Cedars-Sinai investigators have developed a simpler and more accurate method of estimating body fat than the widely used body mass index, or BMI. Called the relative fat mass index, or RFM, it was chosen from more than 300 possible formulas after validation against a database of 12,000 US adults.
"We wanted to identify a more reliable, simple and inexpensive method to assess body fat percentage without using sophisticated equipment," said the study leader, Dr Orison Woolcott, of Cedars-Sinai.
While the BMI is commonly accepted, many medical experts in the field of obesity consider it to be inaccurate because it cannot distinguish among bone mass, muscle mass and excess fat. BMI also does not account for the influence of gender — women generally have more body fat than men.
The new formula developed at Cedars-Sinai is called the relative fat mass index, or RFM, and it uses only height and waist circumference measurements.
"Our results confirmed the value of our new formula in a large number of subjects: Relative fat mass is a better measure of body fatness than many indices currently used in medicine and science, including the BMI," Woolcott said.
For the first time, researchers examined more than 300 possible formulas for estimating body fat using a large database of 12,000 adults who participated in a health and nutrition survey conducted by the US Centres for Disease Control and Prevention.
In the next step, investigators calculated the relative fat mass for 3,500 patients and compared the results to the patients' outcomes from a specialized, high-tech body scan called DXA, widely considered one of the most accurate methods of measuring body tissue, bone, muscle and fat. The patients' RFM results corresponded most closely with the precision of the DXA body scan.
"The relative fat mass formula has now been validated in a large data base. It is a new index for measuring body fatness that can be easily accessible to health practitioners trying to treat overweight patients who often face serious health consequences like diabetes, high blood pressure and heart disease," said Dr Richard Bergman, the senior author of the study and director of the Cedars-Sinai Sports Spectacular Diabetes and Obesity Wellness and Research Centre.
And the best part, according to Woolcott: "You don't need a bathroom scale to determine your relative fat mass, just a measuring tape."
To determine relative fat mass (RFM), you need to measure your height as well as your waist circumference. To measure your waist, place the tape measure right at the top of the hip bone and reach it around your body for the most reliable result. Next, put those numbers into the relative fat mass equation – making a ratio out of the height and waist measurements. The formula is adjusted for gender.
Relative Fat Mass Formula
Men: 64 – (20 x height/waist circumference) = RFM
Women: 76 – (20 x height/waist circumference) = RFM
More than 93m people – nearly 40% of the US population – are considered overweight, according to the CDC. Obesity is associated with a poor quality of life and premature death from chronic disease.
"We still need to test the RFM in longitudinal studies with large populations to identify what ranges of body fat percentage are considered normal or abnormal in relation to serious obesity-related health problems," Woolcott said.
High whole-body fat percentage is independently associated with increased mortality. We aimed to identify a simple anthropometric linear equation that is more accurate than the body mass index (BMI) to estimate whole-body fat percentage among adult individuals. National Health and Nutrition Examination Survey (NHANES) 1999–2004 data (n = 12,581) were used for model development and NHANES 2005–2006 data (n = 3,456) were used for model validation. From the 365 anthropometric indices generated, the final selected equation was as follows: 64 − (20 × height/waist circumference) + (12 × sex), named as the relative fat mass (RFM); sex = 0 for men and 1 for women. In the validation dataset, compared with BMI, RFM better predicted whole-body fat percentage, measured by dual energy X-ray absorptiometry (DXA), among women and men. RFM showed better accuracy than the BMI and had fewer false negative cases of body fat-defined obesity among women and men. RFM reduced total obesity misclassification among all women and all men and, overall, among Mexican-Americans, European-Americans and African-Americans. 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.
Orison O Woolcott, Richard N Bergman
[link url="https://www.cedars-sinai.org/newsroom/cedars-sinai-investigators-develop-more-accurate-measure-of-body-fat/"]Cedars-Sinai material[/link]
[link url="https://www.nature.com/articles/s41598-018-29362-1"]Scientific Reports abstract[/link]