Broad, Flawed, and Just Plain Useless. Territory’s Senior Manager of Nutrition, Danielle McAvoy on why BMI is a bogus measure of who's fat and who's not.

Body mass index, popularly known as BMI, is a quick calculation used to determine if a person is healthy based on their weight. While BMI can be useful when looking at broad populations, it was never intended to be used for measuring an individual’s health. It doesn't account for racial, ethnic, or gender differences, and it ignores the fact that where the fat is on your body matters more than how much fat there is. There are much better ways to measure health than BMI, but it is deeply entrenched in our health systems, and unfortunately does more harm than good.

BMI is a math hack created by a statistician in the 1800s to determine the size of the “average man.” When the formula first appeared in a scientific journal, the article warned that BMI should only be used for general population studies and not for individuals. But here we are two centuries and countless medical advancements later, still using BMI to label a person as overweight, or obese. BMI affects what health services a person is eligible for, and what an insurance company will cover. It was even used as a factor to bump people up the line for the Covid-19 vaccine. The US Equal Employment Opportunity Commission proposed rules in 2016 that would allow companies to penalize employees via health insurance costs if they don’t hit a certain BMI. A lot rides on this flawed calculation despite the evidence that simple measures like waist size are more accurate predictors of health risk.

The formula for BMI equals weight in kilograms over height in meters squared. The results are categorized as follows:

Less than 18.5 = underweight

Between 18.5 and 24.9 = healthy

Between 25 and 29.9 = overweight

Above 30 = obese 

These ranges were chosen because people prefer numbers that end in 0s and 5s, not because there’s a change in health risk between 24 and 25. A major flaw in the BMI formula is that it can’t tell what percentage of body weight is bone, muscle, or fat. A sedentary person with 20 extra pounds of body fat and a fit, muscular athlete could have the same BMI but very different health profiles. BMI overestimates body fat in muscular people, and underestimates it in elderly and “skinny fat” people, meaning someone who doesn’t look overweight but is metabolically unhealthy. Someone who could improve their health with diet or lifestyle changes may never be encouraged to do so as long as their BMI is “healthy.”

BMI also fails to account for ethnicity, race, and gender. The formula was based on a European male body. Women, Asian, Black, and Latino people don’t carry body fat in the same way as white men do. Recently, the WHO adjusted the BMI cut-offs for people of Asian descent after research showed they have higher metabolic risk at lower BMIs than other populations. Though this is an acknowledgment that BMI is problematic, no other adjustments have been made.

Waist size and waist-to-hip ratio are simple methods that offer a more accurate picture of health because abdominal fat is more correlated to disease than fat elsewhere on the body, which BMI fails to distinguish. 

Governing bodies like NIH, the CDC, and the American Heart Association use BMI as the standard measure of disease risk because it can help predict health outcomes on a broad level. However, a 2016 study looked at the BMI and metabolic health data of 40,000 Americans and found that half of the “overweight” people and one-third of the “obese” people were metabolically healthy.  On the other hand, one-third of the “normal weight” people were metabolically unhealthy. The researchers estimated that 75 million Americans are wrongly classified as healthy or unhealthy by BMI.

Health is far more complex than a simple math equation designed for a European male in a time before cars were invented. It’s time to move past an antiquated, racially problematic formula that does a poor job of assessing health.