Total body size and central fat distribution are important determinants of insulin resistance. The BMI and waist circumference (WC) thresholds in African Americans that best predict insulin resistance are unknown. Our goal was to determine the BMI and WC values in African Americans, which optimally predict insulin resistance. The subjects were African Americans (68 men, 63 women), aged 35 ± 8 years (mean ± s.d.), with a BMI of 30.9 ± 7.5, in the range of 18.5-54.7 kg/m 2 , and with a WC of 98 ± 18, in the range of 69-173 cm. Insulin resistance was defined by the lowest tertile of the insulin sensitivity index (S I ). The Youden index was calculated to determine the WC and BMI thresholds that predict insulin resistance with an optimal combination of sensitivity and specificity. In men the thresholds that optimally predicted insulin resistance were a BMI ≥30 kg/m 2 or a WC ≥102 cm. For women, insulin resistance was best predicted by a BMI ≥32 kg/m 2 or a WC ≥98 cm. In African Americans, insulin resistance (in men) was best predicted by a WC ≥102 cm, and in women by a WC ≥98 cm, or by a BMI value that fell in the obese category (men: ≥30 kg/m 2 , women: ≥32 kg/m 2 ).
In African-American women, OCP use is associated with an increase in markers of cardiovascular risk manifested by increased insulin resistance, glucose intolerance, and elevated TGs.
The Disposition Index, the product of the insulin sensitivity index (S I ) and the acute insulin response to glucose, is linked in African-Americans to chromosome 11q. This link was determined with S I calculated with the nonlinear regression approach to the minimal model and data from the ReducedSampled-Insulin-Modified-Frequently-Sampled-Intravenous-Glucose-Tolerance-Test (ReducedSample-IM-FSIGT). However, the application of the nonlinear regression approach to calculate S I using data from the Reduced-Sample-IM-FSIGT has been challenged as being not only inaccurate but also having a high failure rate in insulin-resistant subjects. Our goal was to determine the accuracy and failure rate of the Reduced-Sample-IM-FSIGT using the nonlinear regression approach to the minimal model. With S I from the Full-Sample-IM-FISGT considered the standard and using the nonlinear regression approach to the minimal model, we compared the agreement between S I from the Full and Reduced-Sample-IM-FSIGT protocols. One hundred African-Americans, (BMI 31.3 ±7.6 kg/m 2 (mean±SD), range 19.0-56.9 kg/m 2 ) had FSIGTs. Glucose (0.3g/kg) was given at baseline. Insulin was infused from 20 to 25 minutes (total insulin dose 0.02U/kg). For the FullSample-IM-FSIGT, S I was calculated based on the glucose and insulin samples taken at -1, 1, 2, 3, 4, 5,6,7,8,10,12,14,16,19, 22, 23, 24, 25, 27, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 180. For the Reduced-Sample-FSIGT, S I was calculated based on the timepoints which appear in bold. Agreement was determined by Spearman correlation, concordance and the Bland-Altman method. In addition, for both protocols, the population was divided into tertiles of S I . Insulin resistance was defined by the lowest tertile of S I from the Full-Sample-IM-FSIGT. The distribution of subjects across tertiles was compared by rank order and kappa statistic. We found that the rate of failure of resolution of S I by the Reduced-Sample-IM-FSIGT was 3%(3/100). For the remaining 97 subjects, S I for the Full and Reduced-Sample-IM-FSIGT were: 3.76±2.41 L.mU -1 .min -1 , range 0.58-14.50 and 4.29±2.89 L.mU -1 .min -1 , range 0.52-14.42, relative error 21±18%, Spearman r=0.97, concordance 0.94, (both P<0.001). After log transformation the Bland Altman limits of agreement were: -0.29 and 0.53. The exact agreement for distribution of the population in the insulin-resistant tertile versus the insulin-sensitive tertiles was 92%, kappa 0.82±0.06. Using the nonlinear regression approach and data from the Reduced-Sample-IM-FSIGT in subjects with a wide range of insulin Corresponding Author: Anne E. Sumner, MD, NIDDK, 9000 Rockville Pike, Bethesda, MD 20892-1612, Phone: 301-402-4240, FAX: 301-435-5873, e-mail: AnneS@intra.niddk.nih.gov. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it i...
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