People driving to work every day are packing on more pounds than their colleagues on trains, buses and bikes, according to a new study from Australia.

“Even if you are efficiently active during leisure time, if you use a car for commuting daily then that has an impact on weight gain,” lead author Takemi Sugiyama of the Baker IDI Heart and Diabetes Institute in Melbourne told Reuters Health.

Among people in the study who got at least two and a half hours of weekly exercise, car commuters gained an average of four pounds over four years – one pound more than people who got to work another way or worked from home.

Of 822 study participants, only those who got enough weekly exercise and never drove to work managed to stave off any weight gain over the course of the study.

Participants who didn’t get enough weekly exercise also gained weight, but how much they gained wasn’t tied to their mode of getting to work, according to results published in the American Journal of Preventive Medicine.

“Simply achieving the amount of moderate physical activity otherwise recommended won’t provide enough compensation to overcome the effect of commuting for a long period of time,” said Lawrence Frank of the School of Community and Regional Planning at the University of British Columbia in Vancouver, Canada.

But there are probably other factors at work that were not considered in the study, noted Frank, who was not involved in the new research.

“People who have longer commutes tend to purchase a lot of their food and run a lot of errands on their way to and from work,” which could influence weight gain, Frank told Reuters Health.

And 80 percent of car trips are non-commuting, he points out.

Previous studies that focused on total time spent in cars per day have also found a link to becoming overweight or obese.

A 2004 study of adults in Atlanta, Georgia, found that each additional hour of time spent in a car each day was associated with a six percent increase in the chances of obesity.

“Commuting is a truly important predictor of obesity,” Frank said.

In Australia, about 80 percent of working adults take a car to work every day – similar to the 86 percent figure in the U.S.

But many of those people don’t have another option, said Sugiyama, who researches health risks in daily life.

“The message is, if possible try to avoid cars, but for many people that sort of choice isn’t available,” he said.

That’s largely due to the layout of cities and suburban areas, according to Frank, a registered landscape architect. “Spreading growth out, and building cities to add cheaper housing at the outskirts is a problem,” he said.

“It’s the responsibility of government to provide public transport to and from work, and design neighborhoods where short walks are accessible to people,” said Sugiyama. “But that’s a long term solution.”