Reduced Insulin Sensitivity in Mexican-Americans From San Antonio With Elevated Incidence of Type 2 Diabetes Compared With Mexicans From Mexico City

The interesting article by Burke et al. (1) in the September issue of Diabetes Care reports a higher incidence of type 2 diabetes among low-income Mexican-American residents in San Antonio, Texas, than in a comparable population from Mexico City, Mexico. This difference was not accounted for by the potential mediating factors examined (BMI; waist circumference; fasting and 2-h post–glucose load insulin and glucose; fasting triglycerides and HDL cholesterol; and percentage of impaired glucose tolerance, impaired fasting glucose, or hypertension).

However, in our opinion, an important consideration is missing: the possible role of reduced insulin sensitivity in the high incidence of type 2 diabetes in San Antonio. Indeed, analysis of the data shown in Table 2 of the paper by Burke et al. indicates that both men and women who were residents in San Antonio had statistically significant higher 2-h glucose and 2-h insulin levels compared with men and women who were residents in Mexico City. The simultaneous increase in insulin and glucose levels suggests the occurrence of reduced insulin sensitivity.

Recently, we published a method that allows the quantitative measurement of insulin sensitivity from the insulin and glucose levels recorded at 0, 1, and 2 h during an oral glucose tolerance test (OGTT), or even (with minimal loss of sensitivity) from data recorded only at 0 and 2 h (2–4). Because the values of insulin and glucose at 0 h (i.e., in the fasting state) and at 2 h during OGTT are given in the article by Burke et al., it was possible for us to calculate insulin sensitivity. Our method is based on the following formula: ISI(gly) = 2/[(INSp × GLYp) + 1], where ISI(gly) indicates the insulin sensitivity index toward glycemia [our method also allows the measurement of insulin sensitivity toward blood free fatty acids (FFAs), or ISI(ffa)] and INSp and GLYp indicate insulinemic and glycemic areas, respectively, during OGTT for the person studied. Because this test is based on the “areas” during OGTT, it can be indicated as ISI(gly)-a. By using basal levels, instead of areas, we can measure the insulin sensitivity in the basal state, or ISI(gly)-b. Both the basal levels and areas are expressed by taking the “mean normal value” as 1 (i.e., by dividing the observed value by the mean normal value). In normal subjects, ISI(gly) is always ∼1, with maximal variations among patients comprised between 0 and 2. ISI(gly), as well as the insulin sensitivity index toward blood FFAs, can be easily calculated through a computer program that is freely downloadable from the following website: http://users.iol.it/francesco.belfiore/index.htm.

As just mentioned, to measure insulin sensitivity with our method, the mean normal values of basal levels and areas of insulin and glucose are required. Because Burke et al. did not include a “normal” group in their study, we used as mean normal values for insulin and glucose those values reported in Table 1 of a previous article by the same authors (5); the values were obtained from 870 individuals of both sexes. Note, however, that the use of different mean normal values would equally affect the ISI(gly) values in both groups (Mexico City and San Antonio residents) and therefore would not alter the difference between them. Results of our analysis are shown here in Table 1.

These values of insulin sensitivity obtained with our index are derived from the mean values (and not from individual values) of insulin and glucose reported in Table 1 of the article by Burke et al. (1). Therefore, they cannot be statistically evaluated. However, because the reduced values of ISI(gly)-a that we found in the San Antonio population were derived from the “areas” that showed statistically significant difference between the two populations, these reduced values should be statistically significant. On the other hand, the ISI(gly)-b is derived from basal values of insulin and glucose, which changed in opposite directions in the two populations, thus annulling each others statistical significance.

Thus, because the value of ISI(gly) in normal subjects is always equal to 1, both of the populations studied have reduced insulin sensitivity. However, compared with Mexico City residents, the San Antonio population shows a more pronounced reduction in insulin sensitivity deduced from the areas during OGTT, i.e., during the absorption period (characterized by high insulin and glucose levels), but not in the basal state. This is true for both men and women. This observation, which in turn may depend on several factors, may contribute to explain the different incidence of diabetes between the two populations.