People who drive to work every day are packing on slightly more weight than their colleagues who use trains, buses and bicycles to get to work, even if they exercise in their spare time, according to an Australian study.

"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," said lead author Takemi Sugiyama of the Baker IDI Heart and Diabetes Institute in Melbourne.

Among people in the study who got at least two and half hours of weekly exercise, car commuters gained an average of 1.8 kilograms (4 lbs) over four years - about half a kilogram (1 pound) more than people who got to work another way, or who 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 now 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.

There are probably other factors at work that were not considered in the study, noted Frank, who was not involved in the 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 influence weight gain, he said.

In addition, 80 percent of car trips are non-commuting.

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.

In Australia, about 80 percent of working adults take a car to work every day. In the United States, that figure is 86 percent.

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

Many of those people don't have another option, said Sugiyama, noting that it's the responsibility of government to provide public transport to and from work, and design neighborhoods where short walks are accessible.

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