[Scott Johnston]

Upwards:

Fasted training still has its place, but as I have maintained for years, it will be most beneficial for low-volume trainers for whom the added stimulus of low glycogen will enhance the AMPk signaling pathway, which is one of the main paths for upregulating many of the genes responsible for aerobic adaptation.  These lower-volume trainers will have plenty of time between sessions to restore glycogen levels.

For higher volume trainers, like you seem to be, the benefits of fasted training may not outweigh the drawbacks.  High-volume trainers, those doing in excess of 8-10 hours of aerobic work in a week, are already giving a big stimulus to the AMPk pathway.  Low glycogen levels will delay recovery and if this condition persists due to too much fasted training, you risk overtraining.

Using fasted training in your base period where intensity is kept low and doing, say, the first 1-2 hours of a long run in a fasted state before beginning normal fueling are strategies used by Tour de France riders and some of the top triathletes in the world.  I have this information from personal communication with those athletes’ coaches.

As with so much in life, nuance plays a big role. Blanket statements concerning training or physiology when speaking about as complex an organism as  humans seem short-sighted.

Scott