by Jon Robison, PhD, MS
The BMI is a measure of height and weight – specifically weight divided by height squared. It is the predominate measure by which health professionals and governments determine what is and is not a “healthy weight” for a particular individual, thereby informing them if they are “at risk” for morbidity and premature mortality. In reality, however, BMI is not only not a good measure of health, it is actually not a measure of health at all.
The formula itself was created around 1850 by the brilliant Belgian mathematician, astronomer and statistician Lambert Adolphe Jacques Quetelet – and appropriately named The Quetelet Index. Dr. Quetelet was not a health professional and he was not interested in fat or health risk. He was fascinated by the idea of using statistics to draw conclusions about societies – and the “average man.” Some of us will remember the 20th century figure portraying the average family as having 2.4 children. Not only was his formula not health related, it was never meant to be used on individuals, only on populations. As Stanford University mathematician Keith Devlin (the Math Guy on NPR’s Weekend Edition) recently commented, “the absurdity of using statistical formulas to make any claims about a single individual is made clear by the old joke about the man who had his head in the refrigerator and his feet in the fire: on the average he felt fine!” A wonderful expose of the inherent mathematical absurdities associated with the use of this formula can be found in Dr. Devlin’s article Do You Believe in Fairies, Unicorns or the BMI?
The Quetelet Index remained as such until 1972 when Dr. Ancel Keys appropriated it as a proxy for body fat percentage (renaming it the Body Mass Index) in an article in The Journal of Chronic Diseases. The rest, as they say, is history.
So the formula is being used for something for which it was never intended and in a manner that is mathematically indefensible. Are there any other problems? We have been told that the BMI serves as a measure of health because it is a good indicator of body fatness, and therefore a good predictor of health problems and premature mortality. Is this true or isn’t it?
Statistician Dr. Gregory Kline examined this question in an article in The Healthy Weight Journal in 2001. He found that while the BMI can give us a pretty accurate average body fat percentage for a large group of people, on an individual level it is a poor predictor of body fat percentage. For example, Kline showed that in a sample of 1,000 people from Central Massachusetts, for a BMI of 35 the average percent body fat was around 32. However, individuals with a BMI of 35 had a range of body fat percentages from 18 to 47! (Remember the guy with his head in the fridge?) Dr. Kline also encountered the same problem when he used BMI to predict individual fitness or blood pressure concluding that:
“Using BMI to assess degree of adiposity and, more importantly, health risk for an individual is questionable and unwarranted due to the magnitude of error in prediction.”
But wait, there is more! Not only is BMI not a good predictor of body fat, fitness, or blood pressure, it is also not good at predicting mortality or morbidity. In 2006 a large systematic review of the relationship between bodyweight, mortality and coronary artery disease in the esteemed British medical journal The Lancet concluded that BMI was a poor predictor of either. In an accompanying editorial, another physician researcher wrote:
“BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular disease for both primary and secondary prevention.”
Two years later, back in the States in the Archives of Internal Medicine, Wildman et al. analyzed a representative sample of the US population and found that using BMI as a proxy for health resulted in misdiagnosing 51% of the healthy people as unhealthy. Dr David Haslam, clinical director of Britain’s National Obesity Forum got it right when he said; “it’s now widely accepted that the BMI is useless for assessing the healthy weight of individuals.”
So, there we have it. The measure we are using for the supposedly most serious health problem facing us today is mathematically bereft, lacks a theoretical foundation and is a poor indicator of health. According to the Math Guy this realty should come as no surprise as:
“The BMI was formulated, by a mathematician, not a medical physician, to provide a simple, easy-to-apply mathematical formula to give a broad, society-level measure of weight issues. It has absolutely no scientific or medical basis. It is based purely on a crude statistical analysis. It measures a general society trend, it does not predict.”
The sooner the health establishment gets its head out of the sand and owns up to this reality the better. I probably wouldn’t bet much on that happening anytime soon. For now, however, it is at least somewhat comforting to know that the people who really know about these things are willing to lead the way – again quoting the Math Guy:
“Since the entire sorry saga of the BMI was started by a mathematician – one of us – I think the onus is on us, as the world’s experts on the formulation and application of mathematical formulas, to start to eradicate this nonsense and demand the responsible use of our product.”
Come on health professionals – Now it’s our turn!