GLYCEMIC INDEX DIETS

GLYCEMIC INDEX DIETS

Definition

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.”

Origins

Low-glycemic diet 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 people with diabetes. In the 1980s, researchers developed the glycemic index (GI).

Before 1981, carbohydrates were classified as simple or complex. Simple carbohydrates include 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 (e.g., potatoes), 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). They 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 using the glycemic index have come out. 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 1997, epidemiologist and nutritionist Walter Willett of the Harvard School of Public Health developed the glycemic load as a more accurate way of rating carbohydrates compared to the glycemic index. This is because the glycemic load factors in the amount of a food eaten, whereas the glycemic index does not. The glycemic load of a particular food is determined by 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. However, a serving of two cups has 10 net carbs because of its high fiber content, for a glycemic load of seven, which is considered low.

Description

Glycemic index (GI) 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 ranking. Most foods that are rated high on the glycemic index contain high levels of carbohydrates. Some people with diabetes use the GI as a guide in selecting foods and planning meals. The GI ranks foods based on their effects on elevating blood sugar 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 GI is a ranking of carbohydrate foods that individuals with diabetes can use to manage their disease. The ranking is based on the rate at which carbohydrates affect blood glucose levels relative to pure 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. The lower the GI of a food, the slower the rate with which it is absorbed into the bloodstream.

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 individual consuming the food. These factors naturally affect each food's glycemic index rank. In addition, differences exist in various glycemic indices 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. It is determined by having ten or more healthy people eat a measured quantity of a digestible carb, usually white bread. The rise in their blood glucose level is measured for the next two hours. The rise 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 eaten. A high GI is considered to be 70 and greater, a medium GI is 56–69, and a low GI value is 55 or less. A related value is glycemic load (GL). Glycemic load is calculated as follows: GL = GI x the amount of available carbohydrate in a 100 g serving/100. In general, low-carb diets recommend a glycemic load of 80 or less. A high glycemic load is considered to be 120 or more.

The following is the GI for a few common 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

The GI is 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, sautéed, or fried, for example) and what other foods are consumed with it.

The following foods are acceptable on a low-glycemic index diet:

• cornflakes
• oats, barley, and bran cereals
• citrus fruits to slow emptying of the stomach
• a variety of vegetables, especially salad vegetables
• wild rice or brown rice instead of white rice
• whole grain breads
• al dente whole grain pastas
• reduced sugar desserts
•  

Function

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 insulin 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 control of blood glucose. 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.

Athletes may also use GI diets to prepare for athletic competitions or to recover from training. Low GI is often favored before an event, while higher GI aids in the replenishment of glycogen stores.

Benefits

There is conflicting scientific research on the benefits of a low-glycemic index diet for both people with diabetes and people trying to lose weight. Glycemic index diets may help people with diabetes 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. Research published in the January 2010 issue of Sports Medicine found that eating a low-glycemic meal prior to prolonged exercise may have some merit, though this effect may be minimized if carbohydrates are consumed during the activity. Regardless, 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 one gram per minute of exercise. Following exercise, moderate-to-high GI foods enhance glycogen storage.

Precautions

If an individual has health concerns, a low-glycemic index diet should be undertaken only under 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.

Risks

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 gain weight.

Research and general acceptance

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 people with diabetes 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 people with diabetes. Not all physicians and endocrinologists (medical specialists who treat disorders of the glands, including diabetes) subscribe to the Association's position.

Resources

Beale, Lucy and Julie Alles. The Complete Idiot's Guide to Glycemic Index Snacks. New York: Alpha, 2011.

Raffetto. Meri B. The Glycemic Index Diet for Dummies. Indianapolis, IN: Wiley Pub., 2009.

Smith, LeeAnn. The Everything Glycemic Index Cookbook. 2nd ed. Avon, MA: Adams Media, 2010.

Foster-Powell, Kaye, Susanna H. A. Holt, and Janette C. Brand-Miller. “International Table of Glycemic Index and Glycemic Load Values: 2002.” American Society for Clinical Nutrition 76, no. 1 (2002): 5–56. http://www.ajcn.org/content/76/1/5.full (accessed August 9, 2012).

O'Reilly, John, Stephen Wong, and Yajun Chen. “Glycaemic Index, Glycaemic Load and Exercise Performance.” Sports Medicine 40, no. 1 (January 2010): 27–39. http://dx.doi.org/10.2165/11319660-000000000-00000 (accessed August 9, 2012).

Harvard Health Publications. “Glycemic Index and Glycemic Load for 100þ Foods.” Harvard Medical School. http://www.health.harvard.edu/newsweek/Glycemic_index_and_glycemic_load_for_100_foods.htm (accessed August 9, 2012).

Harvard School of Public Health. “Carbohydrates and the Glycemic Index,” Carbohydrates: Good Carbs Guide the Way. The Nutrition Source, Department of Nutrition, Harvard University. http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/carbohydrates-full-story/index.html#glycemic-index (accessed August 9, 2012).

Higdon, Jane, and Victoria J. Drake. “Glycemic Index and Glycemic Load.” Linus Pauling Institute Micronutrient Information Center, Oregon State University. http://lpi.oregonstate.edu/infocenter/foods/grains/gigl.html (accessed August 9, 2012).

Mayo Clinic staff. Glycemic Index Diet: What's Behind The Claims. MayoClinic.com. August 24, 2011. http://www.mayoclinic.com/health/glycemic-index-diet/MY00770 (accessed August 9, 2012).

MedlinePlus. Carbohydrates. February 8, 2012 http://www.nlm.nih.gov/medlineplus/carbohydrates.html (accessed August 9, 2012).

University of Sydney. Glycemic Index. http://www.glycemicindex.com (accessed August 9, 2012).

Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL 60606-6995, (312) 899-0040, (800) 877-1600, amacmunn@eatright.org, http://www.eatright.org.

American Diabetes Association, 1701 North Beauregard St., Alexandria, VA 22311, (800) DIABETES (342-2383), askADA@diabetes.org, http://www.diabetes.org.

Center for Food Safety and Applied Nutrition (CFSAN), U.S. Food and Drug Administration, 5100 Paint Branch Pkwy., College Park, MD 20740, (888) SAFEFOOD (723-3366), consumer@fda.gov, http://www.fda.gov/Food/default.htm.

National Diabetes Education Program, One Diabetes Way, Bethesda, MD 20814-9692, (301) 496-3583, http://www.ndep.nih.gov