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Body mass index: What it can (and can’t) tell you about your health

Body mass index: What it can (and can’t) tell you about your health

Doctors often use BMI to determine if you’re at a healthy weight. But it’s not always accurate. Here’s why.

You recently checked the results of your yearly wellness exam. And near your height and weight, you may have seen another number. That’s your body mass index, or BMI.

Your BMI is a measurement of your body fat based on your height and weight. Doctors widely use BMI as a tool to screen for unhealthy weight. It also helps doctors assess your risk of developing chronic conditions related to excess body fat like heart disease and type 2 diabetes, among others.1 In 2019, for example, doctors used BMI to assess patients’ body fat and related diseases at about a rate of 85% for commercial health plans and 88% for Medicaid plans.2 

Knowing your BMI can help your doctor put you in one of four weight categories: obese, overweight, normal weight or underweight.1

But your BMI is just one screening tool your doctor can use to figure out your risk of diseases linked to high body fat.And on its own, BMI doesn’t always tell the whole story about the connection between your weight, body fat and health, say experts.1

Read on to learn what your BMI can and can’t tell you about your health.

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How does your doctor calculate your BMI?

Your BMI is simply your weight (in kilograms) divided by the square of your height (in meters).3 You can use this BMI calculator to do the math for you.4 So, for example, if you’re 5-foot-10 and weigh 150 pounds, your BMI is 21.5.4

Here’s how the four BMI weight categories connect to your BMI number, according to the National Heart, Lung, and Blood Institute:1

  • A BMI below 18.5 is considered underweight.
  • A BMI of 18.5 to 24.9 is considered normal weight.
  • A BMI of 25 to 29.9 is considered overweight.
  • A BMI of 30 or greater is considered obese.

If you have a BMI of 21.5, that means you’re considered to be at a normal weight.

What can my BMI tell me about my health?

Doctors use your BMI to screen for certain health problems. “It’s a good starting point,” says Stephen B. Johnston, MD. He is a cardiologist at USMD, part of Optum, in Fort Worth, Texas. But it can’t just end there. BMI is a quick, easy way for providers to get a snapshot of one of your health indicators, and then they can investigate further. Research has linked BMIs of 30 or higher (i.e., obesity) with certain long-term conditions such as:5

If you find out that you have a high BMI after your physical, talk to your doctor. They can tell you what your number means for your personal health profile. They will also consider other risk factors you may have for chronic conditions, such as high cholesterol or high blood sugar.1

It’s important to know that having a high BMI doesn’t mean you will definitely get these diseases. And the opposite is true, too. You can have a normal BMI and still have conditions such as heart disease and diabetes.

What can’t my BMI tell me about my health?

Your BMI number may be a helpful first look for doctors, but it has some flaws. One of the biggest: It measures your weight, but does not directly measure your level of body fat, according to the Centers for Disease Control and Prevention.5 And it’s body fat, not weight, that is linked to your risk of developing the diseases mentioned above.

Other limitations of BMI include:

  • It can underestimate the amount of body fat a woman has compared to a man, even if their BMIs are the same.1
     
  • It can underestimate the amount of body fat in certain people, especially older adults and children.6
     
  • It is not always an accurate measure for very fit athletes, who may have a lot of muscle and little fat. They may still have a high BMI because muscle weights more than fat.1,6
     
  • It is not always an accurate measure of body fat in people of certain racial or ethnic backgrounds. Research shows that people with the same BMI can actually have higher or lower amounts of body fat, depending on their race/ethnicity.5,7

BMI also does not factor in where you carry your weight. Having excess fat around your belly (apple-shaped) puts you at a higher disease risk than if that fat is around your hips and butt (pear-shaped).8

Plus, there are many other things besides weight that affect your overall health. These include your genes, dietary choices, exercise habits, mental health and whether you smoke.8

So, let’s say your BMI is 21.5, but you smoke and rarely exercise. In that case, having a BMI of 21.5 doesn’t necessarily mean you’re in better health than a fit person with a BMI of 29 who has better lifestyle habits. You may still be at a higher risk of heart disease or certain cancers than the person with a higher BMI.1

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I have my BMI number. Now what?

Whatever your BMI number is, it can help tell you something about your health. Your doctor can help explain that to you and make sure you’re making the healthiest choices.

But your BMI can’t tell your doctor everything. And some major health organizations are changing their views on BMI. In 2023, for instance, the American Medical Association (AMA) called for a new approach to figuring out how a person’s weight affects their overall health.Their suggestion: Doctors should not rely on BMI alone to figure out whether a patient is at a healthy weight. BMI has issues, the AMA says, because it does not account for differences across gender, age, and different racial and ethnic groups. Some people naturally have bigger bodies — but that doesn’t necessarily mean they’re unhealthier.9

Remember: Belly fat is another risk factor for obesity-related medical conditions, not just your weight. So, another screening tool your doctor might use to evaluate risk is to measure abdominal fat, which is the distance around your waist (waist circumference). It can be used alongside BMI and your family history to help figure out disease risk.1,10 You may be at higher risk of conditions such as heart disease and type 2 diabetes if:

  • The distance around your waist is more than 40 inches (for men).
  • The distance around your waist is more than 35 inches (for nonpregnant women).

Bottom line: The next time you get a physical exam, your doctor will likely measure your height and weight and calculate your BMI. It’s a quick way for them to better understand your overall health. But it’s not the only way.

Sources

  1. National Heart, Lung and Blood Institute. Assessing Your Weight and Health Risk. Accessed October 19, 2023.
  2. National Committee for Quality Assurance. Adult BMI Assessment (ABA). Accessed October 19, 2023.
  3. Centers for Disease Control and Prevention. Body Mass Index: Considerations for Practitioners. Accessed October 19, 2023. 
  4. National Heart, Lung and Blood Institute. Calculate Your Body Mass Index. Accessed October 19, 2023.
  5. Centers for Disease Control and Prevention. About Adult BMI. Last reviewed June 3, 2023. Accessed October 19, 2023.
  6. National Library of Medicine, MedlinePlus. Body mass index. Last reviewed July 25, 2022. Accessed October 19, 2023.
  7. Nature. Different correlation of body mass index with body fatness and obesity-related biomarker according to age, sex and race-ethnicity. March 1, 2023. Accessed October 19, 2023.
  8. Cleveland Clinic. Body Mass Index. Last reviewed May 9, 2022. Accessed October 19, 2023.
  9. American Medical Association. AMA: Use of BMI alone is an imperfect clinical measure. June 14, 2023. Accessed October 19, 2023. 
  10. Centers for Disease Control and Prevention. Assessing Your Weight. Last reviewed June 3, 2022. Accessed October 19, 2023.

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