Regarding the mass and energy equation, and disregarding intestinal nuclear reactions:
mass_in = mass_out + mass_stored
energy_in = energy_out + energy_stored
food + water input + oxygen input = pee + poop + sweat + CO2 output + weight_gain
food energy value (when burned with oxygen) + work_input (massage therapist, etc) = (poop + pee) energy value + work_output + waste heat
I hate to ask this, but have any of the diet and nutrition experts made people poop into a calorimeter and burned it to see what the output energy was? If not, this thread has enough people for a rough stab at a study. As we cook our poos and record the results with an iPhone ap we'll write, should we post the inflow versus outflow or keep that kind of private?