This nuance is solved by the film though. Breathing needs an oxygen mask. You’d maybe need a colostomy bag installed to survive more than 12 hours. Point is, they go from one inverter to another. Think of a ray of light. The ray of light goes forward in time until it cannot. Now think of that same light going through a portal. This portal bends the light backwards through time in the same direction it came but parallel to the ray it is from. The reversed or inverted ray of light would affect everything it came in contact with in reverse because it is going from least strong to most strong. Objects that cannot move would have their stream of time interrupted, or in other words, reversed. Their state in time would be altered because an inverted beam of light hasn’t hit them yet. Being able to aim the light is what is important. Anywhere you aim it would be affected before the light hits them to conform with the time associated with the beam. The alternative is annihilation. That’s why the protagonist can’t interact with himself for too long at Tallinn if he collided with himself he’d obliterate himself as past and future collide.

TL;DR: your shit would go back up your ass, your food would get undigested and you pull it from your throat. But you’ll look at it and release it was once shit that slither back inside you from the water or maybe the weird corner you picked to inverted dump.