Ignoring water and focusing on the solid mass, more than half of your feces is composed of bacterial corpses and waste. The rest is about one quarter plant matter you couldn’t digest mixed with some complex carbs that weren’t fully broken down, and the remaining quarter is divided between protein and fat that wasn’t fully digested/absorbed; the exact fraction depends a lot on the person and their diet.
Back to the top though, healthy stool is 50%-75% water, so really your feces consists primarily of water, bacterial biomass, and indigestible fiber. Even if the remaining bits could be *perfectly* digested and absorbed, you’d still have the same need to poop.
Simple answer: No it’s not possible. There’s a lot more going on than “food that our body couldn’t digest or use” going on. Even with “human optimized” foods like Soylent or its EU variant JimmyJoy you still have waste, with the addendum of making your stomach and intestinal tract lazy because you don’t eat solid foods making “going back” to real food an actual pain.
We are not able to get to 100% but there have been a lot of research into low residue diets. This is used as part of some medical therapies as well as by the military. It mainly consists of reducing the intake of fiber as well as a few other adjustments. And you are right that it is not something you would want to maintain for long periods of time. But almost all food have at least some fiber, cellulose, or other hard to digest substances. And if the food does not have it the bacteria in our gut will make these things. So it is impossible to have a diet that reduces your feces to nothing.
You will always need to poop.
People still poop even when they are on [Total Parenteral Nutrition (TPN)](https://ameripharmaspecialty.com/total-parenteral-nutrition-tpn-faq/). TPN patients can no longer digest food at all, temporarily or permanently. Instead of eating or drinking, they have a nutritional solution injected directly into a vein. Some patients do some oral food with the TPN, but many are not putting any food or drink at all into their mouth, stomach, or intestines. And yet they still occasionally poop to get rid of waste products like dead cells.
It always perplexes me when people claim that meat is difficult to digest, in the context of “I’m not feeling well, I’ll eat oatmeal instead”. It’s actually quite the opposite. If you ate nothing but meat you’ll still defecate, but your body is so efficient at digesting it, there’s much less waste and you’ll only go every few days. And no, it isn’t constipation.
When I was breastfeeding my brand new infant, she didn’t poop for like 2 days. I was getting worried, but a lactation consultant (who was also an RN and had 6 kids of her own) told me not to over worry because the breast milk was being completely digested, so there was not much left to poop out. The next day she pooped (much my relief).
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