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A new study finds BMI scores for people over 40 may be less accurate. SolStock/Getty Images
  • A new study finds that the body-mass index may be less accurate for people over age 40.
  • As people age, their body composition of fat and muscle also changes.
  • Researchers found through in-depth scans some people over 40 with lower BMI may have had high levels of fat that would classify them as obese.

A new study finds that body-mass index standards for obesity may need to be adjusted for people over 40.

The new research was presented this week at the annual European Congress on Obesity in Venice, Italy. Researchers found that as people age their body composition changes. In the study, some people over 40, who had BMI scores below what would be considered obese, had high levels of fat compared to muscle.

The research, from a cross-sectional study conducted by the University of Rome “Tor Vergata” and the University of Modena and Reggio Emilia in Italy and the Beirut Arab University in Lebanon, looked at data from 4,800 adults — more than 61% of them women — between the ages of 40 and 80 who were referred to the Division of Clinical Nutrition at the Department of Biomedicine and Prevention at the University of Rome “Tor Vergata.”

Individuals excluded from the research included people who were pregnant, on any medication that affects weight or body composition, or had preexisting conditions associated with weight loss or severe psychiatric disorders.

Study participants were divided by the WHO BMI index:

  • not underweight or overweight/obese (BMI 18.5-24.9): 1,087 participants
  • overweight (25.0-29.9): 1,826 participants
  • obese (30.0 and above): 1,887 participants

Researchers then used a type of body scan called dual x-ray absorptiometry (DXA) scans to examine body fat percentages.

They used an index that divided participants by age- and gender-specific obesity cut-off points:

  • 40–59 years: Body Fat Percentage (BF%) of 40% or higher for females and BF% of 28% or higher for males
  • 60–79 years: BF% of 42% or higher for females and BF% of 30% or higher for males

According to the WHO BMI standards, about 38% of men and 41% of women in the study had a BMI of 30 or higher, which would qualify them as obese.

However, they compared those measurements to the body fat percentage. They found that according to the body fat percentage measurement, about 71% of the men and 64% of the women qualified as obese.

Researchers concluded for people aged 40 and older, a BMI of 27 was a better standard for obesity, as opposed to the current World Health Organization standard of 30.

Obesity is a chronic condition defined by an excess of fat in a person’s body, and it is linked to multiple health conditions including increased risk of type 2 diabetes, heart disease and cancer.

For over a century, BMI has generally been used as a way to measure obesity. In the U.S., having a BMI of 30 or more is categorized as obese.

But, this measurement does not consider body composition or the amount of muscle versus fat a person has.

In addition to potentially being less accurate for anyone with more muscle mass, some experts say that age-related muscle loss and increases in body fat can mean that BMI is not an ideal way to determine weight status — or overall health.

According to the Centers for Disease Control and Prevention, “anyone with a BMI between 25 and 29.9 would be classified as overweight and anyone with a BMI over 30 would be classified as having obesity. However, athletes may have a high BMI because of increased muscularity rather than increased body fatness.”

Mir Ali, MD, bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, CA, who was not involved in the research, told Healthline that BMI is used so often simply because it’s easy.

“The problem with looking at BMI, that’s what we’ve been using for many, many years, is it’s not as accurate as some other measurements. The big advantage of BMI is [that it’s] easy to calculate. The problem with measuring obesity is you need a scan, like a DXA scan, or some other type of way to accurately measure the amount of fat somebody has — that’s harder to do, it’s harder to get insurance to cover that it’s harder to get patients to go and get that done,” Ali said. “It’s a reasonably accurate measure of somebody’s overall weight. It’s essentially factoring out a person’s height; it unfortunately doesn’t take into account body composition. So somebody who’s extremely muscular may look like they have a high BMI because of their height versus weight, but they don’t need to lose fat.”

Melanie Murphy Richter, a registered dietitian nutritionist and the director of communications for the nutrition company Prolon, who was not involved in the research, echoed this sentiment, adding that BMI gets used a lot because it’s such a quick measurement that doesn’t require any extra physical assessment, lab tests, or blood work.

“BMI is painfully limited in its ability to detect actual health risk, both in that the type of weight a person carries is critically different, and where specific types of fat on your body can be more indicative of your health risk than your weight overall,” Richter said. “Additionally, visceral fat — the most dangerous type of fat that lives around your vital organs — cannot be accurately determined from a BMI calculation. In fact, people with low BMIs with high visceral fat content can be at much higher risk for disease than someone with a high BMI but low visceral fat.”

Weight discrimination has become a larger part of cultural conversations about body positivity and body-shaming. And medical professionals are generally trained to look at weight as a sign of overall health, which can result in people feeling stigmatized in healthcare settings.

Newer guidelines recommend a different perspective. Research from 2017 published by the Centers for Disease Control and Prevention (CDC) recommends emphasizing physical activity for all people, regardless of their BMI.

Richter said that the idea of lowering the threshold for obesity with older adults can be helpful to correct some of the flaws of BMI measurement. But RIchter noted that the lack of ethnic and racial diversity in the study, along with the very real stigmas that come with ideas around obesity, means there are larger issues to examine about weight and overall health.

“Pervasive weight stigmas and the widespread idolization of unrealistic body ideals are very real sources of body shame, body dysmorphia, and the development — and exacerbation — of unhealthy relationships to food and also to oneself. Feeling unworthy in your body or amongst your friends and communities can cause irreparable damage to a person’s self-worth that can then cascade into harmful food patterns and eating disorders that are dangerous and life threatening,” Richter said.

Ali added that while the Italian study was interesting, there are aspects of the findings and methods that are hard to apply to the healthcare system in the U.S.

“It’s difficult to apply this kind of measurement practically — this was done in Italy, where you know, there’s more socialized medicine, it may be easier to get these [kinds] of studies on people,” Ali said. “Here in the US, it’s not as easy to get a DXA scan and get that covered by insurance or get patients to actually go and do it. ‘They’re like, why do I need this? What is that going to change?’”

A new study presented this week in Italy suggested that WHO BMI standards for obesity should be lowered for adults between the ages of 40 and 80.

BMI is used widely in the U.S., where a BMI of 30 or above would indicate obesity.

Experts caution that BMI has a number of flaws, such as not measuring muscle mass, and that people over 40 generally begin to lose lean muscle and add fat as they age.