(CNN) Fast food often gets the blame for calorific meals, but new research has found that your favorite chain restaurants are dishing up meals with even more excessive calories, worsening the global obesity epidemic.

Overly large portions and food laden with sugar and fats are regularly being served in sit-down eateries, as well as fast food outlets, according to two studies published in the BMJ on Wednesday.

Contrary to popular belief, the problem -- which experts say is fueling obesity, pushing up rates of diabetes, heart disease and other serious illnesses -- is also not confined to the United States, the studies say.

Although the poor nutritional content of fast food has been well-documented, there has been less research into the energy content of more traditional "full-service" or sit-down restaurants.

One UK study analyzed the calories in 13,500 main meals served in 27 UK restaurant chains, of which 21 were full-service restaurants and the remainder serving fast food. A US study measured calories in the most frequently ordered meals in 116 restaurants across five countries -- Brazil, China, Finland, Ghana and India -- comparing them against the United States.

The calorie content of the most popular meals in British and international restaurant chains is excessive, and only a minority meet public health recommendations, the researchers found. In the global study, only China was found to serve "significantly less energy than restaurants in the US."

Fast food, it concluded, contained on average 33% fewer calories than more formal restaurant meals.

The UK's health agency, Public Health England, recommended that lunches and evening meals should not exceed 600 calories (also measured as kcal). But almost none of the restaurants surveyed reflected this.

"We found that the number of calories in the average restaurant meal was more than the number of calories in the average fast food meal," said Eric Robinson, senior lecturer in psychology at the University of Liverpool, who led the UK study. "A lot of people would expect there to be more calories in fast food, but our findings were counter to that."

'Fast food comes out as the good guy'

The average main meal dish at a fast food chain in the UK contained 751 calories, compared with an average of 1,033 at sit-down restaurants. Only 11% of full-service dishes served at the 21 restaurants studied -- including Pizza Express, Zizzi and Wagamama -- met the recommended limit of 600 calories, though that rose to 17% in fast food meals.

These restaurants did not respond to requests for comment.

The worst-performing of the British fast food outlets, KFC, had an average of 987 calories per meal, but it fared better than more than half of the sit-down restaurants. Meanwhile, an average meal at Burger King (711 calories) proved less calorific than every one of the traditional restaurants surveyed.

The worst offenders were named as Hungry Horse, where the average meal contained 1,358 calories, followed by Sizzling Pubs (1,269 calories) and Flaming Grill Pubs (1,232 calories).

Sizzling Pubs' parent company, Mitchells and Butlers, said in a statement, "We share nutritional information online for guests to access and we are very mindful of our role in helping guests make knowledgeable decisions about what they eat and drink while dining with us.

"We also regularly work with our suppliers to explore ways we can reduce sugar levels in our dishes and have also committed to the Public Health England Sugar Reduction programme."

The other restaurants did not respond to requests for comment.

The international study, which didn't name specific restaurants, found an average of 809 calories in main meal dishes served by fast food chains and 1,317 calories in restaurant chains. Only the Chinese served fewer calories than Americans: 1,045 calories in full-service, compared with 1,362 in US eateries, and 561 in fast food restaurants, as opposed to 969 in America. Indian meals tended to be higher in calories; restaurants in the remaining countries were comparable to those in the United States.

Once again, a small minority of meals met the 600-calorie recommendation, with 94% of full-service dishes and 72% of fast food dishes exceeding this limit.

Robinson said that although there is considerable debate in Britain about forcing restaurants to display calorie content, only 9% of the meals his research covered met the maximum.

"So even if you want to know about calories on the menu, you are not left with many meals to choose from," he said. "Restaurants need to serve fewer calories. There are a lot of meals that have far too many calories, whether it's due to portion sizes, ingredients or cooking methods.

"The bigger issue it relates to is that we are living in a food environment that encourages and causes people to eat more calories than they need," he said. "If a government is serious about trying to reduce obesity and the number of people who die young because of poor diet and obesity, then the food environment is something that needs to be addressed and tackled."

Susan Roberts, senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, was the lead author of the international study. "Until now, fast food has been criticized all the time, but in this study, fast food comes out as the good guy," she said. "It was still bad, but it had 33% less calories than standard restaurants."

Potential steps to be taken include reducing portion sizes or even introducing proportional prices for restaurant meals, Roberts said. In other words, people would be required to pay for the portions they ordered rather than a one-size-fits-all approach.

"I think that most of us would be delighted to be able to go into a restaurant and order the size that we actually want. But that would mean that restaurants would have to actually recognize that they are part of the problem, which I think they will deny," Roberts said.

The authors of both reports said no firm conclusions could be drawn, as both were observational. However, they also warned that their findings may not fully reflect the reality, as they did not take in to account drinks, appetizers, desserts or side orders.

'Step up and take responsibility'

Caroline Cerny, lead of the Obesity Health Alliance, a coalition of more than 40 British organizations and charities, noted that "regularly eating excess calories over time leads to weight gain."

"Children these days are growing up in an obesogenic environment, and it is becoming increasingly difficult for families to make healthy choices. That's why we welcome government proposals to introduce mandatory calorie labeling on menus in restaurants, cafes and takeaway. But we also need the whole food industry, including restaurants, to step up and take responsibility for making healthier eating easier, by reducing calories and sugar in their meals by 20% in line with government targets."

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Tam Fry, chairman of the UK's National Obesity Forum, welcomed the findings but said he thought Public Health England's 600-calorie recommendation was "unrealistic".

"We have all become so accustomed to eating so much more now that I think a more realistic maximum would be about 800 calories," he said. "The restaurant industry has got to clean up its act, but I fear it won't do it voluntarily because they feel they will lose custom. So there must be some kind of regulation to bring them into line, or else they will be named and shamed."