whenever you starve or excercise the physiology and physiol. chemistry switches to katabolic: gluconeogenisis from even aminoacids, from lactate (Cori's-cycle), etc. and a-Hydroxbutyrate-Prod. from Lipids Beta-Oxidation.
Therefore for that time muscle growth should stand still for a while, all hormones support this feature in all organs, even in those, we do not know by now, what hormons influence them and their peptide sequence.
Shortly: When five lions identify you as THE delicious dinner of TODAY, it is wiser to release all energy possible, than to make muscles and fatty tissues grow. ;-)
On the other hand myostatin release is HIGHER following appropriate exercise compared to "couch potato"-ed muscles at rest, that is the physiological hormone explanation of exercise-rest cycling in rehab. or athletes training.
(Sports Med. found out earlier than description of Myostatin ;-) )