The Skinny on BMI

After a few week hiatus, I thought it’d be good to get back to a nutrition-related topic.  This is a sneak peak of what I’ll be publishing in my Nutrition 101 column in May’s Oasis Magazine.

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The Skinny on BMI

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Most people with even a passing interest in health and nutrition are familiar with the term Body Mass Index or BMI.  BMI is commonly cited in news reports and used by health care practitioners as a measurement for assessing health.  The take away message by the general public?  The higher someone’s BMI, the fatter, and subsequently less healthy, you are.  But is it all really that simple?

What is BMI?

For any of you that have plugged your particulars into an online BMI calculator, you’re already aware that you only need two measurements to calculate your BMI – your height and weight.  The formula is actually quite simple: weight in kilograms divided by height in meters squared (or kg/m2).  The current classification system is commonly used:

Snap 2015-04-03 at 10.54.01

From what we know, health problems can arise when people fall outside the normal BMI range.  By extension for optimal health it stands to reason that everyone should be trying to attain a normal BMI, right?  Not so fast!  What would you say if I told you that BMI was never intended to be used to as a tool for assessing an individual’s health?  Let me explain.

BMI’s origins

The BMI measurement was developed in 1832 by Adolphe Quetelet in his attempt to classify the “normal man” and had nothing to do with weight-related disease1.  It wasn’t until 1972 that BMI’s use in the health care industry really began to take off.  This was when an obesity researcher named Ancel Keys published a study2 demonstrating that of all the health indices of the time BMI correlated most closely with body fat percentage, which itself was a challenge to measure and a strong predictor of mortality.  Cut-offs were eventually created and in 1998 the National Institute of Health in the US adopted the table above for use by health practitioners.

Limitations and Alternatives

The biggest problem with BMI comes when translating the results of a population onto an individual.  It may be true that at a population level people with a BMI of 30 or higher have a much higher risk for developing diabetes or cardiovascular disease but you can’t say that this is also true for any one person because an individual’s health status is dependent on many factors.  In fact Dr. Keys himself counselled against using BMI in this manner.  There is also mounting evidence that the existing BMI categories don’t translate particularly well to many different populations.  For example, BMI overestimates obesity in African Americans and people with high muscle mass.

A much better indicator of health status is body fat percentage.  This is now easier to measure than ever before.  Many gyms today have equipment that can measure someone’s percent body fat by simply standing on machine that looks like a fancy scale.  If measuring your percent body fat isn’t available another simple measure has been shown to be better than BMI – waist circumference.  It’s easy, cheap and correlates better with abdominal fat levels, which is an important factor for determining someone’s health.

The Bottom Line

So is BMI something that should be completely done away with?  I wouldn’t go quite that far.  BMI is a fine tool for tracking the health of a population.  As a society we should be trying to create policies that shift more people into a healthy BMI range, whatever that range may be.  However, the practice of using BMI as the primary justification for counselling someone to lose (or for that matter gain) weight needs to be retired.

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References

1.  Eknoyan G. (2008). Adolphe Quetelet (1796-1874) – the average man and indices of obesity.  Nephrol Dial Transplant. Jan; 23(1):47-51.
2.  Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL.(1972). Indices of relative weight and obesity. J Chronic Dis. Jul 1;25(6):329-43.
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