BMI – is it just a number?

BMI, or body mass index, has long been used as a way to assess body weight in the United States. The federal government uses the calculation to track obesity rates nationwide, and according to this scale, 42.4 percent of American adults age 20 and older are obese, notes the Centers for Disease Control and Prevention (CDC).

Adults can measure their BMI by taking their body weight in pounds, dividing that value by the square of their height in inches, and multiplying the result by 703, or using an online calculator. The numbers are then used to determine weight categories:

  • Below 18.5 is underweight
  • 18.5 to 24.9 is normal
  • 25 to 29.9 is overweight
  • 30 and over is obese

If this formula seems complicated and somewhat arbitrary, that’s because it is.  Body fat percentage, not BMI, is largely responsible for a person’s health. Too high a number can result in obesity related complications, including diabetes, heart disease and increase your risk for many cancers. By relying on a person’s overall weight, BMI may overestimate body fat in athletic, muscular individuals or those with large or tall frames, labeling them as overweight or having obesity when they are not at increased risk. 

However, the reverse is also true, you can have a “normal” BMI but still have a high percentage of body fat—and the associated medical complications. More than half of U.S. adults currently considered to have a normal BMI actually have a high body fat percentage (more than 30 percent fat for women.) 

The way in which fat is distributed in your body has major health implications, yet it’s not taken into account by the BMI calculation. Belly fat, or fat that accumulates around the waistline and abdominal organs, greatly heightens a person’s risk of heart disease, type 2 diabetes, and other obesity-related complications, as well as death. But a person could easily fall into the “normal” BMI category (18.5 to 24.9) and have a waist circumference that puts them in a risk health category.

Individuals with “normal,” or non-obese/non-overweight BMIs, but with a large waist circumference (known as central obesity) are much more likely to die prematurely compared with people who are deemed obese via BMI but don’t have a large circumference. For women, central obesity is defined as a waist circumference greater than 35 inches for women. Waist circumference should be measured at the smallest area of one’s waist, just above the belly button. 

Waist-to-hip ratio can also be used to estimate risk.  This number divides waist circumference by hip circumference (measured around the widest part of your buttocks). Anything below 0.9 is considered healthy. A ratio of 1.0 or above is correlated with a two to three times increased risk of dying. Someone with a larger waist than hips will have a waist-to-hip ratio of 1.0 or above.

Understanding Your Risk: Talk with your healthcare provider to come up with a plan to assess your risk- this may include your BMI, waist circumference, hip measurements, blood pressure and lab results.

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