BMI vs. Body Fat Percentage: A Primer

[First of all, I heard the word “primer” on the radio today and the host pronounced it like “PRIMM-er” rather than “PRIME-r,” which is how I’d always imagined it was said.  Does anyone have the authoritative last word on this?]

Anyhoo, I had a client head over to my #AskAmanda page on Facebook today (every Wednesday, BTW) and ask me a great – and often confusing – question:

How is muscle tone accounted for in BMI readings?

BMI, for those of you who haven’t caught on to this medical buzzword, stands for Body Mass Index.  This measure is easily calculated using a formula with two variables – your height and weight.

BMI is useful in medical settings because it helps establish a range – an estimate, if you will – of categories that help medical professionals zero in on potential risk factors.  For example, a BMI of over 40 (this would look like a woman who is 5’2″ but weighs 220 pounds) means you are categorized as morbidly obese, which is correlated with several health problems including ventilatory disorders, circulatory congestion, and unexplained sudden death (!).

Here’s the problem with BMI, though – let’s say you’re a bodybuilder.  You are 5’7″ tall and weigh 200 pounds (completely possible on a frame with a lot of muscle!) – this puts your BMI at 31.3, which reads as “obese.”  That said, you may have so little body fat that you are not remotely at risk for excess-fat-related health problems, such as heart disease or diabetes – but on paper, you fall into that same unhealthy category due to your BMI.

This is why I am always harping on clients to get their body fat measured.

Body fat percentages are exactly what they sound like – a ratio of the fat you carry on your body relative to the lean mass (muscle, bone, and fluid).  They can be measured using hydrostatic weighing (the best choice – if you’re ballin’ and have access to a water displacement tank), calipers (available at most gyms, and very reliable), bioelectrical impedance (the quickest, easiest method), and even good old-fashioned measuring tape.

As you can imagine, two people can have the same BMI but vastly different body fat percentages and overall appearances – the pictures below make it clear that the number on the scale (and thus, the number in your BMI calculation) doesn’t matter at all if your body fat is in a healthy range – and for most people, they’d prefer a higher number if it means more lean mass.

The reason I get on such a soapbox about this is because clients come to me with either one or two numbers on their mind – their body weight, almost always, and their BMI, if they’ve been told it’s “high” or “obese” or “at risk” – and they lose focus on the actual changes that need to take place in their bodies for better health.

Long story short, whether you are someone worrying about your weight on the scale, or someone who has always had a “healthy” weight but lacks lean muscle (the “skinny fat” issue, which I’ve written a whole post about in the past), it’s time to put BMI behind you and get a solid measure of your body fat percentage (below is an example of a body fat chart so you can evaluate your progress) for a true, meaningful read of your physical fitness.

What’s your opinion on BMI*?  Is it useful at all or do you rely more on BF%?

*a quick note – I actually wrote my Master’s thesis using BMI as a central variable because it is useful in policy recommendation due to estimating power.  That said, for individual case-by-case clients, I broadly favor BF% for the reasons I outlined above.

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