How BMI compares to body fat percentage in determining obesity

Many people with “normal” body mass indexes may still be obese, new research suggests.

The study, which will be presented Friday in Chicago at ENDO 2023, the annual meeting of the Endocrine Society, adds to a growing body of evidence that highlights the flaws of using BMI to measure obesity.

Obesity, in the simplest terms, refers to when a person has an excess of fat in the body.

BMI — which is calculated using a person’s height and weight — is the standard way to determine whether someone is obese or is overweight. A BMI of 30 and above falls into the obese category for most people; 25 to under 30 is considered overweight. It’s ubiquitously used by medical professionals to determine who is at risk for Type 2 diabetes and heart disease and who qualifies for anti-obesity medications and weight loss surgery.

According to the new study, BMI misses a lot of cases of obesity. Researchers looked at data from almost 10,000 US adults collected from 2011 to 2018 and found that almost 36% had obesity based on their BMI. Use of a different measure of obesity — a person’s body fat percentage — put that figure at 74%. The study has not yet been published in a peer-reviewed journal.

“The main message here is that we vastly underestimate the prevalence of obesity using BMI and there are many people with normal BMI who still have obesity,” said Dr. Aayush Visaria, an internal medicine resident at Rutgers Robert Wood Johnson Medical School, who led the study.

The findings come as the American Medical Association also takes steps to move away from relying solely on BMI to assess health and obesity, citing “numerous concerns with the way BMI has been used to measure body fat and diagnose obesity.” On Wednesday, it adopted a new policy advising health care professionals to use other measures — such as where fat is distributed in the body, waist circumference and how much of a person’s weight comes from fat — along with BMI.

In the new research, Visaria used several measures of obesity in addition to BMI, including body fat percentage and something called android-to-gynoid fat ratio. Here’s how these different tools stack up.

How is BMI calculated?

The World Health Organization cutoff for obesity for most people is a BMI of at least 30, and at least 25 for people of Asian descent. The lower cutoff is because Asians are more likely to develop obesity-related conditions, including high blood pressure and diabetes, at lower BMIs, according to the National Institutes of Health. Visaria used a cutoff of 27.5 for Asian adults, which is the standard some medical organizations use instead of 25.

BMI is calculated by dividing a person’s weight, in kilograms or pounds, by their height, measured in meters or feet, squared. Its simplicity makes it an extremely accessible tool — many online calculators allow a person to simply enter their height and weight — but it has limitations.

Namely, “we’re not sure what that weight is made up of,” said Dr. Jaime Almandoz, medical director of the Weight Wellness Program at UT Southwestern Medical Center in Dallas, who was not involved with the new research.

How much a person weighs doesn’t only depend on how much fat is in the body; there’s also the weight of muscle, bone, water and organs. BMI does not parse this out.

“People with the same BMI, the same pants size and the same belt notch have different amounts of fat and muscle,” said Dr. Vijaya Surampudi, an assistant director of the UCLA weight management program in Los Angeles, who was also not involved with the new research.

People who have more muscle and less fat burn more calories while at rest and are considered to have a healthier body composition, but because muscle also weighs more than fat, a very muscular person’s BMI can skew higher.

“There are people with a BMI over 30 who are weight lifters who are actually very healthy,” Visaria said.

How is body fat percentage calculated?

Body fat percentage looks at how much a person’s weight comes from fat.

It can be measured using a type of X-ray called a Dexa scan, which can distinguish between bone and soft tissue, like fat, in the body.

Total body fat percentage greater than or equal to 25% for males, or 32% for females, constitutes obesity, according to the American Society of Bariatric Physicians.

Being able to determine how much of a person’s body is fat makes this measurement more accurate than BMI, but the technique comes with its own drawbacks.

While there are less cumbersome ways than a Dexa scan to calculate body fat percentage, specialized equipment is still required. That can include special scales or calipers that pinch and measure the thickness of a person’s skin folds, both of which can be purchased for use at home. Some wearable devices are also able to estimate body fat percentage.

But these tools still leave a key question unanswered: where is the fat stored in a person’s body?

“Where we store fat infers a lot of health consequences,” Almandoz said.

How is android-to-gynoid fat ratio calculated?

And then there is the third measure used in the study, called the android-to-gynoid fat ratio.

The measurement looks at how fat is distributed through a person’s body and, most importantly, how much of that fat is visceral fat. The scan compares the amount of fat around the belly (known as android fat) to the amount of fat around the hips and thighs (known as gynoid fat), spitting out a ratio.

Visceral fat — fat stored deep in the belly, around the organs — is associated with a number of health consequences, including Type 2 diabetes and heart disease.

“When you’re thinking about body composition, you want to know your metabolic risk. The big question here is, how much fat is around the organs,” Surampudi said.

While visceral fat carries risks, fat stored around the legs may actually be protective against metabolic disease, Visaria said.

Although there are no set guidelines for obesity cutoffs using this measurement, he said, the general rule of thumb is that anything over 1.0 constitutes obesity.

The android-to-gynoid ratio is similar to another common measurement, the waist-to-hip ratio, which compares the circumference of a person’s waist to the circumference of the hips, both of which can be measured with a tape measure.

Which is the best?

Despite BMI being an imperfect tool, Almandoz said, it should not be completely thrown out. Instead, he said, new BMI measurements need to be fine-tuned to include cutoffs that are tailored to not just different sexes, but also take a person’s age and race into account.

“We need to have more specific BMI cutoffs based on this data so we can make the tool better, rather than saying, ‘it’s not great so let’s not use it,’” he said. “The alternatives may not be practical for clinical care in low resource settings.”

Any measurement of body composition needs to be able to connect results, whether that be the amount of fat a person has or where that fat sits, to specific health risks, he said.

Dr. Jack Resneck Jr., a past president of the American Medical Association, said in a statement that some physicians still find BMI “to be a helpful measure in certain scenarios.”

“It is important for physicians to understand the benefits and limitations of using BMI in clinical settings to determine the best care for their patients,” he said.

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