We were talking about shytting at the gym the other day and the feeling   was to try to  avoid it or at least don't make a habit of it ....  why? because a common theme, was religiously taking a shyt right before leg training  and then eventually  developing some kind of ass related issue,either anal fissures or rhoids. One poor guy actually got a blood clot in a rhoid after he had took a shyt and started his leg training doing squats with very heavy weight  and ended up on his belly for a week in serious pain, had to go sick from work . Another guy, this was a policeman which I suspect will give some people a guilty pleasure... his ass actually exploded when he was attempting a max squat, he had just taken a shyt right before and his a$$hole obviously could not take it. The floor by the squat rack looked like the floor of a meatlocker. 
I understand liking feeling 'empty' and there is nothing worse than feeling like you need a shyt while training, but my advice get into a habit of shytting first thing in the morning and not hanging around on the toilet. If you are a serious trainer, you will be putting strain on your ass muscles like every where else, plus all the supplements can have effects on shytting as well. Be kind to your a$$hole.