Fasted cardio is simply exercising without any food in your body.
The first problem with this statement is that in this age of plenty, not many people are ever truly fasted as their most recent meal probably wasn’t that long ago (even if they’ve slept!).
The second problem is that if you’re reading this then you’re probably interested in some sort of quite intense training. Science tells us that the harder we train (higher heart rate, moving faster, not able to talk) the more we need energy quickly, or we’ll slow down and not be able to maintain that intensity. If you haven’t got immediately accessible fuel and you’re relying on last night’s meal, it’s a slower process to provide energy for what you want to do and you’ll probably stop sooner rather than later.
Thirdly, there’s a difference between a meal and a light snack in terms of how it feels in your stomach when you’re exercising, so don’t try eat a heavy meal right before training.
There’s a grain of truth in this whole idea, but like so many things around diet, people have missed the wood for the trees:
If you don’t have any food in you to burn then you’re body will go to look for stored fuel (fat and muscle) to burn for energy BUT the overall intensity of the session will undoubtedly be lower than if you were fed so you’ll not be burning as much energy for the hours (and days) after the session.
So you can burn a bit more in the short term, but less longer term, by being hungry…
Or you can fuel yourself to be able to train harder and reap the benefits of a larger burn over a longer period of time.
All that being said, if you feel ill when you exercise on an empty stomach you should eat before training, but if you feel more energised then keep doing what you’re doing!
The amount you’re eating over the day and the week matter more than an individual meal.