Is BMI an Accurate Predictor of Health?

If you've ever talked with a healthcare provider about your weight, you likely heard them mention your body mass index (BMI). 

The BMI scale categorizes your weight as underweight, normal, overweight, or obese. Healthcare providers may use your BMI to help determine whether your weight puts you at risk for certain health conditions.

However, BMI only considers two measurements: Weight and height. It might seem unfair that your health risk is based partly on a number that only accounts for those two factors. After all, BMI doesn't measure things like heart or lung health. It also cannot tell a healthcare provider if you exercise or eat a nutritious diet.

That being said, is BMI an accurate predictor of health? To fully understand, it's essential to know why healthcare providers even use BMI and how it became a tool in health and medicine. 

Here's what you should know about BMI, how it was developed, and whether it's something you should consider.

What Is BMI?

BMI is a screening method for how your weight might relate to disease outcomes. BMI is your weight in kilograms divided by the square of your height in meters. You may see your BMI expressed with the units kg/m².

Generally, as your BMI increases, so does your risk for certain health conditions, such as:

• Heart disease
• High blood pressure
• Type 2 diabetes
• Gallstones
• Breathing problems
• Certain types of cancer

But on its own, BMI cannot determine someone's body fat percentage and does not provide a complete picture of your overall health.

Where Did BMI Come From?

During the 1830s, Belgian mathematician Adolphe Quetelet was one of the first to develop BMI. Quetelet set out to apply math to the physical characteristics of the human body and intended to do so in terms of a population's average. Quetelet never advocated that people use the ratio, then named the Quetelet Index, as a general measurement of someone's weight status.

The Quetelet Index was later renamed BMI in 1972 by researcher Ancel Keys, PhD. Keys coined the term after publishing a study in the Journal of Chronic Diseases. In the study, Keys collected data from more than 7,000 men based in the United States, Finland, Italy, Japan, and South Africa. Based on that data, Keys showed that the formula could optimally measure obesity.

In 1985, the National Institutes of Health (NIH) began using BMI to define obesity. And as of 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

BMI Can Help Predict Health Risk

As for how researchers developed those four specific BMI cut-off points, that can be traced back to the 20th century. In 1942, Metropolitan Life Insurance Company used data to create height and weight tables. 

Plus, a statistical analysis helped determine the risk factors that the company included in its insurance plans. Essentially, the Metropolitan Life Insurance 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 D. Hensrud, MD, MS, associate professor of nutrition and preventative medicine at the Mayo Clinic College of Medicine in Rochester, Minn., told Health.

And as of December 2022, people can calculate their BMI to determine risk factors pertaining to their health. The Centers for Disease Control and Prevention (CDC) provides a BMI calculator and chart to 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 exercise physiologist, molecular biologist, biostatistician, and senior advisory board member for the National Academy of Sports Medicine, told 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," explained Dieter. "The same is true with BMI. The higher your BMI, the more at risk you are for a poor health outcome."

Limitations of BMI

Dr. Hensrud and Dieter agreed that BMI works well as a measure for the health of the population as a whole, especially in the high BMI categories. But BMI is not always as helpful 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," noted 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," explained 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 ["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," said Dieter.

BMI also doesn't indicate where your fat is stored. The location of your body fat can also reveal some important facts about your overall health.

"Having more visceral fat around your belly puts you at greater risk for dyslipidemia, increased blood pressure, elevated blood glucose, and cardiovascular disease," noted Dr. Hensrud.

Limitations of BMI for Special Populations

The risk factors sometimes associated with BMI are more common among specific populations than others.

Generally, Dieter noted that you should "talk to [a healthcare provider] if you're concerned about your BMI or if you feel misrepresented."

Race and Ethnicity

Asian people and Asian Americans may have increased health risks associated with low BMI. The Joslin Asian American Diabetes Initiative (AADI) provides BMI cutoff points different from the NIH.

Black people might also experience inaccuracies associated with BMI. For instance, some evidence suggests that Black people tend to have less body fat than white people. However, BMI does not take into account body fat percentage. So, even though Black people might have less fat and higher muscle mass than others, they may be misclassified as overweight.

Age and Sex

Age and sex might also affect BMI's ability to gauge health risk accurately. At the same BMI, women tend to have more body fat than men. Also, people tend to lose muscle mass as they age. So, at the same BMI, older adults usually have more body fat than younger adults.

Physical Activity

Even your fitness regimen can affect your BMI's health prediction accuracy. Athletes may have a high body weight not because of their body fat but because of increased muscle mass. In that case, BMI can misclassify them as overweight or obese.

BMI and Obesity

Despite the measurement's flaws, Dr. Hensrud and Dieter warned against disregarding BMI completely. 

"The vast majority of the population falls into the overweight [or obese] category with a BMI of 25 to 30 [kg/m²]," explained Dieter. "They're walking around with excess weight, and research shows they're at greater risk for metabolic and cardiovascular diseases."

People who have obesity are at increased risk of several health conditions, including:

• High blood pressure
• Heart disease and stroke
• High LDL ("bad") cholesterol, low HDL ("good") cholesterol, or high levels of triglycerides
• Type 2 diabetes
• Gallbladder disease
• Osteoarthritis
• Sleep apnea and other breathing problems
• Chronic inflammation and increased oxidative stress
• Certain types of cancer
• Mental illnesses, such as depression and anxiety

Dr. Hensrud also said that people with high BMIs generally have a much greater mortality risk. 

Overall, Dr. Hensrud and Dieter agreed that while the equation may not be as helpful individually, 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," noted Dieter.

Alternatives to BMI 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 recommended considering the following methods.

Body Fat Analyzers

There are affordable, easy-to-use body fat analyzers, including skinfold calipers and bioelectrical impedance analyzers. Skinfold calipers and bioelectrical impedance analysis are some of the most commonly used methods to measure body fat percentage.

Skinfold thickness is determined by pinching a fold of skin at a given site of your body using a tool known as precision thickness calipers. Those tools measure the average thickness of the subcutaneous adipose tissue, the fat that lies beneath your skin. Then, you can use the skinfold thickness measurements in formulas to estimate body fat percentages.

Someone with the experience and skill to perform the test correctly should conduct a skinfold measurement. You may be able to find body composition testing at some gyms and healthcare facilities.

Bioelectrical impedance analysis is a portable, non-invasive method that introduces a passage of low-level current into the body. The resistance impedance to the current is then measured to calculate fat and lean body mass.

When compared to each other, research has found no significant difference in body fat percentage between skinfold calipers and bioelectrical impedance analysis.

Waist Circumference

Excessive abdominal fat can increase your risk for obesity-related conditions. That risk increases if your waist circumference is more than 35 or 40 inches for women and men, respectively. 

According to the CDC, to correctly measure your waist circumference, follow these steps:

1. Stand and place a tape measure around your middle, just above your hipbones.
2. Make sure the tape is horizontal around the waist.
3. Keep the tape snug around the waist, but do not compress the skin.
4. Measure your waist just after you breathe out.

Waist-to-Hip Ratio

This measurement can zero in on how much visceral fat is distributed in your midsection. Visceral fat cushions the organs in your abdomen.

The waist-to-hip ratio can provide a more accurate measurement of your mortality and disease risk. For example, one study published in 2018 in Medicine (Baltimore) showed that a high waist-to-hip ratio increases heart attack risk.

To obtain the measurement, divide your waist circumference by your hip circumference. A waist-to-hip ratio of 1.0 or higher is considered "at-risk" for adverse health consequences for both men and women.

Weight-to-Height Ratio

To get this measurement, divide your waist circumference by your height. 

One study published in 2014 in PLoS One found that a ratio of more than 0.52 increases the risk of diabetes, heart disease, stroke, and shorter life expectancy. The researchers stated that keeping your waist circumference less than half of your weight decreases that risk.

Keep Track of the Five Metabolic Risk Factors

All of those 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 to monitor your health and risk for disease. 

The five metabolic risk factors that can raise your risk for heart disease and other health problems, such as diabetes and stroke, include:

• High triglyceride level
• Low HDL cholesterol level
• High blood pressure
• High fasting blood sugar
• Large waistline

A Quick Review

BMI is a calculation based on a person’s weight and height that screens for weight categories that may lead to health problems. However, it does not diagnose an individual's body fat percentage or overall health.

BMI may help express health risks, especially for populations. But it is not always as valuable on an individual level. Some limitations of relying on BMI are related to race, gender, age, and athletic experience.

There are other methods for measuring health risks. In particular, keeping a close watch on blood pressure, cholesterol, blood sugar, and triglycerides is essential for monitoring your health and risk for disease.