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Assume that I have some way to introduce nutrients into the body of a healthy human but I do not have any method of changing their biology. No surgical procedures nor special "drugs" that would affect the normal functioning of a human body.

Would any of the 2 following methods lead to a human not needing to "go number 2"?

  • Getting nutrients in fully liquid "soups".

  • Getting intravenous injections of some form of serum that contains all needed nutrients for the body to be healthy.

If not is there a plausible way a human could achieve this?

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    $\begingroup$ Welcome to Worldbuilding, Makogan! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon Feb 15 at 22:16
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    $\begingroup$ I normally go once a day every day, but one time I went every other day for a period, and even managed 5 whole days without defecating. I'm sure a healthy person with the right type of digestive conditions could manage a week or two, which is a tiny fraction of a human lifetime. Beyond that some type of advanced medicine and special diet would be necessary to stretch out for months or years without defecating. $\endgroup$ – M. A. Golding Feb 16 at 17:08
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    $\begingroup$ Decades ago I heard hearsay about a person with intestinal blockage who eventually vomited feces. Would a recurring version of that be acceptable? $\endgroup$ – Emilio M Bumachar Jul 26 at 11:46
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The tricky problem is bile.

If a person is fed and watered completely by vein (this is done routinely; total parenteral nutrition) the guts can atrophy but they still make mucus and bacteria still live in them. Even a person on TPN must poop occasionally.

A person might have all of the intestines removed and have the stomach be a dead end - sometimes that can happen with an obstetric catastrophe or certain tumors. The remaining problem as regards excretion is then the liver. The liver makes bile as part of its job and it is not going to stop. That bile has to go somewhere or it builds up in the liver; you get jaundiced and then eventually die. If you do not have intestines and are living in TPN there must be some route established for the bile to drain. One way is to have a drain placed.

Your OP precludes surgery of this sort and so as posed, the question is no. The guts are not going to completely quit making stuff while they are still alive.

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Yes.

People can live without colon, or small intestine, or stomach (however, there is probably no case of anyone surviving without all of them). In those cases, digestive system gets a "shortcut", which results in more frequent and more liquid "#2".

With fully liquid "soups", some material would still has to be excreted from the other end of the system. With IV injections, excretion is not necessary, so, your goal is achievable, at least in theory.

However, there are 2 practical problems:

  • We can not just leave digestive system non-functioning and presume that it would not cause any trouble. Some maintenance (or surgical removal) of stomach and intestines would become necessary;
  • Long-term IV feeding is unsustainable at the current level of medical science - people develop problems with veins, infections and liver failures.
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  • $\begingroup$ Do people develop these medical issues because of the lack of stimulation to the digestive system? or is there something about the contents of common IV serums (e.g lack of certain proteins) that could theoretically be replaced assuming an advanced enough technology? $\endgroup$ – Makogan Feb 15 at 22:54
  • $\begingroup$ @Makogan there is probably no problem with feeding solutions. Problems are because we need a catheter to plug to a live vein; and the liver (a vital organ we can't dismiss) which has a direct connection to digestive tract (what Willk had mentioned in his answer). $\endgroup$ – Alexander Feb 15 at 23:39
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The real question is, for how long?

Short-term: Sure. A few days or even weeks is doable and shouldn't cause any lasting harm.
Long-term: Only if you have to. There are consequences to your health.

A friend of mine lived for years with nothing but a central line and drinking water.

She had profound medical problems that caused her gut to shut down. She could not digest food and she got down to the 80-something pound range before her doctors finally took action.

A central line is an IV. It's just like a regular intravenous line in your arm (or sometimes leg) except it's placed (usually) in the chest. A PICC line is another possibility. It starts in the arm and is threaded further toward the chest.

These types of IVs allow larger volume of materials through them. The downside is they're harder to put in and they run a high risk of infection. But they're pretty stable. My friend could tie off her tubing, cover the site well, and go out, even do sports (non-contact sports like kayaking).

She was fed with all the macro and micro nutrients she needed. She drank water to provide additional fluids (and because it's really awful not to have any water as your mouth and throat get uncomfortably dry).

Periodically, she had to take oral antibiotics to kill off bad bacteria growing in her gut. It is not healthy to go without oral food that long. She would follow the antibiotics with probiotics. She also had to eat small amounts now and again so her gut wouldn't completely shut down.

Remember, your gut is the main component of your immune system. So, sure, you can live off of IV fluids but you need a functioning gut.

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