Researchers found most meals served in the 123 restaurants they included in their analysis exceeded total calorie recommendations for a single meal.
Published in the Journal of the American Academy of Nutrition and Dietetics, the study found that some meals even exceeded total daily calorie recommendations.
"These findings make it clear that making healthy choices while eating out is difficult because the combination of tempting options and excessive portions often overwhelm our self-control," says senior study author Susan B. Roberts, PhD, of the Jean Mayer United States Department of Agriculture (USDA) Human Nutrition Research Center on Aging (HNRCA) at Tufts University in Boston, MA.
Overeating is a key contributor to obesity; eating too much food can cause an energy imbalance, where more calories are taken in than used, leading to weight gain.
Large fast-food chains are often criticized for their unhealthy and oversized food offerings. Roberts and colleagues say such restaurants are an easy target because they disclose nutritional information for each of their offerings.
But in their new study, the team found the problem is not confined to large fast-food chains; local restaurants are also guilty of serving meals that are too big and contain too many calories.
92% of meals exceeded total calories recommended for a single meal
To reach their findings, the researchers analyzed the calorie content of 364 popular meals served across 123 local and large-chain restaurants in three US cities - Boston, MA, San Francisco, CA, and Little Rock, AR - between 2011-2014.
American, Chinese, Greek, Indian, Italian, Japanese, Mexican, Thai and Vietnamese restaurants were included in the analysis.
The team calculated the calorie content of each meal by comparing them against USDA food database values, and calorie content was also compared with daily and individual meal calorie recommendations for humans.
The USDA base their daily calorie recommendations on a person's gender, age and how physically active they are; a moderately active woman aged 19-30, for example, is recommended to have around 2,000-2,200 calories daily, while 2,600-2,800 calories daily are recommended for a moderately active man of the same age.
The researchers found that 92% of the meals they analyzed - including those from local restaurants - exceeded the total number of calories recommended for a single meal. What is more, some of these meals exceeded total daily calorie recommendations.
American, Chinese and Italian meals contained the highest amount of calories, coming in at an average of 1,495 per meal.
"Favorite meals often contain three or even four times the amount of calories a person needs," notes Roberts, "and although in theory we don't have to eat the whole lot, in practice, most of us don't have enough willpower to stop eating when we have had enough."
Diners should be able to choose portion sizes
But according to Roberts, this lack of willpower has a biological explanation: the "cephalic phase of digestion" - a Nobel prize-winning theory discovered by Russian scientist Ivan Pavlov more than 100 years ago - where the sight, smell, thought or taste of food stimulates appetite, making it hard to resist when it is put in front of us.
"All we have to do is see and smell food and our sympathetic nervous system revs up, insulin secretion drops blood glucose and our stomach relaxes - the goal of these physiological changes being to prepare us to eat all the food within reach," explains Roberts. "So we order our favorite dishes because that is what tempts us, and then we eat more than we need because the portion is too large."
In order to combat this biological mechanism that tempts us to overeat, study coauthor William Masters, PhD, professor of food economics at the Friedman School at Tufts, suggests diners should have the option of ordering meals in portion sizes that suit them:
"Standard meals are sized for the hungriest customers, so most people need superhuman self-control to avoid overeating. There is a gender dimension here that is really important: women typically have a lower caloric requirement than men, so on average need to eat less. Women, while dining out, typically have to be more vigilant."
Giving diners the option of choosing smaller portion sizes might encourage restaurants to adjust their default portion sizes toward what the average consumer requires, speculates Masters, rather than catering for what the hungriest diner wants.
"Customers could then order anything on the menu in a more appropriate size, and be able to eat out more often without weight gain," he adds.
Last month, Medical News Today reported on a study that found diners who paid less for an all-you-can-eat buffet felt more guilty and uncomfortable after eating than those who paid a higher price.