If you are looking for proof that Americans' vegetable habits lean towards french fries and ketchup, the U.S. Department of Agriculture has it: Nearly 50 percent of vegetables and legumes available in the U.S. in 2013 were either tomatoes or potatoes. Lettuce came in third as the most available vegetable, according to new data out this week.

And while the USDA's own dietary guidelines recommend that adults consume 2.5 to 3 cups of vegetables a day, the agency's researchers found that only 1.7 cups per person are available.

"The dietary guidelines promote variety," Jeanine Bentley, a social science analyst at the USDA's Economic Research Service, tells The Salt. "But when you look at it, there isn't much variety. Mostly people consume potatoes, tomatoes and lettuce." (The data technically tally domestic production and imports, then subtract exports, but researchers commonly use them as a proxy for consumption.)

The federal dietary guidelines do not recommend relying primarily on potatoes, tomatoes and lettuce for most of our vegetable needs. They prescribe a varied mix that includes dark leafy greens, orange and yellow vegetables, and beans—along with those potatoes and tomatoes. And they want us to eat them because they help reduce the risk for heart disease, stroke and some cancers as well as help keep us at a healthy weight.

So the vegetables that are available don't really match what we're supposed to be eating. What about what we are actually eating?

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Some 87 percent of adults failed to meet the vegetable intake recommendations during 2007-2010, according to recent survey data from the Centers for Disease Control and Prevention. The survey found a lot of variation state to state — with 5.5 percent of people in Mississippi getting enough vegetables to 13 percent in California meeting the recommendations.

Most people are likely to be eating tomatoes and potatoes, but as the USDA has noted, we often get them in the not-so-nutritious forms of french fries and pizza. About one-third of potatoes, and two-thirds of tomatoes, were bound for processing — think chips, sweetened pizza sauce and ketchup.

All these numbers beg some questions: Do our lopsided habits mean that Americans are merely eating what's on offer, a kind of supply-side theory of diet? Or are all those potatoes and tomatoes crowding out spinach and Brussels sprouts because they're what consumers demand?

"We have a serious disconnect between agriculture and health policy in our country," said Marion Nestle, a leading nutrition researcher and author at New York University. "The USDA does not support 'specialty crops' [like vegetables] to any appreciable extent and the Department of Commerce' figures show that the relative price of fruits and vegetables has gone up much faster than that of fast food or sodas." So while Americans are told to eat fruits and vegetables for their health, the government has meanwhile mostly just subsidized other crops that end up in cheaper, less healthy processed food. "Price has a lot to do with this," she adds.

Although this week's USDA report focuses on the limited variety of vegetables available to American shoppers, other agency data suggest that the country simply doesn't offer enough vegetables, period. A 2010 study in the American Journal of Preventive Medicine estimated that the U.S. vegetable supply would need to increase by 70 percent — almost entirely in dark leafy greens, orange vegetables and legumes — in order for Americans to meet recommended daily allowances at the time.

With a dietary landscape like that, it's entirely possible that Americans are choosing potatoes and tomatoes, at least for now, says Lindsey Haynes-Maslow, a food systems and health analyst at the Union of Concerned Scientists.

"What I see here with lots of potatoes, tomatoes and lettuce ... [is] that people are used to these items, and habits are hard to break," says Maslow, adding that relying mostly on the potatoes, tomatoes and lettuce "doesn't cut it," nutrition-wise.

Still, she says, "If more Americans got used to eating more fruits and vegetables they might be demanding more of it," she says. "But it's really hard to demand something you've not grown up with."

That's why behavioral economists are so keen to figure out how to nudge kids to try and develop a taste for more vegetables — they're researching everything from financial incentives to arranging food differently on the lunch line. And there's some hopeful news in that department: The CDC recently reported that, since the passage of the Healthy, Hunger-Free Kids Act of 2010, there's been a big increase in the number of schools serving two or more vegetables and whole grain-rich foods every day.

Most interesting of all, that food isn't just on kids plates: It's getting eaten, too. A Rudd Center for Food Policy and Obesity study of kids' lunch habits following the passage of the bill found that kids ate more fruit, threw away fewer vegetables and ate more of their now-healthier entrees, too.

Tracie McMillan is the author of The American Way of Eating, a New York Times bestseller, and a senior fellow at the Schuster Institute for Investigative Journalism at Brandeis University. You can follow her on Twitter @tmmcmillan.

Copyright 2015 NPR.

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