The fat-burning zone is 60%–70% of your maximum heart rate
(220 minus your age). For optimal fat-burning results, you are
advised to keep your heart rate within this range for more than
30 minutes.
10.6.10
Do I have to keep my heart rate
within the fat-burning zone in order to
burn fat?
The body burns the greatest percentage of fat when your heart
rate is in the fat-burning zone. If you increase your workout
intensity beyond that range, your body will burn a smaller
percentage of calories from fat. That's why we recommend
keeping your heart rate within the fat-burning zone for best
results.