New research provides the best evidence yet linking public transportation usage with lower obesity rates.

(Photo: Oli Scarff/Getty Images)

The idea that mass transit helps fight obesity makes intuitive sense. Taking the bus or train usually means walking to your final destination, thus providing an automatic period of pound-shedding daily exercise.

But while studies have pointed in that direction, doubters have wondered whether the effects are overstated. Perhaps, the argument goes, people who take public transportation are simply healthier than those who habitually drive. Or even if they do walk from the station, it may be their longer commute gives them less time to do intense exercise when they get home.

Well, a new study using county-level data takes such factors into account—and still finds a strong link between mass transportation usage and lower obesity rates.

"Our research suggests that investing in convenient and affordable public-transit systems may improve public health by reducing obesity, thereby providing more value than previously thought," co-author Sheldon Jacobson, a University of Illinois professor of computer science, said in releasing the results. The paper is published in the journal Preventive Medicine.

Get to the gym, or get on the bus—your choice.

Jacobson and colleagues Zhaowei She and Douglas King came to their conclusion after studying county-level data on both mass-transit usage and vehicle ownership. "This approach focuses on people forced to use public transportation due to traffic constraints," they write.

Using data from the Centers for Disease Control and Prevention, they noted the percentage of each county's residents that are obese, as well as the percentage that take part in "leisure time physical activity" such as running or various sports.

The 2009 National Household Travel Survey provided data on the percentage of the county population that uses public transit at least two or more days per week, as well as the average number of vehicles per household. The researchers also factored in such variables as household income, education, the percentage of people with health insurance, and whether the area has a rail or subway line.

Their key finding: "A one percent increase in country population usage of public transit is associated with a 0.2 percent decrease (in the obesity rate)."

That's a big deal. The researchers note that, for those who are employed, "increasing public transit usage may be an equally effective strategy in losing weight in comparison to increasing leisure-time physical activity."

Get to the gym, or get on the bus—your choice.

Now, studies like this cannot prove causation. But, as the researchers note, their work provides "further empirical support for encouraging public transit usage as an intervention strategy for obesity."

Remember that advertising campaign insisting that patronizing Subway would lower your weight? Well, the message appears to true—so long as you're talking not about a sandwich shop, but an actual subway.