Why Do Doctors Use BMI?

The abbreviation BMI stands for body mass index. It's a calculation doctors and healthcare providers use when referring to your weight in relation to your height. It's also a screening instrument to determine whether you need further evaluation for risk of obesity and related health problems. Although BMI isn't sensitive enough to tell the difference between a muscle-bound heavyweight boxer and an elderly grandmother, it is a convenient tool that gives your healthcare provider a quick snapshot of your level of body fat.

BMI Calculation

The formula for determining your BMI -- your weight in kilograms divided by the square of your height in meters -- was developed in 1832 by Belgian astronomer and social scientist Adolphe Quetelet. He wasn't trying to measure obesity, but BMI has become the standard way Western medicine defines obesity. If your BMI measures greater than 25, you're considered overweight. If it's greater than 30, you're considered obese.

BMI Controversy

Studies in adults and children, such as those published in the June 2008 and March 2009 "International Journal of Obesity," respectively, show that BMI works for people with very high levels of body fat. It is not a good calculation for those who are not obese. That's a big problem, considering its job is to help your doctor know if you're at risk for obesity or to assist in determining whether you could be headed for illnesses such as heart disease or metabolic syndrome. In these studies, the BMI measure labeled children as obese when they had normal body fat and failed to identify elevated body fat levels in many adults.

Sensitivity of BMI

Part of the reason for BMI's controversy is that it's a one-size-fits-all approach tool. It doesn't distinguish between a person who weighs more because of lots of lean muscle mass and a person who weighs more because of extra fat. The BMI also doesn't take into account how your body fat is distributed. For example, you have higher risk for disease if you have extra belly fat than if you have more fat around your hips. The authors of the study in the "International Journal of Obesity" also said BMI presents more problems with men and the elderly.

BMI Bottom Line

Public health officials ranging from the Centers for Disease Control and Prevention to the World Health Organization, or WHO, accept the BMI as a screening tool and not as a way to diagnose a health problem. For example, the WHO says: "BMI provides the most useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults. However, it should be considered as a rough guide because it may not correspond to the same degree of fatness in different individuals." Your doctor will use additional procedures like waistline measurements and skinfold tests before determining whether you are obese and need treatment. The BMI remains a cost-effective, quick and relatively simple way to gauge your risk for obesity and other health problems.