The study’s findings also suggest that getting more people to use mass transit is as effective at lowering obesity rates as getting them to engage in leisure-time physical activity.
“As local communities seek to allocate public funds to projects that will provide the most benefit to their residents, our research suggests that investing in convenient and affordable public transit systems may improve public health by reducing obesity, thereby providing more value than had been previously thought,” said Sheldon H. Jacobson, the study’s senior author and a professor of computer science at the University of Illinois, in a released statement.
As almost everyone knows by now, the United States is in the midst of an obesity epidemic. More than one-third of American adults (36.5 percent) are obese, according to the Centers for Disease Control and Prevention (CDC). The annual medical cost of obesity to the U.S. economy is estimated at $150 million. People who are obese incur almost $1,500 more in medical expenses each year than people who are not obese.
Minnesota’s obesity rate is currently at 26.1 percent.
Unraveling confounding factors
Past research has found that communities with higher mass transit use tend to have lower obesity rates. But it hasn’t been clear if those two factors are directly connected. Other factors might explain the link.
For example, people who are obese may simply prefer automobiles to mass transit. Obesity, therefore, could be a cause of lower mass transportation use, not a result of it. Or people who are obese may not live in communities with easily accessible mass transit. Indeed, other research has found that people who are obese tend to prefer living in more sprawling neighborhoods, where mass transit often isn’t available.
In addition, it’s possible that there’s a “substitution effect” between transportation-related and nontransportation-related physical activity. People who take mass transit may be too tired when they return home to engage in additional physical exercise. An overweight person may therefore prefer driving rather than taking public transit to work — even if he or she wishes to lose weight.
For the current study, Jacobson and his colleagues analyzed county health and transportation data from across the United States.
After controlling for factors known to influence obesity and people’s choice of transportation — such as household income, poverty rate, education level, access to health care, and leisure physical activity — they found that for every 1 percent increase in the number of residents who frequently use mass transit, the obesity rates in that county dropped by 0.2 percent.
“Our research suggests that, in addition to benefits to the environment and greater access to transportation for residents, community-level investments into public transit systems may also benefit public health by reducing obesity rates,” said co-author Douglas King, a lecturer in industrial engineering at the University of Illinois, in the released statement.
The study also found that, among people with jobs, the effect of mass transit on obesity rates is essentially the same as the effect of increased leisure-time physical activity on such rates. Specifically, the data revealed that for every 1 percent increase in the number of residents who engaged in leisure-time physical activity, the obesity rates in that county dropped by 0.18 percent.
“For the employed subpopulation, increasing public transit usage may be an equally effective strategy in losing weight in comparison to increasing leisure time physical activity,” the researchers write.
Limitations and implications
The current study shows only a correlation — not a direct causal link — between increased mass transit use and decreased obesity rates. Other factors not controlled for by the researchers may therefore explain the study’s results.
Still, the findings support other research on this topic, including an earlier study by Jacobson and King that found an association between reducing daily driving — even by a mile a day — and a reduction in weight.
“The choice to ride public transit instead of driving can create an opportunity for physical activity,” Jacobson said. “For example, when someone rides a bus, they may begin their trip by walking from their home to a bus stop before boarding the bus. Then, once they get off of the bus, they may still need to walk from a bus stop to their destination. Alternatively, if they had driven a car, they might simply drive directly from their home to their destination and eliminate the walking portion of the trip.”
FMI: You’ll find an abstract of the study on the Prevention Medicine website, but the full study is behind a paywall.