Body mass index (BMI) and body fat are very different measures of obesity/health.  Unfortunately they are all too often used interchangeably. Last year my Doctor told me I was borderline Overweight (based on BMI).  Based on height and weight measures, I do weigh “too much” for my 5’2″ stature.  BUT, I’m not fat nor am I unhealthy. My body fat is 25% at 43 years old. Politely I let her know that BMI wasn’t an applicable measure of obesity/fitness for me and this is why . . .
Body mass index (BMI) is used to determine if a person is overweight. Generally speaking, for the mass population, it’s “related” to body fatness and is inexpensive and easy to perform on a large group of people.  However, for someone who is active – this measurement is meaningless.  Because BMI is based solely on height and weight, there is no accounting for muscle mass.  We all know that muscle weighs more than fat.  An athlete who is short and has a lot of muscle mass (low body fat), may fall into the overweight or obese BMI category because the BMI standards say she/he is too heavy for her/his height. Even if that extra “weight” is muscle mass, it doesn’t matter for BMI calculations.  Using myself as an example, being 5’2″ tall and weighing 142lbs, my BMI is 26.  When I did my bodybuilding show, I was 5’2″, 137 lbs; my BMI was 25.  According to the measure of BMI – I’m overweight! Now let’s consider a more accurate indicator of overweight. Calculate your BMI HERE.
Body composition is a measure of your percentage of body fat compared to your lean muscle mass. Body fat is the percentage of your body that is fat. This can be measured by several different methods, but the easiest is biolelectrical impedance. It sounds worse than it is, but it requires you to stand on a device that measures the resistance to flow of a super small current passing through your body. This speed is entered into an equation that calculates the percentage of your body weight that is due to fat. Body composition does take into consideration your muscle mass – not just height and weight.  Although more accurate than BMI, the measurement of body fat does require a trained individual and it is more time consuming.

In the graphic, both ladies have the same BMI, but vastly different body shapes. The lady on the right has significantly more muscle so she would have lower body fat than the lady on the left. According to BMI, both are the same measure of “obesity/health”. Again, for someone who has muscles, BMI is meaningless.  For more reading, the New York Times published an article on the difference between BMI and body fat. Very interesting.

BOTTOM LINE: BMI is a gross estimation of overweight.  It is NOT indicative of the true measure of fat in the body. Rely on body fat measures rather than BMI as your indicator of fatness.

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