NEW YORK: Think you are being good by opting for a "healthy" restaurant over a fast food joint? Maybe not, say US researchers, who found that eating in restaurants that bill themselves as healthy can make you fatter.

A series of new studies from New York's Cornell University found that people who spurned fast food in favor of restaurants which advertised their food as healthy often treated themselves to higher-calorie side dishes, drinks or desserts. The research, published in the October online version of the Journal of Consumer Research, found people tended to underestimate by 35% how many calories were contained in so-called healthy restaurant foods.

"We found that when people go to restaurants claiming to be healthy, such as (sandwich chain) Subway, they choose additional side items containing up to 131% more calories than when they go to restaurants like McDonalds," Brian Wansink, director of the Cornell Food and Brand Lab, said in a statement.

Wansink and co-author Pierre Chandon, a marketing professor at international business school INSEAD in France, said simply asking people to reconsider restaurants' health claims prompted them to better estimate calories and cut down on side dishes.

"In estimating a 1,000 calorie meal, I've found that people on average underestimate by 159 calories if the meal was bought at Subway than at McDonald's," says Wansink.