It boils — not burns — down to this: During the same amount of time you don't use more calories at lower exercise intensities. If you're trying to lose weight and you have only 30 minutes to work out, you would burn fewer calories walking at a moderate pace compared to walking at a fast pace.
Working out at higher intensities may cause you to burn a lower percentage of fat, but since you burn more total calories, you still use more fat calories. (See Following a Cardio Plan for Weight Loss to get tips on what exercises can help you shed pounds.)
Low- to moderate-intensity exercise can burn a significant number of calories over a period of time. If you aren't fit enough to push yourself to work at a high intensity, or you have a physical weakness that prevents you from doing so, you can still burn a lot of calories by doing low-intensity workouts for a longer period of time.
Myth: Running, cycling, or other cardio activities are more fat burning once you've been doing them for more than 15 or 20 minutes.
The Truth: Technically, once you've been exercising for 15 or 20 minutes, your body has made the shift to using a higher percentage of fat for fuel. But again, if you're trying to lose weight, it's about the total number of calories burned, not necessarily the fuel source.
For example, say that at rest you burn up to 60 percent fat. When you enter the initial phases of intense exercise, the ratio changes. You may now burn only 30 percent fat because your body is using quick-energy carbohydrates. Once the exercise is sustained, the body switches back to using a higher percentage of fat to fuel the movement (up to 75 percent fat).
In this aerobic phase of exercise, a higher percentage of fat is being used for energy. But if you aren't working out for a very long period, you may still burn more total calories and, therefore, more fat calories working out harder.
Put another way, if burning as many calories as you can is the best way to lose weight, even a dummy can figure out which activity of the following is going to give the best results (answer: jogging and sprinting), even though their fat-burning quota is on the low end of the ratio.Activity Calories Burned Fat Percentage Calories from Fat
Watching TV for 20 minutes 40 calories 60 percent 24 calories
Walking for 20 minutes 100 calories 65 percent 65 calories
Jogging & sprinting for 20 minutes 250 calories 40 percent 100 calories