A series of new studies from New York's Cornell University found that people who spurned fast food in favour 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 percent 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 percent more calories than when they go to restaurants like McDonalds," said Brian Wansink, director of the Cornell Food and Brand Lab.
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.
As it takes an energy imbalance of 3 500 calories to put on one pound in weight, that extra 159 calories could lead to weight gain of almost five pounds over a year for people who ate at Subway twice a week.
He said the studies helped explain why lower calorie menus and choices at fast food restaurants had not had the expected impact in cutting calorie intake and tackling rising obesity.