BMI Not Always a Good Indicator of Metabolic Health, Study Shows

  • A new study tracked rates of BMI obesity for 20 years and found that many obese people were actually metabolically healthy.
  • BMI has been noted as an inherently flawed health metric, but is still widely used in the medical community as a quick, easy means of assessing an individual’s health status.
  • Experts recommend doctors analyze health from a more well-rounded perspective, including BMI, body composition, and metabolic health.

A person’s body mass index (BMI) is often used as an indicator of their general health, but that’s not always the most accurate tool, a new study found.

The JAMA study, published March 9, tracked rates of BMI obesity from 1999 to 2018, and found that many obese people with elevated BMIs were actually metabolically healthy, meaning they didn’t have elevated blood pressure, blood sugar, bad cholesterol, or triglyceride levels.1

The general prevalence of people with metabolically-healthy obesity (MHO) increased from 3% to 7% during the 20-year period. Individuals with MHO now make up a larger proportion of those with obesity; though the percentage of MHO increased, the levels of metabolically-unhealthy obesity and general levels of obesity also went up.1

“We define obesity based on BMI, and that inherently is flawed,” Mohini Aras, MD, obesity medicine specialist at the comprehensive weight control center at Weill Cornell Medicine and who was not involved in the study, told Health. “People with the same BMI can have markedly different weight-related medical comorbidities and health risks.”

BMI and Metabolic Health

Researchers looked at ten cycles of National Health and Nutrition Examination Survey (NHANES) data from 20,430 participants between 1999-2000 and 2017-2018.

Researchers looked at 7,386 participants with obesity, or a BMI over 30, and assessed their metabolic health. Over the 20-year period, the study found that BMI obesity rates increased from 28.6% to 40.9%. The number of those with metabolically-unhealthy obesity also jumped during this period, from 25.4% to 34.3%.

Both these categories also increased within the general population during this timeframe.

“The prevalence of [MHO] has increased among obese people. The new news is that we’re more obese—there are more people who are metabolically unhealthy obese, and there are more people who are [MHO] in the last 20 years,” Sam Klein, MD, director of the Center for Human Nutrition and of the weight management program at the Washington University School of Medicine in St. Louis, told Health. He was not involved in the study.

Interestingly, while metabolically-unhealthy obesity and general obesity saw a more steady rise, MHO jumped sharply between 1999 and 2006, but has only increased marginally since then. Experts aren’t quite sure why.

Body Mass Index (BMI) is a dated health measure that does not account for several factors, like body composition, ethnicity, race, gender, and age.

This said, BMI is still used in the medical community as an inexpensive and quick method for analyzing potential health status and outcomes.

Of the four metrics used to determine metabolic health—elevated blood pressure, cholesterol, blood sugar, and levels of triglycerides (a type of fat) in the blood—only two decreased during the survey period. In the late 1990s and early 2000s, many people were concerned with cutting out fat from their diets and many were treated aggressively for high cholesterol and triglycerides, Dr. Aras explained. This could explain why MHO increased so quickly from 1999 to 2006, she posited.

While positive changes in public health patterns have occurred, the incidence of high blood pressure has stayed stagnant and the percentage of high blood glucose levels has increased substantially among the BMI-obese population since 1999.

Researchers also found that men, older people, white people, those with a higher income, those with a BMI closer to 30, and those with private insurance saw larger MHO increases.

Though the mechanisms behind the MHO’s rise is unclear, Dr. Klein said, we do know that all kinds of BMI obesity have increased in the U.S. between 1999 and 2018, even though BMI at face value doesn’t tell us the whole story about the health of that population.

The Problem with Tracking Weight and Health

In addition to its findings on obesity rates, the study also illuminates some of the challenges that come with trying to measure weight and health. Every person’s experience with weight is different, Dr. Aras noted, so making broad conclusions about an entire population can be challenging.

“We don’t know the whole story just based off of BMI,” Vijaya Surampudi, MD, assistant professor of medicine at the Center for Human Nutrition at UCLA Health, told Health.

BMI is based on a person’s weight and height, which can sometimes indicate whether they’re at a healthy weight. But that’s certainly not always the case.

“You just have a weight and the height, but it doesn’t give you any information of how weight is deposited on the body,” observed Dr. Surampudi, who was not involved in the study.

A BMI over 30 is considered obese. Using this metric, somebody tall and muscular—like Dwayne “The Rock” Johnson, Dr. Klein said—would be considered obese. This is obviously uncommon, but still illustrates some of the errors inherent in the tool.

BMI aside, judging whether a person is healthy is challenging. The four criteria that these researchers used to determine a metabolically-healthy person are not universally agreed upon, which means that these definitions of health can vary from person to person.

“There [are] no standardized definitions for [MHO],” Dr. Klein clarified. “Depending on the definition that’s used, depending on the population being studied, you can get a large variability in the prevalence of [MHO] in different studies.”

Despite the challenges of judging weight at a public health level, looking at body composition can be a useful tool when determining an individual’s health, Dr. Surampudi explained. This allows doctors to look at the amount of muscle and different types of fat in the body, giving a much clearer picture of overall health.

More to Learn About People’s Experiences with Weight

Though the disconnect between metabolic health and BMI is clear, more work needs to be done to figure out why some people with obesity are healthier than others, noted Dr. Klein.

Men, white people, older people, and privately insured people saw more increases in MHO. This could suggest that MHO is a consequence of better medical advice and having more time to exercise and eat well, Dr. Aras reasoned.

The type of obesity a person has seems to have something to do with the different types of fat in their body—whether they have more fat around the organs, or in other places, such as under the skin. Though Dr. Klein noted it’s also unclear if those differences cause better or worse metabolic health, or if they’re yet another effect of something else.

“If we can understand what protects [people with MHO] from the adverse effects of excess body fat, and what predisposes other people to adverse metabolic effects of excess body fat, we would go a long way into developing better treatments to improve the health of people with obesity, without even changing their body weight,” explained Dr. Klein.

Another large factor at play is doctors’ ability to determine whether someone has MHO or metabolically-unhealthy obesity. For example, people with MHO could be told to lose weight when in actuality, Dr. Klein said, it would do little to improve their health.

While BMI isn’t always an indicator of health, obesity can affect some people’s ability to move and breathe well and can put them at risk of other diseases, such as type 2 diabetes. Since consequences of weight is different for each person, Dr. Aras recommended those with higher BMIs be evaluated to see what effect, if any, their weight is having on their health.

“Diet and activity are the foundation of this,” she said. “It’s not necessarily the number on the scale.”


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