How did ancient people know they needed to eat salt?

So, I am not a historian, but a former nurse, I have a passion for nursing and medical history, and would love to take a stab at this question...

I would say that it is difficult if not impossible to give a definitive answer to your question, as salt use pre-dates society, historical documentation, and possibly modern man. However, I will speak of some relative points, from a somewhat medical lens.

Some background...

People have known about the need for salts for a very long time. There is hard evidence of salt production dating back to at least c. 6000 BCE, archaeologists have uncovered salt mines in Bulgaria and the Natron Valley (Egypt) dating to the neolithic period, and there is evidence to suggest that salts have been some of the first and most widespread trading commodities.

Salts are important for several reasons. Yes, beings do need to eat salts (aka electrolytes) in order to survive, but in the days before refrigeration salt was also a key preservative, and is where most of our salt intake comes from (both historically and in the modern day). Though we will never know exactly how and when humans realized that salts are essential to life or a key preservative, it seems to be something that pre-dates man. We observe wild animals harvesting salts and traveling large distances to consume salt from known sources. Humans crave salts. There has been some modern studies to understand the psychobiological and physiological reasons that that humans crave salts (and generally salt intake is generally much greater than required for survival).

There are many salts that are essential to the human diet (chloride, potassium, sodium, being the main salts required to maintain cellular function and electrolyte balance)... example, humans need about 16g of sodium a day to maintain body function, (about 2 tsp or 10 ml) and some of this comes from naturally from food (veg and meat), and most of it comes from eating processed food (which is the main dietary source of salts is as a food preservative).

Salt Deficiencies...

It is generally rare to have a electrolyte deficiency in people are living with an adequate diet, and water intake (generally high sodium levels are expected). In the modern west, people who are hospitalized are from electrolyte deficiency are generally elderly and the condition is often related to water/salt balance (ie dehydration), and co-morbidites (related to vomiting that causes dehydration which causes electrolyte imbalance). It is generally secondary, or tertiary to a greater issue (starvation, disease, etc).

If an isolated hyponatremia (low sodium levels) occurs, it manifests first a stupor and lethargy then progresses to muscle spasms --> seizures --> coma --> then death (you are not just “stuck down”, you do not simply drop dead, there is a slow progression that takes days to weeks). Without understanding the underlying cause of any pathology, people would have an understanding many things from observing cause and effect (similar in nature to how herbal remedies developed before medical science was able to provide healers with information on pathophysiology and treatment).... If someone is experiencing stupor or lethargy the first cure would likely be to get them to rest, eat and drink. You do not have to be a medical profession to see the correlation, even if you do not understand that it is the salt and water that brings the body back into homeostasis, you understand that food and drink makes you feel better.

The symptoms of mild hyponatremia are very different than let's say the course of a virus. Even lay-healers were able to tell the difference between certain symptoms, and may have had some knowledge of cause and treatment (for some common pathologies). It is unknown to what extent prehistoric healers would have understood such things. In general severe dehydration, severe vomiting, old age and starvation come first, extreme hyponatremia doesn't really occur on it's own (generally speaking, there is too much salt in the diet, not too little, and historical methods for preserving food used much more salt than we use today). Historically if you are at risk of dying from hyponatremia, you were already at risk from dying from starvation, disease, old age first.

On the science of electrolyte deficiencies...

The medical understanding of electrolyte deficiencies is relatively new, Svante Arrhenius discovered Ionic disassociation (how salts act chemically when dissolved in water) for which he won a Nobel prize for in 1904. Though I do not have an exact citation for when the understanding of the pathology of electrolyte imbalance was discovered in a medical sense, it would be with/after breakthroughs in the understanding of biochemistry in cellular function.

In summery....

Salt production dates back to prehistoric times, and salt use predates man.... it has always been known of and eaten, even if it was not understood why it is physiologically important.

Humans crave salt, and we are only really beginning to understand this in a scientific/medical way.

Salts were and are used in preserving food and for flavor additives, (not just as an essential nutrient). This is where most salt intake comes from.

Prehistoric healers had knowledge of disease and cures, even if they did not understand the scientific/medical reasons for either.

Morbid salt deficiencies are generally secondary to other diseases, old age, starvation and dehydration.

Understanding of medical salt requirements have only developed over the last century, but people have understood the need for salt for much longer, generally related to food preservation (when the use of salt started was prehistoric, though we do not know the specifics).

I hope this is helpful, (and acceptable for this sub). Feel free to add to this or ask follow up questions....

Some Sources

Sizer & Whitney, “Nutrition Concepts and Controversies” 2nd canadian ed. 2011

Mosby's Dictionary of Medicine, Nursing and Health Professions. 9th ed. 2012

Morris & Johnson, “Salt craving: the psychobiology of pathogenic sodium intake”, 2008

Cirillo, Capasso, Di Leo & De Santo “A History of Salt” 1994

https://www.scientificamerican.com/article/processed-food-a-two-million-year-history/

https://www.health.harvard.edu/heart-health/take-it-with-a-grain-of-salt

http://articles.latimes.com/2012/nov/01/science/la-sci-sn-oldest-european-town-20121101

note: I have pulled a bit from memory of my school days, the rest of my texts are in storage.