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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
up vote 27 down vote accepted

In developed countries we usually consume enough salt (sodium to be exact) without actually adding table salt to food. Everything can become toxic when consumed in excess - even water - and when we frequently add more salt to foods, we tend to consume sodium in potentially harmful excess. That's what your friends are referring to.

However, salt (sodium) is one of the most essential substances your body needs to stay alive, for several reasons. One of the main purposes of sodium is the upkeep of blood's osmolarity (i.e. concentrations of osmotically active compounds. Higher salt concentration on one side of a permeable membrane attracts water to that side - 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.

Delayed update to pick up some side aspects of your description: 1) There are of course things that humans do which have absolutely no value to them whatsoever. Anything that plays into the feel-good-reward-circuit in our brain can become such an unhelpful habit. Take smoking and drug consumption as examples. 2) About evolutionary relevance: Being a key player in maintaining body function, evolution selected for instictively liking salt. Simultaneously, most people will not like food that is extremely salty - a protective mechanism against excess.

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I described "one of the main purposes"; the other main purposes that come to mind are: 1) Upkeep of membrane potential - all cells utilise gradients of Na and K ions to maintain a voltage differential between their interior and the extracellular liquid, enabling various cell-sustaining processes including energy generation. 2) Nerve signalling - neurons utilise Na ions to transduce targeted voltage spikes. – Armatus Aug 15 '14 at 10:36

I agree with Armatus, however I feel his answer incomplete.

Sodium (along with chlorine) is the most important ion in the blood if we are talking about osmolarity and water transport. Water wants to move to the high osmolarity places so its transport is mainly controlled by the transport of sodium through selective sodium channels. This is not just about blood osmolarity, e.g. the absorption of water from the guts, the filtration of blood in the kidney, the sweating, etc... all major water transport in the body is mediated by sodium transport. So sodium is essential.

Your friends are right about too much sodium can harm you. Just try out something: eat something salty, and wait a few minutes. You will be thirsty, because sodium concentration increased your blood osmolarity, and so your body released hormones to restore osmolarity, and these hormones make you thirsty. After you drunk water, your blood pressure becomes higher until the kidney get rid of the extra sodium. High blood pressure is bad to your heart. This is just one way too much sodium can harm you...

Your friends probably meant that if you don't have a lot of training and so you don't sweat a lot, you don't need crystalline salt in your food, because your food already contains enough sodium.

It is an interesting question why do we love extra salt despite the fact that our food contains enough of sodium. I don't know the answer of that. It certainly makes foods tastier, but I cannot find or find out anything about why this is evolutionary good. I think it is something similar to narcotics or sugar, it has a (small) hedonic value, despite that it isn't necessary good...

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Evolution is still basically a random "pseudo-process" with pruning. Even if a trait were negative, it will only be removed from the population if it is harmful enough in the environment you live in. Most people don't like to drink seawater, and other ready sources of salt were not readily available in our ancestral environment (the same with other mammals, who also tend to crave salt). Historically, you were probably more likely to suffer from a lack of salt, rather than too much salt; which also means that there'd be a benefit to a "craving for salty food". – Luaan Mar 14 at 8:52
    
Look into how long would a human live with no salt in there body. And im not talking about the table salt. – user5434678 Mar 16 at 5:36
    
@TheVoid Where should we look into that? Do you have any human experiments? Mice can live at least 3 months with hyponatremia. physiolgenomics.physiology.org/content/43/5/265.short – inf3rno Mar 16 at 15:35
    
@TheVoid I found something: acep.org/Clinical---Practice-Management/… "Severe hyponatremia may result in cerebral edema, encephalopathy and death. " , "1986 study in which 27% of previously healthy women undergoing elective surgery died after developing acute hyponatremia (averaging 108 mmol/L). " I did not find the study, but I think this must have been relative fast, because they weren't able to treat these patients. – inf3rno Mar 16 at 15:39
    
Salt adds flavour to food. That is why. – Neil Meyer Apr 5 at 16:01

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