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Dune (novel) describes a body suit called Stillsuit:

a stillsuit is a "body-enclosing garment" of Fremen design which performs the "functions of heat dissipation and filtering bodily wastes", as well as retaining and reclaiming moisture

For this question, I am interested into reclaiming property of this suit. The book argues about the fact that the losses were virtually only due to exposed skin.

However, I am wondering if the human body is actually consuming (through biochemical reactions) any water or it is used entirely as a dissolvent or maybe as a catalyst), so that its loss entirely depends on the ability of reclaiming it externally.

Question: Does human organism consume water or is it only used as solvent?

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Humans and all other animals actually are net producers of water ("metabolic water") by oxidizing sources of energy including fats, proteins, and carbohydrates - effectively the opposite of the chemical reactions that photosynthetic plants use to turn water and atmospheric CO$_2$ into biological materials and oxygen gas. There are also many metabolic reactions in humans that consume water (in particular hydrolysis reactions), but it is less than the amount produced.

However, water is indeed important as a solvent, and there are non-negligible losses through evaporation (especially when sweating) and an important role of water in the kidneys to assist in flushing waste.

If one was able to reclaim all evaporative losses and excretion losses, no additional water would be necessary. However, the energy requirements to do so go up exponentially the closer you get to 100% reclaimation.

The International Space Station has several water recovery systems to reduce the incredibly expensive cost of supplying enough water for humans on long missions in space. These systems include both urine purification and atmospheric reclamation but they do not approach 100% capture.

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  • $\begingroup$ evaporation from the lungs and urine are both big source of loss, so the suits would do a lot. a suit has some advantages over the space station. $\endgroup$ – John Jun 29 '18 at 21:19
  • $\begingroup$ @John The main advantage would be insulation, in that if you keep someone in a humidified environment evaporation would be decreased, but that doesn't solve the problem of evaporation in the lungs unless breathing is also contained in the suit, and I can't think of any advantages a suit would have over a space station in terms of urine recovery. The advantage the space station has, on the other hand, is that it is large enough to carry the necessary equipment, has available power, etc. $\endgroup$ – Bryan Krause Jun 29 '18 at 21:22
  • $\begingroup$ "The main advantage would be insulation, in that if you keep someone in a humidified environment evaporation would be decreased" 'Insulation' generally refers to heat, not humidity. And keeping someone in a humid suit in a desert seems like a really bad idea, and would likely just cause them to sweat more. A better to reduce water loss would be if the suit can keep the person cool. $\endgroup$ – Acccumulation Jun 29 '18 at 22:44
  • $\begingroup$ @Acccumulation The only way sweat cools you is if you allow it to evaporate, so if a Fremen suit is collecting all water it can't be allowing the wearer to cool via sweating anyways. Insulation need not refer only to thermal insulation. $\endgroup$ – Bryan Krause Jun 29 '18 at 22:46
  • $\begingroup$ @Acccumulation the suits are described as trapping and collecting exhaled water,urine, and a large percentage of sweat. It is basically a nearly sealed environment suit designed with complex special heat exchangers. $\endgroup$ – John Jun 30 '18 at 3:29

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