The once simple subject of carbohydrate has become increasingly complex. As the diet wars rage on, sugars and starches continue to be the focus of diabetes nutrition experts. Should we be eating high carbohydrate, low carbohydrate, or no carbohydrate at all? Which meal plan is the yellow brick road to good health, optimal blood glucose, and a better body weight for our clients?
Up the yellow brick road.
The glycemic index (GI) has been proposed as a nutrition approach for improving blood glucose control in individuals with diabetes. It is a scientifically based method of ranking foods from 0 to 100 according to their potential for raising blood glucose. Research on the glycemic index began in the late 1970s and it continues to be extensively tested and refined by scientists such as Dr Jennie Brand-Miller in Sydney, Australia, and Dr Thomas Wolever in Toronto, Canada. The glycemic index concept has been widely adopted in diabetes management in Australia, New Zealand, Canada, the United Kingdom, and France, but remains controversial in the United States. Although research results are mixed, clinical studies in individuals with diabetes have shown that low GI diets improve glucose control by decreasing HbA1c, fructosamine, postprandial blood glucose excursions, the incidence of hypoglycemia, and blood lipids.1-4 A number of epidemiological studies have shown that intake of high glycemic index foods is positively associated with an increased risk of type 2 diabetes5 as well as coronary heart disease.6 By learning more about the glycemic index, diabetes educators can help their clients better understand the relationship between food choices and blood glucose, thus achieving improved glycemic control.
DIETARY DOGMA DISPROVED?
For many years, the accepted dietary dogma for diabetes was to avoid sugary foods (the so-called simple carbohydrates) because they were absorbed quickly by the body and produced a more rapid and larger rise in blood glucose. Complex carbohydrates (or starches) were encouraged because it was believed they were more slowly absorbed, resulting in a smaller rise in blood glucose levels. After a closer look at the scientific evidence, the American Diabetes Association concluded in their 1994 Nutrition Recommendations7 that the most Important factor is the total amount of carbohydrate consumed rather than the source.
The results of research on the glycemic index have challenged many long-held nutrition theories. Over 600 foods have been tested in Individuals with and without diabetes and assigned a glycemic index number based on their potential to raise blood glucose.8 Some selected foods and their GI values are shown in Table 1. Pure glucose, which produces the greatest rise in glucose levels, is assigned a glycemic index of 100. Every other food is ranked on a scale from 0 to 100 according to its actual effect on blood glucose levels when eaten as an equal amount of carbohydrate. For example, when scientists determine the GI of a certain food, they feed an amount of food containing 50 grams of carbohydrate to a volunteer. This may consist of 2 slices of sourdough bread, 1 cup of baked beans, or 20 jellybeans. Each of these foods contains 50 grams of carbohydrate and is equivalent to the amount of carbohydrate in 3 tablespoons of pure glucose powder. The research volunteers eat their test foods and their blood glucose levels are measured every 15 to 30 minutes over the next 2 hours. The blood glucose levels are plotted and used to construct a blood glucose response curve for the 2-hour period. The volunteers' response to the food being tested is compared with their response to 50 grams of carbohydrate from pure glucose to determine a glycemic index number.9
Carbohydrate foods that break down quickly during digestion, such as most flaked breakfast cereals, have the highest glycemic index value. Carbohydrates that break down slowly, such as most fiber-rich breakfast cereals, release glucose gradually into the bloodstream and have low glycemic index values. Many so-called complex carbohydrates such as bread and potatoes are digested and absorbed quickly and have higher GI numbers than moderate amounts of sugary foods. For example, a baked potato has a GI of 93, while chocolate candy pieces have a GI of 33. The explanation for this difference lies in the structure of the food. The starch molecules in the potato are fully gelatinized, which makes them easier for digestive enzymes to attack. Therefore, the carbohydrate is rapidly digested and absorbed. Other key factors that influence GI include the starch composition, the physical form of a food, the food fiber, the acidity of a food, and the methods by which a food is cooked and processed. In addition, foods that contain sucrose have a lower GI because sucrose is a disaccharide made up of both glucose and fructose, and the fructose portion has little effect on glycemia. Because fructose makes up almost half of sucrose, the GI of sucrose-containing foods is lower because they contain less pure glucose. Another factor at work in chocolate candy pieces is the fat content of chocolate. Fat slows down the rate of stomach emptying, thereby slowing the digestion of the carbohydrate. High-fat foods tend to have lower GI numbers than their low-fat versions.
What's Your GI IQ?
Glycemic Index of Selected Foods
|Food||Amount||Glycemic Index||Carbohydrates, g|
|Jelly beans||10 large (1 oz)||80||26|
|White-skinned mashed potatoes||1/2 cup, cooked||70||20|
|Banana||1 medium, raw||55||32||Macaroni||1 cup, cooked||45||52|
|Skim Milk||1 cup||32||12|
|Red kidney beans||1/2 cup, boiled||27||20|
GLYCEMIC INDEX AND DIABETES: HANDLE WITH CARE
Of course, this does not mean that diabetes educators should recommend chocolate candy pieces in place of baked potatoes. Glycemic index information should be used in the context of the healthy eating guidelines for overall good diabetes nutrition. Potato chips may have a lower GI than a baked potato, but their fat content should be considered as well. Foods containing refined sugar may have an intermediate GI, but the empty calories from sucrose could be put to better use in a variety of healthier food choices. Potatoes have a high GI value, but there is still a place for them in individualized meal plans.
According to GI research, the glycemic index of a food remains the same whether 1 ounce or 1 pound of it is consumed because the numbers are a relative ranking comparing 1 food to another. In theory, a client can eat less of a high GI food and more of a low GI food and end up with the same blood glucose response. Just because a food has a low GI does not mean people with diabetes can eat unlimited portions of it without affecting their blood glucose levels, weight, and blood lipids. In other words, portion size still counts. Until more is known about the glycemic response to food, the amount of total carbohydrate eaten should still be the main consideration, particularly when calculating an insulin dosage for an upcoming meal.
Critics of the glycemic index point out that the complexity of the GI concept may make it very difficult to apply in day-to-day dining situations.10 Early in the course of glycemic index research, some scientists were skeptical that the GI of individual foods could be used to predict the GI of a mixed meal. Research1 has since revealed that the glycemic index of a meal consisting of a mixture of carbohydrate foods can be calculated. However, this is not easy for most individuals because of the need for food composition figures or a nutrient analysis program. In general, studies show that when a food with a high glycemic index is combined with a food with a low glycemic index, the complete meal has a glycemic index in the intermediate range. To further complicate matters, other factors may need to be considered when putting the glycemic index concept Into practice. For example, the premeal glucose concentration can affect the rate of gastric emptying, thus influencing the glycemic response to a food. At high glucose concentrations (>170 mg/dl), gastric emptying is delayed; when blood glucose levels are low, a rapid gastric emptying rate occurs.10 Other factors that affect the blood glucose response include stress, time of day, recent physical activity, health status, and length of time since the previous meal. Using glycemic index information correctly requires much more nutrition savvy than casually thumbing through a reference book and labeling foods good and bad on the basis of their GI value.
Helpful Tips for Putting the Glycemic Index Into Practice
GLYCEMIC INDEX: FROM THEORY TO PRACTICE
Should you use the glycemic index approach with your clients? One of the researchers of the glycemic Index concept, Thomas Wolever, states "As originally intended, the glycemic index was meant to supplement the information in food tables, not replace it."12 Glycemic index information should be used along with the guidelines for a low fat, consistent carbohydrate diet that currently are recommended for diabetes, keeping in mind that the amount of total carbohydrate consumed is also significant. Several tips for putting the glycemic index into practice are summarized in Table 2. Clients who are working with the glycemic index should measure blood glucose levels frequently (premeal and postmeal, if necessary) and be alert for the need to modify diabetes medications.
Glycemic index information is already available on the Internet, in cookbooks, and on some food labels, in Europe and Australia. Although the American Diabetes Association currently feels that there isn't enough evidence to support the inclusion of the glycemic index in their new nutrition recommendations, research is ongoing in Australia into incorporating glycemic index information into the exchange system. A long-term, multicenter trial may be necessary to end the controversy surrounding the use of the glycemic index. In the meantime, it is best to advise clients to maintain a consistent carbohydrate intake while considering lower glycemic index choices within food categories.
1. Giacco A, Parillo M. Rivellese AA, et al. Long-term dietary treatment with increased amounts of fiber-rich low-glycemic index natural foods improves blood glucose control and reduces the number of hypoglycemic events in type 1 diabetic patients. Diabetes Care. 2000;10:1461-1466.
2. Jarvi AE, Karlstrom BE, Granfeldt YE, et al. Improved glycemic control and lipid profile and normalized fibrinolytic activity on a low-glycemic index diet in type 2 diabetic patients. Diabetes Care. 1999;22:10-18.
3. Frost G, Wilding J, Beecham J. Dietary advice based on the glycaemic index improves dietary profile and metabolic control in type 2 diabetic patients. Diabetic Med. 1994;11:397-401
4. Fontvieille AM, Rizkalla SW, Penfornis A, et al. The use of low glycaemic index foods improves metabolic control of diabetic patients over five weeks. Diabetic Med. 1992;9:444-450.
5. G. Salmeron J, Ascherio R, Rimm EB, et aI., Dietary fiber, glycemic load and risk of NIDDM in men. Diabetes Care. 1997;20:545-550.
6. Liu S, Willett WC, Stampfer MJ, et al. A prospective study of dietary glycemic load, carbohydrate intake and the risk of coronary heart disease in US women. Am J Clin Nutr. 2000;71:1455-1461.
7. American Diabetes Association. Clinical practice recommendations 2001. Diabetes Care. 2001;24:S44-S47.
8. Foster-Powell K, Miller JB. International tables of glycemic index. Am J Clin Nutr. 1995;62:871S-890S.
9. Wolever TM, Jenkins DJ, Jenkins AL. eI al. The glycemic index: methodology and clinical implications. Am J Clin Nutr. 1991;54;846-854.
10. Point/counterpoint. Glycemic Index: pro and con. Nutr Today, 1999,34:64-88.
11. Chew I, Brand JC, Thorburn AW, et al. Application of glycemic index to mixed meals. Am J Clin Nutr. 1988;47:53-56.
12. Wolever TMS The glycemic index: flogging a dead horse? Diabetes Care, 1997;20:452-456.
Brand-Miller J., Wolever TMS, Colagiuri S, Foster-Powell K. The Glucose Revolution: The Authoritative Guide to the Glycemic Index. New York: Marlowe & Co; 1999.
Foster-PowelI K, Brand-Miller J, Colagiuri S, Wolever TMS. The Glucose Revolution Pocket Guide to Diabetes. New York: Marlowe & Co; 2000.
www.glycemicindex.com The University of Sydney, Australia, glycemic index Web site. It includes GI tables, data, and research, as well as information about the GI symbol, programs, and books.
www.mendosa.com The Web site of David Mendosa, a freelance medical writer, who has gathered a wealth of information about the glycemic index.
This article originally appeared The Diabetes Educator, the official publication of the American Association of Diabetes Educators, Volume 27, Number 4, July/August 2001, pages 505-508 and is reprinted here by permission of the author.
Go back to Home Page
Go back to Diabetes Directory