MEXICO CITY >> Mexico put its schoolchildren on a diet at the beginning of the year. But as often happens with New Year’s resolutions, there are many ways to cheat. Here is some of what is allowed for sale in schools under new guidelines that are intended to combat childhood obesity: lollipops, potato and corn chips (in multiple guises), and cookies (complemented by marshmallow or chocolate filling).

But the message is getting through, sort of. Portions verge on the miniature; sugar is limited; the chips are baked, not fried; and soft drinks are banned in elementary schools.

“My doctor told me that I had to drink water to look after my health,” said Santiago Daniel Torres, a bulky 14-year-old. Gone are the grease-drenched sandwiches and fried pork rinds that he used to buy. “They banned them,” he said as classmates wandered by clutching foil packets of cookies and chips. “More water, that’s better.”

By all measures, Mexico is one of the fattest countries in the world, and the obesity starts early. One in three children is overweight or obese, according to the government. So the nation’s health and education officials stepped in last year to limit what schools could sell at recess. (Schools in Mexico do not provide lunch.)

The officials quickly became snared in a web of special interests led by Mexico’s powerful snack food companies, which found support from regulators in the Ministry of the Economy. The result was a knot of rules that went into effect on Jan. 1.

“What’s left is a regulatory Frankenstein,” said Alejandro Calvillo, Mexico’s most vocal opponent of junk food, particularly soft drinks, in the schools. “They are surrendering a captive market to the companies to generate consumers at a young age.”

Mexican officials argue that the new rules are successful, even though parts of the original proposal have been relaxed.

“We managed to do the most important things, which was to pull out the soft drinks and to get the composition of foods changed,” said Dr. Jose Angel Cordova, Mexico’s health minister. He estimates that one-third of Mexico’s health care spending goes to fight diseases related to obesity.

The snack food companies’ concerns may go beyond their sales in Mexican schools, said Cordova. If Mexico sets a precedent, he said, other governments may follow.

“We had to negotiate and negotiate, and it suddenly got complicated,” Cordova said. “They tried to drag out the timing until finally we just imposed and we applied the rules.”

The education minister, Alonso Lujambio, said the new rules had removed 90 percent of fried foods from schools. “That is a very aggressive change,” he said.

But he stopped short at a suggestion that all junk food should be banned from schools. “The central issue is to educate children to exercise moderation in what they eat and emphasize healthier products,” Lujambio said.

It is a high-minded approach at odds with the scene during a recent recess period at a downtown Mexico City middle school.

When the bell rang at 10:50 a.m., children streamed onto a tiny patio, where Marisela Beltran was selling chicken sandwiches.

Mindful of the new guidelines, Beltran has been experimenting with healthier foods, bringing oranges and once offering a salad of chopped nuts, raisins, lettuce and apples. It was not a popular offering, said her nephew Francisco Peralta, who sells the school’s packaged snack food.

“When we bring things like that to the patio, they attack me in there,” he said gesturing at his closetlike store, where cookies, bran bars and juices were displayed on wooden shelves.

The food companies, including multinationals in Europe and the United States, say their new portfolio of school snacks are evidence that they are committed to combating the problem. But they also complain that they are forced to compete with street vendors who gather outside school gates to sell inexpensive junk food to children as they head home.

Difficult as the problem may be, at least one school principal has found a simple solution. The snack food salesmen “come knocking at the door, and we just say no,” said the principal, Maria Teresa Zamorano.

Since she took over at Estado de Quintana Roo Elementary School in a working-class neighborhood of Mexico City in August, Zamorano has remade the recess menu.

On one day recently, there was a hot meal of rice and tortillas, prickly pear leaves with eggs and onions, and squash with soft white cheese. Her students could choose among fresh cucumber, jicama, watermelon slices and cooked corn kernels. For dessert, there were popsicles and miniature cups of gelatin.

At the end of the school day, the children poured out of the gates onto a narrow street cluttered with vendors selling candy, chips, nachos and ice cream. Many bought a snack for the walk home.

Still, they have not forgotten the lessons from school.

“Almost all of the girls eat fruit,” said Leticia Garcia Gutierrez, 11. Then she added: “Sometimes we eat candy. But that’s because we’re kids.”