BMI: Is it reliable?

Body mass index is one means of determining whether an individual is at an appropriate weight. Individuals of abnormally high or abnormally low BMI are at risk for various diseases and disorders, making BMI an important diagnostic tool in medicine and personal health monitoring. However, BMI isn't always a reliable measure of health.

BMI

Your BMI is essentially a modified ratio of your weight to your height. You can calculate your BMI by dividing your weight in kilograms by your height in meters, squared. Alternately, you can multiply your weight in pounds by 703, and divide that number by your height in inches, squared. BMI values less than 18.5 correspond to being underweight, while values of 18.5 to 24.9 indicate a healthy weight. Individuals with BMI greater than 25.0 to less than 30.0 are considered overweight. The American Heart Association defines obesity has having a BMI of 30.0 or greater and extreme obesity has having a BMI of 40.0 or greater.

Validity

For individuals of relatively normal activity level and body habitus -- that is, those who are neither particularly fine-bones and slender nor particularly muscular -- BMI is relatively valid in assessing weight status, according to a 2008 article in the "International Journal of Obesity." As such, BMI is one common tool used by fitness coaches and physicians to quickly assess a client's or patient's weight status.

Reliability

While your height won't vary from day to day, your weight can -- and can even vary within a given day, depending upon your hydration level, what you're wearing and how much you've eaten. As such, BMI values are most reliable when used to assess weight status in terms of trends over time; BMI variations within a given day -- and even within a given week -- aren't meaningful. However, as your weight changes over time, your BMI will reliably reflect the change in weight status.

Concerns

With regard to two specific subsets of the population, BMI fails rather notoriously, notes the article in "International Journal of Obesity." Individuals with a lot of muscle mass, such as body builders, may have high BMI because of the density of muscle tissue. Even if they have very low body fat, these people can have a high BMI, incorrectly indicating obesity. Further, very inactive but slender individuals, most commonly women, sometimes appear healthy or even underweight based upon BMI. However, because fat is less dense than muscle, it's possible for them to have unhealthy levels of body fat despite appearing lean and light.