Mexico City - Mexico's 8 percent tax on high-calorie snacks has been successful in reducing junk food purchases, but only by a small amount and only among poor and middle-class households, a study said Tuesday.

The report published in the online journal PLOS-Medicine showed an average reduction of 5.1 percent in purchases of items subject to the tax, which was implemented in 2014. The reduction equaled only about 25 grams (0.88 ounces) per month per person.

Poorer households bought 10.2 percent less junk food, while medium-income households bought 5.8 percent less, according to bar code analyses based of consumer-tracking data. Higher-income shoppers showed no impact at all from the new taxes. The tax applies to processed foods having more than 275 calories per 100 grams of product.

The study did not indicate whether families reduced calorie intake, bought healthier foods or simply switched to cheaper street food - important factors for Mexico, which is plagued with high obesity rates.

The study was conducted by researchers from Mexico's National Institute of Public Health and the University of North Carolina, Chapel Hill, who said future studies "should explore how these shifts are linked to changes in the nutritional quality of the overall diet."

Nutritionist Julieta Ponce of the Center for Dietary Orientation said the study's finding only open the debate on how to substitute empty calories in Mexicans' diet for better-quality food. For example, the junk food reductions noted in the study could be replaced by eating a single tortilla or a third of a cup of beans.

Ponce said the question is how to return such foods as fresh fruits and vegetables to the average Mexican's diet on a larger scale.

"Obesity and adverse metabolic effects are also the result of the loss of the traditional diet, not just the consumption of junk food," Ponce said. "At the moment, Mexico lacks a broader policy on eating well. For the moment, there are just emergency measures to limit the damage."

The study is in line with a paper released late last year suggesting another Mexican tax on sugary soft-drinks had achieved an average 6 percent reduction in some purchases. That tax is 1-peso (5 cents) per liter.

Critics of the fast-food tax have suggested that poorer Mexicans, faced with higher pass-along prices on processed snacks sold by major companies, may simply switch to non-taxed similar foods sold by unlicensed street vendors. The country's streets, especially outside schools, parks, bus stops and subway stations, are crowded with vendors selling no-brand or home-made snacks that often have salt, sugar or fat counts as high as name brands.

Even name-brand chips are sometimes made worse at street stands, where vendors will open a bag of chips and douse them in an infinite variety of salty, sugary or spicy sauces.

Supporters of the tax agree that those practices, known as junk food "substitution," could blunt the gains of tax-induced reductions, but insist they won't erase the improvements.

"There will be a discussion about 'substitution,' but I don't think any (street) product has the distribution network that these (name-brand) products have," said Alejandro Calivillo, director of the advocacy group The Power of the Consumer. "There could be substitution in some cases, but I don't think it would be anything significant."

Mexico is among the fattest countries in the world. Just under one-third of adults are obese, according to the U.N. Food and Agriculture Organization. Seven out of 10 Mexicans are overweight and the country has surpassed the U.S. in obesity rates, mostly due to a diet of fatty foods and sugary sodas, according to a United Nations report.