06/17/2011 06:20 pm ET Updated Aug 17, 2011

Glycemic Index and Diabetes: The Jury's Still Out

Contrary to popular belief, all carbohydrates are not equal when it comes to how much a food elevates a person's blood sugar. The glycemic index (GI) was created in an effort to rank foods by the effect they have on an individual's blood sugar levels after they eat that food. The index basically measures the difference in response between the food in question and a reference food -- usually a specific glucose solution or a single slice of white bread. The measure of glycemic load (GL) takes into account the amount of the carbohydrate consumed, as well as the food's GI.

There's been a lot of talk about low-glycemic-index diets being better for helping diabetics control their blood sugars, but the studies that have been done tend to be small and of short duration, and results of these studies have been mixed. Other studies show a relationship between the risk of developing diabetes and GI or GL (or both), while others do not. This lack of fully understood science hasn't stopped the development of commercial diets based on the glycemic index, which include the South Beach Diet and Nutrisystem Nourish. The following are just a couple of studies that illustrate how the research into blood sugars, diabetes and glycemic index is still not very well understood.

Back in 2008, researchers in Canada decided to improve on past studies by designing a larger, more long-term study to compare the effects of a low glycemic index diet with a high cereal fiber diet (published in the Journal of the American Medical Association). Their goal was to see if the hemoglobin A1C (a measure of diabetic control) improved on either diet, and as a secondary goal, they also looked at whether that diet helped improve the participants' cholesterol scores as well.

There were 199 men and women who enrolled in the study. All had Type 2 diabetes and were taking oral medications to control their diabetes. Their hemoglobin A1C values at the start of the study were between 6.5 percent and 8 percent, and none had significant cardiovascular, kidney or liver issues, nor were they being treated for cancer. Just over 85 percent of the participants were overweight.

The participants were randomly assigned to one of two dietary interventions:

1. Low GI diet: emphasized low GI breads such as pumpernickel, quinoa and flaxseed; large flake oatmeal or bulgur/flax hot cereals; white pastas and rice; and legumes such as beans, peas and lentils. Included fruits such as apples, pears, oranges, cherries and berries.

2. High cereal fiber diet: emphasized brown (whole-grain) breads, crackers, breakfast cereals, and rice; potatoes with skin; and legumes. Included fruits such as bananas, mangos, grapes, watermelon and canteloupe.

In both diets, the participants were asked to avoid French fries, chips and pastries such as pancakes, muffins, doughnuts and white rolls or bagels, and were to avoid those fruits not specifically permitted by their dietary intervention.

Interestingly, the overall goal of these two treatments was to maintain a similar overall amount of fiber in the diet while reducing the glycemic index in the low GI diet by 10 to 20 percent.

After six months, the researchers compared the participants' blood pressures, cholesterol scores, and hemoglobin A1C values to their results at the start of the study. Both groups reduced their hemoglobin A1C by 0.5 percent for the low GI diet versus 0.18 perent for the high cereal fiber diet. The low GI diet participants also improved their cholesterol scores more than those on the high cereal fiber diet.

That said, although their goal was to maintain a similar overall amount of fiber in both diets, in actuality the low GI diet averaged 4.6 grams of fiber per day more than the high cereal fiber diet, meaning that the low GI diet was higher in soluble fibers (the types of fiber found in apples, pears and many vegetables) while the high cereal fiber diet was higher in insoluble fibers (those in whole grains). This would appear to show that a low glycemic index diet is slightly better at helping Type 2 diabetics control their blood sugars -- and when I say slightly, the difference is just about one-third of 1 percent, not really clinically significant.

More recently, a study published this year in the British Journal of Nutrition took another look at dietary GI and GL and the risk of Type 2 diabetes. The researchers made use of information gathered in a 12-year cancer prevention study in Finland, which included more than 25,000 male smokers between the ages of 50 and 69 years of age. At the start of the study, the participants completed a demographic questionnaire and their height and weight were measured. They also responded to a detailed diet history questionnaire asking how often and how much they ate of 276 foods over the previous year.

Using those questionnaires, the researchers were able to calculate each participant's nutrient intakes as well as GI and GL. They also calculated the intake of carbohydrates as a percentage of their overall caloric intake and then that percentage was broken out for low-, medium- and high-GI foods. The researchers grouped the men into five levels of increasing dietary GI and five levels of increasing dietary GL.

At the end of the study, the researchers compared the diets of those 1,098 men who had developed Type 2 diabetes with the diets of those who did not. Interestingly, when the researchers adjusted for age and intervention group, high GI and GL corresponded with lower risk of type 2 diabetes. However, when they also adjusted for other variables, that relationship disappeared.

Because they had calculated the percentages of caloric intake for low-, medium- and high-GI carbohydrates, they were able to analyze what difference substituting medium-GI carbohydrates for high-GI carbohydrates might make. That substitution of medium-GI foods for high-GI carbohydrates correlated with a reduction in risk of Type 2 diabetes.

On the other hand, substituting low-GI carbohydrates for high-GI carbohydrates made no difference in the risk of Type 2 diabetes, while substituting low-GI carbohydrates for medium-GI carbohydrates actually increased the risk of Type 2 diabetes.

Granted, dietary recall is subject to the vagaries of memory, but the extremely large sample of more than 25,000 men makes it less likely that such errors would significantly skew the conclusions. That said, the researchers noted that this particular study, showing no relationship between GI and the risk of Type 2 diabetes, is in line with other studies that include only men. Studies including only women, on the other hand, show some association between GI and diabetes. (One theory they put forward to explain this difference is that women may tend to eat more low-GI foods that might help protect them from diabetes in other ways.)

Glycemic index is certainly a useful tool, but it does not (yet) appear to be the single best answer for controlling blood sugars or preventing the development of Type 2 diabetes. Whether you have Type 2 diabetes or not, we do know that a diet high in both soluble and insoluble fiber is going to be better both for your blood sugars and your cholesterol. Eat more vegetables and fruits and choose whole grains and legumes to get both soluble and insoluble fibers in your diet -- the best of both worlds.