Research at University of Illinois suggests that weight-loss campaigns that promote exercise may actually cause people to eat more.
People who viewed posters suggesting that they "join a gym" or "take a walk" ate more food after looking at the posters than people who saw similarly designed posters prompting them to "make friends" or "be in a group," the researchers found.
Subliminal words about being active had a similar effect on study participants, said psychology professor Dolores Albarracín, who led the research.
"Viewers of the exercise messages ate significantly more (than their peers, who viewed other types of messages)," she said. "They ate one-third more when exposed to the exercise ads." Those exposed to subliminal words about activity during a computer task ate about 20 percent more than those exposed to neutral words, she said.
The study, which appears in the journal Obesity, builds on previous research by Albarracín that suggests that general messages to be active can prompt people to behave in a variety of ways, some of which may have negative consequences.
Those designing public health campaigns are in the habit of trying to change one behavior at a time, Albarracín said. They should be aware that "whatever they communicate is likely to influence not only the behavior they had in mind but other behaviors that might be somewhat remotely linked," she said.