Glycemic Index and Type 2 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 sugars. The Glycemic Index (GI) was created in an effort to rank foods on 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.

Naturally the Glycemic Index has been of great interest to those pursuing research in diabetes prevention and treatment, but the results of these studies have been mixed. Some studies show a relationship between diabetes risk and GI or GL (or both), while others do not. The 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.

A study published in the British Journal of Nutrition (2011;105(8):1258-1264) takes 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 over 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 1098 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 - which showed a reduction in risk of type 2 diabetes. On the other hand, substituting low-GI carbohydrates for high-GI carbohydrates made no difference, while substituting low-GI carbohydrates for medium-GI carbohydrates actually increased the risk of type 2 diabetes.

What this means for you

The authors of this study note that this study, showing no relationship, is in line with other studies that include only men, while other studies including only women have shown some association between GI, GL and diabetes. One theory they put forward to explain this difference is that women may tend to eat more low-GI foods that would have other properties that help to reduce their risk of diabetes. In any event, it's pretty clear that the jury is still out on GI, GL and type 2 diabetes risk. Which is not to say that these measures can not be helpful - only that more studies need to be done.

First posted: April 27, 2011