The idea behind fasted cardio is that you exercise first thing in the morning before you eat anything in order to burn fat instead of glycogen. The concept is sound but let's take a look at the science behind it to see if it actually works. Fasted Versus Fed Cardio We're always told how important it is to eat before you work out; this is so you have plenty of glycogen to fuel your workout. The issue here is that during fed cardio, you're burning carbs. When you exercise during fasted cardio, you're burning fat. That's not theory, it's fact. Fasted cardio helps your body become efficient at using fat for energy. Also, your metabolism and hormones are all in the perfect balance to promote fatty acid mobilization. Here's the science behind why fasted cardio burns more fat. Growth Hormone Levels are High Through the night, your body is creating growth hormone, which has a direct negative correlation with insulin levels. In other words, when insulin is low, growth hormone levels are high and vice versa. Growth hormone is extremely catabolic to fat cells and effectively breaks down fat. Your Insulin Levels are Low Since you haven't eaten in 10 hours or so, your insulin levels are low which allows for the mobilization of fat. When insulin levels are high, such as they are after you eat, this can't happen. Your Blood Sugar is Low This can cause problems for you during exercise especially if you're diabetic. If you're not, you may have a bit less energy but if you don't have a readily-available supply of sugar in your body to use as fuel, there's no other option other than to resort to a back-up source of fuel: fat. All of these conditions add up to make your fasting body a fat-burning machine. Exercising before you eat also trains your body to burn fat more efficiently. There are a few different ways that you can switch from burning glycogen to burning fat and exercising aerobically and this is one of them. Fasted cardio actually burns more fat than fed cardio.