Your body fat percentage is killing you, not your BMI, study suggests

In the latest study to question the usefulness of the body mass index, researchers have found that the percentage of one’s body fat is a better predictor of future death than BMI.

“Body mass index has no statistically significant relationship with all-cause mortality,” the University of Florida team reports in the July edition of the journal Annals of Family Medicine.

Instead, the researchers found that adults with a high body-fat percentage were nearly twice as likely to die from any cause over 15 years than those with a healthy body fat range, and more than three times likely to die from heart disease.

By contrast, a BMI indicating overweight or higher wasn’t associated with a statistically significant higher risk of death from any cause.

“This is the ultimate Coke versus Pepsi test. And BMI failed,” the study’s lead author, Arch Mainous, a professor and vice chair of research in community health and family medicine, said in a news release.

The BMI has been increasingly facing a moment of reckoning, with criticism growing that the body composition measure is a flawed and overrated proxy for determining a person’s health.

The BMI divides a person’s weight by the square of his or her height, and then, based on a range of numbers, lumps people into one of four categories: underweight, healthy weight, overweight or obese.

It’s been endorsed by the World Health Association and other health associations the globe over as the standard measure for body composition, the U of Florida team wrote.

However, the BMI doesn’t account for bone or muscle, meaning people with muscular physiques can be classified as overweight or obese.

It also doesn’t take into account where excess fat is deposited. Visceral fat or belly fat that wraps around the internal organs and increases the risk of diabetes and heart disease is more dangerous than fat accumulated around the hips.

For their study, the researchers analyzed data from 4,252 people aged 20 to 49 enrolled in a national U.S. health and nutrition survey. All had a technician measure their height, weight and waist circumference. Their body-fat percentage was also assessed via “bioelectrical impedance analysis,” or BIA, which works by applying a small, harmless current through the body, measuring resistance as it passes through the body’s tissues. It estimates total body composition, including fat-free mass and total body water.

Data collected from the health survey were linked to death certificate records. The researchers looked at the “mortality status” — alive, or deceased — of each participant after 15 years.

Adults with a higher body-fat percentage (27 per cent or more in men; 44 per cent or more in women) were 1.78 times more likely to die from any cause than people with lower body fat percentages.

Those with a high body-fat count were also 3.62 times more likely to die from heart disease.

However, a BMI in the unhealthy range — 25 or higher — was not associated with a higher risk of death from any cause, compared with adults in the healthy BMI range.

“Now remember, using BMI did not flag any risk at all in this younger population, which isn’t one we typically consider to be at high risk for heart disease,” the study’s senior author, Dr. Frank Orlando, told CNN.

“Think of the interventions we can do to keep them healthy when we know this early. I think it’s a game-changer for how we should look at body composition,” he said.

The study used BIA technology decades old “and that still had stronger associations with mortality when compared with the BMI,” the researchers wrote.

“Current BIA models provide reproducible results in less than one minute,” making their adoption easy for busy family medicine clinics, they said.

“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 patient who can benefit most from obesity and body fat reduction strategies to prevent a wide variety of diseases,” Mainous told Medscape .

At-home bathroom scales, fitness apps and smartwatches have also started to incorporate BIA technology though they aren’t as precise as office-based machines.

A high waist circumference — more than 44 inches in men and more than 35 inches is women — which doesn’t depend on height or weight also did better than BMI in predicting risks.

The study has several limitations, including that it focused on mortality as the outcome. While “this is the strongest and most definitive health outcome” the researchers didn’t consider morbidity, meaning other health complications other than death.

The authors are urging more and larger studies across different populations to determine healthy body fat percentage ranges, and at different ages.

“Once these standards are validated, it is likely that measuring BF% with BIA will become standard of care,” they said.

“These data will drive better discussions in the doctor’s office as well as public health initiatives with the goal of improving the health of all.”

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