Do Glycemic Index, Glycemic Load, and Fiber Play a Role in Insulin Sensitivity, Disposition Index, and Type 2 Diabetes?

In this issue of Diabetes Care, Liese et al. (1) report on the relation of dietary glycemic index, glycemic load, carbohydrate, and fiber intake to measures of insulin sensitivity, insulin secretion, and adiposity in the Insulin Resistance Atherosclerosis Study (IRAS). The evaluation was confined to those with normal or impaired glucose tolerance and did not include diabetic individuals. As the authors state, there has been no previous large epidemiologic study relating glycemic index and glycemic load to direct measures of insulin sensitivity and insulin secretion, whose dysfunction are the two hallmarks leading to type 2 diabetes.

There are some caveats to this study that must be pointed out. First, it is a cross-sectional study looking at one point in time. Longitudinal studies are certainly preferable. Second, it is an observational study and interventional studies are more valuable, though much more difficult and expensive to carry out. Third, the food frequency questionnaire used in this study was not specifically designed to test for glycemic index and glycemic load. While it has been validated as an overall instrument, it has not been validated for reproducibility and reliability as an appropriate glycemic index instrument, and this needs to be done by the IRAS group. This field has been dogged by the inaccuracy of dietary records and the difficulty in calculating dietary glycemic index and glycemic load levels of individuals from their reported intake of foods. Validation of experimental instruments is crucial. Fourth, the minimal model was instituted using 30 plasma samples to calculate insulin sensitivity (2). This study used 12. While this seems reasonable in a large epidemiological study, the reliability is likely to …