Insulin Sensitivity Does Not Account for Differences in Type 2 Diabetes Incidence Between San Antonio, Texas and Mexico City, Mexico

We would like to thank Belfiore and Iannello (1) for their interesting questions regarding our study (2). We agree that lower insulin sensitivity in the San Antonio, Texas population could have contributed to the elevated incidence in this population compared with the Mexico City, Mexico population. Drs. Belfiore and Iannello calculated their insulin sensitivity index using the mean fasting and 2-h insulin values from our study because the raw data were not available to them. To address this question, we calculated the two indexes from the individual fasting and 2-h insulin values. Two indexes were calculated: ISI-a, a measure of insulin sensitivity based on the areas under the oral glucose tolerance test curve and ISI-b, a measure of insulin sensitivity in the basal state. As seen in Table 1, ISI-a was significantly higher in Mexico City, whereas ISI-b was not, thus confirming the calculations of Belfiore and Iannello. As noted by Belfiore and Iannello, these associations, which parallel the fasting and 2-h insulin values in Mexico City, could reflect impaired insulin secretion rather than enhanced insulin sensitivity.

Controlling for either ISI-a or -b did not attenuate the odds ratios reflecting the excess incidence of diabetes in San Antonio (Table 2). Thus, these indexes of insulin sensitivity do not account for the difference in incidence between the two cities. However, because these indexes may not be perfectly correlated with insulin sensitivity measured by a more definitive method, e.g., the euglycemic clamp, we do not dispute that differences in insulin sensitivity may account for part of the incidence difference between the two cities.