Last month in this space I asked Slate readers to help me understand what's going on with obesity—specifically, the explosive growth in obesity over the past 10 years. I'm still digging out from under the excessive weight of the e-mail deluge (several thousand), but here's my preliminary report.
The least helpful contributions are the ones that explain, often in painstaking detail, that obesity is caused by some combination of eating too much and exercising too little. Well, duh. The question is why eating and exercise habits have changed so dramatically at this particular time.
Some readers speculate that nothing has changed other than the definition of obesity. Nice try, but you're wrong: All the numbers in last month's column were based on a fixed definition. (You are obese if your weight in pounds exceeds 4.25 percent of the square of your height in inches.) Others attribute the obesity explosion to the aging of America. Another nice try, but it doesn't fit the facts. Obesity is growing within every age group. There are more obese 40-year-olds today than there were 10 years ago; you can't blame that on aging boomers. (A lot of my e-mail conflated the question "What causes obesity?" with the question "Is obesity a bad thing?" The latter is a separate question, and I plan to come back to it in this column in the near future.)
The angriest e-mail comes from readers who believe they've been duped into eating too many carbohydrates and not enough fat. The Agriculture Department's ubiquitous "food pyramid" recommends six to 11 servings a day of bread, cereal, rice, and pasta—a diet certain, in the minds of my irate correspondents, to trigger insulin reactions that prevent you from burning calories and/or cause you to crave between-meal snacks. At the same time, the food pyramid discourages you from eating the fats that would assuage your hunger. (This circle of ideas, which began with Dr. Atkins' Diet Revolution, is very trendy these days.) So the idea is that the government is handing out exactly the wrong advice, everyone's following it, and nobody's noticed how counterproductive it is. Well. Maybe.
The second-angriest letters come from those who are sure we're getting fatter because of growth hormones in our meat. I won't pretend to know enough science to analyze this one, but I will note that an unusually high fraction of these e-mails are tinged with unmistakable hysteria.
Many of my correspondents surmise that life is busier and more stressful now than 10 years ago, so that we exercise less and eat more haphazardly. That makes sense if indeed life is busier and more stressful, but I have no idea whether that's true. Here's one bit of evidence to the contrary: The average work week is no longer now than in 1990. I'm willing to believe that the average Slate reader's life has gotten busier and more stressful, because the average Slate reader is more likely than the average American to be raising children and in the midst of a demanding career. But it would be rash to overgeneralize.
Quite a few of you mention suburban sprawl, which means more driving, less walking, and less time for meal preparation. I am skeptical, because obesity is growing rapidly in every part of the country but sprawl is still localized. Still, it would be interesting to know whether obesity is growing fastest in the most sprawling areas.
Those were the most common responses. Now for some of the cleverest:
Joseph Taylor makes the simple economic argument that eating is a form of entertainment, and it's gotten cheaper relative to (say) water-skiing; ergo people choose more food and less water-skiing. And he points out that his theory is eminently testable: We could investigate whether obesity is correlated with food prices across geographic regions. Pat Reimann points out that food has gotten not just cheaper but also more varied (consider the uncountable number of new flavors of Ben and Jerry's), which again raises the value of eating relative to other forms of leisure.
Ariel Kaminer starts with my own favorite theory and gives it a novel twist. I argued that medical advances have lowered the health cost of obesity, so people rationally choose to be fatter. Kaminer rejects the rational choice model but points out that the same medical advances have kept a lot of obese people alive, well, and included in the statistics.