It has been a topic for discussion among runners since treadmills were invented but more recently some scientific studies have been done that have proven that you can indeed burn more calories while running outside compared to running inside on a treadmill. However, you can burn just as many calories on a treadmill as you can outdoors, it will just require you to run on the treadmill for a longer period of time or at a higher intensity (or both if you really like pain).
Why running outdoors burns more calories than running on a treadmill?
In a nutshell running outdoors is just more intense by providing more resistance. If you’ve ever run on a treadmill for a period of time and then one day gone outside to run the same distance you will see that it feels a lot more strenuous running outdoors.
When you run indoors on an electric treadmill, the belt assists your legs by pulling them back compared to when you run outside where you must push off against the ground with your legs without any help. To go along with the greater resistance of running outdoors, there is also the resistance from the wind. Another aspect of running outdoors that makes it more intense is that when you run on a treadmill you are in an air conditioned home or gym while when you run outdoors it is usually much warmer.
If you are only running to stay in shape then a treadmill will work just fine. On the other hand if you are a competitive runner, running outdoors with the assistance of a GPS watch is the only way to prepare you for the intensity of an outdoor running competition.
Studies have shown that treadmills can reduce injuries by 50% compared to running outdoors and the weather is not always accommodating so a treadmill will allow you to continue your running program. Just remember that if you do run indoors that you will need to run at a faster pace or a longer distance to get the same intense workout that you are used to outdoors.