What is BMI?

There’s a lot of talk about BMI in the news and in fitness magazines, etc. But, what is it and why should we care about it? There are BMI charts posted in my exam rooms (not my doing) that intrigue everyone. I always catch patients plotting their own heights and weights on the chart, which serves as a nice lead-in to discussing the uncomfortable topic of weight.

BMI stands for Body Mass Index. It is meant to be a simple numerical representation of how “fat” your body is, a ratio of your weight to your height. It is calculated as:

BMI = kg / m^2

or = [mass (in kilograms) / height (in meters) squared]

Here’s an easy BMI calculator where you can enter your data in feet/inches and pounds rather than using metric measurements.

What do the numbers mean?
BMI < 25 – Ideal
BMI 25-30 – Overweight
BMI > 30 – Obese

For most body types, this value has moderate usefulness. It’s especially useful to be able to tell someone that their morbidly obese body habitus far exceeds the expected values as above and to give them a goal. However, it is of absolutely no value in individuals with a fair amount of muscle mass. This equation obviously does not differentiate between weight from fat vs. weight from muscle. Therefore, an individual with a large muscle mass could appear obese on this chart despite having a body fat percentage of less than 10%, for example.

I quickly throw this calculation out the window when confronted with a high value in someone who has a fair amount of muscle mass. Unfortunately, I fear that insurance companies will start to track more data like this and will inappropriately punish individuals like this who have abnormal BMI’s due to a large amount of lean body mass.

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