Measuring Obesity

Part 2: The Limitations of Body Mass Index

The problem of obesity is so complex from a scientific standpoint and so highly-charged in the public discussion that it’s hardly surprising that there are divergent opinions. My previous article prompted several responses arguing that “obesity” should be considered a “disease.” But let’s leave this question aside for the moment.

Before one can address this question in any serious manner, one has to examine the measure that is being used to gauge obesity – in this case body mass index, or BMI, which is calculated as weight (in kilograms)/(height in meters)2.

The problem I was pointing out is that a BMI of 30 or above used to define obesity is only a crude proxy for adiposity, or fat content. Properly defined, obesity is the excess accumulation of body fat. BMI is widely used because it is easy to calculate and inexpensive, since all it requires is measurement of weight and height, and avoids the much more costly and time-consuming methods used to measure fat content of the body.

BMI has come into widespread use by clinicians and receives enormous coverage in the media, but with scant attention to its limitations. BMI is useful and it is correlated with the risk of developing certain diseases, most prominently diabetes, heart disease, and several cancers. However, one cannot afford to ignore its limitations, which are considerable.

First, BMI does not distinguish between body fat and lean mass (everything other than fat – i.e., bone, muscle, water, etc.). Two people can have the same BMI but very different body fat content. Furthermore, while misclassifying some people as obese who are not obese by more accurate measures, using this criterion actually misses a substantial proportion of people with excess body fat.

This is because where the fat has accumulated and what type of fat are both important, and BMI tells us nothing about this. BMI cannot distinguish between abdominal fat vs. lower body fat (often referred to as “apple” vs. “pear” shape). Waist circumference, or waist girth, does a better job than BMI of capturing abdominal fat, which is more deleterious to health than lower body fat. But waist circumference is less widely used. Finally, neither measure can distinguish between visceral fat (considered to be most deleterious to health) and the more benign subcutaneous fat.

To define something as a disease, one needs clear-cut and specific criteria that correctly classify those with the disease as HAVING the disease and also correctly classify those without the disease as NOT having the disease. A yardstick which misclassifies a substantial proportion of the population clearly has a problem.

Before running off to classify something as a disease, especially using a tool that declares that a third of the adult population has this disease, one has to pause to look critically at the tool that is being used to make that determination. While BMI is useful, it is far from being a precise instrument for measuring health-related adiposity in the general population.

Geoffrey Kabat is an epidemiologist at the Albert Einstein College of Medicine who has studied a wide range of lifestyle and environmental exposures in relation to cancer and other chronic diseases. In addition to his scientific work, he is interested in risk perception and the public understanding of science. He is the author of Hyping Health Risks: Environmental Hazards in Daily Life and the Science of Epidemiology and writes the "Risk-omics" column at Forbes. This article is reprinted with his permission.

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