Fast-food lovers would scoff down around 10 per cent fewer calories if shop menus listed each item's energy content, new research suggests.
In the online experiment, those who were shown generic fast-food menu boards displaying energy content ordered burgers, chips and other items that, on average, contained 490 kilojoules less energy than the orders of those shown menus without the energy listings.
And when energy content was joined by red/orange/green traffic-light labelling to indicate healthier and less-healthy items, the orders contained 500 kilojoules less energy than the comparison group shown the standard menus.
One of the researchers, Maree Scully, of the Cancer Council of Victoria, told the transtasman Obesity Society conference in Auckland that the online experiment, involving more than 1000 people in Australia, found that energy labelling alone, or with traffic-light colourings, led to meal orders containing significantly less energy.
"Thus, the introduction of mandatory disclosure of kilojoule information on menus at fast-food chain outlets would be a positive policy initiative."
Co-researcher Jane Martin, of the Obesity Policy Coalition, said a number of Australian Governments - but not Victoria's - had laws requiring fast-food chains to list energy content of items on their menus. Even in states where they didn't yet have to, chains were now doing it anyway.
"The fact that they are doing it voluntarily says they don't see it as too much of a threat to sales."
"I think it would be good to have traffic lights as well. I think that would encourage reformulation and would probably encourage healthier options.
"In places like Starbucks ... where you are buying a coffee you might think twice about what size you get, or you're buying something that you might think sounds healthy like a berry smoothie.
"Some of those products sound very healthy but they are quite energy dense because they might be high in sugar, they might even be relatively high in fat if they have got milk in it or cream."
Ms Martin said energy listings at the point of sale - rather than on food containers the buyer saw only after the purchase - "help to empower consumers in ways that help them make better choices".
McDonald's in New Zealand has listed energy content and other nutritional data on its tray mats, food packaging and website for several years but spokesman Simon Kenny said it had no plans to list energy content on menu boards.
The chain would instead wait for the outcome of the Government's review of food labelling, which he understood was due to be completed by early next year at the latest.
2070kJ (kilojoules) Big Mac
1660kJ McDonald's large French Fries
1680kJ Subway Meatball Marinara 6-inch sandwich
2155kJ Starbucks grande whole milk white chocolate mocha
Sources: company websites
Median daily energy intake
Source: Ministry of Health