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The pros and cons of measuring obesity with BMI

U.S. Army

Finding one’s body mass index (BMI) is as simple as typing in height and weight measurements into a BMI calculator. Those easy results, though, can sometimes be misleading, which is why the widespread use of BMI is troubling, according to a doctor at the Mayo Clinic.

Dr. Francisco Lopez-Jimenez joins us on "Take Care" this week. He's division chair of preventive cardiology at the Mayo Clinic. Lopez-Jimenez said the concept of having a standard definition of obesity is relatively new in medicine, originating about 30 years ago. This introduction of BMI was generally a good thing, he said.

“Historically, obesity has been recognized as a medical condition for many, many years, and I think it was an important step to give it a name, give it a definition,” Lopez-Jimenez said. “By having a way to diagnose, I think we can increase the awareness.”

BMI soon became ubiquitous, being used for blanket definitions of obesity and health when it was never meant to do so.

“The main problem is that something that was used in the population basis … turned into a measurement to be used individually, which wasn’t the original intention,” Lopez-Jimenez said.

Lopez-Jimenez said BMIs on the extreme ends -- very underweight or obese -- can be fairly reliable measurements for health levels.

“The problem is the middle ranges,” he said. “That’s where the measurement can give a significant amount of error … It’s a mistake to consider the results of the BMI in the intermediate range as something that you have to act upon.”

The middle range brings out one of the main problems with using BMI as a standard definition of health: it is only based on weight and height. Lopez-Jimenez said many things factor into a person’s weight, like muscle mass, bones, fat and water, and that can mean two people with the same BMI have vastly different body types and health levels.

“If you get 100 people with a BMI that is exactly 25, you will see a big range of body fat percentage,” he said.

The unreliable nature of BMI is why researchers like Lopez-Jimenez are trying different angles to look at health, like waist circumference and waist-to-hip ratios. This is because, though BMI was a good first step, he said it is important to accurately measure body fat.

“To address obesity as a whole, I really believe we have to move to the next step, which is measuring body fat,” he said. “Measuring obesity by measuring the fat directly is something that is relatively simple.”