Biology in the News Explained

The continuing misuse of Body Mass Index

Some researchers (Flegal et al., 2005 and 2007) have claimed, using data for Body Mass Index (BMI) and death rates, that it is somehow beneficial to be “overweight.” This is the type of announcement that will always make headlines in the popular press. As usual, these studies are fraught with flaws that would only be accepted in a publication about humans, and are meaningless in the terms that really matter to all of us, which is what decisions individuals should make about their lifestyles in order to have the best chance of living long, healthy lives.

The major reason why studies like these are meaningless is because of the arbitrary nature of definitions: “underweight” = BMI <18.5; "normal" = BMI 18.5-25; "overweight" = BMI 25-30; and "obese" = BMI > 30. There may not be much reason to quibble with the extremes on this scale. As expected, the majority of “excess deaths” were associated with the extremes in weight – both “underweight” and “obesity grade 2″ (BMI>35). But of course what makes the headlines is the fact that over the time period studied, “overweight” people had fewer excess deaths of most diseases than “normal” people.

The first question anyone should ask is, what exactly does BMI measure? BMI is nothing more than a ratio of your weight by your height (squared). It takes nothing else into account. As originally conceived, the BMI was not intended for assessing individuals medically. It has become a popular measure in the media because it is somewhat useful for describing population-level trends – such as the well known trend in the U.S. toward more obesity over the last few decades. BMI data in this context can show us that there is something about the lifestyle of Americans which is causing us to gain weight, and in a rational world would lead to measures to provide plenty of decent food and opportunities for reasonable levels of physical activity for everyone in the country.

But for an individual, BMI is not particularly useful, because it does not take into account the individual variation in body type (for example, bone density or muscle density, which contribute far more to weight than fat), not to mention activity level or overall conditioning, or even gender. All the talk of BMI has emphasized calculating one’s own BMI, for example at the federal government’s CDCsite, which, based on the arbitrary, government-created ranges listed above, spits out an assessment that you are “normal,” “overweight,” etc. Of course the caveat on most of these sites is given that for “highly trained athletes” BMI may be “high”; it is implied that this condition is sufficiently rare that the great majority of us need not take it into account.

On any football team, even at the lowest level (such as my NAIA college), every player would be considered “overweight,” and many are classified as “obese.” Perhaps this is a straw man, but I would challenge someone who seriously believes in the BMI as an individual health measure to calculate it for anyone who regularly goes to a gym or just has more than moderate physical activity. Based on my own experience, I suspect that a lot of Americans who ended up in the “overweight” category were fit; hence, the “earth-shattering” results that it is beneficial to be “overweight.”

Of course, these papers are indeed using the typical huge sample sizes to make their point, so aren’t they using BMI correctly, to study general population trends? If only they said so in the discussion, or at least included the caveat about the arbitrary divisions in BMI classes, one could more easily put these papers into the proper context, which is that there are health risks associated with the extremes of BMI. But of course they don’t. The truth is that only you and your doctor can assess your personal health risks and the benefits to changing your lifestyle (if that is even realistically possible for most people). The media saying to everyone, “hey, it’s better to be overweight!” are missing the point. If you are interested in maintaining or improving your health, stop using BMI calculators. Just eat well and get enough exercise to feel good.

References

Flegal, K.M., B.I. Graubard, D.F. Williamson, and M.H. Gail, 2005. Excess deaths associated with under weight, over weight, and obesity. Journal of the American Medical Association 293:1861-1867.

Flegal, K.M., B.I. Graubard, D.F. Williamson, and M.H. Gail, 2007. Cause-specific excess deaths associated with under weight, over weight, and obesity. Journal of the American Medical Association 298:2028-2037.

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