Study: Current BMI Obesity Guidelines May Be Inaccurate for Those Over Age 40

A new study suggests that for people over 40, the standard classification of obesity as a body mass index (BMI) of 30 may need to be adjusted. Instead, older adults could have obesity if their BMI is over 27.

The findings come from research presented on May 15 at the European Congress on Obesity, a conference held in Venice, Italy.

The study authors looked at a group of adults in Italy and recorded each person’s body fat percentage and their BMI. They found many people over age 40 had obesity based on their high amounts of body fat, even if they didn’t reach the traditional BMI threshold for the disease.

BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. It’s been a widely-used tool since it was first introduced in the 19th century, but in recent decades, experts warn it may not be very useful to determine a person’s metabolic health.

As people age, their body composition and fat-to-muscle ratio tend to change, which could explain why using BMI to determine obesity is inaccurate for this age group, experts said.

“This new BMI cut-off recognizes the physiological differences between middle-aged and older adults and younger populations,” said study author Marwan El Ghoch, MD, chief of the Department of Food and Dietetics at Beirut Arab University and associate professor at the University of Modena and Reggio Emilia in Italy.

“It is likely that the changes in body composition across the lifespan can lead to higher obesity at a lower BMI,” El Ghoch told Health. “What applies for young adults may not apply also for middle age and older adults.”

For this study, El Ghoch and his research team looked at data from 4,800 adults between the ages of 40 and 80, about 60% of whom were women. The participants were from a cross-sectional study conducted by the University of Rome “Tor Vergata,” the University of Modena and Reggio Emilia, and Beirut Arab University in Lebanon.

Participants were grouped into categories based on BMI standards from the World Health Organization (WHO). There were 1,087 people with normal body weight, 1,826 with overweight, and 1,887 with obesity.

Researchers then found each person’s body fat percentage via a body scan called dual-energy X-ray absorptiometry (DXA), and used a model to identify obesity based on body fat.

The analysis found that around 38% of men and 41% of women had a BMI over 30, meaning they met conventional WHO BMI criteria for obesity.

“However, when assessing our sample according to body fat percentage, around 71% of the men and 64% of the women were determined to have obesity,” said El Ghoch. “For this reason, we concluded that classifying obesity with only a BMI [more] than 30 is not enough, without accounting for body composition.“

The results of the study found that the most appropriate BMI cut-off point for identifying obesity for middle-aged and older adults was 27.08 kg/m² in women and 27.36 kg/m² in men. El Ghoch and his colleagues found this metric has a high degree of accuracy as well, with a nearly 90% chance of detecting the disease.

In the U.S., it’s estimated that about 45% of adults ages 40 to 59 and 43% of adults over 60 have obesity. Lowering this BMI threshold for obesity could mean a much larger swath of the population actually has the disease.

However, the study does have its limitations, El Ghoch explained. It was conducted in one area of Italy, so in order to generalize the findings, expanding the research into other populations is necessary. In addition, the authors did not look at dietary habits, physical activity patterns, and sleep health, all of which can increase obesity risk and may interact with age-related differences.

As illustrated in the new research, many in the medical community agree that, at its core, BMI is not always a good tool to assess a person’s health.

“BMI only says how big someone is, but does not say anything about body composition such as fat versus muscle,” Rekha Kumar, MD, endocrinologist and associate professor of clinical medicine at Weill Cornell Medicine, told Health. “Recent research has undermined the validity of BMI as the end-all-be-all tool to assess obesity.”

Unlike body composition tests, BMI does not take into account where fat is stored in the body. Therefore, it cannot account for the variation in fat distribution among different body types and gender.

BMI “can be stigmatizing to patients in larger bodies who are healthy” if they’re deemed overweight or obese unnecessarily, Kumar said.

On the other hand, BMI can also miss obesity cases, particularly in older adults. As people age, their body composition changes, and their metabolic health can decline, even if their overall weight remains stable or decreases.

“This shift in body composition can lead to a situation where older individuals may have a higher proportion of body fat relative to muscle mass, which can contribute to obesity even at a lower BMI,” John Whyte, MD, MPH, chief medical officer at WebMD, told Health.

“We also get shorter as we age, typically losing half an inch per decade after age 40,” Whyte added. “Many times, people don’t actually get their height measured for the calculation, so that can skew results.”

Simply, health is multidimensional, and one number cannot paint a full picture of a patient’s health, Kumar said.

“I think this research will encourage other tools to assess body composition rather than just BMI,” she explained. “We need affordable tools and scans that actually tell us about body composition, but both excess fat and low muscle are not healthy.”

Since people over 40 tend to lose lean muscle and add fat as they age, it’s important to seek out methods for managing weight and metabolic health beyond just BMI.

Whyte recommends tracking obesity status via waist circumference measurements, assessments of physical fitness, and body composition analyses, which are included in some home scales.

“These approaches provide a more comprehensive view of an individual’s health beyond just their weight or BMI,” he added.

But despite the criticism of BMI, it may still be helpful for middle-aged and older people to know what their BMI is, and to discuss it with a healthcare provider if that number is approaching or over 27.

This research is unlikely to lead to immediate changes in BMI guidelines, Whyte explained, but it does contribute to the ongoing dialogue about the limitations of BMI as a sole indicator of health.

“Over time, as more evidence accumulates, there may be adjustments to guidelines or recommendations regarding the use of BMI in healthcare settings,” said Whyte. “I don’t think we throw it away completely—and other tools also have limitations. The key is it shouldn’t be the only measure.”