As we try to figure out how to curb an unhealthy increase in obesity, one of the factors under consideration is the built environment. Those who in live in places where few destinations are within walking/biking distance, public transit is limited, and the environment is unfriendly to pedestrians and cyclists may find it harder to get the recommended amount of physical activity. Studying the built environment’s effect on obesity is challenging, though. People who live in pedestrian-friendly areas with good public transit may be more likely to get enough activity and less likely to be obese than their counterparts in car-dependent places, but that doesn’t demonstrate causation. What if people who hate physical activity naturally gravitate toward auto-dependent areas, and walkers move to places where it’s easy to get around on foot?

This is why I was excited to see a study by researchers from the University of Pennsylvania, Drexel University, and the RAND Corporation that takes advantage of a natural experiment to study causality (via Ryan Avent). It was published in the August 2010 issue of the American Journal of Preventive Medicine, and the full text is available for free online.

The natural experiment took the form of a new light rail transit (LRT) line in Charlotte, NC. Researchers conducted two phone surveys with residents living within a one-mile radius of the new line; participants were first interviewed 8-14 months before the LRT opened, and again 6-8 months after the system became operational. The study ended up with a total of 489 respondents who lived in the area and agreed to be interviewed at both times.

Participants were asked for their height and weight (which was then used to calculate BMI) and whether they met weekly recommended physical activity (RPA) through walking. The American College of Sports Medicine and American Heart Association recommend vigorous activity 3 times a week, ≥20 minutes a time; or walking 5 times a week, ≥30 minutes a time. (The study also covers residents’ perceptions of the built environment – the whole thing is worth a read for anyone interested in this topic.)

The authors found that “there is a significant association between LRT use and reductions in BMI over time.” The details are as follows:

Specifically, LRT reduced their BMI by an average of 1.18 kg/m2 compared to similarly situated non-LRT users over a 12-18 month follow-up period. For a person who is 5′5″, that is equivalent to a relative weight loss of 6.45 lbs. Use of LRT is also associated with a reduced odds of becoming obese (OR=0.19, 95% CI=0.04, 0.92). LRT users were 81% less likely to become obese over time. The odds of increasing one’s physical activity through vigorous exercise to meet RPA levels is also associated with LRT use (OR=3.32, 95% CI=0.81, 3.63) but was only significant at the p<0.10 level. The association between LRT use and meeting weekly RPA levels of walking was in the positive direction (OR=1.36, 95% CI=0.39, 4.73) but not significant.

There are many reasons to shift more of our transportation from single-occupancy vehicles to public transportation, including reducing pollution and congestion and improving quality of life for those who can’t drive (whether due to age, physical limitations, or inability to afford a car). If such a shift can also play a role in reducing obesity, that makes the case for transit even stronger. I hope in the coming years we’ll see more new transit lines added, and more researchers taking the opportunity of these natural experiments to study how public transportation affects public health.

Citation: MacDonald JM, Stokes RJ, Cohen DA, Kofner A, Ridgeway GK. The Effect of Light Rail Transit on Body Mass Index and Physical Activity. Am J Prev Med 2010; 39(2):105-112. doi: 10.1016/j.amepre.2010.03.016.