Amy Sancetta/AP
Wal-Mart Supercenters give us more junk food, cheaper. (Amy Sancetta/AP)

Our collective waistline has been expanding for decades. The adult obesity rate in the United States surged from 13 percent in 1960 to a whopping 35 percent in 2012 — with the most pounds added during the ’80s, according to the Centers for Disease Control and Prevention.

Meanwhile, the first Wal-Mart store opened in 1962. The first Sam’s Club debuted in 1983. The first Wal-Mart Supercenter came in 1988.  And today, thousands of warehouse-style grocery destinations offer bargains in bulk to shoppers across the country.

Coincidence? Maybe not. A new paper argues that food distribution methods have contributed to America’s obesity crisis.

“We live in an environment with increasingly cheap and readily available junk food,” said Charles Courtemanche, assistant professor of economics at Georgia State University. “We buy in bulk. We tend to have more food around. It takes more and more discipline and self-control to not let that influence your weight.”

Courtemanche co-authored a paper with researchers from the University of Iowa, University of Virginia and University of Louisville that was released this week and that examines how different economic factors drive obesity in America.

One of the biggest culprits: more cheap goods sold in bulk. “Greater availability of these stores reduces travel time to obtain food, presumably increasing weight,” researchers wrote.

The density of an area’s Supercenters can significantly affect the obesity rate, according to the study. Opening an additional store per 100,000 residents increased an area’s average body mass index by 0.24 units, or 10.8 percent of the sample obesity rate, the study found. The overall effect: “These estimates imply that the proliferation of Wal-Mart Supercenters explains 10.5 percent of the rise in obesity since the late 1980s.”

As the stores became ubiquitous, Americans started swelling at a faster rate — “a slow uptick to this exponential shape,” Courtemanche said.

To compound matters, people who never set foot in a superstore could still be affected by the chains’ popularity. Stores like Target, for example, were pressured to offer similar deals — not unlike this five-pound bag of popcorn for $17.99 — to keep up with the competition.

“There are market-wide impacts that make junk foods cheaper.” Courtemanche said.

Obesity, defined as a body mass index of at least 30, increases one’s chances of developing heart disease, diabetes, high blood pressure and strokes. America has one of the highest rates in the world, according to the Organization for Economic Cooperation and Development — and that’s bad for taxpayers. The epidemic has already cost us $190 billion in medical expenses, research shows. Half the burden is covered by Medicare and Medicaid.

Economics sheds light on shifting calorie incentives. But we can’t blame the local Wal-Mart Supercenter(s) for tantalizing our senses with, say, Banana Split Oreos. We can’t blame coupons that enable us to snag two packages for the price of one.

“It’s a matter of self-control,” Courtemanche said. “I’d say: Buy your healthy food in bulk and your junk food in smaller quantities.”

Update:

"It's clear that their research did not consider any of Walmart’s commitments to make healthy eating easier in the U.S.," Wal-Mart spokesperson Tara Greco said in an e-mail. "Four years ago, Walmart unveiled a comprehensive effort to provide its customers with healthier and more affordable food choices. Since then, the company has made significant progress in product reformulation, building stores in food deserts and providing access to fresh fruits and vegetables, as well as increasing charitable support for nutrition programs that help educate consumers about healthier food solutions and choices."

*This post has been updated to include comment from Wal-Mart.