Fasted cardio isn’t just cardio on an empty stomach. It’s cardio done in a fasted state: when your body is no longer processing food and insulin levels are low. It’s typically done first thing in the morning before breakfast, since you can be certain you will be in a fasted state having not eaten for 8+ hours.
The most common reason to do fasted cardio is if your goal is to burn body fat. Because you’re exercising without fuel, your body uses the fat which it has stored up to provide you with the energy to workout, and therefore this leads to fat loss.
However, there are a few downsides to fasted cardio. Research shows that the longer your cardio sessions are, the more they impair strength and muscle growth. Your body will start to burn protein which will come from your muscle mass. You can combat this by taking BCAA’s (branched-chain amino acids) before you work out to essentially shield your muscles. It’s also useful to pair resistance training with your fasted cardio in order to preserve muscle mass.
Another drawback is that your workouts won’t be as high a quality as if you were to do a cardio session after fuelling your body with food. Especially when you first start doing fasted cardio, you may feel less focused and like you have less energy.
Remember: in order for fasted cardio to be effective in burning fat, you need to pay attention to your diet. Make sure you get in your carbs, such as potatoes, whole-grain rice and bread, fruit and veg, and proteins like milk, eggs and chicken, as after fasted cardio your body will better absorb these - protein and carbohydrates will go back into the muscles to help them repair and refuel, instead of going into fat stores. In order to burn fat, no matter what type of exercise you’re doing, you also need to be in a calorie deficit, which means you need to consume less calories than you’re burning.