Part of the reason, some think, is because they don't have access to, the money to buy, or the desire to eat fresh fruits and vegetables.
That's the idea behind initiatives like the "One more a day pledge" (whose slogan sounds like the pledge-taker might already be choking on carrots: "I pledge to eat ... and help my family eat ... at least ONE MORE fruit or veggie every day." [ellipses sic])
Produce is less calorically dense than grains, meat, and fat, so increasing its consumption might indeed make sense as an obesity-fighting strategy—that is, if eating more fruits and vegetables caused people to compensate by eating fewer cookies and french fries.
Unfortunately, though, we don't really eat that way: We'll have a tossed salad—and thena Chipotle Quesarito. At least, that's what RAND health economist Roland Sturm found in a new paper he co-authored with Ruopeng An, a health policy professor at the University of Illinois at Urbana-Champaign.
"Conventional wisdom is an awful guide for policy," Sturm told me. "The consumption of fruits and vegetables has increased during the obesity epidemic."
Differences in diet, such as eating more Cheetos and fewer cucumbers, help explain why some individuals are more obese than others, Sturm said. But they don't explain why obesity has grown across all populations in nearly all U.S. states over the past few decades.
The study, published in the journal CA: A Cancer Journal for Clinicians, found that while college-educated people are still less likely to be obese than their less-educated counterparts, they're still fatter than they used to be:
Increase in body mass index over time for people of various levels of education (CA: A Cancer Journal)
And the BMIs of the uber-healthy Coloradans, who regularly appear on "healthiest states" lists, have been rising over time, just like those of Mississippians have:
Prevalence of overweight over time in California, Colorado, and Mississippi. (CA: A Cancer Journal)
Today, people eat about 30 more pounds of vegetables and 25 more pounds of fruit per year than they did in 1970, according to Sturm's calculations.
Unfortunately, they're eating more of everything else, too: The average adult consumed about 2,100 calories in 1970, but in recent years that number has risen to more than 2,500.
Average daily per capita calories, adjusted for waste (CA: A Cancer Journal for Clinicians)
Attempts to discourage the consumption of certain macronutrients also don't seem to work. Historically, people have simply eaten less of the forbidden substance and more of the others. During the low-fat craze of the 90s, for example, fat consumption dipped, but carbohydrate intake skyrocketed. And after the Atkins diet took off in 2000, people simply swapped carbs back in for fat:
Change in macronutrient consumption over time. (CA: A Cancer Journal for Clinicians)
"Preventing obesity is not about eating more food, regardless of how many nutrients it provides," Sturm and An write, "but consuming less energy or expending more."
Past research on the produce-obesity issue has been mixed: A 2003 study of a large sample of children found that eating more fruits and vegetables had no significant impact on weight. Around the same time, a different study of middle-aged nurses found those who ate more produce were less likely to become obese.
Sturm emphasized that his study is different because it's looking at top-level changes over time, not disparities between groups of people.
A recent Lancet study found that rich and poor countries alike are now struggling with obesity, and that there have been "no national success stories" in stemming the epidemic. So while it's definitely a problem that, say, poor American women tend to be fatter than richer women, another frightening trend is the overall rise of large waistlines over time.
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