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Is it possible with some chemicals as intake for humans to completely convert the food/chemical to energy to survive?

This is similar to when we are talking about clean energy where water and some gases are the only byproducts with release of energy.

Is this possible in real humans that chemicals / food plus chemicals are converted into only energy and water?

And could this water waste then be excreted through body heat?

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    $\begingroup$ Thanks. What about a symbiosis? Is this allowed? The good old (cheap) nanobot or some bacterium that takes care of waste products? Btw what about e.g. hair, fingernails, sweat, skin cells and so on. The human body will technically always excrete something, you don't want this to stop, right? $\endgroup$ – Raditz_35 Apr 19 '18 at 10:33
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    $\begingroup$ "And could this water waste then be excreted through body heat?" makes no sense. If you want to convert mass to energy, then by E=mc^2 a fraction of milliliter would kill your human. $\endgroup$ – Mołot Apr 19 '18 at 10:35
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    $\begingroup$ you are using mixed up concepts: without excretion doesn't mean "expel only water and gases" nor "convert all mass to energy". Please clarify $\endgroup$ – L.Dutch - Reinstate Monica Apr 19 '18 at 10:47
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    $\begingroup$ Food is "chemicals". Everything is "chemicals". The question would clearly benefit from a straightforward description of the intended effect. For example: "I would like to have metahuman characters who breathe ordinary air, drink water and eat ambrosia and nectar, and output only water and carbon dioxide. In particular, I don't want my metahumans to output urea or uric acid, and the ambrosia and nectar must not leave any residue to be output as fecal matter." $\endgroup$ – AlexP Apr 19 '18 at 11:38
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    $\begingroup$ Digestive tract can't digest everything. If a human has swallowed a golden ring, or a ceramic crown, do you want those to be turned to energy/gases as well? $\endgroup$ – Alexander Apr 19 '18 at 17:11
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Kinda, it not like how your thinking, let me explain. In order to never poop or excrete waste you need to first understand the two types of metabolism Catabolism and Anabolism. Basically Catabolism is the breakdown of stuff for energy and anabolism is the construction of stuff needed by the cells. So based on this their are two options, either your humans use up everything they eat in high energy reactions in their biology using synthesis to transform any toxic or unusable compounds. Or they use the strategy that fungi do and absorb food on the outside and not take in the bad stuff allowing them to successfully never poop.

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No.

The human body has many different elements in its composition. Not all of those can become gaseous, or part of gas molecules, at room temperature. For example, the calcium in your bones, or the iron in your blood. We are constantly losing these through excretion and replacing them via food intake.

Some of the elements that do become gaseous at room temperatures are extremely poisonous and we would die if we excreted them this way, such as the chlorine which the stomach uses to make its acid. From the wiki:

Chlorine reacts with water in the mucosa of the lungs to form hydrochloric acid, destructive to living tissue and potentially lethal.


And could this water waste then be excreted through body heat?

In another question in this site, I did a calculation for how much energy a couple bacteria would release if their mass was completely turned into energy. The result is that it would release enough energy to increase the temperature of a human adult by around 1194 degrees Kelvin (approximately +2149F). A bacterium is considerably less massive than a droplet of water. I will leave the rest up to your imagination.

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    $\begingroup$ The body could get rid of chlorine by expelling it as a gaseous compound, e.g. refrigerant R12, CCl2F2 $\endgroup$ – Gary Walker Apr 19 '18 at 18:18
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    $\begingroup$ @GaryWalker R12 is a poison, an irritating agent. It was banned for most uses around the world. $\endgroup$ – Renan Apr 19 '18 at 18:24
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    $\begingroup$ R12 was not banned because it was toxic or an irritant, it was banned due to its potential damage to upper atmospheric ozone. Everything is poisonous if the concentration is high enough, R12 is safe at concentrations well beyond levels that true poisons are safe. LC50 for R12 is 760,000 ppm for 4 hr tests on rats -- this is remarkably non-toxic. $\endgroup$ – Gary Walker Apr 22 '18 at 16:53
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Another factor here: The digestive system has loads of bacteria in it. IIRC something like 25% of your feces is such bacteria. There are also medical foods meant to be extremely easily digested--look over the ingredients list almost all of it is obviously going to be absorbed very quickly (it's basically pre-digested, ready to be absorbed.) I would be surprised if a gram per day wasn't absorbed--but you still defecate a lot more than that when eating such stuff.

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