The calorific cost of a 590ml soft drink? How about a five mile walk.

Signs telling children how far they will have to walk to burn off the calories in their bottle of soft drink are an effective way of encouraging them to choose a healthier option.

Researchers at the School of Public Health at John Hopkins University in the US found that 40% of people who saw such signs when they went to buy a drink decided to make a different selection.

They said their findings added to the argument that simply putting a calorie count on a product may not be enough.

For six-week stretches between August 2012 and June 2013, study leader Sara Bleich and her colleagues installed signs in shops in low-income areas.

The signs, four in all, presented a key fact about the number of calories in a 20oz (590ml) bottle of soft drink, sports drink, or fruit juice: That each bottle contained 250 calories, had 16 teaspoons of sugar, would take 50 minutes of running to work off those calories, or would take five miles to walk the calories off.

They saw 3,098 drink purchases in the stores by 12- to 18-year-olds and interviewed 25% of them afterwards on whether they saw and understood the signs.

Of the 35% who said they saw the signs, 59% said they believed them and 40% said they changed their behaviour as a result.

When compared with purchasing behaviours when there was no signage, the most effective sign, Ms Bleich said, was the one which told shoppers they had to walk five miles to burn off the drink calories.

Overall, the number of sugary drink calories bought went from 203 calories to 179 after people saw the sign showing the exertion need to burn off the calories.

Regardless of which sign was posted, the percentage of adolescents who chose to buy no beverage at all rose from 27% to 33% over the study period. Water purchases, meanwhile, rose from 1% to 4%.

“People don’t really understand what it means to say a typical soda has 250 calories,” said Ms Bleich.

“If you are going to give people calorie information, there’s probably a better way to do it. What our research found is that when you explain calories in an easily understandable way, such as how many miles of walking needed to burn them off, you can encourage behaviour change.

“This is a very low-cost way to get children old enough to make their own purchases to drink fewer sugar-sweetened beverages and they appear to be effective even after they are removed.”