One number can help researchers compare the health of people all over the world.
And, if you use it correctly, it can give you an insight into your own health, doctors say.
Body mass index, or BMI, is a measure calculated from height and weight that places a person into one of four categories: underweight, normal, overweight or obese.
In a few seconds online, you can crunch your numbers and figure out where you fall — from 20 to 25 is considered normal, and 30 or above is obese. For example, a 5-foot, 10-inch man who weighs 170 pounds would have a normal BMI of 24.
While BMI can’t tell the whole story of your health, said Dr. Steven Heymsfield, a researcher at the Pennington Biomedical Research Center, it can point you in the right direction.
“BMI is really just stage one,” Heymsfield said. “And then what you really want is to find out what the translation of that BMI is to health.”
Over the past three decades, BMI has become a well-known — and sometimes controversial — measure.
The formula for BMI is a person’s weight in kilograms divided by height in meters squared. First developed in the 1800s as a way to understand the relationship between height and weight, in the 1970s a prominent researcher compared multiple measures that sought to figure out a person’s body fat, and body mass index was the most accurate, simple measure.
It became widely used in the 1980s as a tool for public health, Heymsfield said, when organizations such as the National Institutes of Health and the World Health Organization began using it to compare different populations.
Examining data, doctors found that there was a U-shaped curve associated with BMI.
On the low-end of the BMI measurement — 18 and lower — people tended to be too thin and had a lower life expectancy. At the high end were the obese, people who had higher risks of diabetes, cancer and heart disease.
“I think what people need to realize is that its real usefulness is on a population level,” Heymsfield said. “It tells us how the obesity epidemic has developed and how it’s spreading and how it varies between states and cities.”
On a personal health level, the formula has its limits.
“It’s very useful on a global worldwide scale,” Heymsfield said. “When you come down to individuals and you start burrowing down, that’s where the wrinkles are.”
The wrinkles pop up mostly in muscular athletes. According to BMI, most professional football and basketball players appear obese, said Heymsfield, who regularly examined the New York Knicks basketball team in the 1990s.
“They all had BMIs over 30 — all of them,” Heymsfield said. “They all had body fat percentages in the single digits. Some of these guys were enormously strong.”
For older people, the BMI should not be strictly followed either, Heymsfield said.
A mostly healthy person who has lived into his or her 70s can be a little overweight, he said. As people in their 70s naturally lose muscle and their appetites generally shrink, becoming frail is a major health concern, he said.
“The bottom line is if you’re a little overweight and you’re 70, I wouldn’t spend a huge amount of time dieting it off like when you’re 30,” he said. “But I would definitely try to maintain your level of physical activity and fitness.”
For most people, calculating their own BMI can be useful, Heymsfield said.
“When you get down to your own number you have to understand what it is telling you,” Heymsfield said. “It can help you sort of place yourself on that spectrum, but it can’t really tell you your body fat very accurately or your health risks very accurately.”
For example, anyone with a BMI over 30 who is not an athlete should be concerned, he said.
If you’re in a “gray zone” where your BMI seems high, but you appear to be healthy otherwise, Heymsfield advises you to get a basic medical exam.
“The reason that’s useful is you will see at that point what your real risk is,” he said. “You will see if you have high blood pressure, abnormal cholesterol or these other risk factors.”