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A few friends of mine told me that salt provides zero nutritional value to us, and in fact can harm our bodies. Now, these guys are medical students, and being an engineering student myself, I decided not to argue with them. The rest of this question assumes that this fact is true, so if it's not, you can just go ahead and call me out now...

So here's my understanding of things: we 'like' doing things because of our instincts, which have slowly become refined over millions of years. For example, I 'like' eating foods with fat in it because my instinct compels me to do so. Fat is 'good' for my body, since it provides a lot of energy (obesity problems aside).

So are there certain things, such as eating salt, that are not in fact beneficial in any way, and we only do these things because we were trained to as children? This is the only thing I could come up with, but it's not a very satisfying explanation for a few reasons. First, I think humans have been eating salt for a long time. This would mean that most likely, it is actually our 'instinct' to eat salt. Also, salt is eaten in every culture today, which has the same implication.

So is there some better explanation?

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Look into hyponatremia (low Na+) and hypokalemia (low K+) as evidence that the body does in fact need both nutrients. –  jonsca Jun 5 '12 at 4:15
    
But does salt help prevent those? And if so, could this be reason enough for us to like eating salt? –  Hassan Jun 5 '12 at 4:32
    
Well, the proper balance of salt and water does (though hypertonic saline is not always given to hyponatremic patients), so it was meant more as a counter to your friends' assertion rather than an answer to the question. I would guess that the cravings do have something to do with maintaining that salt/water balance, but I do not know for sure. –  jonsca Jun 5 '12 at 4:39
    
@jonsca Okay thanks for the info, you learn something everyday. –  Hassan Jun 5 '12 at 4:47
    
Welcome. That first comment wasn't meant to be chiding at all, I just think some of the key to what you are looking for might be within those two pathologies. –  jonsca Jun 5 '12 at 4:49
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up vote 14 down vote accepted

Your friends are referring to the principle that everything can be toxic, if consumed in too high concentrations. Salt is one of the most essential substances your body needs to stay alive, for several reasons.

One of the main purposes of salt is the upkeep of blood's osmolarity (i.e. concentrations of osmotically active compounds. You know, higher salt concentration on one side of a permeable membrane attracts water to that side etc - I'm sure you've heard that before). There are numerous systems in your body to make sure the osmolarity of blood is correct. If they fail and blood becomes hypo or hypertonic, your cells will be sucked dry or pumped full of water and in either case, burst and die.

Look up the renin-angiotensin-aldosterone system for example: when the kidney filters blood, it reabsorbs or lets through water depending on the current blood osmolarity; leading to higher or lower amount of higher or lower concentrated urine. You drink lots, your blood is diluted, it becomes less tonic, kidney registers that and lets water through more, you urinate more. There are many more elements involved there, including blood pressure, nerve signals stimulating thirst or hunger of different kinds, some hormones etc.

As you can see, there's a reason why the basic infusion given in hospitals to replace lost blood quickly isn't just water but normal saline.

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