Putting calories on food labels and menus "is not a panacea", say researchers, who analyzed the best evidence to date on the policy's effects.

25 research reviews Calorie labeling in many countries found that people chose to eat slightly fewer calories (the equivalent of two almonds), but even this modest effect helped prevent adults from gaining weight as they aged.

New rules on calorie labeling are coming into force in the UK in 2022 as part of the government's plan to encourage people to make healthier choices.

The government said it would continue to assess the impact of outdoor calorie labelling, including on people with eating disorders.

Restaurants, cafes and takeaways with more than 250 staff must now display the calorie content of meals on their menus.

For Imara, a 27-year-old human resources assistant from Birmingham, the change has been a positive part of her weight loss journey.

"It means I'm able to stay on track, stay within my calorie deficit and still go out for a nice meal with friends or my husband and still have a good time without worrying about going off track.

"Sometimes I'll just eat a meal and not think about the calories," she said.

"But if there's a day or week that I really want to stay on track, then I'll look at the calories and see if it's in line with my goals."

The study examined the impact of calorie labeling in supermarkets, restaurants and other food outlets in countries including the United States, United Kingdom, Canada and France.

The study found that people chose and purchased an average of 11 fewer calories (or 1.8%) fewer foods over a 600-calorie meal.

Senior author Dr Gareth Hollands from University College London said this had a "small but potentially meaningful impact on people's food choices" but "we shouldn't expect miracles".

He said calorie labeling "could play a useful role" alongside other measures such as taxes on food and drink, marketing restrictions and reformulation - allowing manufacturers to reformulate foods to make them healthier.

Dr Natasha Clark of Bath Spa University, who launched the project at the University of Cambridge, said the evidence was "strong".

As calorie labeling becomes more common, this effect could make a "meaningful contribution" to public health.

However, charities say the policy could make people with eating disorders feel more anxious about food, potentially triggering a relapse.

Milly Goldsmith, 27, is a lifestyle influencer from London who battled anorexia nervosa and body dysmorphia from the age of 13 to 18.

She said calorie counting was what "fueled" her eating disorder.

"I really feel like it just reinforces these eating habits and makes it a toxic way of looking at food."

Although Millie is in recovery, she describes a constant battle between her "eating disorder brain" and her "logical brain," which doesn't focus on calories in meals.

The review acknowledges that there is a "lack of data" on possible harms, including mental health effects, and the authors recommend future research to assess this.

In a bid to encourage healthier eating, the government has announced plans to ban junk food advertising and calorie labeling on TV and online in the UK.

This is part of a wider plan to address high rates obesity.

A spokesman for the Department of Health and Social Care said: "We are also giving local authorities greater powers to block unhealthy takeaway applications near schools and stop fast food giants targeting pupils."

The Scottish Government has been consulting on whether restaurants and cafes must display calorie levels.