People who switched from driving to work to using public transport, cycling or walking instead lost significant amounts of weight, a study has found.
Research led by the University of East Anglia saw 4,000 participants describe their usual main mode of transport for their daily commute and provide details of their height and weight, which was used to calculate their body mass index (BMI).
Researchers then used a series of analyses to see if changes in mode of transport were linked to changes in weight over a two-year period.
In the first part of the study, which included 3,269 respondents, 179 people had stopped driving to work and were either walking or cycling (109) or taking public transport (70).
They tended to be younger and less likely to have access to a car than those who continued to drive.
Those who chose to walk or cycle instead tended to have a lower household income and a shorter commute, which became shorter still after making the switch, while those who opted for public transport were significantly more likely to be more highly educated.
Switching from a car to walking, cycling, or using public transport was associated with an average reduction in BMI of 0.32 kg/m2 after taking account of other influential factors - equivalent to a difference of around 1kg a person, on average.
The longer the commute, the stronger the association, with a reduction in BMI of 0.75 kg/m2 (equivalent to a weight loss of around 2kg) associated with journeys of more than 10 minutes, and 2.25 kg/m2 associated with journeys of more than 30 minutes - equivalent to weight loss of around 7kg, on average.
In the second analysis, which included 787 people, 268 switched from active to passive travel. Some 156 stopped walking or cycling and 112 switched from public transport (usually a bus or coach) to the car.
Those who stopped walking or cycling to work were significantly less likely than those who stopped using public transport to be in a managerial or professional post. They also tended, on average, to have a shorter commute, which lengthened after the switch.
Those who had previously used public transport, on the other hand, had a short commute after the switch.
But switching to a car was associated with a significant weight gain of around 1kg per person (or 0.34 kg/m2 ) after taking account of other influential factors.
"Our observation that switching from private motor transport to active travel or public transport was associated with a reduction in BMI, even in a relatively short time period of under two years, suggests that a shift in the proportion of commuters using more active modes of travel could contribute to efforts to reduce population mean BMI," the authors concluded.
"Combined with other potential health, economic and environmental benefits associated with walking, cycling and public transport, these findings add to the case for interventions to promote the uptake of these more sustainable forms of transport.
"If large numbers of people could be enabled to take up active travel to work, for example through environmental and policy interventions in the transport and planning sectors, the benefits for population health may be larger than those of alternative interventions targeted at producing larger individual health benefits for relatively small numbers of people."
Philip Insall, director of health at the charity Sustrans, said: "With one in six deaths being linked to physical inactivity, lack of exercise is as dangerous as smoking.
"This is why the new government needs to urgently invest in walking and cycling. Making our roads safer so that people feel able to get out of their cars and on to bikes will have a radical impact on obesity rates - making for a happier, healthier population which places less strain on an already stretched NHS."