Although I’ve discussed this issue a number of times over the years, every now and then a new study comes out that provides further evidence of the limitations of body mass index (BMI) as a measure of health, or even adiposity (level of fat in the body).
Recall that BMI is the most common metric used to assess body weight status, and to identify the presence of overweight and obesity. While it is great when used in epidemiological studies across thousands of people, it’s a pretty lousy measure on an individual basis. So why does it keep being used? Its relatively easy and inexpensive to measure height and weight – hence, the measure persists in clinical practice.
In this cross-sectional study the authors assessed the BMI, body fat percentage, and cardiometabolic risk factors of 6123 (924 lean, 1637 overweight and 3562 obese classified according to BMI) Caucasian subjects (69% females) between the ages of 18 and 80 years.
What did they find?
First, 29% of subjects classified as normal weight and 80% of individuals classified as overweight according to BMI had a body fat percentage within the obese range. Thus, on an individual basis BMI tends to consistently underestimate a person’s adiposity. This data implies that there are many individuals who don’t weigh that much on an absolute scale, but a large proportion of their weight is composed of fat tissue. These are people who may look thin, but tend to be soft, with little muscle tone. In a clinical setting, these folks may easily be overlooked by their physician due to their “normal weight.”
Conversely, approximately 5% of individuals classified as overweight and 0.2% of those classified as obese by BMI actually had low levels of fat mass. These individuals would be the bodybuilders of the bunch – high absolute body weight that is composed mostly of muscle mass. This is often the line of argument used to illustrate how ineffective BMI is at measuring adiposity. And yet, as I’ve criticized before, the misclassification in this direction appears to occur only rarely. This simply suggests there are simply few body builders around. More importantly, any physician should be able to conclude that the patient in front of them with a BMI of 31 kg/m2 is not actually obese when their biceps are bigger than their waist.
Finally, when compared to individuals who were actually lean (both on BMI and body fat), those with a high levels of adiposity, regardless of their BMI (normal weight, overweight or obese BMI) had poorer cardiometabolic profiles, including elevated blood pressure, blood glucose and lipid levels, as well as markers of systemic inflammation.
So how can a physician determine whether a patient with a normal or overweight BMI is actually obese? Measuring waist circumference certainly seems to help. This one measure can help distinguish those with a low versus high adiposity despite similar BMIs.
Even better, physicians could start paying less attention to weight or adiposity altogether, and evaluate more relevant markers of a patient’s health, including metabolic factors, psychological status, mobility, etc.
Reference: Gomez-Ambrosi et al. Body mass index classification misses subjects with increased cardiometabolic risk factors related to elevated adiposity. International Journal of Obesity (2012) 36, 286–294; doi:10.1038/ijo.2011.100; published online 17 May 2011