Opting for mass transit as opposed to driving is not only better for the environment but also for your health and waistline.

The metro, tube, subway, train or whatever you call it is an easy way to integrate exercise into your daily routine. On June 30, John M. MacDonald, PhD, a criminologist and co-author of a new study to be published in the August edition of the American Journal of Preventative Medicine, explained to National Public Radio (NPR), a nonprofit membership US-based media organization, "commuters who took light rail to work regularly were 6.45 pounds lighter than the folks who drove."

The metro "could add an extra mile a day [of walking] for the average rider. Over the course of a year, that mile a day will translate into substantial loss of weight.

"People who used the system also were less likely to become obese over time," added MacDonald.

SocialWorkout.com, a social network fitness site, tested this very theory in March with one of their interns, referred to as the ‘Wired Intern' in an effort to motivate ‘9 to 5ers' to get fit and show how many calories can be burned per hour during a 9am to 5pm workday.

The intern spent the day racing through rush hour twice to get to work by metro and again taking it to school and a yoga class. The chart showed the day packed with lunching, jokes and work burned 2495.29 calories with the biggest burning hours taking place while hopping on the train and doing yoga - all were almost at the same level. Take a look: http://www.socialworkout.com/2010/03/25/wired-intern-does-9-5-and-burns-surprising-number-calories-0

The next time you think a taxi or driving is easier and more convenient, try thinking about the calories you will burn and money saved by running for the metro, up and down the stairs.

Smartphone applications by MetrO (free) can help you navigate the world's metro lines.

MacDonald's study to be available in August here: http://www.ajpm-online.net/current