because when you eat your body uses up energy to digest that food. 50% of your energy everyday is wasted digesting food. in your case you are transferring that energy to other things
that is why intermittent fasting people say they don't get sleepy in the afternoon and can sleep less at night and still feel rested anyway. when you over eat like 90% of guys do you end up being one of those perma bulkers in the gym who are 'big' and can 'lift heavy'.. but they look like shit
also ADRENALINE. we evolved to have energy while we had to find food. we didn't evolve to find food then have energy
it is funny to me how you accidentally found this out, now you can simply use this new thing as a weapon. unfortunately most people on these forums and in the gym have no fucking clue about this concept. they just know how to stick steroids in their ass and eat a lot. but if you listen to our podcasts we actually talk a lot about all this.
When i prolong fast for days or weeks I actually have my BEST WORKOUTS for those reasons.
so my advice for you starting out is to intermittent fast, make sure you hydrate and add some electrolytes to your water, THEN you can workout, then you eat a meal. that is the correct order that you should do it for top energy