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The Skinny on Body Mass Index

Brian Turner

Posted on March 08 2016

Body mass index (BMI) is a measure of bodyweight that is useful for classifying the health risks of bodyweight. Body mass index is based on the concept that weight should be proportional to height. BMI is a fairly accurate measure of the health risks of bodyweight for average (non-athletic) people, and it is easy to calculate and rate (BMI = weight (kilograms)/ height2 (meters). Because BMI doesn’t distinguish between fat weight and fat-free weight, however, it is inaccurate for some groups. For example, athletes who weight train have more muscle mass than average people and may be classified as overweight by the BMI scale. Because their “excess” weight is in the form of muscle, however, it is healthy. Further, BMI is not particularly useful for tracking changes in body composition— gains in muscle mass and losses of fat. Women are likely to have more body fat for a given BMI than men. BMI measurements have also over- and underestimated the prevalence of obesity in several ethnic groups. If you are an athlete, a serious weight trainer or a person of short stature, do not use BMI as your primary means of assessing whether your current weight is healthy. Instead, try simple methods for measuring body fat such as bioelectrical impedance or skinfold testing.


(The New York Times, April 15, 2014)