Yes, to some degree, having a higher BMI has been associated with a lower risk of death. But interpreting these new findings to mean anything more than that, and precisely that, is dangerous.
The counterintuitive findings that people who are overweight live longer, published today in the Journal of the American Medical Association, couldn't have been more perfectly timed, coming as it has right when people are resolving to be healthier -- which involves, for many, losing weight.
In many places where this story has been picked up, including The New York Times, The Wall Street Journal, and Time, the implication seems to be that the push to get people down to a "healthy" weight has been overblown.
But that being overweight is associated with increased lifespan isn't new. It's called the "obesity paradox," and studies documenting it have lead to widespread speculation about the potential "protective benefits" of excess body fat.
For some health advocates, the implication is downright offensive. Walter Willett of the Harvard School of Public Health, for example, lost his cool this morning on NPR, declaring, "This study is really a pile of rubbish and no one should waste their time reading it."
But the study's author, Katherine Flegal of the Centers for Disease Control and Prevention, mounted a solid defense: "It's statistically significant." Those three words carry weight -- if an association has been found to be significant, it tells us that if nothing else, we need to acknowledge that the results are in some way legitimate and warrant our attention.
The findings are without doubt interesting, which on its own makes the study worth reading. The problem is that despite the grandness of the meta-analysis -- it takes into account over 3 million people! -- it still has an extremely limited scope. It looks at BMI, and only BMI, in relation to death, regardless of cause. It's impossible to report on its baseline conclusion without taking into account substantial caveats.
This can be said of any study where one thing is found to be associated with another, but in this case the findings are particularly amenable to being mistakenly interpreted as instructions, along the line of: "You should gain weight to live longer." In this case, its real value is that it highlights the problems we always run into when attempting to talk about weight and healthy living.
Aside from the obvious limitations -- people who pass away after a lengthy period of disease, for example, will likely be thinner than they might have been had they died unexpectedly -- the study fails to take into account any of the various other measures used to assess health. It ignores blood pressure, blood sugar, and cholesterol -- high levels of all are directly associated with a variety of chronic conditions and diseases -- not to mention mental health and life satisfaction scores. As another large-scale study recently pointed out, longevity isn't everything. The population as a whole is living longer than it was twenty years ago, but the number of those years spent in poor health are increasing as well.
That BMI is an imperfect measure of body size is emphasized here as well. The simple calculation of height and weight ignores gender, age, and muscle mass -- I remember being hopelessly confused the first time a guy told me his goal was to gain weight. Where on the body fat is located is important as well (belly fat, for example, poses a greater health risk than excess weight that's more evenly distributed). A BMI in the "overweight" range, from 25 to just below 30, encompasses a broad sweep of body diversity: A frequently cited argument is that Michael Jordan, at his prime, would have been classified as overweight. By almost any other measure but BMI, we would almost certainly put him in the range of ideal health.
Just as BMI glosses over such variance, so, too, have news outlets reporting on the study. Instead of illustrating the findings with bodies that look like this:
A less conventional rendering of an "overweight" person could have been just as appropriate:
While in the most basic of ways, it makes sense to pay attention to the number on the scale, it only gives us one metric of health that, if not understood in context, is basically useless. If we could get used to looking at weight more holistically, in terms of overall health, the link between BMI and longevity wouldn't be so shocking.