Abstract P028: Higher Waist Circumference Identifies Insulin Resistance in Middle-age African-American Women Compared to Standard Metabolic Waist Thresholds for Europeans.
Authors: Harman Jane L , Buxbaum Sarah G , Hairston Kristen G , Sumner Anne E 
Published: July 2021

Introduction
Waist circumference thresholds that predict insulin resistance (IR) or metabolic syndrome may differ by race and ethnicity. Notably, a recent analysis of data from young African American (AA) adults (age 35±8) indicated that for AA women, the waist circumference (WC) predicting the top tertile of IR (mean HOMA-IR=3.0) was higher (≥98cm) than the IDF and NCEP-ATPIII threshold (≥88cm) for central obesity of metabolic syndrome.

Hypothesis
This analysis tested the hypothesis that in middle-aged AA adults, the WC predicting IR would be similar to that found in young AA adults: in AA women, a higher WC would predict IR than the standard WC threshold for metabolic syndrome.

Methods
Participants were AA adults attending the first examination of the Jackson Heart Study (2000-2004). After restricting analysis to non-related persons, non-diabetic, ages 35-80, BMI 18.5-55, with non-missing HOMA-IR, the analytical data set comprised 2269 persons (1374 women); age 55±11 (mean±SD); HOMA-IR 3.6±2.2; BMI 31±6.4 kg/m
2
; WC: women: 98.3±15.6cm, men: 100.2 ±13.7cm. By ROC analysis, WC was a significantly better predictor of HOMA-IR than BMI for women but not men, although the predictive Area Under the Curve (AUC) was greater for men than women for both variables: AUC for WC vs.BMI: 0.78 vs 0.77 for men (p=.13); 0.75 vs 0.72 for women (p<.02). Due to lack of international standardization for insulin measures, IR was defined as the upper tertile (≥3.8) or upper quartile (≥4.4) of HOMA-IR. Mean HOMA-IR for the upper tertile was 5.9. For various thresholds of waist, the Youden index (sensitivity + specificity - 1) was calculated, as well as Pos Pred Value (PPV), Neg Pred Value (NPV), and diagnostic accuracy (DA) (prevalence*sensitivity + (1-prevalence)*specificity). The WC threshold that maximized the Youden index was considered the most appropriate for determining IR.

Results
The Youden index for predicting IR by WC maximized at 96cm for women and 104cm for men, with IR defined either as the upper tertile or the upper quartile of HOMA-IR. Sensitivity, specificity, PPV, NPV, and diagnostic accuracy were necessarily affected by the definition of IR that determined prevalence; using the upper tertile of HOMA-IR to define IR, optimized WC thresholds had DA =0.66 for women and 0.74 for men.

Conclusions
The WC thresholds for identifying the top tertile of IR (96cm women; 104cm men) were comparable to those found for younger AA adults in a previous study (98cm women; 102cm men), notwithstanding the much higher mean HOMA-IR (5.9) of the upper tertile in this middle-aged group, compared to that (3.0) of the younger AA adults of the earlier study. For middle-age AA women, this analysis confirmed the finding of the earlier study of younger AA women, in that the waist circumference threshold identifying insulin resistance in AA women may be higher than that defining metabolic syndrome-related waist circumference in European women.