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This human population developed in a mild climate region, with fair abundance of food, water and shelter. However, though they have the skills and the means to explore forth, the population is unable to leave the region, because as soon as it does, it becomes "homesick".

No, not only missing mommy and friends, but really because their bodies are unable to synthetize a particular substance which is vital for their well-being and that is plenty available in their home wells and springs water, but not elsewhere.

Therefore, as soon as they leave the region and have no access to the local water (except some reserve of water that they can carry along), they simply get sick.

They initially report fatigue, difficult to focus and communicate, then they further develop difficulties to coordinate their movements until even breathing becomes difficult and death is inevitable.

If they fail to return home they perish within a few days. Only few of them have been reported to survive more than two weeks away.

We are not able to examine the water in loco, can you help understanding what could this substance be?

Important I am more interested in a substance which can be water soluble and essential for life. I am open to adapt the symptoms and/or the time span for the death, if a suitable substance is given in the answer.

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  • $\begingroup$ Humanoids? Human level intelligence? Animals? Earth? Earth like? $\endgroup$ – Mołot Mar 21 '17 at 7:54
  • $\begingroup$ I really hope it's Kandrona rays $\endgroup$ – Vylix Mar 21 '17 at 7:56
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    $\begingroup$ @Mołot, humans on Heart (edited the question to make it clear) $\endgroup$ – L.Dutch Mar 21 '17 at 7:59
  • $\begingroup$ This might be a little less opinion-based if you tell us about the symptoms experienced by these beings when "sick". Also, can you define the amount of time further? Is this about hours, days, weeks? $\endgroup$ – a CVn Mar 21 '17 at 8:45
  • $\begingroup$ @MichaelKjörling, I tried to cover your remark in the edit $\endgroup$ – L.Dutch Mar 21 '17 at 8:58
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Lithium.

https://www.nytimes.com/2014/09/14/opinion/sunday/should-we-all-take-a-bit-of-lithium.html

Lithium is known as a treatment for bipolar disorder AKA manic depression. Somehow the lithium levels out the highs and lows and allows affected individuals to lead a more normal life.

Lithium also occurs naturally in groundwater to a greater or lesser extent depending on the locale. The linked article reviews evidence that lithium in drinking water is correlated with less suicide and less violent crime on a societal level.

Like anything else, some individuals will be more susceptible to a given effect and some less. Maybe the homesick humans here are extremely susceptible. Genetic drift, or some other benefit of a state akin to bipolar disorder has left these humans requiring consistent lithium intake. The ancestral population got away with this because of where they happened to live: a site with high lithium levels in the water.

Without the lithium, their mood cycle begins to increase in amplitude. Some people get increasingly depressed - "homesick" and risk suicide if they do not get back. Others become more and more manic - sick in a different and more energetic way.

Lithium in the water is real and its affect on mood (bipolar disorder being one extreme) is real. I think bipolar disorder offers more narrative possibilities than something like congenital adrenocortical insufficiency and salt (sodium chloride) wasting - also a real thing, and treatable by high sodium intake, but the sickness manifestation is just low blood pressure and feeling terrible.

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    $\begingroup$ Lithium: filling your body batteries since Stone Age ;) $\endgroup$ – L.Dutch Mar 21 '17 at 14:17
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    $\begingroup$ That was the first thing that came to mind as I was reading the OP. +1 $\endgroup$ – Fayth85 Mar 21 '17 at 20:46
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Electrolytes

Electrolytes are important because they are what cells (especially nerve, heart and muscle cells) use to maintain voltages across their cell membranes and to carry electrical impulses (nerve impulses, muscle contractions) across themselves and to other cells. Kidneys work to keep the electrolyte concentrations in blood constant despite changes in the body. For example, during heavy exercise, electrolytes are lost in sweat, particularly in the form of sodium and potassium. These electrolytes must be replaced to keep the electrolyte concentrations of the body fluids constant.

Hyponatremia (low sodium levels):

In hyponatremia, the level of sodium in blood is too low.

  • A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics.
  • Symptoms result from brain dysfunction.
  • At first, people become sluggish and confused, and if hyponatremia worsens, they may have muscle twitches and seizures and become progressively unresponsive.

Although ensuring adequate electrolyte levels should really be a fundamental part of the crew's nutrition and health planning...

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  • $\begingroup$ You may want to update your answer in light of OP's revision 5. $\endgroup$ – a CVn Mar 21 '17 at 9:11
  • $\begingroup$ @MichaelKjörling Darn it, I'm a Worldbuilder, not a doctor...! And I really hate changing an answer that already has up-votes. All the OP has to do to get an answer is to Google the symptoms to find likely causes. $\endgroup$ – Snow Mar 21 '17 at 9:29
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    $\begingroup$ @Pete, your answer is fine. $\endgroup$ – L.Dutch Mar 21 '17 at 10:37

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