The effect of decision rules on the choice of a body mass index cutoff for obesity: examples from African American and white women

Background: Ethnic differences in the relation of body mass index (BMI; in kg/m(2)) to morbidity and mortality have led investigators to question whether a single cutoff for obesity should be applied to all ethnic groups. Objective: The effects of using 4 different outcomes and 3 different measures of effect as criteria for comparing BMI cutoffs were shown with the use of data from 45- to 64-y-old African American and white women. Design: Data were from the Cancer Prevention Study I (CPS-I) and the Atherosclerosis Risk in Communities (ARIC) Study. The outcomes were mortality (9211 deaths), diabetes (757 cases), hypertension (1518 cases), and hypertriglyceridemia (1264 cases). The measures of effect were incidence rate, rate ratio, and rate difference. The BMI in African American women that was associated with a risk equivalent to that of white women with a BMI of 30 was estimated, Results: There was no significant association between BMI and mortality in African American women. The BMI in African American women that was associated with a risk of diabetes equivalent to that of white women with a BMI of 30 was 28.0-34.5, depending on the measure of effect. For hypertension, the equivalent risk in African American women occurred at a BMI of < 18-38, depending on the measure of effect. There was no BMI at which African American women had an incidence rate or rate ratio for hypertriglyceridemia that was as high as that of white women with a BMI of 30. Conclusion: BMI cutoffs associated with equivalent risk across ethnic groups differ widely depending on the outcome and the risk estimate.