Is the current BMI obesity classification appropriate for black and white postmenopausal women?

Objective: To evaluate the relation between body fatness (% Fat) and body mass index (BMI) and to evaluate the validity of the BMI standards for obesity established by the NIH in older black and white postmenopausal women. Research methods: Height, weight, BMI, and % Fat, assessed by DXA, were determined for 296 healthy, independently living women ranging in age from 50 to 80 years ( M +/- s.d.; 64.4 +/- .8 years). Results: Per NIH guidelines, 32% were classified as obese (>= 30 kg/m(2), mean BMI 28.1 +/- 5.5 kg/m(2)). In contrast, using the % Fat criterion of 38% advocated by Lohman to define obesity, 47% of our sample was obese (mean % Fat 37.3 +/- 6.2%). A moderately high curvilinear relation existed between BMI and % Fat (R =0.82, SEE =3.57 % Fat, P < 0.05). Race added meaningfully to the prediction of % Fat (P < 0.05) such that for the same BMI, black women will have 1% lower body fatness than white women. Based on a % Fat >= 38 as the criterion for obesity, receiver operating characteristic (ROC) analysis, performed separately by race, indicated that the currently accepted BMI cutpoint for obesity produced low sensitivity (9% and 61% for black and white women, respectively). Alternatively, BMI values >= 28.4 kg/m(2) for black women and >= 26.9 kg/m(2) for white women to define obesity maximized classification accuracy. Conclusion: We conclude that current BMI categories may not be appropriate for identifying obesity among postmenopausal women. Furthermore, the relation between BMI and % Fat is different in black compared to white women but remains constant from the sixth through the eighth decade of life.