Body Fat Beats BMI in Predicting Death

Body fat percentage (BF%) may be a better predictor of all-cause and cardiovascular mortality in individuals aged 20-49 years than BMI, according to a new study published in the Annals of Family Medicine.

Researchers suggested that clinicians more widely adopt the tool used in the study to measure BF%, a bioelectrical impedance analysis (BIA), and said the measurement will become standard of care in the future.

“For essentially the same price as a scale to weigh newborns or a machine to sterilize instruments, a machine to reliably and validly assess body fat percentage will allow a practice to accurately target the patients who can benefit most from obesity and body fat reduction strategies to prevent a wide variety of diseases,” said Arch G. Mainous III, PhD, a professor and vice chair for Research in the Department of Community Health and Family Medicine at the University of Florida in Gainesville, Florida, and lead author of the study.

Experts have been vocal about abandoning BMI for more precise measures because the calculation cannot distinguish muscle mass from body fat. Previous findings have shown BF% is a more accurate reflection of current health status and a predictor of well-being later in life. Still, BMI remains widely used in clinical settings.

To assess how BMI and BF% are associated with 15-year risk for mortality, Mainous and his team analyzed data from a population-based cohort of 4252 adults (33.64% women) in the 1999-2004 National Health and Nutrition Examination Survey.

The data included the participants’ BMI and BF% obtained from physical examinations. BF% was measured using a BIA device that assesses fat-free and fat mass.

The researchers defined an unhealthy BF% as greater than or equal to 27% in men and greater than or equal to 44% in women.

The researchers found that BF% was more accurate in predicting 15-year all-cause and heart disease mortality than BMI. Participants with an unhealthy BF% had a 78% increased risk for all-cause mortality (adjusted hazard ratio [HR], 1.78; 95% CI, 1.28-2.47; P < .001) and were about 2.6 times more likely to die from cardiovascular disease (adjusted HR, 3.62; 95% CI, 1.55-8.45; P = .003) over the study period than were those with a healthy BF%.

The researchers did not find a statistically significant association between BMI and the outcomes, including heart disease mortality (adjusted HR, 2.227; 95% CI, 0.833-5.952; P =  .110).

The finding that BF% showed stronger associations with mortality than BMI was striking considering the study used BIA technology from 25 years ago, said Frank A. Orlando, MD, clinical associate professor and medical director at UF Health Family Medicine at the University of Florida and an author of the study. BIA machines today produce results in < 1 minute, while previous iterations of the technology entailed application of electrodes and took up to 5 minutes to produce a calculation.

“BIA machines are reliable and valid and can be integrated into a clinical practice to provide body composition measurements that can stratify young adults for mortality risk,” Orlando said.

BIA is generally more accessible to clinicians than dual x-ray absorptiometry (DEXA) scans, Orlando said.

“DEXA scans have not supplanted BMI because it’s expensive and difficult to use,” Orlando said.

A new DEXA scanner costs a practice between $16,000 and $45,000. BIA machines range from $129 to nearly $16,000, an expense Orlando said is well worth it.

Clinicians may need to work with leadership at their practices to support the purchase of BIA machines and could use the study findings to convey the value these devices provide, said Ryan Singerman, DO, the section chief for Primary Care at Parkview Health in Fort Wayne, Indiana, who was not involved in the study.

“Not everyone is well-versed in the medical evidence or equipped to convince management of the need,” Singerman, whose practice uses BIA, said.

Kyle J. Thompson, MD, a bariatric surgeon at Intermountain Health in Salt Lake City, Utah, said while BMI is still the clinical go-to, some patients are gaining awareness of BF% and may seek out clinicians who use the measure. Offering BF% analysis could bring more business, he said.

“By offering this service, you may increase the number of referrals you receive and see an increase in the volume of other services you provide, such as medical weight loss or bariatric surgery,” Thompson, who was not involved with the study, said.

But with BMI as the current standard, no consensus exists on the definition of healthy BF%, which has implications for patient care.

“Minimal training is required to interpret the results produced by BIA, but there is still a significant amount of variability in how these results are used to guide clinical care,” Thompson said.

Mainous said that the next step in replacing BMI with BF% is reaching a consensus on the definition through large, prospective trials.

“Once these standards are validated, measuring BF% with BIA will then become the standard of care,” Orlando said. “Until then, clinicians can still use BMI alongside BIA in the office to augment risk stratification using published thresholds for healthy body fat percentage.”

None of the study authors reported relevant financial disclosures.