Body mass index (BMI) is a way that doctors measure your risk for certain health conditions that are linked to being overweight or having obesity.
The benefit of BMI is that it’s simple. “It's easy to measure and standardized. It's just height and weight. Nothing else,” says Seth Kipnis, MD, director of bariatric and robotic surgery at Hackensack Meridian Jersey Shore University Medical Center in Neptune, New Jersey.
BMI is a convenient screening tool, but it can’t provide a clear picture of your overall health.
What Is BMI?
To figure out your BMI, divide your weight in kilograms by the square of your height in meters. You can also use an online BMI calculator.
Then, you classify your weight by the number you get:
- Underweight: BMI less than 18.5
- Ideal: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class I obesity: 30 to 34.9
- Class II obesity: 35 to 39.9
- Class III obesity: BMI higher than 40
The concept of BMI dates back to 1832. A Belgian mathematician came up with the formula to try to find the ideal body weight of white French and Scottish men. In the 1970s, a physicist used it to analyze data and gave it the name of “body mass index.”
Hundreds of years later, BMI is still used by doctors across the U.S.
How Do Doctors Use BMI?
BMI allows your doctor to quickly estimate your risk for certain health conditions that are linked to being underweight or overweight.
For instance, a low BMI can put you at risk for issues like:
- Anemia
- Immune system issues
- Malnutrition (not having enough vitamins and minerals to stay healthy)
- Osteoporosis
- Infertility
A high BMI is linked to:
- High blood pressure
- Heart disease
- Arthritis
- Sleep apnea
- Gallstones
- Mental health conditions like depression
- Type 2 diabetes
- Some types of cancer
A high or low BMI signals your doctor to keep a closer eye on your health.
Health plans also use BMI to decide if they will pay for some medical treatments.
What Are the Flaws of BMI?
Despite being widely used, BMI isn’t perfect. Among its flaws:
BMI isn’t precise.
For instance, “Tall people will have lower BMI. Short people will always have a high BMI. Muscle weighs more than fat. Muscular people will have a higher BMI. Football players who are young, very fit and healthy all will have high BMIs,” Kipnis says.
Many important factors, like your gender, race, genes, and age play a part in your weight, but BMI doesn’t factor them in.
BMI doesn’t map your body fat.
BMI can’t show where fat is inside your body.
“It’s important to recognize where the fat is distributed,” says Hany Takla, MD, a weight loss surgeon at the Orlando Health Weight Loss and Bariatric Surgery Institute in Orlando, Florida. “What we call visceral fat is a more dangerous type of fat accumulation that predisposes to more medical conditions.”
Visceral fat expands your belly size and surrounds your organs. Too much of it can change how well your body functions. A waist greater than 40 inches for men and 35 inches for women puts you at higher risk for many health issues.
BMI can cause stigma and bias about larger bodies.
Feeling ashamed or embarrassed that your BMI is high could keep you from going to the doctor as much as you need to. It also makes you nearly three times as likely as someone with a “normal” BMI to feel that you're not getting proper medical care.
BMI is not for everyone.
For instance, it shouldn’t be used if you are:
- Under 18 or over 65 years old
- Pregnant
- A serious athlete or body builder
- Managing a health condition that causes muscle wasting
Can You Have a High BMI and Still Be Healthy?
There’s a lot that BMI can’t tell your doctor, like how healthy your meals are or how often you exercise.
One study of 40,000 people found that nearly half of people with an “overweight” BMI, 29% with an “obese” BMI, and 16% with a BMI of type 2 or 3 obesity were “metabolically” healthy.
But obesity, and how it affects your body, is complex. If you have a high BMI and work out regularly, you may be able to keep your blood pressure and cholesterol within healthy ranges and decrease your risk for type 2 diabetes. But over a long time, obesity can still raise your risk for health issues.
Better Ways to Measure Your Health
BMI shouldn’t be the only screening tool that your doctor uses to assess your health.
They should also look at your:
- Blood pressure
- Blood glucose
- Cholesterol levels
- Liver function
Your waist circumference (pants size) for men and hip circumference for women can also be helpful. “Men and women have different patterns of obesity — apple and pear shaped bodies, respectively,” Kipnis says.
Waist or hip circumference takes into account your visceral fat and can help predict your risk of conditions that are linked to it.
“Stamina and exercise tolerance can also be a measure of body health,” Kipnis says. “Can you walk up a flight of stairs without becoming short of breath? Or at all?”
An imaging test, like a dual-energy X-ray absorptiometry (DXA) scan, can give more precise measurements of your body fat, as well as your bone density and lean tissue. But it’s expensive and not every medical center has the equipment.
BMI in the Future
“BMI doesn't need to be replaced, but it needs to have different expectations based on variables,” Kipnis says. “People are different. Charts need to reflect that, and this would help manage expectations when managing weight loss methods.”
When screening you for chronic diseases, your doctor needs to consider a number of different factors besides your height and weight.
Doctors could also get better at explaining what BMI means, especially if you have a high number.
“Being overweight or obese is related to a lot of things, like genetics, medications, psychological issues, stress, and sleep, and this is all proven by science,” Takla says.
Once you know that your weight can be connected to many things that are out of your control, “then [you] can actually seek help to get control over it,” Takla says.
