Sodium and human physiology
Why do we need sodium and how much do we actually need?
Sodium is the principal cation in the extracellular fluid and its main functions are related to blood volume maintenance, water balance and cell membrane potential; it is also essential for acid-base balance and nerve conduction. More than 95% of ingested salt is absorbed by the gut and, as part of the electrolyte regulatory system, urinary sodium losses increase when the body has surplus sodium. Often, the main excretion of sodium is by sweat.
Briefly, an increase in plasma osmolality occurs when the plasma becomes more concentrated; this can be produced either by dehydration or by excessive salt intake, producing stimulation of osmoreceptors and the sensation of thirst. When the thirst centre in the hypothalamus is stimulated, more water is taken into the intestine and subsequently enters the bloodstream. Simultaneously, endocrine systems stimulate the kidneys to retain more salt and water.
The endocrine system can also work in the opposite direction. High salt intake can cause high blood volume, and this leads to hormonal release which causes less salt retention by the kidneys and more salt excretion in urine.
During prolonged exercise or hot weather, blood flow through the skin is adjusted and water and salt may be lost from the body through sweating in order to cool the body down and to maintain deep body temperature.
The physiological need for sodium, in acclimatized adults, is about 165-230 mg sodium per day. The human species evolved in the warm climate of Africa, a salt-poor continent, on a low-salt diet (as low as 50 mg sodium per day). Humans have come to depend on the physiological retention of the limited salt naturally present in food (Elliot and Brown, 2006).
There are some pathological conditions that could cause hyponatraemia (sodium concentration in the serum that is lower than normal). However, depletion of body sodium in healthy individuals is very unlikely, because people tend to eat excessive amounts of sodium, beyond their physiological needs (Denton, 1982).