A new study says that the United States is exporting its obesity epidemic--to Mexico.

According to "Exporting Obesity," a recent paper from the Institute for Agriculture and Trade Policy, a Minneapolis-based think tank, U.S. agricultural products and policies are feeding growth in Mexico's obesity rate. The paper asserts that opening up trade avenues with the North American Free Trade Agreement, or NAFTA, allowed the U.S. to send even more of its massive quantities of corn, soybeans, sugar, meat, and other foods to Mexico. An influx of cheap calories helped to push Mexico's obesity rate upward.

"I think that certainly U.S. farm policies encourage farmers to produce largest volume of food as cheaply as possible and to rely on export markets," says Karen Hansen-Kuhn, international program director at the IATP. The trade shifts that took place since the start of NAFTA in 1994, not to mention America's high obesity rate, make the U.S.-Mexico example an interesting case study. "We can trace the very direct changes that have happened, the fact that U.S. corn exports quadrupled since NAFTA began."

Longstanding U.S. subsidies promote overproduction of crops like corn and soybeans, the think tank paper claims, making those commodities cheaper and bringing to market a massive supply of byproducts like corn syrup and soybean oil that are calorie-dense but not very nutritious. Corn, much of which is used as a feed grain, also promotes increased meat production.

These factors may have contributed to Mexicans' ballooning waistlines. According to the IATP paper, Mexicans' average daily energy obtained from fat grew 28.9 percent from 1988 to 1999, the period during which NAFTA was negotiated and implemented. Soda consumption in Mexico also grew by 37.2 percent from 1984 to 1998.

According to the most recent data from the Organization for Economic Cooperation and Development, Mexico's obesity rate is 30 percent, second among member countries only to the America's 33.8 percent and a significant increase from 2000, when Mexico's obesity rate was less than 25 percent. By another measure, Mexico is even fatter than the U.S.: 69.5 percent of Mexicans are overweight or obese, compared to 68 percent in the United States.

It's not just Mexico that's gaining weight. Obesity is now an epidemic on a global scale. A 2010 OECD study found that Mexico and five other emerging economies—China, Brazil, Russia, South Africa, and India—are all facing the growing obesity epidemic.

This may be an example of what some experts call "nutrition transition"—a phenomenon in which low- and middle-income countries become less healthy as they gain in prosperity. This can mean that malnutrition and obesity can go hand in hand.

"What's interesting is in the developing countries, in the initial stage of nutrition transition, rich people are more likely to become obese," says Frank Hu, Professor of Nutrition and Epidemiology at the Harvard Public School of Health. Given time, he says, obesity becomes more prevalent among poorer populations.

It's not just raw, cheap commodities that are to blame. The exporting of sugary sodas or fast foods by U.S. corporations is also boosting obesity, says Hu. "When they sell their products in developing countries, it certainly directly or indirectly contributes to energy balance and unhealthy dietary patterns, as well as increased obesity." But he adds that pinning down the degree to which this occurs is nearly impossible. "It's very difficult to quantify the exact magnitude of the contribution by American companies or by American policies."