MEXICO CITY - Researchers say that Mexico's 8-per cent tax on high-calorie snacks has been successful in reducing junk food purchases, but only among poor and middle-class households.

A study published Tuesday in the online journal PLOS-Medicine showed an average reduction of 5.1 per cent 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.

But in a country plagued with high obesity rates, the study did not indicate whether families reduced calorie intake, bought healthier foods or switched to cheaper street food.

Researchers from Mexico's National Institute of Public Health Future and University of North Carolina, Chapel Hill suggested future research "should explore how these shifts are linked to changes in the nutritional quality of the overall diet."