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Glycemic index diets rank carbohydrates based on their ability to affect blood glucose (sugar) levels. These diets generally consider foods high in carbohydrates, such as bread, sugar, and pasta, as bad and low carbohydrate foods, such as meat, fish, and dairy products, as good.
Low-glycemic diets concepts were first developed in the 1960s and were originally designed for individuals with diabetes. At that time, the prevailing medical attitude was that a diet emphasizing well-balanced foods while paying special attention to carbohydrates (carbs) and avoiding carbohydrate-rich foods, helped to control blood sugar and insulin levels. This came after a number of medical studies linked eating foods high in carbohydrates with elevated blood glucose levels in diabetics. In the 1980s, researchers developed the glycemic index (GI).
Before 1981, carbohydrates were classified as simple or complex. Simple carbohydrates included fructose (fruit sugar), sucrose (table sugar), and lactose (milk sugar). Complex carbohydrates are also composed of sugars but the sugar molecules are strung together to form longer and more complex chains. Foods high in complex carbohydrates include vegetables, whole grains, and beans. In 1981, researchers David Jenkins and Thomas Wolever of the University of Toronto Department of Nutritional Sciences developed the glycemic index (GI) and published a study suggesting that using the glycemic index of foods was a more accurate way of classifying carbohydrates than the simple and complex system.
Since 1981, dozens of low-carb diets and diet books have come out, using the glycemic index as the primary guiding principle. Among the more popular glycemic index-inspired diets are the Sugar Busters Diet, Zone Diet, Protein Power Diet, Suzanne Somers diet, and South Beach Diet. In general, these diets are similar. Their differences are in the zeal that they limit carbohydrate intake.
In 1997, epidemiologist and nutritionist Walter Willett of the Harvard School of Public Health developed the glycemic load as a more accurate way ofrating carbohydrates compared to the glycemic index. This is because the glycemic load factors in the amount of a food eaten which the glycemic index does not. The glycemic load of a particular food is determined by
(Illustration by GGS Information Services/Thomson Gale.)
multiplying the amount of net carbohydrates in a serving by the glycemic index and dividing that number by 100. Net carbohydrates are determined by taking the amount of total carbohydrates and subtracting the amount of dietary fiber. For example, popcorn has a glycemic index of 72, which is considered high, but a serving of two cups has 10 net carbs for a glycemic load of seven, which is considered low.
Glycemic index diets vary in the specifics but most have one simple rule: people can eat as much food as they want providing the foods have a low glycemic index (GI) ranking. Most foods that are rated high on the GI contain high levels of carbohydrates. Some people with diabetes (diabetics) use the GI as a guide in selecting foods and planning meals. The GI ranks foods based on their effects on elevating blood sugar (glucose) levels. Foods with a high GI tend to increase blood glucose levels higher and faster than foods with a low GI value. The GI is not a measure of a food’s calorie content or nutritional value.
The Glycemic Index
The glycemic index (GI) is a ranking of carbohydrate foods individuals with diabetes use to manage their disease. This ranking is based on the rate carbohydrates affect blood glucose levels relative to glucose or white bread. Generally, the glycemic index is calculated by measuring blood glucose levels following the ingestion of a carbohydrate. This blood glucose value is compared to the blood glucose value acquired following an equal carbohydrate dose of glucose or white bread. Glucose is absorbed into the bloodstream faster than any other carbohydrate, and is thus given the value of 100. Other carbohydrates are given a number.
relative to glucose. Foods with low GI indices are released into the bloodstream at a slower rate than high GI foods.
A number of factors influence the digestion and absorption rate of food, including ripeness, particle size, the nature of the starch, the degree of processing and preparation, the commercial brand, and the characteristics of the diabetic patient, and these factors naturally affect each food’s glycemic index position or rank. In addition, differences exist in the glycemic indeces of foods due to the choice of reference food, the timing of blood sampling, or the computational method used to calculate the glycemic index.
The glycemic index measures the quality rather than the quantity of carbohydrates found in food. Quality refers to how quickly blood sugar levels are raised following eating. The GI is a standard— 10+ healthy people consume a digestible carb (usually white bread), and their blood glucose rise is followed for the next two hours—that is assigned an index value of 100. Other foods are compared to the standard in order to arrive at their ratings. The higher the GI number, the faster blood sugar increases when that particular food is consumed. A high GI is considered to be 70 and greater, a medium GI is 56-69, and a low GI value id 55 or less. In general, low-carb diets recommend a glycemic load (the total GI number in foods consumed per day) of 80 or less. A high glycemic load is considered to be 120 or more.
The following is the GI for a few foods:
- Cornflakes, 83
- Grapefruit, 25
- Watermelon, 72
- Sugar, 64
- Potato chips, 56
- White bread, 70
- Sourdough bread, 54
- Macaroni, 46
- Baked red potato, 93
- French fries, 75
But the GI in not a straightforward formula when it comes to reducing blood sugar levels. Various factors affect the GI value of a specific food, such as how the food is prepared (boiled, baked, sauteed, or fried, for example) and what other foods are consumed with it.
The following recommendations help achieve a daily diet based on low glycemic index foods
- Cornflakes, 83
- Oats, barley, and bran cereals
- Citrus fruits to slow emptying of the stomach
- A variety of vegetables, especially salad vegetables
- Wild rice instead of white rice
- Whole seed breads
- Al dente whole grain pastas rather than white potatoes
- Less sugary desserts
Glycemic index diets have two separate functions. The first is to help individuals with diabetes or insulin resistance syndrome maintain normal and steady blood glucose levels. The second is to aid in weight loss.
The objectives of management in diabetic patients are to reduce hyperglycemia, prevent hypoglycemic episodes, and reduce the risk of complications. For people with diabetes, the glycemic index is a useful tool in planning meals to achieve and maintain glycemic control. Foods with a low glycemic index release sugar gradually into the bloodstream, producing minimal fluctuations in blood glucose. High GI foods, however, are absorbed quickly into the bloodstream causing an escalation in blood glucose levels and increasing the possibility of hyperglycemia. The body compensates for the rise in blood sugar levels with an accompanying increase in insulin, which within a few hours can cause hypoglycemia. As a result, awareness of the glycemic indices of food assists in preventing large variances in blood glucose levels.
There is conflicting scientific research on the benefits of a low glycemic index diet for both diabetics and as a weight loss tool. Glycemic index diets may help diabetics maintain constant levels of blood glucose. By consuming more fruits and vegetables and whole grains rather than processed foods, low glycemic diets encourage higher fiber consumption.
Experts disagree regarding the use of the glycemic index in athletes’ diets and in exercise performance. Insufficient evidence exists supporting the benefit of low glycemic meals prior to prolonged exercise. Nonetheless, a low GI pre-event meal may be beneficial for athletes who respond negatively to carbohydrate-rich foods prior to exercise or who cannot consume carbohydrates during competition. Athletes are advised to consume carbohydrates of moderate to high GI during prolonged exercise to maximize performance, approximately 1 gram per minute of exercise. Following exercise, moderate to high GI foods enhance gly-cogen storage
If an individual has health concerns, a low glycemic index diet should be undertaken with the supervision of a doctor. Doctor supervision of the GI diet is not necessary when the individual is healthy and disease-free. People with diabetes should consult an endocrinologist, who may recommend discussing the diet with a diabetes dietitian.
Eating a diet based solely on the glycemic index of foods can lead to overeating and a weight gain rather than loss. No emphasis is placed on total calorie intake or on the amount of saturated fat content. By basing one’s diet on glycemic index alone, it is still possible to eat excess calories and to, therefore, gain weight.
There is mixed acceptance of glycemic index diets by the medical community. Some studies have shown GI diets can be effective in controlling blood sugar levels in diabetics and in helping people lose weight. Other studies have contradicted these findings. No major studies or research has shown that GI diets are harmful to a person’s health. The American Diabetes Association has adopted a position that there is not enough conclusive evidence to recommend the general use of a low-GI diet for diabetics. Not all physicians and endocrinologists (medical specialists who treat disorders of the glands, including diabetes) subscribe to the association’s position.
In 2006, researchers at the University of Sydney (Australia) found that low-carb, low-GI diets decreased fat mass and reduced the risk of cardiovascular disease.
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American College of Nutrition. 300 South Duncan Ave., Suite 225, Clearwater, FL 33755. Telephone: (727) 446-6086. Website: http://www.amcollnutr.org.
American Diabetes Association. 1701 N. Beauregard St., Alexandria, VA 22311. Telephone: (800) 342-2383.
American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. Telephone: (800) 877-1600. Website: http://www.eatright.org.
American Society for Nutrition. 9650 Rockville Pike, Bethesda, MD 20814. Telephone: (301) 634-7050. Website: http://www.nutrition.org.
Center for Nutrition Policy and Promotion. 3101 Park Center Drive, 10th Floor, Alexandria, VA 22302-1594. Telephone: (703) 305-7600. Website: http://www.cnpp.usda.gov.
Ken R. Wells