Do current guidelines for waist circumference apply to black Africans? Prediction of insulin resistance by waist circumference among Africans living in America

Abstract

Background: To lower the risk of diabetes and heart disease in Africa, identification of African-centred thresholds for inexpensive biomarkers of insulin resistance (IR) is essential. The waist circumference (WC) thresholds that predicts IR in African men and women have not been established, but investigations recently conducted in Africa using indirect measures of IR suggest IR is predicted by WC of 80-95 cm in men and 90-99 cm in women. These WC cannot be used for guidelines until validated by direct measurements of IR and visceral adipose tissue (VAT). Therefore, we determined in a group of African-born black people living in America (A) the WC, which predicts IR and (B) the influence of abdominal fat distribution on IR.

Methods: The 375 participants (age 38±10 years (mean±SD), 67% men) had IR determined by HOMA-IR and Matsuda index. VAT and subcutaneous adipose tissue (SAT) were measured by abdominal CT scans. Optimal WC for the prediction of IR was determined in sex-specific analyses by area under the receiver operating characteristic (AUC-ROC) and Youden index.

Results: Women had more SAT (203±114 vs 128±74 cm²) and less VAT than men (63±48 vs 117±72 cm², p<0.001). Optimal WC for prediction of IR in men and women were: 91 cm (AUC-ROC: 0.80±0.03 (mean±SE)) and 96 cm (AUC-ROC: 0.81±0.08), respectively. Regression analyses revealed a significant sex-VAT interaction (p<0.001). Therefore, for every unit increase in VAT, women had a 0.94 higher unit increase in SAT and 0.07 higher unit increase in WC than men.

Conclusion: Working with a group of African-born black people living in America, we accessed technology, which validated observations made in Africa. Higher SAT at every level of VAT explained why the WC that predicted IR was higher in women (96 cm) than men (91 cm). For Africans to benefit from WC measurements, convening a panel of experts to develop evidence-based African-centred WC guidelines may be the way forward.

Keywords: Africans; insulin resistance; visceral adiposity; waist circumference.

Figure 1

Flow diagram for enrolment. OGTT, oral glucose tolerance test; WC, waist circumference.

Figure 2

Example of automated VAT and SAT scans in African immigrant man and woman of similar age, BMI and WC. BMI, body mass index; SAT, subcutaneous adipose tissue; VAT, visceral adipose tissue; WC, waist circumference.

Figure 3

Metabolic characteristics by glucose tolerance group. Men: solid bars; women: open bars. Group 1: normal glucose tolerant (NGT) (reference group (REF)); group 2: NGT and insulin resistant (IR); group 3: abnormal glucose tolerant (Abnl-GT) and IR; group 4: Abnl-GT and no IR. (A) Body mass index (BMI); (B) waist circumference (WC); (C) visceral adipose tissue (VAT); (D) subcutaneous adipose tissue (SAT); (E) HOMA-IR; (F) insulinogenic index. Comparison with group 1 (NGT): *P≤0.05, **p≤0.01, ***p≤0.001.

Figure 4

Correlations between central fat depots. Men: solid circles, dotted line; women: open circles, solid line. (A) Correlation of WC to VAT, interaction of sex and WC significant p<0.001, equation for men: WC=77.62+0.12*VAT; equation for women: WC=77.43+0.19*VAT. (B) Correlation of SAT to VAT, interaction of sex and WC significant p<0.001, equation for men: SAT=47.56+0.69*VAT; equation for women: SAT=101.44+1.63*VAT. VAT, visceral adipose tissue; WC, waist circumference.