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.
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