Corisol sucks. It’s realised in to your body from mental and physical stress, but here’s the kicker: it leads to the breakdown of muscle for energy and prevents fat burning. So not good.
How do you prevent it?
Avoid crash diets (as well as Glucagon wading in, cortisol will join the party).
Get enough sleep (cortisol levels rise if you are short of sleep).
Train hard for more than two days and your body will give you a good shot of cortisol.
Now you know why they bang on about getting enough sleep to grow big quickly…