Is it necessary to evacuate bowels before any type

Is it necessary to evacuate bowels before any type of yoga practice including meditation. I have been practicing advanced yogic postures and am student in meditation but my bowel evacuation takes five hours and minimum ten cups of tea after leaving bed. This jeopardise my entire practices. Taking ten cups of tea before advanced yogic practices never harmed me but there is no peristalsis till late in the fore-noon. Nothing works. My age is 67 years now. Bowel evacuation before meditation will defininitely increase concentration as per your version but is it advisable not to try to mediate for want of bowel evacuation and leave thereby braham muhurtha period for advanced yogic practices and meditation. I have searched entire net for my quest but no answer except to evacuate bowels before yogic practices and meditation. Please give me an authoritative answer and oblige.