Mexico's so-called junk food tax may actually be working — a bit.

Mexico instituted an 8 percent tax on high-calorie snacks in 2014, a year after a UN report said it had overtaken the United States as the world's most obese country.

This week, the online journal PLOS-Medicine published a study that found a 5.1 percent reduction in purchases of products that fall under the new tax.

But the study also noted that the drop was concentrated among the poor, where the fall in consumption was 10.2 percent. Middle income consumption fell by 5.8 percent, and high income junk food purchases remained unchanged.

The results back charges that fiscal efforts to force healthy eating are actually a regressive tax that hurts those with low incomes. The report also said nothing about whether those buying less junk food are buying healthier alternatives.

The tax applies to processed foods which have more than 275 calories per 100 grams. It was implemented alongside another tax on sugary soft-drinks. A report released last year claimed that the soda tax led to an average 6 percent reduction in consumption.

Activists who promoted the soda tax complained that the authorities were not fulfilling their promises to use the additional tax income to put drinking fountains in all schools.

Other critics have pointed out that such initiatives have no impact on Mexico's street economy, which was not part of the survey either.

Unlicensed vendors are ubiquitous in Mexico's cities selling everything from industrially-produced chips, soda, and candy bars, free from the taxes.

They also sell a wide variety of unhealthy alternatives, for example churros made on the sidewalk. Many also offer factory-made products with additional calories on top such as dorilocos — an opened bag of Doritos covered in things like processed cheese and fat-filled cheap meat.

As well as the junk food tax, Mexico's authorities have embraced more original efforts to combat the country's major obesity problem. The Mexico City metro system introduced a weight-loss initiative last year where those who completed 10 squats prior to boarding a train were given a free ride.