"If fasted state cardio could potentially increase fat mobilization, it’s also potentially more catabolic to muscle tissue. This is due to an increase in cortisol production during fasted exercise. Since cortisol levels are already high in the morning, this could lead to more muscle wasting than during non-fasted cardio.
In fact, cortisol levels could increase muscle breakdown and the use of amino acids as an energy source. This is especially true if high-intensity energy systems work is performed. If an individual uses lower intensity (around 60-65% of maximum heart rate), the need for glucose and cortisol release are both reduced and thus the situation becomes less catabolic." Christian Thibideau
We can agree to disagree. Plenty of articles on this very website with studies that say fasted exercise is NOT necessary to burn fat and not even more effective for that matter. Your diet (overall calorie deficit) is your best tool to burn fat. The weightlifting is the stimulus to hold onto the muscle. Additional steady state low intensity cardio can be used to further the deficit and you can burn more fat that way,
The experts disagree