The body mass index (BMI) has become the most-used means of determining whether we are overweight or not, supposedly by measuring our body fat. Plug your height and weight into a BMI calculator and you’ll get a quick and dirty “definitive” answer. But where did BMI come from and how useful is it really?
BMI was invented in the 1800’s by a Belgian polymath named Adolphe Quetelet. It’s a mathematical calculation dividing a person’s weight by the square of his/her height. He was doing social research on what the “average man” looked like. His research had nothing to do with health. He was researching a lot of things, like a man’s arm strength or the age he marries.
The “pioneers” studying and pushing BMI for profit? Insurance companies. In the early 1900’s, motivated by their bottom line, life insurance companies started doing studies of BMI to show overweight people were more likely to have health problems and die earlier to show their policy holders.
In 1972 a professor and researcher named Ancel Keys published his paper “Indices of Relative Weight and Obesity.” He examined height-weight formulas and determined Quetelet’s was the best. Keys gave Quetelet’s formula the name “body mass index.”
BMI caught on. Instead of being used as studies of population health, doctors began using it as a quick way to measure body fat. But Keys had actually warned in his paper against using BMI for individuals diagnoses, since the equations ignores a lot of other variables, like exercise and ailments like high blood pressure. The Center for Disease Control (CDC) notes that BMI is only one factor to consider in determining weight problems:
BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI. However, to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.
And that’s where the problem lies. BMI is of limited usefulness, and can in fact be misleading for a number of reasons. Body frames aren’t taken into consideration. BMI doesn’t measure actual body fat. Muscle weighs more than fat, which can skew the results. The elderly lose height without weight. Gender is ignored in BMI. Individuals’ medical history is ignored. Women have more body fat than men. According to the BBC, weight around our middle (being apple-shaped) is more unhealthy than weight around our hips and thighs (being pear-shaped).
Measuring body fat is better accomplished by directly assessing body fat—like with the use of calipers or measuring the circumference around a person’s waist. So why is BMI used? According to the CDC it’s because its easier and less expensive than better alternatives:
Other methods to measure body fatness include skinfold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and isotope dilution. However, these methods are not always readily available, and they are either expensive or need highly trained personnel. Furthermore, many of these methods can be difficult to standardize across observers or machines, complicating comparisons across studies and time periods.
In essence, BMI allows us to diagnose ourselves in a way that may not be accurate. Even when we do go to the doctor, our doctors often use the BMI chart as a crutch, an easy way of “diagnosis,” doing little more than we can do ourselves.
If our health isn’t being harmed enough by our society’s misplaced dependence on BMI, our access to medical care is being affected as well. Private health insurers use BMI in their underwriting as a means to raise premium rates or deny coverage entirely. And now our employers are starting to pass on costs as well. A recent NY Times article talked about how employers are more frequently requiring workers who are overweight pay more for their health insurance. And yes, they determine it by BMI. These financial burdens may require workers to settle for less-expensive, not-as-good health insurance coverage or drop coverage altogether.
Basically the BMI is lazy health care. And we deserve better.