Body mass index, or BMI, is one of the most frequently referenced and rarely explained health metrics Americans use. You've probably found where you fall on the mysterious scale using online calculators, and you may have even made assumptions and adjustments based on where on what range that number fell. The power that number has is pretty astounding, actually. BMI is used as a barometer for overall public health and is the basis of just about every obesity statistic you see. Countless people base major health decisions on a pretty simple calculation, often without consulting health professionals, so one would hope that number is reliable.
The science behind the number, or lack thereof, might surprise you. Here are facts about the figure that you should keep in mind the next time you make a health assessment based solely on your height and weight.
1. It measures mass, not fat.
As you've probably already heard, muscle weighs more than fat. This means athletes and other people with higher muscle mass can fall into the overweight category of BMI 25. In addition to not distinguishing between muscle and fat, it doesn't recognize different types of fat which have different impacts on health. A person who carries most of her fat around her stomach is at greater risk of health problems than someone who carries it on her hips, even if both have the same BMI.
2. It misinterprets the meaning of obese.
The Mayo Clinic defines obesity as a disorder characterized by the presence of excessive body fat. Note the medical definition of obesity is based on fat, not weight. While high BMI correlates with high body fat percentage, BMI doesn't directly measure it. One study found that 39% of participants whose BMI fell below the obesity range were obese according to more accurate body fat measurements.
3. It's more misleading for women than men.
According to the authors of the same study, women lose muscle and bone mass—and replace it with fat—more quickly than men do. This suggests the measure gets even less effective for women as they age.
4. BMI was created by a mathematician, not a physician.
In the early 1800s, Belgian mathematician and social scientist Adolphe Quetelet devised the measurement to help the government easily assess the general population's obesity level. Nearly 200 year later, the measurement hasn't evolved nearly as much as our understanding of nutrition and the connection between weight, body fat percentage, and health risk factors.
5. The metric's creator didn't want it to be used to measure fatness.
Quetelet said his formula about weight, not presence of fat, and didn't want it to be used much beyond its purpose as an assessment tool for the government.
6. The boundaries it defines for "overweight" and "obese" are arbitrary.
BMI categorizes people with a figure below 18.5 underweight, between 18.5 and 24.9 "normal," above 25 overweight, and above 30 obese. These numbers are made up. Even if obesity was a result of sheer weight and not fat, these is no concrete reasoning for why the cutoff is at 30, and not 28 or 33.