Spicy food is well known to cause diarrhea in those who don’t partake regularly. OTOH, I imagine that people in regions where spicy (and very spicy) foods is the norm would have regular stool. What’s mechanism for this? Why does spicy food cause loose stool, and why do people who eat it regularly have regular stool?
Actually, although you get used to it to some degree, it is pretty common for people who live in spicy-food regions to have diarrhea on a regular basis. I live in a spicy food area and have not been particularly solid for years. You get used to it.
Spicy food causes diarrhea? Cite? 
Actually… a cite would be nice, lest we are begging the question.
If it has medical basis, it would be on the idea that capsaicin is an irritant, and your body would move it through your digestive system faster. Faster movement of digestible material through your large intestine, and colon, would reduce the amount of water absorbed in that transit, leading to a looser stool.
I grew up in a spicy-food family, the rest of whom were born and raised in a spicy food area, and I think you may be assuming that correlation equals causation here.
Water quality in many (most?) spicy food areas is extremely poor by Western standards (even by Florida standards, and our water is awful). Now, most people don’t drink tap water, but you can’t help swallowing small amounts while brushing your teeth, showering, and so on. I would hazard that this is far more likely to be the cause of regular loose stool (no pun intended) than seasoning.
Certainly in Cameroon. Not sure what the water quality is like wherever you are in China.
This is all pure anecdote, but when I traveled to Tibet and began eating a blander diet, I felt like something was wrong with my system. Then I realized- I was having solid poops for the first time in over a year! I’m pretty familiar with water-born diseases, and usually these produce acute can’t-leave-the-toilet diarrhea, which I had in Cameroon all the time.
But what I’m talking about is a sort of general, daily, runniness paired with occasional and usually pretty manageable urgency, which pretty much everyone I know in Southwest China experiences pretty much all the time. A friend of mine is in the hospital with an anal fissure right now, likely caused by diarrhea. Most of us are pretty careful with our water- we get water distillers, we bleach our vegetables, etc. But the diet is really something else. It’s almost a sure thing, for example, after a night at my city’s unholy spicy duck hot pot that you’ll spend a good chunk of the morning on another kind of pot.
I’ll freely admit that it may be the oil, though the aforementioned duck hot pot is not particularly oily.
Some afferent gut neurons seem to be very capsaicin-sensitive, especially if they’re not regularly exposed to this molecule which is responsible for the heat of hot peppers. Stimulation of these neurons by capsaicin results in more rapid transit time through the bowel, and hence, diarrhea.
An added bonus of rapid transit time is incomplete digestion and absorption of the capsaicin, resulting in the mucus membrane tissues of the terminal gut getting exposed to the same fiery treat. Net result: Burning in, burning out. The latter is also known as “wolf-ass”.
I beg to differ.
I am an indian living in India and eat spicy food . Diarrhea is caused by unhygienic food,not by spices .
In India hotels catering to foreign tourists, do have special menus with minimum spices.
Capsaicin irritates the nerves as described below:
The toxin from E. Coli activates the same neurons in a similar way:
So you’re both right.
But most folks get used to the capsaicin effect with repeated exposure, and don’t have the inflammatory response in the gut anymore. I know I got used to it.
Locally we call it the “Ring of Fire.”
I also live in China, and my opinion is that it is the oil.
Everything seems to be cooked in oil here.
Even the vegetables. Most Chinese fry their vegetables.
Most hot pots, especially the spicy ones, are very oily, IME.
All that oil just makes everything shoot right through you.
I have embodied “serious Wolf-Ass” before. And just for today too! 
Unfortunately with my Acid reflux getting worse over the years I am going the other way and having cutting back on spices, because waking up with stomach acid in your lungs sucks big time.
But hopefully with my weight losing the Acid will become less of a problem and I can go back to Habaneros.
Refluxing capsaicin is even worse, as it does not respond to acid neutralization. I’ve cut out spicy foods close to bedtime as a result. Having essence of habanero flood my nasal passages at night is even less happy than a case of serious wolf-ass.
All of this is anecdotal. But I ate spicy Indian food all my life and never had a problem. In my twenties and thirties I have cut way way down on it and now sometimes I have problems if I eat too much of it. However, it could just be a facet of getting older.
Hm. That would explain the ‘physiological imperative’ after eating tempura or some Vietnamese foods. (I’m not affected that way with Chinese or Indian food.)
My cite is… “Chipotl-away”
I can handle my Scoville units… going in, through, and out!
But can you handle them on your manhood? Capsaicin-induced urethritis has been described in the literature, and not in happy terms.
Heh. 'Round these parts, it’s known as ‘fire in the hole!’ 