The Body Mass Index (BMI) is a number you have probably heard of. These days it is used by doctors, health organizations, and insurance companies everywhere to classify people as underweight, normal weight, overweight, or obese. Have you wondered where this BMI number came from, and if it really is the best way to measure body size for health? Let’s dig into the history of BMI and other measurements like the waist circumference.
Where Did BMI Come From?
BMI was created in the early 1800s by a Belgian mathematician named Adolphe Quetelet. He wasn’t a doctor or a health expert—he was a statistician. Quetelet developed the formula (weight in kilograms divided by height in meters squared) to study general trends in human growth. He never intended for BMI to be used to judge individual health. So, the index started out as a statistical tool.
How Ancel Keys Popularized BMI
The Quetelet Index, as it was called, wasn’t used much until people began doing large population studies. Ancel Keys, in his work on the Seven Countries Study in the 1970s, was examining populations in various countries to see if he could determine the dietary causes of heart disease. He wanted a single index that could be used to compare body sizes across different populations. And for studying trends in obesity at the population level, Keys found that the BMI worked well. It really did give an measurement of obesity at the population level. But even Keys himself warned that BMI was not a perfect measure of individual health.
When BMI Became the Standard for Obesity
Despite its limitations as a tool for measuring individual obesity, BMI gained popularity among researchers as a tool for defining overweight and obesity in their population studies. In 1995, the World Health Organization (WHO) defined obesity in terms of BMI rather than other measurements or tools. Perhaps height and body weight were easier to obtain than waist circumference? Maybe there was less error in the BMI methodology? Maybe it sounded like a better, more complete measurement than waist circumference because it accounts for your body weight at any height. If you are too heavy for your height, well, you are probably overweight or obese. So, the WHO said if your BMI is greater than 25 you are overweight and if your BMI is over 30 you are obese. And there are further categories of obesity as well. It was clear from the research up to that point that obesity at a population level was correlated with negative health outcomes. Clearly we are in the midst of an obesity epidemic, so this isn’t an argument to just accept that some people are just large. That idea is false and dangerous
While the BMI was a simple way of tracking obesity at the population level across the globe, at the individual level it doesn’t take into account muscle mass, bone density or fat distribution. So, there are some people classified as overweight by the BMI who actually are quite healthy.
How BMI Became a Personal Health Metric
Over time, as the BMI was more widely used and supported by medical research and health outcome data, the BMI shifted from not only being population study tool to being used for individual health assessments. Doctors started using it to counsel patients about weight, and insurance companies used it to set premiums. This shift was problematic because BMI does not differentiate between fat and muscle, nor does it consider where fat is stored. A fit athlete with a muscular build could have a BMI over 25 and be classified as overweight, even if they had very little body fat.
More importantly, BMI doesn’t tell us anything about where fat is stored. And that’s critical because belly fat (visceral fat) is far more dangerous than fat stored in the thighs or hips.
The CDC recommends that individuals use BMI as a quick screening tool, but to also include medical history, lab results, physical exam, and health behaviors to assess one’s health as an individual.
Is Waist Circumference A Better Alternative?
Instead of relying only on BMI, a simpler measure of health and obesity is waist circumference. The obesity research also shows that excess belly fat is strongly linked to heart disease, diabetes, and other metabolic issues.
In the Nurses’ Health Study women who had a “normal” BMI (between 18.5 and 25.0 kg/m2) who had a waist circumference greater than 88 cm were 3 times more likely to have a fatal stroke or heart attack compared to women with normal BMI and smaller waists. So, fat around the middle is more dangerous for your health, and BMI doesn’t account for where your fat is located.
Here are the waist circumference cutoffs for healthy weight:
- Men: Less than 37 inches (94 cm)
- Women: Less than 31.5 inches (80 cm)
Some organizations have more lenient guidelines, like 40 inches (102 cm) for men and 35 inches (89 cm) for women. These numbers are too high for the risk of diabetes according to data from the Health Professional Follow-Up Study. So, 37 inches for men (35 inches if you are of South Asian ethnicity) and 31.5 inches for women is the healthy category. If your waist measurement is greater than these cutoffs the evidence suggests you have a higher risk of health problems, regardless of BMI.
Another way to use your waist circumference is to compare it to your height, in the Waist to Height Ratio. A healthy waist to height ratio is less than 0.5. So, if your waist is more than half of your height, you are too thick around the middle.
How to Measure Your Waist Correctly
- Locate the Measurement Point: Find the midpoint between the bottom of your last rib and the top of your hipbone.
- Position the Tape Measure: Wrap a tape measure around this midpoint, ensuring it’s parallel to the floor.
- Ensure Proper Fit: The tape should be snug but not compressing the skin.
- Measure After Exhaling: Take the measurement after breathing out normally.
The Takeaway
BMI was never meant to be used for personal health assessments. It is very useful as a tool for studying populations but applying statistical tools to individuals only works for those who are very typical and labels some individuals the wrong way. The real issue isn’t weight alone—it’s fat distribution, especially belly fat.
If you want to know if you are overweight you can skip the BMI chart. Instead, grab a tape measure and check your waist size. It’s a far better indicator of your risk for chronic diseases and a more useful tool for personal health management.
