--- > The type of fuel your body uses to create energy changes when you increase your intensity. In zone 1 and 2, the main energy source used is fat. In zone 3, 4 and 5, you’re using carbohydrates as a primary fuel source. - [View Highlight](https://read.readwise.io/read/01has6wv18remqg5bsvx6b2bfs) --- > Many runners train at a pace that is too high for their current fitness level. Their training intensity is in zone 3, 4 or 5, so they primarily use their limited supply of carbs for energy instead of stored body fat. > Heart rate training flips the script on that method, showing runners it’s okay to run slower. In fact– almost all of us are running *too fast.* Slowing down leads to better fitness, less injury, and faster race times in the long term. Over time, it allows you to build a healthy and sustainable relationship with running instead of the constant crash-and-burn cycle that most runners experience through too much high-intensity exercise. - [View Highlight](https://read.readwise.io/read/01hashvbr7n123d9pn0500vnzt) ---