Do You Believe in Fairies, Unicorns, or BMI?

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Got a call from an upset client earlier,

She’d gone to the doctors for a check-up, you know, women’s stuff!

Anyway, she rang me quite upset. You see the nurse measured her BMI and came back to her and said…

Wait for it…

She was over-weight!

Now that might not come as a surprise to you, but it really shocked this girl, because quite clearly she most definitely isn’t over-weight.

In fact her body fat percentage is only 18%

Which for a female puts her into the ‘athlete’ category on the ACE body fat chart (the most commonly used guidelines for body fat classification) (1). We knew this as she’d recently completed my FFS programme, where we measure body fat percentage.

But for a few moments it really upset her, well until I’d explained to her just how deluded BMI really is. You see, the BMI or body mass index formula uses height and weight to determine a person’s amount of body fat.

BMI classifies persons in four different weight ranges: underweight, normal, overweight and obese.

There are a couple of issues with this, first up – the BMI formula was devised in the 19th Century by Adolphe Quetelet, a  Belgian statistician who came up with the Quetelet Index of Obesity, which basically measured obesity by dividing a person’s weight by the square of his or her height.

It was later termed ‘BMI’ in a study published in the Journal of Chronic Diseases by Ancel Keys in 1972 and then went on to become a widely accepted method of categorising obesity, particularly in the NHS.

So we’re using an equation that was defined in the 1800s and brought to accepted practice over 40 years ago to determine if someone in the 21st century is overweight? When we have far more advanced techniques to determine a person’s body fat?

The second and biggest issue with BMI is it doesn’t take into account body composition.

So someone (like the girl earlier) whose weight may initially seem a little high for her size has a body composition that is muscular and lean.

When a person loses fat and ‘replaces’ it with muscle, they may not ‘lose’ overall weight and as BMI does not distinguish between lean muscle mass and fat it will classify him or her as overweight.

According to the American Council on Exercise, when assessing someone with a lean muscular physique BMI may cause frustration and confusion about their body!

Kind of what happened this morning!


——— Top tip coming up ———

Unless you’re noticeably carrying excess weight (particularly centrally), don’t stress over BMI readings. Find a trainer who can accurately measure your body fat percentage (the best way is by skin fold callipers) and use that as your measure.

Movie stars Johnny Depp, Brad Pitt, Will Smith and Tom Cruise have all scored as obese – imagine what Arnold Schwarzenegger would have scored at his prime?

Till next time


PS – The girls on my FFS, Female Fitness & Strength programme all have their body fat percentages taken regularly to plot their progress. Find out more >>>HERE<<<




(1)      Ace Lifestyle & Weight Management Consultant Manual, The Ultimate Resource for Fitness Professionals. American Council on Exercise; 2009.



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Marti Whitaker, EzineArticles Basic Author