When you visit your doctor, he or she will probably measure your height and your weight. Then a Body Mass Index (BMI) will be calculated and placed on your chart. The BMI has actually become one of the standard “vital signs” required at a doctor visit over the last five years. This was not something that physicians started measuring on their own, it is a required measurement most health insurance plans insist upon before they will pay for the visit. BMI is an interesting and arguably worthless measurement. It was developed by a Belgian physicist by the name of Adolphe Quetelet some time between 1830-1850. Quetelet was a “Social Physicist,” trying to combine probability and statistics with the study of sociology (1). BMI was originally called the “Quetelet Index” and was designed specifically to measure averages among large populations in sociology or epidemiological studies (2).
BMI = mass (kg) / [height (m)]²
However, it wasn’t Quetelet that got BMI placed on the medical chart. BMI was made popular by the infamous Ancel Keys (the same Ancel Keys responsible for the flawed Seven Countries Study) in his July 1972 article published in the Journal of Chronic Disease. Ancel Keys, himself, explicitly stated that BMI was designed specifically for population studies, and inappropriate for evaluation of individual health (2). But, because of the ease of measurement, and the fact that Life Insurance companies had been using BMI to set your insurance premiums since the 1970’s, Health Insurance companies adopted it as a measure of overall health. In 1985, because BMI found great favor in epidemiological research, the National Institutes of Health (NIH) adopted it as the method to identify and define obesity in patients. And, in 1998 the NIH identified the BMI cutoffs – 25 for overweight, 30 for obesity – as easy numbers that could be remembered and used to counsel patients on weight reduction and health.
The problem with BMI is that it doesn’t actually identify a person with excessive fat accumulation. BMI is a height to weight ratio. It doesn’t account for fat at all. Using the NIH guidelines, the Terminator, with an estimated BMI of 31, would be considered obese. (You try telling the Terminator that he is obese, and see what happens.) Because, muscle weighs twice as much when compared to an identical volume of fat, Mr. Schwarzenegger (or anyone with increased muscle mass), will have a higher scale weight. This raises the BMI calculation, giving a false indication of increased health risk.
I was recently asked about a study published in The American Journal of Medicine that was recently commented on in Scientific American regarding BMI vs Muscle Mass as a predictor of longevity. The assumption is that just because your doctor measures BMI, it must be a great tool predicting health outcomes. The assumption is absolutely wrong. We know from multiple studies, including the article sited above, that increase muscle mass increases overall health, decreases the likelihood of insulin resistance and diabetes, reduces the risk of heart disease, and extends longevity. How can a measure of height to weight predict longevity? It can’t. The only reason BMI is on the medical chart, thanks to the NIH, is so that the physician gets paid. 
In actuality, the most effective way of measuring a persons health is to simply measure body fat. This can easily be done by measuring waist circumference with an inexpensive tape measure. It can also be done with a simple bioelectrical impedance measurement. The gold standard for measuring body fat is to strip you down naked and dip you in a tub of water, measuring the water displacement. (I have very few patient’s that will return to my office after doing that, so we don’t use it very often, but it is effective).
The answer to the question is “NO,” the Terminator is NOT obese. And, if he shows up in my office, I’m not going to comment about his BMI.
1. Eknoyan, Garabed (2007). “Adolphe Quetelet (1796–1874)—the average man and indices of obesity”. Nephrology Dialysis Transplantation 23 (1): 47–51.
2. Keys, Ancel; Fidanza, Flaminio; Karvonen, Martti J.; Kimura, Noboru; Taylor, Henry L. (1972). “Indices of relative weight and obesity”. Journal of Chronic Diseases 25 (6–7): 329–43