Sodium ions (Na+) and chloride ions (Cl-) contribute to the maintenance of concentration and charge differences across cell membranes. Potassium (K+) is the principal positively charged ion (cation) in the fluid inside of cells (30 times higher concentration than outside), while sodium (Na+) is the principal cation in the fluid outside of cells (10 times higher concentration than inside). The concentration differences between potassium and sodium across cell membranes create an electrochemical gradient (membrane potential), which is critical for nerve impulse transmission, muscle contraction, heart function, and the transport of nutrients and metabolites in and out of cells (3, 4). A large proportion of the body's resting energy expenditure is devoted to maintaining the membrane potential by ion pumps in the cell membrane, pumping sodium out of the cell in exchange for potassium (sodium–potassium pump: 3 Na+/ 2 K+ - ATPase).
Uptake (absorption) of sodium in the body’s small intestine (following the stomach) plays an important role in the absorption of chloride, amino acids, glucose, and water. Similar mechanisms are involved in the re-uptake (reabsorption) of these nutrients after they have been filtered from the blood by the kidneys. Chloride, in the form of hydrochloric acid (HCl), is also an important component of gastric juice, which aids the digestion and absorption of many nutrients (2, 5).
Because sodium is the primary determinant of fluid in blood vessels (plasma) and fluid between cells, a number of mechanisms that regulate blood volume and blood pressure work by adjusting the body's sodium content. In the circulatory system, pressure receptors sense changes in blood pressure and send signals to the nervous system and/or glands to affect sodium regulation by the kidneys. In general, sodium retention results in water retention and sodium loss results in water loss (4, 5).
The European Food Safety Authority (EFSA), which provides scientific advice to assist policy makers, has confirmed that clear health benefits have been established for the dietary intake of chloride in contributing to:
· normal digestion by production of hydrochloric acid in the stomach.
Authored by Dr Peter Engel in 2010, reviewed and revised by Dr. Volker Elste on 20.09.2017.