We wash our skin to get rid of greasiness and oils, but we can’t wash out gastrointestinal tract in the same way. How do our bodies remove that “greasy” coating from eating greasy foods from our GI tract?

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When you eat something greasy, you can feel it on your lips and around your mouth. That can get washed away if it’s external to your body with soap and water. What I’m dumbfounded by is how our bodies can seemingly “wash away” that greasiness from our mouths, throat, stomach, intestines, etc… How is it done?

In: Biology

As with soap and water, it’s chemistry most likely.
Washing with soap, if I recall correctly from high school chemistry 3 decades ago, leaves some sort of open “handle” for things to grab on and hold while the solvent (water) cleanses the offending substance away. There may be something to do with changing the surface tension of the water itself as well, if my memory can be trusted.
That doesn’t answer your question though: if I remember all of that with any semblance of correctness, you’ve got to give the grease something to grab onto, so that natural liquids act as the solvent to expel the waste. I hear that intestinally, oatmeal does a great job “rotorootering” your digestive tract. Not the little pouches with chemically flavours you nuke up in a minute, but the real boil it to just this side of paste stuff that generations of folks have poured all types of sugar and milk on to slurp down.

Soap solubilizes fatty substances and water washes it all off (soap is soluble in both, due to it having hydrophobic and hydrophilic parts). Our GI system is similar, it has enzymes that break down the fatty molecules into simpler smaller parts (not much though) and then bile is secreted into the duodenum which solubilizes those components similar to soap, then they get absorbed into the blood (the epithelial cells process the fat and package it into spheres where the wall is made of protein, cholesterol, phospholipids, etc, called chylomicrons, which go to the blood and are processed further by liver and so on). We also have peristaltic movement, which is a rhythmic mechanical wave of smooth muscle contraction that goes from your esophagus all the way to your colon, and this, along with whatever the GI tract secretes (saliva, acid, bile, enzymes etc) moves along the food to your rectum.

Your GI tract washes itself with acid. The contents of your stomach are so acidic that your body refreshes the lining with new cells quickly, a whole new lining every 5-7 days.

In the GI tract you don’t need to wash it away: the acids and enzymes are capable of breaking down most of the fat and grease. And the bits that can’t be completely broken down just get flushed out without any extra effort, since the ever-regenerating slimy texture of the GI tract makes it impossible for any excess lipid residue to around for long.