I'm not familiar with much of the rigorous science on gut stuff, a lot of it is fairly new. I will say that the Cue Action Reward model of habit formation works the best when you use the strongest cues and rewards, i.e. sex or food. I started training fasted a couple years ago. I didn't realize that I was reprogramming my cue action reward from

• cue: hunger
• action: look for food
• reward: calories

to

• cue: hunger
• action: go exercise
• reward: a ton of calories

to the degree that when hungry my first thought is now to go exercise.