What Your BMI Isn’t Telling You (part 1)

The news is filled with headlines about America’s obesity epidemic. The scientific definition of obesity is a BMI of 30.0 kg/m^2 (which is body mass in kilograms divided by height in meters squared), but is that a helpful definition?

While BMI can provide a general insight into someone’s health and body fat levels, it really isn’t the most accurate measure. For example, the average BMI of a Bronco’s NFL football player is 30 kg/m2 or more[1]; some even have a BMI as high as 40 kg/m2 (which would be considered morbidly obese). \

However, many of these players do not appear to have 30+ extra lbs. of body fat on them. BMI doesn’t necessarily paint the entire picture.

What BMI fails to take into account is body composition, or what actually makes up the weight of your body. Our body weight is made up of four main components: muscle, fat, water, and mineral (or bone) mass.

The overall distribution of these components are more accurate determinants of health. When you gain or lose weight, that weight gain or loss could involve muscle, fat, or water (but generally not bone, which doesn’t change from day to day).

Body fat levels can be measured in several ways, including:

BIA (Bioelectrical Impedance Analysis),

Where an electrical current is used to estimate body fat percentage. This technology is available on a basic level in many body weight scales now, but there are specific procedures that should be followed to obtain an accurate measurement.

Skin Fold Calipers

Where specific measurements are taken using calipers that measure the thickness of fat on a specific body part. Usually 3-7 measurement sites are used.

Measurements should always be taken by a professional and require a degree of precision in order to give an accurate reading. Measurements are then plugged into an equation which estimates body fat percentage.

DXA (Dual-energy X-ray Absorptiometry)

Where two x-ray beams are used to take a cross section of the body. This test is normally performed to measure bone mineral density, but it will also provide a very accurate reading of body fat percentage.

All three of these common methods for measuring body fat percentage will provide far more useful indicators than simply calculating BMI based on height and weight, especially for athletes. Want to know what body fat percentage you should be aiming for? Find out next week!


Chen, A. (2016, February 4). If BMI is the test of health, many pro athletes would flunk. Retrieved

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