quick oats and table sugar.
Some factors can affect the GI content of a food. In general, the more processed the food is, the higher its GI level, but this is not always the case. Fat, protein and fiber are also thought to lower the GI content of a food. Another factor is the ripeness and storage time of a food (fruits and vegetables). The riper a fruit or vegetable is, the higher the GI can be. Fruit and vegetable juice may also have a higher GI in the actual fruit or vegetable, but due to the processing of the fruit or vegetable has a decreased fiber content. Cooking foods longer, such as pasta, may also raise the food’s GI content. Also, the rate at which people digest or how the body responds to carbohydrates also varies, so the GI response from a food may vary from person to person.
Postprandial hyperglycemia is thought to be a risk factor in developing diabetes and atherosclerosis, which can contribute to heart disease. Animal research has also shown that a high GI carbohydrate diet is associated with an increased risk of obesity. The rapid rate of glucose increase in the body may cause the body to release excessive insulin, thereby dropping glucose levels too low, causing a cycle of fat storage, lethargy and increased food consumption, which leads to weight gain. If you already have diabetes and consume high GI foods, you may not be able to secrete or process insulin, causing your blood glucose levels to rise too high and leading to additional medical complications.
It’s important to remember the GI of a food is simply a rating of the food’s carbohydrate content. If the GI is used solely in determining which foods to eat, you may consume excessive fat and total calories. Also, the GI value of a food does not indicate the amount of carbohydrate consumed. Portion sizes remain relevant in managing glucose levels and avoiding excessive caloric consumption. Many nutritious foods also possess higher GI levels than foods that are less nutritious. The use of the GI index needs to be balanced with general health guidelines to encourage moderation in consumption of foods with limited nutrients. Another thing to consider is the combination of the foods consumed. When eating a food with a high GI, it can be combined with other lower GI foods to balance the effects on blood glucose levels.
About this Author
Denise Hamilton is a registered dietitian who has been working in the dietetics field for the past 11 years. She holds a master’s degree in clinical nutrition, and her writing has been featured in the “Your Health” and “Health and Fitness” sections of local New York state newspapers near her place of residence.