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The Skinny on BMI

Separating fact from fiction when analyzing fat

BMI

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While it's a good tool to start measuring your body fat, BMI should not be the only method you use, experts say.

Many people don’t realize that the popular weight analysis tool known as body mass index (BMI) was not intended to be a commonly used measurement of fat; rather, it was invented in the 1800s for use in population studies, CNN reports

But then a 1972 study found that the BMI formula best measured body-fat percentage, as opposed to calculating only weight and height. The study warned against using BMI to calculate an individual's levels of fat, but it still gained popularity among doctors and the general public. National and international government agencies began to use BMI, and it eventually became a uniform standard. 



To calculate your BMI, divide your weight in pounds by your height in inches, and then multiply that figure by 703. A result of 30 or above indicates obesity, between 25 and 29.9 means you are overweight, and if you fall between 18.5 and 24.9, you are considered normal weight. Underweight is considered to be below 18.5. While BMI is a reasonable indicator of body fat, it shouldn't be the only diagnostic tool used to analyze the relationship between health and weight, according to the Centers for Disease Control and Prevention.

"Some people who are labeled overweight by BMI have a broader frame or more muscle," Courtney Peterson, an assistant professor in the University of Alabama at Birmingham's Department of Nutrition Sciences, told CNN. Additionally, BMI may underestimate body fat in an older person who has lost muscle tone. 

The dangers of "skinny fat"

BMI can also underestimate the health threats for people who are “skinny fat.” This describes those who overall may seem fit but have large bellies. A study in the journal Frontiers in Public Health found that using BMI alone missed 50 percent of cases of people who had dangerous levels of fat. Study co-author Paul Laursen prefers to use the term "overfat," instead of "overweight," because it is the fat, rather than the weight, that hurts your health — particularly if the fat is stored around the waist.

Hospitals and doctors have advanced machinery to measure fat composition, and there are other methods you can use at home to calculate fat. An old-fashioned tape measure can gauge the circumference of your body at your belly button. If your waist circumference is half your height or less, you have a healthy fat level. But if you are over that number, your health could be at risk. 

For an even easier method, take a look at your hip-to-waist ratio. "If the waist is bigger than the hips, it tells me that the risk carried with that weight is much higher for that person for premature death," Francisco Lopez-Jimenez, M.D., an obesity expert at the Mayo Clinic, told CNN.

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