Our expert says:
FItness is one factor that determines how much energy you use. There are others, most importantly, lean body mass and genetics, because some people just have high metabolic rates and so burn a lot more energy than others. But, within each person, if you start doing regular exercise, the number of calories burned per day increases by quite a large amount, depending on what you do. For example, just doing the training session, say 30 minutes jogging, will burn anything between 400 and 600 kCal, depending on how fast your run. This means that your normal energy use is increased by a whole 500 kCal, which is a big jump. Then, as you get fitter and more well trained, what happens is that you use more energy when at rest, and so then, a fit person will burn more calories even when they are not exercising compared to when they were unfit.
As for your observation, that's not the case, as you get fitter, you are in fact able to exercise at a higher intensity, which means that you are able to burn more calories. For example, you may cycle at a power output of 150W when unfit, and as you get fitter, you increase this to 200W, which means you are actually burning 25% MORE calories than before. So with fitness, you can increase exercise intensity and it is the exercise intensity that is a vital factor determining how much energy you burn.
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