I get asked this question a lot, so here’s my take on this based on research as well as personal experience. In short, the best time to do cardio is when your glycogen stores are empty. How cryptic is that? Well, let me explain.
What is glycogen (pronounced glahy-kuh-juhn)? Glycogen is the body’s main source of energy, and it’s created in the body as a result of the digestion of carbohydrates. Carbohydrates are broken down into glucose, fructose and galactose. Glycogen is mainly stored in the liver, but is also stored in skeletal muscles and the brain.
With this being said, I repeat that the best time to do cardio is when your glycogen stores are depleted. When you workout your body will use glycogen for energy. However, in the absence of glycogen, the body will use fat for energy instead. For this reason, doing cardio first thing in the morning before eating is ideal. On the other hand, if you combine your weight and cardio workouts like I sometimes do, then eat a small, balanced meal 45 minutes or so before your do your resistance workout and do cardio afterwards. This way, you will have the energy for weights and will deplete glycogen during your resistance training. Your cardio workout will then use any remaining glycogen and switch to fat for energy.