There is such a thing as negative pranayama which is practiced by some ascetic. Duryodhana the villian of the Mahabharata was an expert at it. This is why he was living at the bottom of a lake without air for weeks after he lost the Battle of Kurukshetra. What I teach has to do with increasing fresh air and reducing used air. The negative pranayama has to do with decreasing fresh air and increasing pollutants. It is a practice but I do not teach it. Pigs thrive on eating what humans evacuate as food waste, so we cannot say that it is not nourishing. Stool is nourishing but if you are not aiming to get the benefits of eating it as the pigs do, then there is no point in eating it.