Ever since it began running its commercials about Jared Fogle’s weight loss in 2000, the Subway restaurant franchise has gained a reputation of being a healthy alternative to typical fast food fare. But is that perception warranted? Sure, perhaps on some level, but probably not to the extent that you assume.

A recent study put those assumptions to the test — not only for Subway, but also McDonald’s, Burger King, Wendy’s, Kentucky Fried Chicken and Dunkin’ Donuts. Published in the journal BMJ, the study polled customers at these restaurants in the New England area — 1,877 adults, 330 school-aged children and 1,178 adolescents — asking them to estimate the calories in the meals they bought.

The results were eye-opening, to say the least. The mean guess for adults, adolescents and school-aged kids was 649, 490 and 562 calories, respectively. In reality, however, the mean caloric content was 836, 756 and 733 calories. More than two-thirds of all participants underestimated the caloric content of their meals, with about a quarter of them underestimating the caloric value by at least 500 calories. To put that in practical terms, if you eat 500 calories more than you burn each day (which you might very well do if you assume you’re eating 500 calories less than you actually are), you’ll gain about a pound per week. Doesn’t sound like much? How does gaining 52 pounds a year sound?

Perhaps brainwashed by Subway’s healthy reputation, adult and adolescent Subway diners underestimated caloric content significantly more than diners at any other restaurant — adults by 349 calories and adolescents by a whopping 500 calories per meal. (School-aged children had similar guesses for all chains.)

It should be noted that 65% of the adult participants in the study who provided height and weight data were either overweight or obese, as were 34% of adolescents and 57% of school-aged children. Only 5%, 2% and 4%, respectively, said they used the calorie information provided in the restaurant to help them figure out what to buy.

(Sources: BMJ, The Atlantic, CaloriesPerHour.com)