BehindTheMedspeak: Does running outdoors burn more calories?

Anahad O'Connor, in his "Really?" column in the May 6, 2008 New York Times Science section, gives it to you straight: Yes, about 5% more — but the advantage is more than offset by an increased risk of stress fractures.

Here's the piece.

The Claim: Running Outdoors Burns More Calories

The Facts: Pavement or treadmill? Most avid runners have a strong preference for one or the other, but how do the two differ in producing results?

According to several studies, the answer is not so simple. Researchers have found in general that while outdoor running tends to promote a more intense exercise, running on a treadmill helps reduce the likelihood of injury, and thus may allow some people to run longer and farther.

A number of studies have shown that in general, outdoor running burns about 5 percent more calories than treadmills do, in part because there is greater wind resistance and no assistance from the treadmill belt. Some studies show, for example, that when adults are allowed to set their own paces on treadmills and on tracks, they move more slowly and with shorter strides when they train on treadmills.

But other studies show that treadmill exercisers suffer fewer stress injuries in the leg. One study published in 2003 in the British journal of sports medicine, for example, analyzed a group of runners and found significantly higher rates of bone strain and tension during pavement running than during treadmill running, particularly in the tibia, or shinbone. This increased strain can heighten the risk of stress fractures by more than 50 percent, the study found.

The Bottom Line: Studies suggest that running on pavement generally burns slightly more calories, but also raises the risk of stress fractures.