So, I have done both. I used to think that if I worked out on an empty stomach I would pass out & waste away my muscles. That is not true at all. It takes adjusting, but you will eventually not feel like total crap & actually feel better than you’ve ever felt during your workouts. More energy, deeper breathing & of course, more fat burning.
Losing muscle. Do not worry about losing muscle, because in a fasted state you begin to produce what is called BHB (beta-hydroxybutyrate). BHB prevents the oxidation of Leucine (an amino acid). When Leucine is not being oxidized, you are not losing muscle.
The science of fasted training. The bro science behind this says that since you have gone a certain amount of time without any calories, you will quickly burn through your glycogen stores leaving you with just fat to burn. This isn’t exactly true. A study was conducted comparing two groups; one group trained while fasted & the other after consuming a certain amount of carbohydrates (Click here for study). Intramyocellular lipids (IMTG) are little fat molecules which live in muscle. In the study they took biopsies of muscle from each participant & they reported that the fasted group had a reduced IMTG level as low as 6% whereas carbohydrate fed group showed no change. The level of glycogen in their muscles were exactly the same in both groups. This demonstrates our body’s amazing capability to efficiently preserve muscle mass & energy in exchange for fat. When those IMTGs are utilized, your body must replenish the stores with your own body fat; which means your stubborn belly fat is utilized to replenish your IMTG stores in your muscles. On top of all of this, they found that those in fasted state could replenish their glycogen stores 3x faster than those who were fed! Because of higher insulin sensitivity, nutrients are absorbed & partitioned much more efficiently when fasted rather than being stored as extra body fat.
Fat burning machine. So, this is the natural cycle of burning fat during fasted workouts. Bottom line, you burn more fat & cling on to muscle in a fasted state compared to a fed state. This study was done with subjects who fasted only overnight, nothing crazy. In the absence of food, your body increases its production of glucagon (the opposite of insulin). Glucagon promotes the formation of the phosphorylated hormone sensitive lipase (pHSL). pHSL is the active form of HSL which functions to burn fat. Simply put, your body has more fat burning stations all fired up & ready to go. Couple that with the increase of mobilized IMTG’s, & you have a natural, fat burning machine.