MYTH: Muscles will turn to fat when you stop exercising regularly. 

 FACT:  Muscles cannot turn to fat.  Muscle and fat (adipose) are two separate and distinct tissues.  They simply do not have the physical capability to change from one type of tissue to another.  In reality, muscles have the unique property of “use it or lose it.”  If you do not use a muscle, it will literally waste away (atrophy).  This process is perhaps best illustrated when someone has to wear a cast on a broken leg.  When the cast is removed, the relatively unused leg muscles are considerably smaller than they were prior to the injury. If muscle could turn into fat, you should see a veritable “fat ball” when you take the cast off of an injured limb, not a significantly atrophied set of leg muscles.  International Sports Sciences Association (ISSA) 8th Edition Text

On the other hand, fat cannot turn into muscle.  The cellular structures are completely different.