There is no cognitive switch by the body on what type of fuel it uses. It is strictly a supply and demand issue and these processes can (and do) run concurrently. If the body needs energy faster than you can oxidize fat then it 'will' turn to the next available source (muscle). The brain can become accustomed to using ketones but will never 'prefer' them over glucose. The same with the skeletal muscles. If demand is low then fat is fine. Ask for too much too quickly and lean mass will suffer. If you start the day with depleted glycogen and choose to run stadium stairs for an hour on an empty stomach you 'will' loose some lean mass (along with bodyfat). Can you spare it? Just my $.02

Lipo