PUTTING calorie information on restaurant menus encourages consumers to cut back - but the effect soon wears off.

Researchers examined the eating habits of customers across 104 outlets of a US fast food chain in Louisiana, Mississippi and Texas in the two years before, and the 12 months after, calorie labelling was introduced.

They found that the average energy intake per meal reduced by 60 calories - or 4% - initially, mostly driven by a decline in people buying side orders or desserts.

One year on, however, this had diminished to a reduction of just 23 calories per meal as “customers initially responded to the novelty of calorie labels but stopped noticing them over time”. The effect appears to wear off fastest in low-income groups.

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The researchers estimate that a 23 calorie cut per meal “roughly translates to a loss of 1Ib over three years in adults”, on the basis that an average American consumes fast food once every three days.

They add: “Although this suggests minimal individual level impact, microsimulation studies indicate that on a population level even such a small decrease in calories could result in tens of thousands of fewer cases of obesity and substantial savings in annual healthcare costs.”

The study, published today in the British Medical Journal, comes amid a push by the Scottish Government and public health campaigners to force restaurants, takeaways and venues such as cinemas in Scotland to display the calorie content for all food and drink.

The move has been touted as a key plank in the fight against obesity, but it is controversial for the food industry who argue that it would place a “disproportionate burden” on small and medium-sized businesses.

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Willie Macleod, executive director for Scotland at UK Hospitality, said: “Many restaurants, cafes or pubs change their menus regularly, some on a daily basis, to incorporate special items and seasonal ingredients.

“Mandatory labelling on menus would present a problem for anyone wanting to change their offer regularly. It would restrict businesses ability to be flexible and would inevitably lead to much less choice for customers.

“The data now seems to show that mandatory labelling would not even do much to influence customers choices so there can be no justification for introducing it.”

It has been suggested the calorie labelling will encourage food outlets to reformulate meals, but the BMJ study found that two thirds of the items on the fast food chain's menu remained exactly the same, while 21% of items actually increased in calories.

Heather Peace, head of public health nutrition at Food Standards Scotland, said: “People have a right to know the calorie content of the food they choose when eating out.

“This is one of the reasons we are recommending that calories are displayed on menus, for example in cafes, restaurants and takeaways, or when buying food ‘on the go’.

“No single measure will tackle poor diet, overweight and obesity in Scotland. As such, we’ve proposed a range of out of home measures which also includes reductions to excessive portion sizes, reformulation of recipes, healthier food and drink choices on children’s menus and in the public sector.”