Time to Ditch the BMI?

Posted on June 22, 2012


BMI is calculated by plotting one’s height and weight.

I’ve been interested in health and nutrition since I started playing high school sports.  During my 3 year stint as a personal trainer, I developed a distinct opinion of the Body Mass Index (or BMI – the most widely used anthropomorphic measurement in medicine):  it’s not a very good tool.

My issue with BMI began here – it doesn’t actually measure a person’s body fat or lean mass, but rather is an assessment of an individual’s weight and how it deviates from what has been determined ‘normal’ for his or her height.  People come in all shapes and sizes, and my clients were no exception.  You may have heard the argument that BMI doesn’t apply to athletes, for example.  I, myself, met the strict criteria for ‘overweight’ (BMI>25) during my brief foray into amateur bodybuilding.  So I abandoned BMI calculations and instead used body fat percentages as goals for my clients.

An inherent, albeit uncommon, pitfall of BMI.

A study published this April in PLoS One assessed the utility of BMI, DXA scans (typically used to assess bone mineral density), and serum leptin levels when measuring an individual’s adiposity.  Researchers concluded that BMI misclassified 25% of men and 48% of women – the effect was exaggerated in older individuals (likely because individuals lose both height and lean body mass as they age).  The authors suggest that use of DXA and leptin measurements “offers the opportunity for more precise characterization of adiposity and better management of obesity.”  This is where our opinions part ways.

Using DXA scans and leptin measurements to assess adiposity in patients would be unnecessarily costly and invasive (and DXA scans pose yet another source of radiation exposure patients simply don’t need).  Given the widespread availability and improved accuracy of bioimpedance devices, point-of-care testing of body fat percentage is made not only simple to implement, but would be cost-effective in any primary care office.

A typical commercial bioimpedance device.

Now, the virtues of the BMI are that it’s a simple, easy-to-calculate, low-tech means of assessing a patient’s body habitus, and it provides a standardized format for physicians to use.  However, skinfold calipers are also cost-effective.  Bioimpedance devices, while not as accurate as calipers, may be a quick, cost-effective way to more accurately describe and track a patient’s body fat, and they don’t require any experience to operate or interpret.  They also provide a means of describing body composition in a way that patients are more likely to understand.

Wouldn’t it be great if you could have a personalized measurement of your lean and non-lean body masses?  A meaningful number you could track as you diet and exercise?  Many of my clients and patients have been discouraged when their weight doesn’t change, or even increases, when they implement a diet and exercise plan – it can be hard to explain to them that body composition is more important than weight!  Likewise, patients may find body fat percentage easier to relate to than the esoteric BMI.

The reality is, BMI won’t be going anywhere for the time being, but it would be interesting to see these alternatives tested in clinical practice to determine if they can in fact help patients improve their body compositions.

Posted in: Fitness, Health, Medicine