Waist Circumference: Better Than BMI for Predicting Health Risks?

Body mass index (BMI, kg/m2) has long been used as a predictor of morbidity and mortality from several chronic diseases, including diabetes, heart disease, and stroke. Evidence from a recent study indicates that when waist circumference (WC) is added to BMI, the two measurements more accurately predict health risk than BMI alone. According to Dr. Ian Janssen and fellow epidemiologists at Queen’s University, Kingston, Ontario, Canada, WC may be a stronger marker of health risk than is BMI (Am J Clin Nutr 2004;79:379).

Data from a large study

To compare the efficacy of both measures, Dr. Janssen and his colleagues used data from the third National Health and Nutrition Survey (NHANES III). The NHANES data was taken from 14,924 adults, who were grouped into categories of BMI and WC in accordance with National Institutes of Health cutoff points. Men and women with WC values less than <102 cm (40 inches) and 88 cm (35 inches), respectively, were considered to have normal waist circumference, while men and women with WC values above >102 and 88 cm, respectively, were classified as having high waist circumference.

Subjects were placed in one of three categories on the basis of BMI: normal weight (BMI 18.5-24.9), overweight (BMI 25.0 to 29.9), or class I obese (BMI 30.0 to 34.9). Odds ratios for hypertension, dyslipidemia, and the metabolic syndrome were compared for overweight and class I obese BMI categories and the normal-weight category before and after adjustment for WC. BMI and WC were also included in the same regression model as continuous variables for prediction of the metabolic disorders.

Results

According to the authors, their results provide compelling evidence that BMI coupled with WC does not predict an increase in obesity-related health risk better than WC alone, when the two values are examined on a continuous scale. Thus, overweight and obese persons have a health risk that is comparable to that of normal-weight persons with the same WC value. However, when WC is dichotomized as a normal or high-risk value according to the NIH obesity guidelines, BMI remains a significant predictor of metabolic health risk. This suggests that the obesity classification advocated by the NIH is misleading and can be improved.

Thus, obesity-related health risks are explained by WC and not by BMI, say the authors. And, for a given WC value, overweight and obese persons have a health risk that is comparable with that of normal-weight persons. Future studies are required to determine whether WC can be used alone as an indicator of health risk.

A dissenting opinion

According to Dr. George A. Bray, of Pennington Biomedical Research Center, Baton Rouge, LA, there is still far too little evidence that waist circumference can be substituted for body mass index as a measure of health risk. In an editorial in the same issue (Am J Clin Nutr 2004; 79:347), Dr. Bray urges clinicians not to consider using WC alone as an indicator of health risk. Instead, he notes that height and weight, the main components of BMI, are important indicators of health status and are easy for health care workers to measure. In addition, he points out that the National Heart, Lung, and Blood Institute clearly recognizes that measuring BMI is only the first step for a clinician making an assessment of health risk and that central adiposity should also be measured. Dr. Bray added that the most practical criteria for measuring central body fat are WC and WC divided by hip circumference, or using a waist-to-hip ratio.