The real story behind how biking uses up calories has several twists and turns. For starters, there’s more than one way that the body burns calories.
When you use your muscles, they start using the oxygen you breathe in to convert fats and sugars, and sometimes proteins, into adenosine triphosphate, or ATP. This is the basic molecule that supplies energy to cells. “You pretty much need a constant stream of ATP even if you're just hanging out. But when you're exercising, you need a lot,” says Rachel DeBusk, CPT, a triathlete coach at Seattle’s Unstill Life.
Depending on how long and intense your workout is, your body might access or make ATP in different ways. “There's some ATP just waiting in your muscles,” says DeBusk. “But when that's used up, you have to make more.”
During short, intense bursts of exercise, your body uses anaerobic metabolism to convert carbohydrates into ATP. During longer, less intense workouts, your body gets ATP from aerobic metabolism, where most of the energy comes from carbs.
If you're biking at a moderate, steady speed and without much resistance, you're mostly using your aerobic metabolism system. This has a lot of benefits. First, it improves how well your heart and lungs work. And it also helps your body to use up glucose efficiently. DeBusk cautions that not using glucose efficiently can raise your risk of pre-diabetes or metabolic syndrome. With moderate levels of cycling, your body also improves its ability to mobilize fats stored in muscle.
If you're biking at a higher speed, or at greater resistance, you'll rely more on your anaerobic metabolism system. Just by the nature of it, this system isn't one that you can maintain for very long. Cycling harder will help your muscle fibers learn how to adapt to demand.
Does how many calories you burn depend on whether you’re biking outside or using a stationary bike? “You can get a great workout in a fitness studio or outside,” says DeBusk. However, biking outside is more dynamic: You have to be aware of your surroundings, and there’s more variety of movement, as you turn to follow roads and paths. There can also be wind resistance and inclines such as hills, and this may burn more calories than indoor cycling, depending on the spin class you do. In a spin class, you can close your eyes and bliss out to music. If your work or family schedule makes it hard to exercise unless it's a scheduled activity, spin classes are a great option.
You also might opt for indoor biking if you’re pregnant, to reduce risk of an accident. “Cycling is a great form of exercise during pregnancy,” says DeBusk. “Many pregnant women find the non-weight-bearing posture reduces pressure in the lower back.” As pregnancy progresses, make adjustments to your seat and handlebars to accommodate changing hip angles, or try using a more padded seat. “Always stay well hydrated, and don’t overheat,” reminds DeBusk.
What does all of this burn down to? The length of time you ride and the intensity are the primary determinants of how many calories you burn. If you're starting with little or no activity, biking 15 minutes a day, or 30 minutes a few times a week, is an excellent way to improve health, and will likely reduce weight. Once you’ve adapted to moderate riding, add some intensity intervals (like repeats of 30-seconds hard, 1-minute easy), which are even better for burning calories.