Physiological roles of salt in the human body
There is no doubt that an adequate intake of salt in the human diet is required to maintain good health. The Institute of Medicine recommends that healthy adults consume 1500 mg of sodium, or 3.8 grams of salt, to replace the amount lost daily on average through sweat and urination.  (Ironically, this recommendation is almost double the amount theoretically consumed by Paleolithic man.) The minimum physiological requirement of sodium simply to sustain life has been estimated to be 500 mg of sodium per day. 

Sodium is a vital nutrient. It’s a major component of extracellular fluid, and is essential for maintaining the volume of the plasma to allow adequate tissue perfusion and normal cellular metabolism. Because sodium is used as an extracellular cation, it is typically found in the blood and lymph fluid. The maintenance of extracellular fluid volume is an important physiologic function of the sodium in the body, particularly in regards to cardiovascular health.

Besides helping to maintain fluid balance and cardiovascular function, sodium and chloride ions also play an important role in the nervous system. Changes in the concentrations of these ions allow neurons to send signals to other neurons and cells, allowing for nerve transmission as well as mechanical movement. Chloride ions provided by salt are secreted in the gastric juice as hydrochloric acid (HCL).  And HCL is vital to the digestion of food and the destruction of food-borne pathogens in the stomach.

If a true sodium deficiency occurs, mammals experience symptoms of hyponatremia such as brain swelling, coma, congestive heart failure, cardiovascular collapse following acute blood loss, and impaired sympathetic cardiovascular adjustments to stress. Animals in a truly sodium-deficient state will seek out salty food and often consume far more sodium than needed to restore homeostasis.  These behavioral changes in response to inadequate salt intake further demonstrate the biological importance of dietary salt.

Evidence about human salt consumption
The human body has adapted complex physiological mechanisms in order to prevent blood pressure fluctuations in response to these variations in sodium intake. Not surprisingly, epidemiological data has revealed an average sodium intake range of 2400 mg to 5175 mg of sodium per day in developed cultures.  Certain isolated groups in areas such as Brazil, Papua New Guinea, and rural African communities have been found to live on sodium intakes of as little as 1150 mg per day. However, despite finding generally low blood pressure in these remote communities, the little evidence that exists on these low salt societies suggests shorter life expectancy and higher mortality rates.

An example from the Intersalt Study, which examined the impact of population-wide salt consumption on blood pressure, is the Yanomami Indians of the Brazillian rainforest, who are known for having far lower average blood pressure than that of Western populations.  Their lifelong low blood pressure has been attributed to their extremely low consumption of salt, and this has been used as evidence to further support the effort to restrict salt from the American diet.

A major problem that arises from using the Yanomami as an example of the salt-hypertension hypothesis is the wide variety of confounding variables that may also affect their blood pressure. The Intersalt Study researchers admit that:

“In addition to low Na+ intake and high K+ intake, other factors that may contribute to the absence of hypertension and lack of blood pressure increase with age among the Yanomami Indians are as follows: their low body mass index and the almost nonexistence of obesity, no alcohol ingestion, low ingestion of saturated fat, high ingestion of fibers, relatively high physical activity, and the several cultural consequences of living in an isolated community without the psychosocial stress of civilization and without a monetary system or dependence on a job.” 
This data suggest there are many reasons the Yanomami have such low blood pressure.  These include high potassium intake, high physical activity, low stress levels, and complete lack of alcohol consumption. Furthermore, although the Yanomami have low blood pressure and nearly nonexistent rates of cardiovascular disease, their overall health outcomes are less than stellar. They are described in ethnographic literature as having small stature, high mortality and a low life expectancy ranging between 29 and 46 years.  Despite these high mortality rates and confounding lifestyle factors, the Yanomami people are still used as a prime example in support of the salt-hypertension hypothesis.

The results of the Intersalt Study did not indicate any clear pattern between the level of salt intake and blood pressure in those countries studied.  And when average life expectancy is plotted against the countries’ average salt intake, the trend shows that higher salt consumption is actually correlated with longer life expectancy. While this correlation does not imply causation, it is interesting to note the compatibility of a high salt diet with a long life expectancy.

As we can see, there is an enormous range in the daily dietary sodium intake of various cultures around the world, ranging from quite low (1150 mg) to fairly high (5175 mg). Additionally, we know that the healthy kidney is capable of adjusting to fluctuating levels of sodium in the diet in order to maintain fluid homeostasis. Finally, we know that hunter-gatherer and Paleolithic diets were very low in sodium, and that salt was rarely, if ever, added to food. Therefore, it would seem that limiting salt in the diet to those levels recommended by the AHA and USDA would not have any significant consequences, and would be an ideal dietary choice when mimicking the diet of our ancestors. However, evidence is mounting to the contrary: a low-salt diet may actually lead to serious health consequences and higher overall mortality, particularly in conditions like heart disease and diabetes.