People eating at fast food restaurants are highly likely to underestimate the amount of calories they are consuming, especially if they order a higher calorie meal, a new study suggests.

While a report by the Food Safety Authority of Ireland last year recommended that all restaurants should display calories on their menus, this is currently not compulsory in Ireland.

The general consensus is that displaying calories helps people to make healthier choices, thereby reducing the risk of overweight and obesity.

While previous research indicated that people often underestimate the calorie content of what they are eating in fast food restaurants, these studies tended to focus on a just a few restaurants. As a result, US researchers decided to look at a much bigger number and include repeated visits by consumers.

They followed the progress of over 1,800 adults aged 18 and older, more than 1,100 adolescents aged between 11 and 20 and 330 school age children aged between three and 15.

The participants visited 89 restaurants belonging to chains such as McDonald's, KFC, Burger King and Subway.

Among the participants, two in three of the adults were already overweight or obese, as was one in three of the adolescents and over half of the school age children.

While many US fast food restaurants are now required to display calories, at the time of the study, none of the restaurants routinely displayed calories.

The study found that the adults were consuming an average of 836 calories per fast food meal, but were underestimating this by 175 calories.

The adolescents were consuming 756 calories per meal, but were underestimating this by a massive 259 calories. Meanwhile the children were consuming 733 calories, but their parents were underestimating this by 175 calories.

Overall, two in three people underestimated the calorie content of their meals, with one in four underestimating them by more than 500 calories.

Those eating higher calorie meals tended to underestimate the calorie content more often than those eating lower calorie meals and the restaurant where people tended to underestimate the most was Subway.

In fact, adults and adolescents eating at Subway underestimated the calorie content by 20-25% more than those eating at McDonald's.

Details of these findings are published in the British Medical Journal.