Scientists probing the effect of including healthy alternatives, like salads, on fast food menus have warned of a powerful side-effect called "vicarious goal fulfilment".

Test subjects who were strict about what they ate were shown to make less healthy food choices so long as they first considered eating a healthy option on the menu, the US research found.

"We find that simply seeing, and perhaps briefly considering, the healthy option fulfils their need to make healthy choices, freeing the person to give in to temptation and make an unhealthy choice," say Gavan Fitzsimons, professor of marketing and psychology at Duke's Fuqua School of Business.

"Adding one or two healthy items to a menu is essentially the worst thing you can do ... because, while a few consumers will choose the healthy option, it causes most consumers to make drastically worse choices."