Why BMI Isn’t an Accurate Measure of Health

Why BMI Isn’t an Accurate Measure of Health

Chances are at some point in your life, you and your doctor have discussed your body mass index (BMI) in relation to your health. Unfortunately, for many people (maybe you, too) these talks can be distressing, considering all the weight that those three little letters carry.

That’s because “the normative approach to healthcare right now is that BMI is one of the distinct measures of health,” says Maria C. Monge, M.D., an eating disorder specialist and the director of Adolescent Medicine at Dell Children’s Medical Center. Meaning: Whether your BMI is “too high,” “too low,” or “just right,” it supposedly says a lot about your physical well-being. However, many researchers are now saying that the BMI is actually a rather poor summary of how you’re doing health-wise.

Objectively, the BMI is a simple number that represents your weight-to-height ratio. It’s calculated by dividing your weight (in kilograms) by your height (in meters) squared. But this takes on a whole different meaning when you place that number in a health context. Here’s how BMI is currently categorized, according to the Centers for Disease Control and Prevention (CDC):

Experts at the National Heart, Lung, and Blood Institute suggest that your BMI is an estimate of body fat, which if, “too high,” will increase your risk of heart disease, high blood pressure, type 2 diabetes, and certain cancers.

The thing is: This oversimplified narrative encourages many health practitioners to see “excessive” body fat as the direct cause of disease rather than something that may be a correlating factor, according to Elizabeth Wassenaar, M.D., M.S., CEDS, a Regional Medical Director at the Eating Recovery Center. In other words, having a higher BMI does not necessarily mean you’ll experience poor health; there are many factors—independent of weight—that can lead to illness.

“Humans are much more complex than a series of calculations and numbers, and the physiology of health, wellness, and un-wellness are very complicated,” says Dr. Wassenaar.

Below, experts explain why the BMI misses the mark as a standard of health and what doctors can do to serve their patients from a weight-neutral perspective.

There are many reasons why the BMI falls short as an overall health assessment. These are the top ones:

“The formula for BMI originated in 1832 from a Belgian mathematician, Lambert Adolphe Jacques Quetelet, who didn’t intend it for medical use,” says Sylvia Gonsahn-Bollie, M.D., the author of . Quetelet came up with this weight-to-height calculation in order to figure out the average body size among European men. Thus, the “BMI was created as a population-based assessment, not an individual assessment,” adds Dr. Gonsahn-Bollie. “In other words, a BMI chart is a reflection of how much a group of people generally weigh, not what one person ‘should’ weigh.”

Still, in 1985 the National Institutes of Health began to use BMI for individual health screenings. While the BMI appears to be a quick and easy screening tool, nowadays, the CDC explicitly states that it cannot “diagnose the body fatness or health of an individual.”

There are studies that challenge the idea that being so-called “overweight” or “obese” means you are unhealthy. For example, after analyzing data from more than 40,000 participants in the government’s annual National Health and Nutrition Examination Survey (NHANES), researchers from the University of California found that nearly half the people classified as “overweight” and more than a quarter labeled “obese” had healthy blood levels of lipids and glucose. This meant they showed no signs of developing conditions like heart disease or diabetes. At the same time, 30% of those labeled “normal-weight” had unhealthy levels of these markers.

Meanwhile, researchers of a 2016 self-reported study discovered that having a higher BMI does not necessarily increase a person’s risk of heart attack. They followed 4,046 pairs of identical twins over the course of 12 years and recorded 203 heart attacks and 550 deaths among the twins with higher BMIs, and 209 heart attacks and 633 deaths among thinner twins. Even those twins with a BMI of 30 or higher (which is technically “obese”), had no increase in heart attack risk.

“What most studies show is that people considered overweight and mildly or moderately obese live at least as long as or longer than people deemed normal weight,” Lindo Bacon, Ph.D., researcher and co-author of , said in a recent Youtube vlog. In fact, having a “‘normal’ BMI can create a false sense of stable health and cause other risk markers to be minimized or ignored,” says Cara Harbstreet, M.S., R.D., L.D., an intuitive eating registered dietitian at Street Smart Nutrition.

Overall, “there's no scientific measurement that shows you that fat will kill you or that fat people are unhealthy universally across the board,” says Maggie Landes, M.D., M.P.H., an anti-diet physician, coach, and host of The EatFluencer Podcast.

It also doesn’t assess your overall composition of fat and muscle, according to Harbstreet. Even if you were to make the conclusion that fat is automatically bad, the BMI doesn’t even measure it well. “It’s believed that this is a reliable measure of body fat, but it is not,” says Harbstreet, as it doesn’t differentiate between fat, muscle mass (which is more dense than fat, by the way, and can make you weigh more), and bone density.

This is the reason why many muscular athletes who are physiologically sound are often labeled as “overweight,” says Dr. Landes. Meanwhile, elderly adults—who tend to have less muscle mass and lower bone density due to aging—are labeled as having “normal” BMI, potentially leaving doctors blind to potential red flags, Dr. Landes adds. For example, a drop in an older adult’s BMI might be a sign of sarcopenia (excessive loss of muscle), which can increase fall risk and mortality risk.

The BMI weight categories are the same regardless if you are a man, woman, 15, or 75. And yet, your sex, age, and various genetic factors can affect you weight in ways that are not indicative of a health issue, according to Dr. Landes. For instance, women naturally tend to have a higher percentage of body fat than men and as a result tend to have higher BMIs. And even though it’s normal “for children between the ages of 3 and 18 to put on weight as they grow, doctors often mistakenly consider them to be obese and therefore “unhealthy.” says Dr. Landes.

BMI is “especially harmful for people of color and those in marginalized bodies,” according to Harbstreet. In 1972, when obesity researcher Ancel Keys suggested that insurance companies and physicians adopt certain BMI categories to assess “healthy” vs. “unhealthy” weight, he based his evidence upon research he conducted on mostly white European and American men.

However, in 2003, JAMA published a large study that proved that higher BMIs are healthier for Black people than previously thought; the analysts concluded that the optimal BMI (i.e. the BMI associated with the greatest longevity) for adults aged 18 to 85 years is approximately 23 to 25 for white people and approximately 23 to 30 for Black people. Meanwhile, researchers that people of East Asian and South descent tend to have higher rates of diabetes at lower BMIs.

In short, American BIPOC and AAPI patients receive medical advice based on BMI weight categories that do not apply to them.

It’s not uncommon for people living in fat bodies to feel judged, shamed, and scrutinized when they go to their regular checkups, according to Dr. Landes. Even if someone has come to their doctor about an issue that has nothing to do with their weight—like acne or a sinus infection—oftentimes when the doctor finds their BMI is high, “it colors every aspect of a doctor’s visit,” she says. “Weight is now the problem and it’s a distraction to the conversation on health.” Oftentimes this means that fat people get less quality care than their thinner counterparts.

Not only do fat people get less quality care, stigmatizing experiences like these discourage many people from seeking preventative medical treatments for years until they get really sick, adds Dr. Landes. Weight stigma, according to some studies, can also cause chronic stress which can impact a person’s heart health. Some researchers believe that these are the real reasons why high BMI correlates with disease.

Because so much of our healthcare system leans on the BMI to “assess” patients, it’s not going anywhere anytime soon. But luckily, there are plenty of Health at Every Size (HAES)-informed doctors and registered dietitians who are showing that there are less stigmatizing and more reliable ways to measure someone’s overall well-being.

“There’s lots and lots of measures we could look at [besides BMI], like blood pressure, resting heart rate, insulin resistance, certain inflammatory markers, sleep hygiene, eating behaviors [...] asking patients about their mental health and food security,” says Dr. Landes. “All that stuff is going to impact your health risk and your quality of life.”

Dr. Landes argues that combining all of these sorts of tests and asking these questions gives the physician a fuller picture of a person’s health rather than jumping to clinical assumptions based on someone’s weight. In fact, barring very specific situations—like when someone needs chemotherapy, dialysis, or general anesthesia—patients don’t need to be weighed to receive quality treatment, she adds.

If you do not want to discuss your BMI with your doctor, you can request that they avoid weight conversations for future appointments and instead order bloodwork (if it hasn’t been ordered already) in order to focus more definitively on health. This can be a tough conversation to have; Dr. Monge suggests bringing notecards so you are prepared with what you want to say.

If your doctor does not comply, and you’re able to switch practitioners, you can look at the HAES directory to see if there’s a physician nearby who understands to look at your health in a more holistic way.

Overall, understand that “the BMI harms our ability to truly understand what factors might impact disease and how [physicians] might be able to support people in living healthier, more fulfilled lives,” says Dr. Wassenaar. “As doctors, we need to shift our focus on improving an individual’s health rather than on weight loss and emphasize the need to address weight bias, discrimination, and the trauma it can cause people.”

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