Thank you for having the courage to speak so candidly about your crap. In his book What's Your Poo Telling You?, gastroenterologist Dr. Anish Sheth calls the pleasurable sensation you describe "poo-phoria."
Poo-phoria occurs when your bowel movement stimulates the vagus nerve, which descends from the brainstem to the colon. The vagus nerve plays a role in several bodily functions including digestion, and regulating heart rate and blood pressure.
When stimulated, it can cause a number of reactions such as sweating, and the chills you describe. It can also drop your blood pressure and heart rate, causing the lightheadedness that "can lend a sense of subli me relaxation," Sheth and his co-author write. If your poop suddenly overstimulates the vagus nerve, it can even cause you to pass out on the john in a horrifying phemonemon called defecation syncope.
The poop authors write that it tends to take a particularly "large mass of stool" to trigger poo-phoria and its vagal-nerve-induced feelings of exhilaration, intense relaxation, and goose bumps. Poo-phoria can be addictive, the authors warn, though they don’t elaborate on how addicts manipulate their poo to make it especially big.
Bottom line: A particularly large bowel movement can trigger the vagus nerve which, in turn, can drop your blood pressure and heart rate, and give you the chills.
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