Why are so many people overweight? 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 then a 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.