That's because when you exercise, your body shunts blood to the areas it thinks you need it: toward the muscles, lungs, and brain, and away from the viscera (your gut).
When you stop exercising, it takes a little while before your body can make the cardiovascular switch back. This is one reason why you just don't feel hungry after your workout.
And it doesn't matter about your calorie debt. For example, if you haven't eaten and go for a run or something, you won't feel hungry during your run. You may feel light headed if you get low blood sugar and your liver can't squeeze out enough back into your blood stream.
So think of exercise as an appetite suppressant!
Also, don't think that because you just went for a run that you need to immediately "fill the tank" again, because your hunger doesn't work that way.
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