I’m NOT talking about cases when you have the stomach flu or food poisoning and contents come out of both ends all day long.
I’m talking about diarrhea alone, without vomiting. Rather it be from IBS, too much fiber, or whatever. I’m talking about those feelings of nausea you have immediatly before you go to the toilet to poop, during the poop, and immediatly after the poop. But after five minutes or so after the diarrhea episode the feelings of nausea quicly disappear. So clearly the nausea is associated with the diarrhea.
I thought part of what causes diarrhea is the body saying “This is bad, lets flush it out!” So the body works to add a ton of fluid to the pipes to flush them out. That’s probably what feels like nausea.
My two thoughts on the matter:
Serotonin receptors in the GI tract will cause feelings of Nausea as well as increase the GI motility, so one possible mechanism would be simply that the serotonin will cause an upset stomach/nausea feeling, as well as speed up the bowels, so you get both feelings. This can often been seen in various tumors that secrete an abundance of seratonin into the bloodstream.
Also, Osmotic Diarrhea can cause massive dips in fluids and electrolytes if you’re just going and going, and those could certainly trigger changes in the Extracellular/Intracellular fluid levels in the body, including in the CSF/Brainstem, and thus the body would feel the signs and feelings of nausea or a desire to vomit while it struggles to correct itself or figure out another solution.
Your intestines are pumping fast and hard to push things along. This pushing processing happens so hard and fast the fluids cannot be absorbed, thus runny bowel movements/diarrhea.
The pushing of the bowels alone is enough to cause nausea.
I think you’re feeling poisoned, and the nausea and the upset sensation associated with diarrhea are both elements of that. The diarrhea isn’t causing the nausea. Something else is causing both.
I don’t feel like eating when I have diarrhea-esque situations, but neither do I feel nauseated. I’m not in the mood to eat anything, but I don’t feel like I’m about to throw up either. My gut feeling, heh, is that I’m not in the minority on this.
That makes two of us. When everybody in my family is yacking it up, I usually get it the last, and spend all weekend on the toilet instead. (Interestingly enough, my younger son, who is 2, may be the same way. The last couple times we have had stomach bugs come through our house, the two of us haven’t gotten sick, or at least if we did it wasn’t the very noticeable pukey kind. And I highly doubt that it’s because my 2-year-old has superior hygiene to my 3- and 5-year olds.)
Also possibly TMI (and more on topic): when my wife needs to go #2, she will often complain of feeling sick at her stomach right beforehand. We just figure she has her wires crossed somewhere. She at one point was “diagnosed” with IBS (had alternating constipation/diarrhea), but that actually a turned out to be a combination of of celiac disease and celiac-caused autoimmune thyroiditis. My guess would be some people aren’t very good at distinguishing the signals when something wants go evacuate their GI tract, and it all sort of feels the same.