Any aerobic activity will burn calories and improve cardiovascular health, but which exercise is best for weight loss depends largely on preferences and limitations. The easy accessibility of biking and running make these exercises two of the most popular. However, both have trade-offs when it comes to weight loss.
If your only criterion is maximizing calorie consumption, then running is the obvious choice. At 755 calories per hour, a 5-mph run more than doubles the energy expenditure of a leisurely bike ride, which burns just 364 calories per hour for a 200-pound man. The number of calories consumed depends primarily on the weight of the person. As an example, a 160-pound man burns 606 calories per hour when running 5 mph (292 when cycling), which contrasts sharply with a 240-pound man's 905 calories per hour (436 when cycling). Total calories burned depends on the distance traveled, rather than the speed at which you run or cycle; however, the faster you go, the more calories you'll burn per hour, because you're covering more distance. As an example, the previously mentioned 200-pound man burns 1,074 calories per hour when increasing his running speed to 8 mph, but he's covering three more miles every hour.
Fat oxidation is the process of breaking down fat molecules resulting in fat loss. This is another area in which running excels when compared to biking. A 2010 University of Cape Town study showed that running significantly increased fat oxidation when compared to cycling at the same intensity levels among male triathletes. This echoed results of a 2003 University of Birmingham study that focused on only moderately trained males.
When comparing equipment requirements of both exercises, running again comes out ahead. You can run virtually anywhere, and all you need is a good pair of running shoes. Biking obviously requires a bicycle and a suitable area in which to bike. In congested cities, this could prove difficult and even dangerous. However, both exercises are weather-dependent unless you can find an indoor track or opt for a stationary bike or treadmill.
Aside from the dangers of wrecking your bike at high speeds, biking is more comfortable and less jarring to the body compared to the continuously pounding effects of running. This is particularly true for overweight people. Running frequently causes pain in the knees, ankles and back, which doesn't happen as often in cycling. For overweight would-be runners, this discomfort makes the exercise less enjoyable and can ultimately reduce their commitment. If you find running too painful or uncomfortable, biking may well reap greater rewards simply because you'll exercise more.