Can Relative Fat Replace Mass BMI in Assessing Obesity?

 My various calculations are: 

My RFM is 24.

waist-to-height ratio is .904

BMI of 28.2

All because my doctor COMPLETELY FAILED AT GETTING ME 100% RECOVERED!

I'm not worried at all about this stuff.

Can Relative Fat Replace Mass BMI in Assessing Obesity?

Earlier this year, the Lancet Commission on the Definition and Diagnostic Criteria of Clinical Obesity concluded that BMI alone is inadequate for assessing excess body adipose tissue, which is both harmful to health and key to diagnosing clinical obesity. The Commission instead recommended that obesity status include additional anthropometric confirmation plus evidence of reduced organ function and/or limitations in daily activities. 

This marks an important and necessary advance, as relying solely on BMI risks misdirecting obesity treatments and inflating already substantial healthcare costs. 

Developing a more accurate tool for identifying clinical obesity is also essential given the persistent stigma surrounding this disease. Obesity as a standalone disease remains controversial both inside and outside the medical community. A clearer, more reliable tool could reduce stigma and expand access to appropriate care. 

Although the Lancet Commission’s recommendations improve diagnostic precision, they may be onerous to implement in busy clinical practices. Therefore, the search for a simple, practical tool that matches BMI’s ease of use while improving its accuracy in identifying excess deleterious body fat remains a priority. 

Searching for Better Measures

Efforts to measure risk from excess adipose tissue began decades ago. In 1998, the National Institutes of Health published the first obesity treatment guidelines, recommending using BMI plus waist circumference in those with BMI 25-35 to better estimate visceral adipose tissue and identify those at an increased risk of complications. It provided a helpful illustration for practitioners. At the time, supporting evidence was limited (graded “C”). 

Twenty years later, stronger evidence supports a more accurate tool, the Relative Fat Mass (RFM) index. Given the Lancet Commission’s call for a better measure than BMI, we believe RFM deserves renewed attention. We also believe that clinical inertia fueled by the stigma surrounding obesity has clouded progress in developing this tool over the past 7 years. 

A Closer Look at RFM

Developed and validated in 2018 using data from the National Health and Nutrition Examination Survey (NHANES), RFM is a sex-specific anthropometric measure of obesity that estimates body fat percentage based on height and waist circumference using the following formula:

• RFM = 64 − (20 × height/waist circumference) + (12 x sex [0 for males and 1 for females])

This simple calculation incorporates waist circumference as a proxy of visceral body fat while accounting for sex-based differences in fat mass. Multiple studies have shown RFM to be a superior and more consistent predictor of cardiometabolic risk and mortality. 

Obesity cutoffs were derived from NHANES (1999-2014) data linking RFM with all-cause mortality. After adjusting for age, BMI category, ethnicity, education, and smoking status, this analysis suggested that higher RFM was associated with substantially increased mortality risk. Women with an RFM of ≥ 40% (40% body fat) and men with an RFM of ≥ 30% (30% body fat) had a 50% higher risk of death compared with women with an RFM of ≤ 35% and men with an RFM of ≤ 25%. Additionally, women with an RFM of ≥ 45% had nearly double the risk of death, whereas men with an RFM of ≥ 35% had more than 2.5 times the risk of death.

RFM has also outperformed BMI in estimating body fat percentage in children and adolescents, measured using DEXA. For children and adolescents aged 8-14 years, a modified RFM for pediatric populations was successfully tested against BMI for age percentiles. Additionally, RFM has been reported as the strongest anthropometric risk predictor of heart failure and type 2 diabetes in prospective cohort studies of community dwelling adults in the Netherlands. 

Putting RFM Into Practice 

The next steps towards replacing BMI with RFM in clinical practice include validating RFM against the Lancet Commission’s protocol for assessing obesity. If validated, widespread clinical adoption will depend upon national and international training in accurate waist circumference measurement, standardizing waist circumference as a vital sign alongside height and other anthropometrics and incorporating RFM into routine obesity evaluations. 

The greatest limitation of RFM compared with BMI is the potential for inconsistent measurement of waist circumference. Counteracting this requires consistent and accurate landmark identification. If RFM more accurately identifies excess adiposity and predicts mortality, it could fulfill one of the Commission’s core criteria by providing two anthropometric measures that correlate with disease. 

Adoption of RFM will require the collaboration of all societies invested in obesity assessment and treatment. 

Why This Matters Now

The past several years has seen the rise of nutrient-stimulated hormonal (NuSH) therapies, better known as GLP-1s. Yet clinical assessment of obesity remains rooted in outdated and stigma-fueled practices, limiting access to these life-saving medicines. BMI alone cannot reliably assess risk and identify those most likely to benefit from treatment. 

With wider adoption of NuSH therapies, rare but serious adverse effects beyond those identified in clinical trials have been reported in broader populations. Improving diagnostic accuracy may be the first step toward ensuring a favorable risk-benefit ratio for NuSH therapies. 

We urgently need a better tool for identifying clinically significant excess adiposity, and RFM is a promising option. The field of obesity medicine and the health of our patient population depends on it.