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Originally Posted by BL1 IMO you need to do at very least 12mins of cardio until your body starts using 'fuel' aerobically instead of anaerobically. If you run at a faster rate you are using your anaerobic system therefore burning more calories but not burning as much fat. Also Lactic acid build-up will be a lot quicker. As PsCarb said aiming for 40 mins is ideal. This is just my opinion so pick holes if it isn't correct !  |
I've read a few different reports on fat-loss research and my understanding is that for the first 20 minutes of "fat-burning" exercise you actually burn around 40% fat, whereas after around 20-24 minutes this shifts to nearer 70%.
As others have said, if fat burning is your goal then you shouldn't be getting out of breath. Doing so indicates you're straying into the cardio zone, where your body will use the quickest and easiest-metabolised sources of fuel available (which unfortunately isn't your stored fat).