Is BMI an Accurate Predictor of Health? Here's Why It's Not Always Useful, According to Experts

But despite its flaws, BMI still has value.

is BMI accurate

If you've ever had a conversation with your doctor about your weight, you likely heard them mention something about your body mass index, or BMI. Your BMI can categorize you as underweight, normal weight, overweight, or obese. Doctors may use BMI to help determine whether your weight category puts you at risk for certain health conditions.

BMI takes just two measurements into consideration: your weight and height. It might seem unfair that your health risk is based in part on a number that only accounts for those two factors. After all, BMI doesn't measure things like heart or lung health. It also can't tell your doctor if you exercise or eat a nutritious diet.

That being said, is BMI actually an accurate predictor of health? To fully understand, it's important to first know why BMI is even used and how it came to be. Here's what you should know about what BMI is, how it was developed, and whether it's something you should go by.

What is BMI, and where did it come from?

On its own, BMI cannot determine someone's body fat percentage or overall health; simply, BMI is a screening method for how your weight might relate to disease outcomes. "The higher your BMI, the higher your risk for certain diseases such as heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers," according to the National Institutes for Health (NIH).

BMI is your weight in kilograms divided by the square of your height in meters (expressed in kg/m²). It's a formula that Belgian mathematician Adolphe Quetelet first developed in the 1830s. He wanted to see how he could apply math to the physical characteristics of the human body and intended to do so in terms of a population's average; he never actually advocated that the ratio (named the Quetelet Index) be used as the general measure of someone's weight status.

The Quetelet Index was later renamed BMI in 1972 by researcher Ancel Keys, PhD. Keys coined the term after he said that his landmark study—which collected data from more than 7,000 men based in the US, Finland, Italy, Japan, and South Africa—proved that the formula was the best measure of obesity.

The NIH began using BMI to define obesity in 1985 and adopted today's standards in 1998. Across all ethnicities, ages, and genders, BMI falls into the following four categories:

  • Underweight: less than 18.5 kg/m²
  • Normal weight: between 18.5 and 24.9 kg/m²
  • Overweight: 25 to 29.9 kg/m²
  • Obesity: 30 kg/m² or greater

As for how these four specific BMI cut-off points were developed, that can be traced back to Metropolitan Life Insurance Company data from 1942. Using height and weight tables, plus statistics used to determine the risk factors they insure people against, the company found that those who lived within their "ideal" weight for their body height and frame lived longer than those above their "ideal" weight. "These weight charts eventually transitioned to BMI because it correlates better with body fat percentage and health outcomes than ideal body weight," Donald Hensrud, MD, associate professor of nutrition and preventative medicine at the Mayo Clinic College of Medicine, tells Health.

The Centers for Disease Control and Prevention (CDC) provides a BMI calculator, along with a chart so that you can see where your BMI lands within the four categories.

"The best way to think about BMI is as a risk-prediction tool," Brad Dieter, PhD, an advisory board member of the National Academy of Sports Medicine and certified nutrition coach with expertise in how diet and exercise influence chronic disease such as obesity, tells Health. "Think about it like age. Regardless of what else is going on, the older you are, the more likely you are to have a chronic health condition. The same is true with BMI. The higher your BMI, the more at risk you are for a poor health outcome."

Is BMI an accurate health predictor?

Dr. Hensrud and Dieter agree that BMI works well as a measure for health of the population as a whole, especially in the high BMI categories, but it's not always as useful on an individual level.

"Someone with a large muscle mass may have a high BMI but low body fat. Their health risks are probably very low," says Dr. Hensrud. "The opposite is also true. You can carry a large amount of body fat in relation to your weight, putting you at greater risk than your BMI indicates."

And just because you don't have a BMI that puts you into the obese or overweight category doesn't automatically mean you're in good health. "Those on the low end of normal to underweight BMI may have greater mortality risk than those with an overweight BMI because other health factors are at play, like an eating disorder, drug abuse problem, or major medical condition," explains Dieter.

Even those with a BMI considered to be normal weight aren't necessarily risk-free. "If a patient's BMI is normal, but they show other risk factors, like high LDL (considered the bad cholesterol) or high triglycerides, we might have a conversation on how to lower their risk for heart disease despite the fact they're not overweight," says Dieter.

BMI also doesn't indicate where your fat is stored. "Having more visceral fat around your belly puts you at greater risk for dyslipidemia, increased blood pressure, elevated blood glucose, and cardiovascular disease," says Dr. Hensrud.

The underestimation or overestimation of risk factors that BMI sometimes causes is especially true for certain populations.

Asians and Asian Americans may have increased health risks at a lower BMI, according to the Asian American Diabetes Initiative (AADI). That's because even if two people have the same BMI, their level of body fatness may differ. And, as the CDC points out, at the same BMI, Asians have more body fat than whites. Because of this, the AADI provides BMI cutoff points different from those of the NIH. Black people might experience the opposite inaccuracy for, at the same BMI, they have less body fat than whites, per the CDC. So even though people who are Black might have a lower fat mass—and higher muscle mass—they may be misclassified as overweight.

Age and sex might also affect BMI's ability to accurately gauge health risk. That's because, at the same BMI, women tend to have more body fat than men. Also, at the same BMI, older people usually have more body fat than younger adults, as people tend to lose muscle mass as they age. Even your fitness regimen can play a factor in BMI's health prediction accuracy. Athletes may have a high body weight not because of their body fat but because of their higher muscle mass, per the CDC. In their case, BMI can misclassify them as overweight or obese.

"Talk to your doctor if you're concerned about your BMI or if you feel misrepresented," says Dieter.

Despite BMI's flaws, both Dieter and Dr. Hensrud warn against throwing the baby out with the bathwater. "The vast majority of the population falls into the overweight [or obese] category with a BMI of 25 to 30 [kg/m2]. They're walking around with excess weight, and research shows they're at greater risk for metabolic and cardiovascular diseases," says Dieter.

Dr. Hensrud also says that, without a doubt, in general, populations at higher ranges of BMI have a much greater mortality risk. They both agree that while the equation may not be as helpful on an individual level, it's still a good tool when combined with other health markers. "Keep in mind BMI is one tool, along with many others, like medical history, lifestyle factors, and genetics, doctors use to evaluate health," says Dieter.

Alternatives for measuring health risk

If you're looking for easy-to-take measurements besides BMI that can indicate your health risk, Dr. Hensrud and Dieter agree on considering the following methods:

Body fat analyzers: There are affordable, easy-to-use body fat analyzers out there, including skinfold calipers and bioelectrical impedance analyzers, which use low-level electrical currents to measure your body fat percentage. While the Mayo Clinic points out that portable body fat anazleyers are not as accurate as other methods, such as underwater weighing (where your body fat percentage is calculated after you're weighed on dry land and then while sitting on a special chair lowered under water) or air displacement plethysmography (where your body fat percentage is measured while you're in a computerized, egg-shaped chamber), they are certainly less expensive and more accessible.

Waist circumference: Excessive abdominal fat can increase your risk for obesity-related conditions, per the CDC. Women are at higher risk if their waist circumference is more than 35 inches, men at 40 inches. To correctly measure your waist circumference, the CDC says you should:

  • Stand and place a tape measure around your middle, just above your hipbones.
  • Make sure the tape is horizontal around the waist.
  • Keep the tape snug around the waist, but not compressing the skin.
  • Measure your waist just after you breathe out.

Waist-to-hip ratio: The measurement can zero in on how much of your visceral fat (aka, the fat that's stored deep in the belly, wrapped around the organs) is distributed in your midsection. Again, central obesity can lead to mortality and disease risk, and the waist-to-hip ratio can shed light on that risk. For example, a 2018 study showed that a high waist-to-hip ratio is associated with a greater risk for heart attack. To get this measurement, divide your waist circumference by your hip circumference. According to the NIH, for both men and women, a waist-to-hip ratio of 1.0 or higher is considered "at-risk" for undesirable health consequences.

Weight-to-height ratio: To get this measurement, divide your waist circumference by your height. Research has found that a ratio of more than .52 is associated with a higher risk of diabetes, heart disease, stroke, and shorter life expectancy. The researchers said their results support the simple message of: to keep your risk lower, "keep your waist circumference to less than half your height."

But again, all of these size-related measurements are just one aspect of measuring your health. You can also keep track of your blood pressure, cholesterol, blood sugar, and triglycerides as part of monitoring your health and risk for disease. A high triglyceride level, a low HDL ("good") cholesterol level, high blood pressure, and high fasting blood sugar—along with a large waistline—are the five metabolic risk factors that the National Heart, Lung, and Blood Institute says can raise your risk for heart disease and other health problems, such as diabetes and stroke. And all can be easily monitored.

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