When Does The Body Start Burning Fat?

Body start burning fat only after 15-20 min of proper heavy workout. Body starts burning fat once it rach the 60-70% of your HeartRate max ( 220-age ). So once it achieve its heart rate it will start burning calories.

I completely agree with you Pallavi.When you exercise for more than 20 minutes, your body starts to get concerned that it will run out of the regular energy source. So, it starts breaking down fat to supplement that energy. If you exercise for less than 20 minutes, your body most likely has not started breaking down fat, no matter how many calories the LCD screen displays. Getting your heart rate up for 30 minutes instead of 20 mean most likely you are burining a good amount of fat for 10 minutes of your workout.