Weight, Weight, Don't Tell Me

I don’t think I have to cite statistics to prove North Americans are getting heavier. But in case you’re unconvinced by words and numbers, a map might help.  Take this illustration of obesity trends (your web browser has to be set up to display animated images) taken from maps at the US Centers for Disease Control and Prevention.

Trends in Canada and the rest of the developed world, are similar, though less extreme. 

Historically, two decades is just the blink of an eye—but it’s been occasioned by a simply unprecedented increase in our girth.  Year-to-year, the trend is basically imperceptible.  But over time, all of that gradual change adds up to massive, even fundamental shift. 

There’s a fairly straightforward explanation for what’s happening here:  on average, people eat more than than they used to, and they exercise less.  In some cases intentionally, and in others accidentally, we’ve engineered daily exercise out of our lives.  Our jobs have become more sedentary, and in many of the neighborhoods built after World War II, walking to school or stores is somewhere between inconvenient and impossible.  And rising incomes, coupled with a system of agricultural subsidies that encourages the overproduction of grains, means that we’ve surrounded ourselves with an abundance of ridiculously cheap high-calorie foods. 

But of those two explanations—falling levels of daily exercise, rising calorie consumption—which predominates?  This National Bureau of Economic Research working paper (pdf link) argues that the bigger of the two culprits is the abundance of cheap, high-calorie food:  82 percent of the recent rise in developed-world obesity, it argues, has been caused by an increase in calorie consumption.  Falling levels of physical activity matters too, but it’s less than a fifth of the story behind obesity trends.  (The website This is Why You’re Fat—an online gallery of ridiculously artery-clogging foods—is apparently onto something.)

That said, we should be careful not to equate weight and health.  A landmark study published in 2005 in the Journal of the American Medical Association actually found that overweight individuals—folks with a Body Mass Index between 25 and 30—have lower mortality rates than folks who are “normal” weight.  And while obesity (BMI > 30) is associated with significantly higher mortality rates than either normal weight or overweight, the impact of obesity on mortality has declined over time.  The idea that being “overweight” may not be a major health risk has sparked quite a bit of controversy about correlation vs. causality—a debate that leaves experts and non-experts alike unsure of what “healthy weight” really means. 

Which takes us back to exercise.  The decline in physical activity may be a relatively minor culprit in expanding waistlines—but no matter what your weight, moderate daily exercise is good for your health. Which, in my mind at least, suggests that redesigning our neighborhoods and communities to facilitate walking and biking may be, in the long run, an important avenue for improving public health.