Copperman said heavy cellphone use can create mindless eating, much as television does. If you're using your cellphone during much of your time awake, you have to sometimes be eating while using the device, she said.
Cellphone use can also affect sleep, study co-author Barkley noted. He said some students have been known to "sleep text" -- sending messages while they're sleeping and not remembering they did it when they wake up.
Copperman said she worries that while this study focused on college students who were about 20 years of age, many of today's elementary school students are just as tethered to smartphones. "This is probably affecting physical activity in younger kids now, too," she said.
For the study, the researchers surveyed more than 300 college students about their cellphone use, leisure activities and physical activity. Then 49 students used a treadmill test to evaluate their heart and lung fitness.
In that group, those who spent a lot of time on their cellphones -- up to 14 hours daily --- were less fit than participants who only averaged about 1.5 hours of use.
The findings took into account factors such as gender, percentage of body fat and "self-efficacy" -- the participant's confidence that he or she could be active in a variety of settings, Barkley said.
High-frequency cellphone users tended to report they were involved in more sedentary activities than were low-frequency users. The researchers said that high cellphone users may also be attracted to other forms of digital media such as television, movies, computers and video games.
However, the research can't conclude that cellphones are causing people to be less fit, Barkley acknowledged. "It's possible that less fit people use their cellphones more," he said.
Copperman offered some practical advice. She thinks parents should monitor not just what their children are doing on their smartphones, but how frequently they are using them. Adults should start monitoring themselves, too, she suggested, noting if they're interrupting physical activity by using their phones, or making their cellphone their dinner companion.
"People should take time from their cellular technology for a better quality of life," Copperman said.
The study appeared online recently in the International Journal of Behavioral Nutrition and Physical Activity.