04/25/2014 12:16 EDT | Updated 06/25/2014 05:59 EDT

Why Body Mass Index Doesn't Measure Up

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Last week the terrific health writer Jane Brody used her weekly column in the New York Times (April 15, Science Times) to criticize overreliance on Body Mass Index (BMI) as an indication of people being overweight or obese. If anything, she was too mild in her criticism of a standard that has been used to hound fat people about their weight rather than them, and all of us, being urged to be as healthy as possible. ("Fat" rather than "obese" is the preferred term by many in the overweight community. I use both).

BMI is a ratio of weight to height (weight in kilograms divided by the square of the height in meters). The BMI was devised in the nineteenth century as a statistical tool and meant to be a way to assess weight in the population generally. However, over time, it became an easy and inexpensive way to target individuals as being too large: greater than 25 considered as overweight; over 30 considered obese; over 40 morbidly so. BMI seems to be a straightforward way to determine if an individual has excessive weight (and how much) and to track the number of those with such issues in the general population and various subgroups.

But there are lots of criticisms of this measure. Here are four: First, by focusing on body mass, this assessment does not differentiate between fat and muscle. The latter weighs more than the former. Many athletic individuals can have BMIs in excess of 25 (or even 30) and not be overweight by any sensible definition.

Second, like many sharp points of demarcation, the cutoffs have an element of arbitrariness to them. The boundaries for who is overweight and obese were suddenly shifted downward in the late 1990s by the US National Institute of Health. Vast numbers of people were suddenly considered overweight and obese, who days before were not so designated, as measured by BMIs.

Third, these cutoff points do not necessarily signal danger to health. A famous Centers for Disease Control study and follow-ups established that the association of deaths and high BMI was much lower than originally asserted. At the same time it was demonstrated that slightly overweight individuals had lower mortality rates and underweight people had higher ones.

Finally, there is the overarching criticism of focusing on weight rather than healthy eating/drinking and exercise. BMI reduces a complex array of factors concerning pounds and health to a single number that may not signify much that is useful concerning either. At the same time, because BMI assigns a single number, the competitive instincts of individuals can tempt them to compare themselves based on that score. Those with lower numbers can claim superior status without regard to how the number is derived (muscle versus fat) and the actual health of any particular individuals (including those with lower BMIs).

Several states in the U.S. have legislated programs to measure the BMIs of school children as part of a strategy to combat obesity. Ontario and other provinces may be taking up similar efforts. They might think again. Skepticism involves all the criticisms of BMI just discussed. In addition, as advocates acknowledge, there has been little systematic assessment of the effectiveness of these programs. And what would effectiveness mean? That BMI scores are lowered? If so, how? That such screening contributed to children eating/drinking better and being more physically active? That eating/drinking better and being more physically active is a good result even if a child's BMI does not decrease or even increases?

Another worry is that, over time, such screening will lead to even more stigmatization of fat children as information about scores leak out as kids are ranked. The scores, themselves, will say little about children's health, eating/drinking patterns, or activity levels. Measuring BMI will promote scorn towards the obese, particularly fat kids: not the intended result but a negative outcome just the same.

BMIs -- a measure that, mostly, doesn't measure up.


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