A lot of people online are claiming to have lost fat from fasted cardio. I don’t think people will ever stop arguing about its effectiveness, I thought I’d set the record straight and leave you both with what I found as far as the research (and there are studies to talk about) and my opinion.
Fasted Cardio 101
The name itself says it all. Fasted cardio is cardio that is done while training in a fasted state. You can do this in the morning (after waking up) or eight to twelve hours after eating. However, you cannot simply do this on an empty stomach and claim you’re training fasted. The sensation of an empty stomach depends on the person- for some, it could be an hour after eating, and in others, three. It takes eight to twelve hours to enter a fasted state, so choosing to do it in the late afternoon post-lunch doesn’t count as being in a fasted state. 
Fasted cardio is said to have originated in 1999 by famous bodybuilder Bill Phillips in his book Body for Life. In it, Phillips describes fasted training as a way to lower blood sugar and deplete glycogen (carb) stores, forcing the body to burn stored fat as energy to power through the workout. Phillips also claims it lowers insulin, which increases the fat burning.  Ultimately, the pre-req for training fasted is being in an actual state of fasting in order to train with low insulin so that the body can burn fat (instead of carbs).
OK, on paper this looks nice, but is there any proof? Let’s find out.
What the Research Says
Researchers intrigued by the concept of fasted training have put it to the test in several studies. In one, training while in a fasted state was shown to increase fat oxidation and lipolysis. In human terms, it increased the breakdown of fat cells for energy (which is the state of lipolysis) and the burning of this new energy by the cells (fat oxidation). The body was able to take stored fat and use it as energy. Make sense?
A study from the British Journal of Nutrition suggests that this type of training burns up to 20% more fat.  Another study (this time from the University of Scranton) also shows it can reduce overall calorie intake for 24 hours post-workout.  One more benefit is that it increases blood flow and fat burning to the abdominals, or, in simpler terms, that stubborn area of fat that many people struggle to get rid of (aka, the belly pooch). 
But here is where the research starts to contradict.
On the one hand, you’ve got one study from the International Journal of Sport Nutrition and Exercise Metabolism showing the benefits of fasted training. In it, one group trained fasted while another trained after eating. While both groups lost weight, the one that trained fasted also lost body fat. 
And on the other hand, fasted cardio was shown to be no better (or different) than fed cardio, burning about the same number of calories. These findings were published by the National Institutes of Health, which compared fasted cardio to post-breakfast cardio. 
Fasted cardio isn’t without its negatives. There are a few downsides to this approach that you need to be aware of.
A major one is that while it does promote fat burning, it sadly also causes you to burn muscle.  This could almost be seen as tragic news for those of you struggling to build (or maintain) lean muscle! Why go through all the effort to have muscles if you’re only going to lose them? Not only that, but muscle has so many benefits outside of strength. They’re highly metabolic, meaning they burn more calories, and keep you from deteriorating with age. While the research shows this doesn’t happen instantly (meaning you’re not going to lose everything in one go), it does lead to less growth and potential loss.
Another side effect is poor appetite control. While this obviously varies from person to person, research shows that fed training results in less calorie intake compared to fasted.  So, if you’re struggling with overeating, consider fueling up over fasting when it comes to cardio.
You also need to consider the purpose of fasted cardio. If you’re trying to slip into a bikini, then it may speed up your efforts. However, if your goal is to be a better athlete or compete in a sport, then the pros far outweigh the cons. Fasted training may backfire on you because you’re burning muscle and running on zero fuel. As sports dietitian Lauren Antonucci explains, “Can I force my body to burn a higher percentage of fat than carbs? Yes, I can. But I think that the more important question is will I see a benefit as an athlete? Recovery from these types of workouts can be very long and not as enjoyable. I’d rather fuel myself optimally and give myself the best chance of recovery. The return on investment [for proper fueling]will make it back to you in how you feel during your workout, how you recover, and over the long term.” 
Does it Work or Does it Not?
Short answer, yes. To a point. You can also try High Intensity Interval Training (HIIT), which also has its benefits. Or, you can just do cardio however you like it and watch your diet. Because at the end of the day, that’s what counts. Training fasted doesn’t keep you from gaining weight if all you’re doing is overeating processed junk.
Personally, I need a little fuel to pump me up before a workout. That doesn’t have to be a full meal; it could be a small snack or a cup of my pre and post workout matcha tea blend (it will be available in my store soon). Fasted training can be done and has its place, but I think my workouts are better when I have some sort of fuel in me. That’s just my body, though. If fasted cardio is your magic bullet, then by all means,, feel free to continue doing. But be aware of the long-term effects. Be sure to follow-up with proper recovery and nutritionto replenish your body! (If you need help on how to re-fuel from post-workouts, check out this post here)
Source: Zuzka Light