Upping the level of activity during a commute to work could help with weight.
They got on the bus/coach, train or metro, and giving up a private car in the study also included switchers to walking or cycling for the full commute.
It is within a couple of years that switching can help commuters shed weight, say the researchers publishing their work online in the Journal of Epidemiology & Community Health.
Leading the study was Adam Martin, health economist at the University of East Anglia's Norwich Medical School in the UK, who quantifies the potential benefit suggested by the results:
"We found that switching from the car to walking, cycling or public transport is associated with an average reduction of 0.32 body mass index (BMI), which equates to a difference of about 1 kg for the average person.
"This might sound like a relatively small proportion of their total weight, but we also found that the longer the commute, the stronger the association," continues Mr. Martin, concluding:
"For those with a commute of more than 30 minutes, there was an average reduction of 2.25 BMI units, or around 7 kg (over one stone) for the average person."
As well as from the University of East Anglia, contributors in the research team came from the University of Cambridge and the University of York. They reached their findings through an analysis of responses from over 4,000 adults in a survey conducted in three annual waves between 2004 and 2007.
The participants in the British Household Panel Survey reported at each of the three data-collection points what their main mode of travel to work usually was.
For calculation of BMI, the commuters gave their height and weight in the first and third years. BMI figures are reached by diving weight in kilograms by height in meters-squared - kg/m2.
A series of analyses were used to see if changes in weight over time were linked to changes in choice of transport means by the workers.
New scientific credibility for 'intuitive' weight loss link
The researchers say that while the link between commuting and weight "may seem intuitive," it has rarely been tested in a longitudinal study using data from a representative national survey. While previous associations have been made in other types of observational study, this study may be more robust.
"The key feature of this study is that we were able to compare changes in weight over time between commuters who had, and had not, changed how they traveled to work," Mr. Martin says.
It was a longitudinal cohort study, so while it can hint at a cause-and-effect relationship between a population variable and a health outcome - in this case, observations between type of commuting and body mass changes - it cannot definitely prove the link.
"Combined with other potential health, economic and environmental benefits associated with walking, cycling and public transport," continues Mr. Martin, "these findings add to the case for interventions to support a larger proportion of commuters taking up these more sustainable forms of transport."
On the size of the health effect observed, from the wider public health point of view, influence from transport choices could be bigger collectively than the sum of individual interventions on weight - as explained by the study authors:
"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."
The benefits of regular exercise are wider than the question of effect against obesity - the American Heart Association recommends we all do a certain minimum level of physical activity for heart health.
The AHA has produced a physical activity graphic representing the options for overall cardiovascular health, which include the option to simply walk for 30 minutes, 5 days a week. There are also ways to exercise in helping to lower blood pressure and cholesterol, say the cardiologists.
On the obesity debate receiving the present study's contribution, questions rage on about the relative contribution to the problem - and its solutions - from activity versus diet. For example, opinion-leaders in the field of sports medicine last month wrote in The BMJ that sugar and carbohydrates are the obesity culprits, not lack of exercise.
April saw the latest annual "walk to work day" in San Francisco. It was organized by champions of walking, Walk SF.