Overnight fasting does leave you in semi glycogen depleted state (dependent on your state of fat adaptation). It will mostly affect liver stores and not much in the muscles (which have been fairly inactive all night). But when you go out for that longer (or high intensity) run first thing in the AM with the lowered liver glycogen levels the muscles’ source of glucose (the liver) is going to be compromised much quicker. That in turn results in depleted muscle stores. The muscles have to find energy some place. So, lipolysis enzymes up-regulate to give more Free Fatty Acids (FFA) which, through beta oxidation produce Acetyl CoA, the precursor for aerobic metabolism. If the glycogen stores are lower, for whatever reason, then you have to find the fuel some place and your body’s natural inclination is to shift to fat. The overnight fasting just allows you to reach that glycogen depleted state a bit sooner in the workout.
From the article:
As you begin your workout, skeletal muscle prefers its own fat and glycogen stores but gets some of its energy from fats and glucose in the blood. The liver breaks down glycogen to ensure that blood glucose levels are maintained to support all the cells of your body. During exercise, a person ‘hits the wall’ or ‘bonks’ when they run so low in liver glycogen that blood glucose levels drop. Muscle glycogen levels will also be low at this point because the muscle has been preferentially consuming it.
These two systems (liver and muscle glycogen stores) are not separate and independent. Glucose liberated from liver glycogen gets used in the muscles. Anyone who has ever bonked hard and then eaten a candy bar can attest to the fact that his muscles suddenly had power again almost immediately. This happened because his blood sugar rose within minutes of eating and the muscles made use of that glucose in the blood. Deplete liver glycogen and muscle depletion will happen sooner.
As far as I can see the above quote negates the author’s earlier assertion that training in fasted state will not improve fat adaptation via muscle glycogen depletion.
This is exactly what we say in many places on this site. And, you do not have to “Bonk” to trigger a fat adaptation response. Muscle fiber glycogen depletion is perhaps the most powerful signal that starts the aerobic adaptation cascade. But……It is only those muscle fibers that become glycogen depleted that see this effect. This may not be 100% true as there is some evidence that nearby fibers give up their glycogen in times of dire need. So peripheral fibers may get some of this aerobic training effect.
As for not eating till the next workout: While that will speed fat adaptation it is going to slow down recovery. In some cases significantly (by days).
His final comments concerning doing a depleting high intensity workout in the AM followed up with a longer low intensity workout in the PM may be fine for one day. But how are you going to feel the next day or two. In my rather extensive experience I have never seen good results come from delaying fueling after a workout. As mentioned above, it will slow recovery and in the end lessen the amount of training the athlete can do.