Including nutritional information on restaurant and cafe menus may help to tackle the growing problem of obesity, a new study suggests.
A new Cochrane Review of evidence found that nutritional labelling on menus may reduce a person's calorie intake.
Researchers found that labelling on menus in restaurants led to a reduction of 47 calories purchased.
Assuming an average meal of 600 kcal, labelling on menus would reduce the number of calories purchased per meal by 7.8%, they said.
But the authors stressed that the quality of evidence reviewed was "low" and they were only "tentatively" suggesting that nutritional labelling on menus in restaurants could be used as part of a wider set of measures to tackle obesity.
They called for more high quality evidence to strengthen the findings.
The Obesity Health Alliance, a coalition of 40 health bodies and charities, said the study showed that clear labelling helps empower people to make informed choices about the food they eat.
It added that restaurant food "too often" can be a large portion "packed with hidden ingredients".
Researchers, who examined a total of 28 studies on food labelling, said they were unable to reach conclusions about the effect of labelling on calories purchased from grocery stores or vending machines because the evidence was limited.
They called for more research to address the "dearth of evidence" from grocery stores and vending machines and more studies to confirm the findings on restaurant menus.
The Review's lead author, professor Theresa Marteau, director of the Behaviour and Health Research Unit at the University of Cambridge, said: "This evidence suggests that using nutritional labelling could help reduce calorie intake and make a useful impact as part of a wider set of measures aimed at tackling obesity.
"There is no 'magic bullet' to solve the obesity problem, so while calorie labelling may help, other measures to reduce calorie intake are also needed."
Caroline Cerny, Obesity Health Alliance lead, said: "Too often food in restaurants or cafes can be a large portion size and packed with hidden ingredients such as salt or sugar so it's very easy to eat more calories than you need.
"It makes sense that when we know the nutritional content of the food we're eating, the more likely it is that we'll make healthier choices. This important research shows that clear labelling of the food we eat out of home, as is increasingly on display on supermarket products, is an important step to empowering people to make informed choices when it comes to eating."
Alison Tedstone, chief nutritionist at Public Health England, said: "With around a quarter of our calories coming from food on the go, including takeaways and deliveries, we need clear and consistent information on menus at the point of choice.
"Some companies are already doing this, but too often menus are an information-free zone - we need bigger and bolder commitments to help people make healthier choices and avoid obesity-related health problems."