During the campaign, President Donald Trump threatened to withdraw from NAFTA, calling it "a disaster since the day it was devised." But last week, he said he will "bring NAFTA up to date through renegotiation."

However, neither Trump nor those on either side of the trade wars has given any hint that they know about one of NAFTA's most distressing consequences: its adverse impact on the health of the Mexican people.

In the early 1990s, Mexico chose to hitch its wagon to the U.S. and Canadian economies. Mexico agreed to eradicate its import substitution, tariff and price-support systems, as well as subsidies of corn.

As the dominant partner in the deal, the U.S. won the right to export and invest more in Mexico without having to reduce its own domestic corn subsidies. Now, more than two decades later, our research shows that the agreement has transformed Mexico's food system and diet, leading to dramatic increases in obesity, diabetes and diet-related diseases.

How did this happen? First, in the decade and a half since the North American Free Trade Agreement was signed, the Mexican agricultural economy, which had provided access to affordable food to most Mexicans, collapsed. This drove half a million people to migrate each year, and millions more were internally displaced.

Farmers who had grown their own food moved to cities where dense populations, aggressive marketing and the ubiquitous availability of American processed food changed the diet of most Mexicans. Between 1994 and 1998, U.S. direct investment in Mexican food and beverage companies quadrupled. In eight years, the number of Wal-Mart stores in Mexico grew from 114 to 561. By 2012, the one McDonald's that had opened in Mexico City in 1985 had metastasized into more than 500 around the country.

Changes in food availability led to changes in diet. Between 1992 and 2000, calories from sugary sodas among Mexicans increased by almost 40 percent; for children, they doubled. In that same period, the cost in pesos per calorie of food tripled, making the low-cost, unhealthy food imported by U.S. food multinationals more attractive, especially to poor people.

The NAFTA diet led to changes in health. Between 1988 and 1999, the prevalence of overweight and obesity in Mexico almost doubled, from 33 percent to 59 percent. The prevalence of diabetes increased by 30 percent. By 2004, diet-related chronic diseases caused 75 percent of all deaths. These direct and indirect costs of Mexico's NAFTA-imposed health burden constitute an ongoing obstacle to economic development, a far larger cost than the more widely recognized war on drugs.

In retrospect, the food exchanges were especially cruel. People in the U.S. had better access to the foods that contribute to good health: tomatoes, peppers, cucumbers, limes, avocados and mangos. U.S. consumption of all these vitamin-rich fruits and vegetables is way up since NAFTA, and still rising. The low wages of farm workers in Mexico ensure that exports stay cheap, increasing access to healthy food here in the United States.

Mexico, on the other hand, became a dumping ground for cheap U.S. soda, fast food and snacks formulated from the subsidized corn, sugar and soy crops that are the building blocks of the disease-promoting processed-food industry. As health-conscious Americans reduce their consumption of soda and fast food, Coca Cola, Kraft and McDonald's have depended on eaters in Mexico and other middle-income nations to maintain revenue, following the path of the tobacco industry in recouping lost profits in developing nations.

Several changes could reduce the health burden that NAFTA has imposed on Mexico. First, the agreement could change to favor small-scale farmers, a benefit for the health, environment and economy of Mexicans. Second, a new agreement could ensure wage and health protections to workers on both sides of the border, benefiting the food supply and food workers in both nations.

For those who believe that fair trade agreements can benefit all, the goal should be a trade agreement that puts the well-being of all North Americans first. A trade agreement that favors sustainable agriculture, labor mobility and a food system oriented toward health — not corporate profits — would be good for us all.

Alyshia Galvez is an associate professor of Latin American studies at the City University of New York's Lehman College and is finishing a book, "Eating NAFTA: Trade and Food Policies and the Destruction of Mexico." Email: alyshia.galvez@lehman.cuny.edu

Nicholas Freudenberg is a professor of public health at CUNY and author of "Lethal but Legal: Corporations, Consumption and Protecting Public Health." Email: Nick.Freudenberg@sph.cuny.edu