Background: Body mass index (BMI) and waist circumference (WC) are routine clinical estimates of obesity. We tested the fidelity of BMI and WC against direct body fat measures in Black (B) vs. White (W) Americans.

Subjects and Methods: We analyzed the relationship between BMI vs. total fat, WC vs. trunk fat, and weight vs. lean fat free mass (FFM) in healthy B and W adults. Assessments included clinical exam, biochemistries and a 75-g OGTT. All subjects had normal fasting and 2-hr OGTT glucose levels. Total fat mass, trunk fat mass and FFM were measured using DXA.

Results: The participants (n=376; 217 B, 159 W) had an age range of 18-65 years and BMI range of 16.8-66.3 kg/m2 (mean 30.1) . The W vs. B values were: age 46.5 ± 10.5 vs. 42.5 ± 10.3 y, P=0.0003; BMI 28.8 ± 6.78 vs. 31.2 ± 7.40, P=0.0015; total fat mass 29.4 ± 13.21 vs. 31.8 ± 13.76 kg, P=0.11; trunk fat mass 14.77 ± 6.96 vs. 15.34 ± 7.50 kg, P=0.48; FFM 50.12 ± 11.9 vs. 53.± 11.74 kg, P= 0.03. There were tight correlations between adiposity measures: total fat and BMI (r=0.90 in W, 0.92 in B) ; trunk fat and WC (r=0.85 in W, 0.82 in B) ; FFM and weight (r=0.83 in W, 0.87 in B) , with heterogeneity by sex and race. White men and women (vs B) tended to have more body fat per unit BMI or WC and less FFM per unit weight. We assessed 3 integrated ratios (total Fat/BMI, trunk Fat/Waist, FFM/Weight) by sex and race. Total fat/BMI was higher (P<0.0001) in W vs. B (men: 0.946 ± 0.159 vs. 0.833 ± 0.210; women: 1.36 ± 0.189 vs. 1.25 ± 0.164) . Trunk fat/waist was higher in W vs. B men (0.294 + 0.043 in W vs. 0.257 + 0.072, P=0.0046) but similar in women. In contrast, FFM/weight ratio showed no race disparities (men: 0.696 ± 0.055 vs. 0.678 ± 0.200; women: 0.586 ± 0.1vs 0.579 ± 0.076) . The proportion with >3 metabolic syndrome markers was 24.5% in W and 15.7% in B.

Conclusion: Among otherwise healthy subjects with a wide BMI range, African Americans had lower body fat than Whites. This finding that argues for ethnic-specific BMI cut-offs for obesity classification.

Disclosure

Z.Liu: None. A.A.Patel: None. S.Dagogo-jack: Consultant; AstraZeneca, Bayer AG, Boehringer Ingelheim International GmbH, Janssen Pharmaceuticals, Inc., Medtronic, Merck & Co., Inc., Sanofi.

Funding

American Diabetes Association (7-07-MN-13) ; NIH, R01 DK067269

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