If you go to your physician’s office and inquire about your weight
status, he or she will measure your height and weight to derive your BMI
(weight in kg divided by height in m squared). Then they will compare
your BMI to that of established criteria to decide whether you are
underweight (<18.5 kg/m2), normal weight (18.5-24.9 kg/m2),
overweight (25-29.9 kg/m2), or obese (>30 kg/m2) . Often times, this
measure alone determines whether or not you receive lifestyle treatment.
But how useful is this measure anyways? What does it tell you about
your health? And finally, how helpful is it to measure when assessing
the effect of a lifestyle (diet/exercise) intervention?
For quite some time I have been meaning to discuss some of the issues
of solely relying on BMI as a measure of obesity and health, and a nice
nudge from our friend ERV was just the motivation I needed to finally get to work.
Before I get into the various limitations of BMI, I must point out
that the measure is quite useful across large populations, as it is well
correlated with the degree of adiposity, and of course it is extremely
simple to measure in clinical practice.
Nevertheless, here are some of the key issues with BMI, particularly when used on an individual basis.
1. BMI does not differentiate between the
Michelin Man and The Terminator
Ok, we might as well just get this abundantly obvious problem out of
the way. I have heard countless times how one buff celebrity or another
(e.g. Tom Cruise,
Arnold Schwarzenegger, The Rock etc.) would be
classified as overweight or obese according to their BMI due to their
excess amount of muscle. Yes, this is absolutely true. BMI is a measure
of relative weight; fat mass and muscle mass are not distinguished.
Here’s what is equally true: the large majority of the general
population with a BMI in the overweight or obese range does not look
like Jerry Maguire or the Terminator. Also, if you seek advice from your
physician about your “excess weight”, in case you have body
dysmorphia
and cannot yourself decide, they will quickly be able to assess whether
your excess weight is due to your bulging muscles or your rolls of
adipose tissue. So while this is an obvious problem, I would argue not
the main issue.
2. BMI does not differentiate between apples and pears
For over 60 years, we have known that independent of how heavy a
person is, the distribution of their body weight, or more generally the
shape of their body is a key predictor of health risk. It is now well
established that individuals who deposit much of their body weight
around their midsection, the so called apple-shaped, are at much greater
risk of disease and early mortality in contrast to the so called
pear-shaped, who carry their weight more peripherally, particularly in
the lower body. Thus, two individuals with a BMI of 32 kg/m2 could have
drastically different body shapes, and thus varying risk of disease and
early mortality.
Fortunately, a very simple measure allows you or your physician to
decide whether your elevated BMI is of the apple or pear variety: waist
circumference. Current thresholds suggest that a waist circumference
above 88 cm in women and 102cm in men denotes
abdominal obesity.
Interestingly, for the same BMI level, those individuals with an
elevated waist circumference have a greater risk of diabetes,
cardiovascular disease, mortality, and numerous other health outcomes.
Thus, as studies from our laboratory have consistently suggested, waist
circumference may be a more important measure of obesity and health risk
than BMI. Currently, most researchers would agree that waist
circumference should be measured along with BMI to adequately classify
obesity-related health risk.
You can measure your own
waist circumference by using a tape measure
and wrapping it around your abdomen, at the level of the top of your hip
bones. Make sure you measure at the end of exhalation, without sucking
in your gut – you’re only fooling yourself!
3.
BMI does not always budge in response to lifestyle change
Given the number of papers my supervisor, Dr. Ross, and I have
published on the topic, I would argue this is the biggest drawback of
using BMI: it doesn’t always change even though you may be getting
healthier. This is particularly so if you adopt a physically active
lifestyle, along with a balanced diet, but are not necessarily cutting a
whole lot of calories. This lack of change in BMI or body weight is all
too often interpreted as a failure, resulting in the disappointed
individual resuming their inactive lifestyle and unhealthy eating
patterns.
However, as we have argued most recently in a paper in the Canadian
Journal of Cardiology, several lines of evidence suggest that weight
loss or changes in BMI are not absolutely necessary to observe
substantial health benefit from a healthy lifestyle. Thus, an apparent
resistance to weight-loss should never be a reason for stopping your
healthy behaviours.
First, it is well established that increasing physical activity and
associated improvement in cardiorespiratory fitness are associated with
profound reductions in coronary heart disease and related mortality
independent of weight or BMI. Second, exercise (even a single session)
is associated with substantial reduction in several cardiometabolic risk
factors (such as blood pressure, glucose tolerance, blood lipids, etc.)
despite minimal or no change in body weight. Third, waist circumference
and abdominal fat (arguably, the most dangerous fat) can be
substantively reduced (10-20%) in response to exercise with minimal or
no weight loss. In fact, significant reductions in fat mass often occur
concurrent with equal increases in muscle mass in response to physical
activity – equal but opposite (and beneficial!) changes which are not
detected by alterations in body weight on the bathroom scale, and thus
BMI.
So in the end, while BMI surely has its strengths in ease of use and
pretty good reliability in large populations, on an individual basis,
the greater focus should be on healthy behaviors: physical activity and a
healthy diet. And if you must measure something, check your waist
circumference.
Peter Janiszewski
Ross
R, & Janiszewski PM (2008). Is weight loss the optimal target for
obesity-related cardiovascular disease risk reduction? The Canadian journal of cardiology, 24 Suppl D PMID: 18787733
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