Health food is extremely popular in America yet obesity levels continue to rise. A new study in the Journal of Consumer Research explains that paradox.

It turns out that when consumers see a healthy choice, be it drinks, deserts or food, they end up consuming 131% more calories.

“In our black and white view, most food is good or not good,” explain Pierre Chandon (INSEAD, France) and Brian Wansink (Cornell University). “When we see a fast-food restaurant like Subway advertising its low-calorie sandwiches, we think, ‘It’s OK: I can eat a sandwich there and then have a high-calorie dessert,’ when, in fact, some Subway sandwiches contain more calories than a Big Mac.”

In one study, Chandon and Wansink had consumers guess how many calories are in sandwiches from two restaurants. They estimated that sandwiches contain 35% fewer calories when they come from restaurants claiming to be healthy than when they are from restaurants not making this claim.

The result of this calorie underestimation" Consumers then chose beverages, side dishes, and desserts containing up to 131% more calories when the main course was positioned as “healthy” compared to when it was not—even though, in the study, the “healthy” main course already contained 50% more calories than the “unhealthy” one.

“These studies help explain why the success of fast-food restaurants serving lower-calorie foods has not led to the expected reduction in total calorie intake and in obesity rates,” the authors write.

What should people and health agencies do" In the final study, the researchers show that encouraging people to examine whether the restaurant’s health claims actually apply to the particular food they ordered eliminates the “health halo” effects.

As they explain: “More generally, we need to think about food not just qualitatively (as in “good food – bad food”) but also quantitatively (as in “how many calories are in this meal"”).”

Pierre Chandon and Brian Wansink. “The Biasing Health Halos of Fast Food Restaurant Health Claims: Lower Calorie Estimates and Higher Side-Dish Consumption Intentions” Journal of Consumer Research: October 2007.