How the country challenged the junk-food industry and became a global leader in the battle against obesity.

Carlos Monteiro got his start in medicine in the 1970s as a pediatrician working in poor villages and slums in the state of São Paulo, Brazil. His patients were hungry, and it was written on their bodies: Many were anemic, underweight, and stunted. Today, Monteiro is a professor of nutrition at the University of São Paulo’s School of Public Health, a stately building surrounded by lush gardens. It’s a long way—figuratively, at least—from the shantytowns where he trained. His career has done a 180, too. Monteiro’s early research focused on malnutrition, but now he’s mostly occupied with the opposite problem: Brazilians, like most of their neighbors in the Americas, have gotten fat.

This story was produced in collaboration with the Food & Environment Reporting Network (FERN), a non-profit investigative journalism organization.

Over the course of his career, Monteiro, a lanky man with salt-and-pepper curls, has seen a public-health crisis emerge. In the mid-1970s, less than 3 percent of men and 8 percent of women in Brazil were obese. Today, almost 18 percent of adults are obese and more than half are overweight, according to the Ministry of Health, and the rates of chronic, diet-related diseases like diabetes and some cancers have grown. Monteiro has spent years parsing the data on what Brazilians eat; the most salient change he’s seen is the shift from eating foods you can prepare in an ordinary kitchen to what he calls “ultraprocessed products”—highly palatable admixtures of synthetic flavorings and cheap commodity ingredients that require little, if any, cooking. In other words, instant noodles, soda, and processed meats are edging out staples like beans and rice, cassava, and fresh produce.

“The local food system is being replaced by a food system that is controlled by transnational corporations,” Monteiro says. Monteiro, who takes a broad view of nutrition, says this dietary deterioration doesn’t just harm bodily health but also the environment, local economies, and Brazil’s rich food traditions. “We are seeing a battle for the consumer,” he adds.

Over the last 30 years, big transnational food companies have aggressively expanded into Latin America. Taking advantage of economic reforms that opened markets, they’ve courted a consumer class that has grown in size due to generally increasing prosperity and to antipoverty efforts like minimum-wage increases and cash transfers for poor families. And as sales of highly processed foods and drinks have plateaued (and even fallen, in the case of soda) in the United States and other rich countries, Latin America has become a key market. Between 2000 and 2013, soda sales in the region doubled. At the same time, the sales of ultraprocessed foods increased by nearly 50 percent, even as they rose just 2.3 percent in the United States and Canada.