The good news is that running period burns fat! If you want to get more specific about energy sources while running, I'll break it down for you simply. The more oxygen a person is able to take in while performing exercise, the more that energy requirement comes from fat stores, ie, slow running, lower heart rate. The faster a person runs, thus elevating the heart rate, makes the body have to use a more accessible energy source: Carbohydrates. Both types of running burns fat, but the longer, slower running uses mainly fat for energy while performing said activity. However, the recovery time is short, and one goes back to normal fat burning soon after activity ceases. The latter type produces a longer recovery; heart rate stays elevated longer, allowing fat stores to be used for up to 24 hours after the activity ceases. But all of this depends on the intensity and duration of running. Here's an example:
Slow run - 45 min to 1 hour at a heart rate of 60% - 70% of age related Maximum.
Fast run - 20 min to 40 min at a heart rate of 75% -85% of age related maximum.
To calculate heart rate range use this formula:
220 - age = heart rate max
take your max and multiply by the % to get your age related range. See below for an example
220- 20 (age) = 200 (heart rate max) x 65% = 130 beats per min
220- 20 (age) = 200 (heart rate max) x 75% = 150 bpm
So, if you were running in the 65% -75% heart rate range, your target would be 130-150 bpm. That range would produce fat burning mostly while running. If you ran faster, producing a higher heart rate, and for a shorter duration, you would utilize mostly carbohydrate stores. But you would start burning fat during your recovery as your heart rate came down. For more exercise advice, visit
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I hold a B.S. in Exercise Physiology, and am a Certified Strength and Conditioning Specialist (CSCS)