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Body mass index

Does BMI accurately determine body fat?

Your health is so much more than the numbers on a scale. Body mass index (BMI) is a widely use measure to determine if a person is at a healthy weight, but it may not paint the whole picture.

BMI is calculated by your weight divided by your height squared. What it doesn’t take into account is how many of those pounds are fat and how many are lean muscle tissue.

“It’s not the best indicator of how much body fat and how much muscle one has,” says Karen Andry, R.D., a bariatric coordinator at Piedmont. “But it’s a universal number to measure morbid obesity and obesity.”

BMI calculations:

Experts look at these numbers when determining how likely a patient is to suffer serious chronic illness, like diabetes, glucose intolerance, high blood pressure or coronary artery disease:

  • Less than 18.5 = underweight

  • 18.5 to 24.9 = healthy weight

  • 25.0 to 29.9 = overweight

  • 30.0 and above = obese

“The higher the number, the greater the correlation with death,” says Andry.

Other ways to determine your risk for serious disease

However, BMI isn’t the only way to measure a person’s risk for serious disease.

1. Waist circumference can also indicate if a person is at risk for serious conditions. A person is at high risk for serious illness if their waist circumference is: 35 inches or greater for women and 40 inches or greater for men.

2. Underwater body weighing. “The only true, accurate way to test a person’s body fat thickness is underwater body weighing,” says Andry. This method, also called hydrostatic weighing, determines a person’s body composition (body fat vs. lean muscle mass). A person is weighed outside the water, then lowered underwater in a large tank while their underwater weight is measured.  

3. Metabolic cart. Hospitals, such as major burn centers, also use a metabolic cart which measures a person’s carbon dioxide output and oxygen input to determine their body fat and lean muscle tissue percentage.

While other measurement tactics are more accurate, they are not commonly used. BMI can give you a good starting point for discussion with your physician about your health risks.

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