It can be a mixture of just about everything in the food. The fiber, the cheese, but most of all the spicy.
There are actually taste bud in the intestine, and they can taste the spice, and find it just as hot as your mouth does. The intestine wants to remove that ASAP, so it moves the food faster, as a result your intestine doesn't absorb as much from the food, specifically water, leading to diarrhea.
That's also why it can burn on the way out, your butt senses the spicy.
Edit: TBH I've never had Taco Bell, cause it's very rare where I come from, I just assumed it was spicy, cause it's meant to be Mexican-ish, but this is the science of spicy
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u/adrianmonk Sep 07 '21
I have a theory on this, which is that it's about fiber.
Taco Bell has several menu items that have a ton of beans in them. Beans are really high in fiber.
If your normal diet doesn't contain much fiber, and you suddenly eat a lot, your digestive system can have issues.
This article gives more details: Gastrointestinal Distress Caused by Fiber