New Equation Bests BMI at Estimating Body Fat Mass

Lab Medicine

A new relative fat mass (RFM) equation based on height-to-waist-circumference ratio better predicts the percentage of whole-body fat in men and women than body mass index (BMI), new research indicates.

The RFM equation also resulted in fewer instances of misclassification for obesity categories in both sexes and in all ethnic groups tested. 

"BMI is widely used to assess body fatness, despite its limited accuracy to estimate body fat percentage. Thus, simple and low-cost alternatives to BMI with better diagnostic accuracy for obesity in both sexes would be of considerable importance," write Orison Woolcott, MD, and Richard Bergman, MD, both with the Sports Spectacular Diabetes and Obesity Wellness and Research Center at Cedars-Sinai Medical Center in Los Angeles, California.

"In the population studied, the suggested RFM was more accurate than BMI to estimate whole-body fat percentage among women and men and improved body fat-defined obesity misclassification among American adult individuals of Mexican, European, or African ethnicity," they report.

However, obesity expert Lee Kaplan, MD, Ph.D., director of the Obesity, Metabolism and Nutrition Institute at Massachusetts General Hospital in Boston told that although the RFM estimate of body fat mass might prove helpful as a research tool, he doubts it will be more useful than BMI in real-life clinical practice.

He notes that although BMI is used to identify patients who are obese, clinicians should consider what the effect of excess weight is on a patient's health — and the RFM measure, like BMI, does not do that.

"As a clinician what you really want to know is, 'What is the clinical impact of the obesity?' That's what you care about," Kaplan elaborated. The question then becomes, Is a higher RFM associated with worse outcomes? This is the same question physicians need to answer if they use BMI to guide clinical decision-making.

"The fact that RFM is more consistent [in predicting whole-body fat percentage] than BMI in men and women is interesting, but not necessarily all that clinically important," Kaplan reiterated.

NHANES Surveys

To develop and validate RFM, the authors used two sets of data from the National Health and Nutrition Examination Survey (NHANES). They used the NHANES survey conducted between 1999 to 2004 for model development. Percentages of whole-body fat were measured by dual-energy X-ray absorptiometry (DXA) in both models.

They considered over 350 anthropometric measures in order to arrive at a simple, anthropometric linear equation more accurate than standard BMI for estimating the percentage of whole-body fat in men and women of a variety of ethnic groups.

"Height/waist equation, named as the relative fat mass, was the final model selected because of its simplicity (it requires only two common anthropometrics), it was superior to BMI in predicting body fat percentage among men, had similar predicting ability relative to BMI among women, and had overall better performance than BMI among women and men, independently," Woolcott and Bergman write.