Its one of those things that I dont think is just as simple as “if you do fasted cardio you’ll burn fat, or if you do fasted cardio you’ll lose muscle.”
There are many other factors that are going to affect the outcomes.
I think an extremely important part of the whole equation is carbohydrates and BCAA’s. They both have a protein sparing effect, but the Carbs will potentially take you out of the fat burning phase.
At any given time, if you are in a deficit, you have to assume you are burning both fat and muscle (or protein). You can alter this ratio but you can never really eliminate one or the other. The only goal should be to burn a higher percentage of fat than muscle. If you are burning 75% fat, and 25% muscle, and when you are building you are building 75% muscle, and 25% fat, then you are moving in the right direction.
So the goal of any cardio would be to alter the ratio of fat:muscle loss in a deficit. The trick IMO is though that in any given day you may be in a deficit at times, and a surplus at other times. This is why it is possible to build muscle and lose fat at the same time. Sure not the same instant in time, but the same day.
From what I understand, in the morning, when you are fasted, cortisol is high, and the need for carbohydrates is elevated. You could probably almost argue that doing anything regardless of intensity is going to cause muscle loss with fasted morning cardio. But the intensity will likely affect the rate and ratio of fat:muscle loss.
There are probably a few different options that would work quite well. You could do a shorter more intense HIIT style workout, but drink a shake during or right at the end of the workout containing CARBS and BCAA’s, or you could do a longer lower intensity steady workout and avoid the carbs during the workout, and then move into normal eating after.
I know im kinda rambling on here, because I dont clearly understand this entirely, but its one of those things that nobody probably truly understands.
We all know that in training there are so many different methods that are effective, and I think nutrition is the same way. Im sure a lot of top nutrition specialists like Berardi would advice against stuff like fasted cardio, and intermittent fasting, etc. but you cant deny the results people have been getting for years even decades.
***What got me to give fasted cardio a second thought was a transformation log I saw on bodybuilding.com . It may or may not be real, but the guy got amazing results in a very short time. (i’ll try to post it after if I can find it. He seemed to imply that he felt fasted cardio was a big factor in his success.
Another interesting thing he did, was perform his resistance training after his fasted cardio. I could see how this would work if you payed attention to the timing of nutrients. A 30-60 minute fasted cardio session, followed by a carb + BCAA shake during a resistance training workout would probably burn a good amount of fat, and if you continued to fuel recovery after the workout you could probably hold onto a large amount of muscle while burning a large amount of fat. I’ll post up that link if I can find it though.[/quote]
Im not trying to debate whether FASTED cardio is catabolic or not. Im just trying to discuss whether or not HIIT (not fasted) is catabolic while on a keto diet.