Training: Body Use Fat As Fuel Source.
There’s an 80-20 rule with training. About 80 percent of your training should be in an easier zone. Only a small fraction—20 percent or even less—should be at higher intensities to maximize results – Matthew Laye.
At different heart-rate zones, your body uses different fuel sources to create energy for your cells, which drives particular reactions in the body. For example, at lower heart rates (Zones 1 and 2), your body primarily uses fat as a fuel source for your muscles. (Burning fat is also more energetically efficient than burning glucose; that, along with the size of our fat stores, is part of why we can perform Zone 2 exercise for so long.) At heart rates that take you above your ventilatory threshold, or VT1 (typically Zones 3, 4, and 5), when your breathing rate increases along with your heart rate and you can no longer talk comfortably, your body starts to use more glycogen (stored glucose) from your muscles to supplement the fat.