MEXICO CITY — The contentious negotiations over the fate of the North American Free Trade Agreement have veered into one of the world’s most pressing health issues: fighting obesity.

Urged on by big American food and soft-drink companies, the Trump administration is using the trade talks with Mexico and Canada to try to limit the ability of the pact’s three members — including the United States — to warn consumers about the dangers of junk food, according to confidential documents outlining the U.S. position.

The U.S. stance reflects an intensifying battle between trade officials, the food industry and governments across the hemisphere. The administration’s position could help insulate U.S. manufacturers from pressure to include more explicit labels on their products, both abroad and in the United States. But health officials worry that it would also impede international efforts to contain a growing health crisis.

Obesity has at least doubled in 73 countries since 1980. Many public health officials, worried about the rapid spread of highly processed foods, have found hope in a new tactic: the use of vivid warnings on foods with high levels of sugar, salt and fat.

Officials in Mexico and Canada — along with governments in Brazil, Peru, Uruguay, Argentina and Colombia — are discussing options like the use of colors, shapes and other easy-to-understand symbols that warn consumers of health risks. They were inspired in large part by Chile’s introduction of stringent regulations in 2016 that include requirements for black stop-sign warnings on the front of some packages.

But the Office of the U.S. Trade Representative, which is leading the NAFTA talks on the U.S. side, is trying to head off the momentum. It is pushing to limit the ability of any NAFTA member to require consumer warnings on the front of sugary drinks and fatty packaged foods, according to a draft of the proposal reviewed by The New York Times.

The U.S. provision seeks to prevent any warning symbol, shape or color that “inappropriately denotes that a hazard exists from consumption of the food or nonalcoholic beverages.”

Some experts have likened the fight over food labeling to that over tobacco — and the fierce if ultimately unsuccessful opposition and lobbying that industry waged to prevent the imposition of health warnings on packaging. The Trump administration’s position on food labeling reflects the desires of a broad coalition of soft-drink and packaged-foods manufacturers in the United States.

The Grocery Manufacturers Association, a food industry trade group that sits on the advisory board to the trade talks, says it favors voluntary labeling programs. The group says it “supports a modernized NAFTA that will ensure standards are based on science, minimize unnecessary trade barriers, and benefit consumers in all three countries.”

The organization is fighting to keep Chile’s model from being adopted more widely. Roger Lowe, a spokesman for the group — whose board members include executives from Coca-Cola, PepsiCo and Mondelez International, which owns brands like Oreos, Chips Ahoy and Ritz crackers — said it was concerned about the “evidence and impact” of Chile’s laws.

Emily Davis, a spokeswoman for the U.S. Trade Representative, said she could not comment on what she called “alleged negotiating documents.” In general, she said, “the United States supports science-based labeling that is truthful and not misleading.”

Proponents of more explicit labels said the Trump administration’s proposal and the corporate pressure behind it hold the potential to handcuff public health interests for decades.

“It is one of the most invasive forms of industrial interference we have seen,” said Alejandro Calvillo, founder of El Poder del Consumidor, or Consumer Power, a health advocacy group in Mexico that was illegally targeted with government spyware when it fought for a soda tax in Mexico. “The collusion between the industry and the government is not only at the level of spying — it reaches the level of the renegotiation of NAFTA and the nation’s own policy against obesity.”

The U.S. proposal conflicts with the guidance from Mexico’s national health institute and the World Health Organization. Both have recommended that Mexico pass regulations to help combat diabetes, which claims 80,000 lives a year there. That is one of the highest rates in the world — and more than double the record number of homicides that the nation suffered in 2017.

Mexico’s Ministry of Health, which is directly involved in the trade negotiations, said it was reviewing the U.S. proposal with the nation’s health authorities. Public health experts have hailed Chile’s rules as a new standard. They include a ban on the use of cartoon characters like Tony the Tiger, but the package warnings are considered the most aggressive of the tactics.

“We have shown that a simple message and a symbol is enough to communicate that you should be consuming less of certain foods,” said Dr. Camila Corvalán, a nutritionist at the University of Chile who helped develop the logos. “There’s nothing misleading about a warning logo, and clearly this is what worries the industry.”

Food companies have been forced to take note. Over the past two years, more than 1,500 products have been reformulated to make them healthier and to avoid having to carry a warning logo, according to AB Chile, a food industry association.

But passage of the regulation in Chile did not happen without a fight. Eleven countries, led by the United States, raised issues with the proposal before the World Trade Organization.

The Chilean government successfully argued that the measures were a necessary tool to fight the nation’s mounting obesity crisis. Today, Chile’s success has inspired nutrition advocates around the world, including those in Mexico.

“The fact that the industry is freaking out is reassuring, but at the same time it’s worrisome that the U.S. government is trying to defend the position of the food industry,” Corvalán said.

All told, at least 23 countries use some version of front-of-label consumer education. Some of the warnings already adopted or proposed include black boxes or red octagons that draw attention to foods regulators deem unhealthy, using less intense imagery but the same approach as cigarette packaging.

Still, public health experts consider most of the labels other than those required by Chile to be relatively weak or ineffective.

“Chile’s warnings are the new frontier,” said Alexandra Jones, a lawyer at the George Institute for Global Health in Australia. “They represent a potentially much more effective public health intervention: warn people away from the ubiquitous junk foods.” Heading off pressure for more explicit warnings through the NAFTA negotiation is especially appealing to the food and beverage industry because it could help limit domestic regulation in the United States as well as averting a broad global move to adopt mandatory health-labeling standards.

“It kind of kills a law before it can be written,” said Lora Verheecke, a researcher at the Corporate Europe Observatory, a group that tracks lobbying efforts. “And once you put it in one trade agreement, it can become the precedent for all future deals with future countries.”

In most cases, trade law allows governments to retain the right to make rules in the interest of public health, experts say, but the proposal by the United States appears to be aimed at curbing that.

Jones of the George Institute said research found that trade policy had also been used to try to block efforts to adopt warnings in Ecuador, Peru, Thailand, Chile and Indonesia. Chile has moved forward as has Ecuador, but with a less aggressive labeling system, Jones said.

Thailand and Indonesia “appear to have been deterred,” she said, adding, “We call this ‘regulatory chill.'”

One reason that the warning labels are seen as so vital to the efforts to curb obesity is that consumers appear to heed them.

Mexico’s current labeling rules allow for — but do not require — the display of daily intake recommendations of salt, sugar and fat. But they are “indecipherable to consumers” and “totally useless to people,” Jones said.

Government researchers at Mexico’s National Institute for Public Health recently found that only 17 percent of consumers bothered to look at the front-of-pack labels mandated by law.

In separate research, scientists asked college students to try and crack the current labeling system, which, to use effectively, requires mathematics.

“These college kids couldn’t even do it,” said Dr. Simón Barquera, director of health research and nutrition policy at the Mexican public health institute. After starting a campaign several years ago to impose a tax on soda, Barquera and two other backers of the soda tax were targeted by sophisticated spyware sold only to governments on the explicit understanding that it be used strictly against terrorists and criminals.

Mexicans drink on average 167 liters — more than 44 gallons — of soda a year per person, eclipsing what are considered high consumption rates in the United States. In some remote areas of the country, soda is more readily available than clean drinking water.