Partially correct. A percentage of the shit is dead RBCs, so even if you manage to eat optimally, your body will still have to get rid of dead RBCs and they tend to die out frequently. One way that you can in theory test this would be to give someone an IV injection with all the vitamins and minerals they would ever need to exist comfortably. So in essence, they will be put through a fast. Therefore, with all the nutrients entering the body via bloodstream, bypassing the digestive system, it is possible if there are bowels to be voided then it would possibly be RBCs. If I am incorrect, then the person would not shit at all until they began eating food and started shitting out indigestible components + bile.