Salt is vital for a number of key roles in our body. Fluid balance, nerve transmission, stomach acidity and nutrient absorption are the three most notable.
The body is made up of fluid compartments. Most water is in our cells and is called intracellular fluid. The rest is the extracellular fluid or ECF. This is made up of the interstitial fluid (between cells in our tissues) and the plasma in our blood. The interstitial fluid and plasma run into each other in the leaky capillary beds. When we eat a salty meal the osmolarity of our blood rises. This then directly raises the osmolarity of our interstitial fluid.
Water in our cells is separated from interstitial fluid by plasma membrane. The main way that water can escape is by osmosis. After a salty meal, the increased osmolarity of the interstitial fluid causes water to leak out from the cells. This ultimately boosts our blood volume, diluting the saltiness of the blood.
The amount of sodium in our blood needs to be in a tight range between 136-146 mmol/L. If our blood sodium gets too low then the osmolarity of the interstitial fluid is lower than that inside the cells and water seeps into the cells causing them to swell. In serious cases of low blood sodium, called hyponatraemia, the brain swells up leading to coma and ultimately death if untreated. That is why it is important to consume enough salts during long distance endurance events.
Nerve conduction occurs after a change in nerve cell membrane permeability allows positively charged sodium ions to flood through the membranes reversing their polarity. The action potential thus produced is responsible for all our thoughts and movements.
Lack of sodium can lead to cramps. These are normally the cramps experienced towards the end of long endurance efforts. Cramps experienced at other times seem more associated with other factors(2).
Stomach acid & nutrient absorption
The chloride component of salt ensures that our stomachs can form enough hydrochloric acid (HCl). Insufficient HCl in the stomach allows more harmful bacteria through to our guts, which leads to increased illness. The level of acid in the stomach is also critical in ensuring we can digest protein efficiently.
The sodium component of salt helps us absorb glucose and protein lower down in the guts. It can also help us absorb water more effectively. It is notable that UNICEF supplies a solution made up of salt and glucose in order to rehydrate and nourish infants at danger of death from cholera and diarrhoea in the developing nations.