Mexico has seen a slight reduction in high-calorie snack purchases since the country implemented a junk-food tax in 2014, but it's unclear whether overall public health has improved.
"Slight" is right: The average reduction comes out to only 5.1 percent, with higher-income households showing no change in food shopping habits, according to a study published in online journal PLOS-Medicine.
Poorer households bought 10.2 percent less junk food, and medium-income households bought 5.8 percent less.
The results come from a study of bar code analyses of consumer tracking data (most high-tech sentence I've ever written) by PLOS-Medicine.
What the study couldn't catch are purchases of cheap and high-calorie street foods, which are extremely popular in the country.
It's unclear, then, whether the decrease of in-store purchases of junk food means that families are eating healthier or that they're simply buying more junk food from street vendors.
Critics of the tax believe the higher prices aren't causing healthier eating; they say households are buying street foods that are often made worse than name-brand choices with extra sugar and salt.
Nutritionist Julieta Ponce from the Center for Dietary Orientation said the study prompts a much-needed debate in Mexico, one of the most obese countries in the world, about making better-quality food a norm.
"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, according to The New York Times. "At the moment, Mexico lacks a broader policy on eating well."
The study's results align with findings that another Mexican tax on sodas reduced certain soft-drink purchases by 6 percent. Many U.S. cities, such as Philadelphia, are implementing similar ideas.