Professor Dolores Albarracin, a psychology expert at the University of Illinois, said that weight-loss campaigns that promote exercise may actually cause people to eat more than they need - possibly negating the positive effects of the activity.
She revealed people who were shown posters advising them to join a gym or take a walk subsequently ate "significantly more" food than those who were shown similar posters urging them to make friends or be in a group.
The professor, whose findings are published in Obesity journal, confirmed: "They ate one third more when exposed to the exercise ads."
Professor Albarracin noted that policymakers should be aware that "whatever they communicate is likely to influence not only the behaviour they had in mind but other behaviours that might be somewhat remotely linked".
Health experts recommend that we exercise for at least 30 minutes, five times a week.