eli5 – if it takes 12 hours to digest food, how come eating certain foods can cause an immediate bowel response?

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eli5 – if it takes 12 hours to digest food, how come eating certain foods can cause an immediate bowel response?

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Your body is talking to the brain all the time. The tounge says “hi brain, I just tasted a LOT of caffiene get ready” and your brain says “thanks, that probably means I need to stay awake to make sure I can run away from predators. I can do that easier if I have less poop to run with. Hey bowels, any poop to get rid off?” and the bowels go “yep, I’ll start getting ready to poop”.
Also, 12 hours is a good estimate for SOME food to be FULLY digested, but different foods will go through your stomach quicker and be mostly digested in your intestines while some foods stay in the stomach for a long time

It depends on what it is. Compounds in coffee stimulate muscles in the GI tract, some studies say it’s a coincidence, others that it stimulates acid production in the stomach. The actual answer is a bit unknown and depends what source is used.

For things like spicy foods or alcohol which have disproportionally fast responses it is because your digestive system is connected. Normally, liquid is absorbed in the large intestine. If something disturbs the entire system like in the stomach it kind of hits the “[We need to drop the load](https://youtu.be/LympZCCRRCU?t=8)” button. This pushes the entire system along and evacuates whatever was being absorbed in the large intestine for immediate release.

Immediate release is beneficial if something is irritating since that means the GI tract has space to move it along as fast as possible.

Sometimes your body decides that it needs to get rid of whatever is in your bowels NOW. It can be due to illness. It can be due to physical distress such as bloating or irritation. Whatever the reason your body can and will trigger the muscles in your intestines/colon to go in to emergency evacuation mode. Normally when you digest food it takes time and the body can slowly draw the moisture out. The reason diarhhea is more liquid is because there wasn’t enough time for that to happen plus the body may be releasing extra liquid in to your digestive track to help facilitate the evacuation of your bowels. Thats why its a good idea to rehydrate, your body has just wasted a bunch of water it normally wouldn’t.