Are Junk Food Habits Driving Obesity? A Tale Of Two Studies

More than 36 percent of American adults and 17 percent of youth under 19 are obese, according to the latest figures from the Centers for Disease Control and Prevention.

Scientists still don't fully understand what got us here. And sometimes, the answers they've come up with turn out to be wrong. Consider the changing advice on fat, which has been amended of late from its days as a dietary demon.

By now, it would seem that the link between the obesity epidemic and the consumption of high-calorie, low-nutrition foods like sodas, cookies and fries is well-established. But as two recent studies show, researchers are still probing the mechanics of that connection.

Broadly speaking, both studies explore the connection between junk food and weight — though they do so using different data sets from two different populations (adults and kids).

Let's start with the finding that seems most counterintuitive: For most of us, junk foods may not be what's driving weight gain. That's what behavioral economist David Just and his colleagues at the Cornell University Food and Brand Lab concluded in a paper in the journal Obesity Science & Practice.

The researchers looked at data collected in 2007-2008 from a nationally representative sample of roughly 5,000 U.S. adults as part of the National Health and Nutrition Examination Survey (NHANES), including information on weight, height and eating habits. Junk food was defined as fast food, soda and sweets.

Some of that data set had been used in a 2013 CDC study that found that heavier Americans were indeed getting more of their daily calories from fast food. But the Cornell researchers wondered what would happen if they excluded the people on the extreme ends of the weight spectrum — those who are clinically underweight and the very morbidly obese.

And they found that once those groups were eliminated, there was no association between body mass index and how much fast food, sugary sodas and sweets people consume.

The finding, which applies to 95 percent of the population, "was really counterintuitive — not what we expected at all," Just tells The Salt.

But if fast food isn't driving the obesity epidemic, what is? "I suspect we're eating too many calories from all foods," Just says. He points to data from the USDA's Economic Research Service showing that Americans, on average, now eat 500 calories more daily than they did around 1970, before the obesity epidemic took off.

To be clear, Just isn't saying that you can eat all the junk food you want with no consequence. "You increase your consumption of these things, yeah, you're going to put on weight," he says. "But that's not to say that is the differentiator between those who are overweight and those who aren't." And if that's the case, Just says, instead of targeting junk foods in the war against obesity, maybe we should be preaching the gospel of moderation and portion control with all foods.

Sure, that's good advice in general — but it may not mean we can let junk foods off the hook.

Eric Finkelstein, an associate professor at the Duke Global Health Institute at Duke University, notes that the data the Cornell researchers used is only a snapshot of what a cross-section of Americans were eating at a single moment in time. So it's possible, for example, that the overweight and obese people included in the study reported eating less junk foods because they were trying to lose weight.

"I'd lend a lot more credence to studies that follow change [in eating habits and weight] over time," Finkelstein tells The Salt.

And, over time, he says, the evidence suggests strongly that even modest increases in the consumption of certain foods will result in long-term weight gain. He points to a 2011 study in the New England Journal of Medicine that looked at data gathered over decades on 120,000 U.S. adults. Over a four-year period, an extra daily serving of potato chips was associated with weight gain of 1.69 pounds, the study found. That may not sound like much, but for most adults, that's how the pounds add up — gradually, over time, at an average rate of about a pound a year.

Morgan McCloy/NPR

And problem foods will pack on the pounds for kids, too. Last week, Finkelstein and his colleagues published a similarly detailed breakdown of the links between weight gain and certain foods in children. The researchers turned to data on more than 4,600 kids from the Avon Longitudinal Study of Parents and Children, an ongoing study in the U.K. that has tracked the same set of children — with records on their height, weight and food intake — since their birth in the early 1990s.

Once again, potato chips raised red flags.

As the researchers reported in the journal Health Affairs, over a three-year period, every 25-gram serving of potato chips (a little under an ounce) that kids ate daily was linked to about a half-pound of excess weight gain. (Basically, that's defined as weight beyond what a child should weigh for his or her height and age.)

Again, half a pound doesn't sound alarming, "but if you're also getting an extra half a pound from burgers, and half a pound from french fries, these things add up. And some kids are eating more than a serving" daily, Finkelstein says.

Other foods the study linked to excessive weight gain included "kid food" staples — like breaded and coated fish and poultry (think fish sticks and chicken nuggets) and french fries — and processed meats, butter and margarine, desserts and sweets.

That's important, because some 31 percent of American and 38 percent of European kids are now overweight or obese — and the pounds we gain as kids often stay with us through adulthood.