Introduction

Potassium is an essential dietary mineral that is also known as an electrolyte. The term electrolyte refers to a substance that dissociates into ions (charged particles) in solution making it capable of conducting electricity. The normal functioning of our bodies depends on the tight regulation of potassium concentrations both inside and outside of cells.

Food Sources

The richest sources of potassium are fruits and vegetables.

Minerals from plant sources may vary from place to place because soil mineral content varies geographically.

Some important food sources of potassium:

Tomato paste	Spinach	Raisins	Radish	Orange juice
Passion fruit	Papaya	Banana	Red bell pepper	Apricot

Recommended Dietary Allowance (RDA)

The European Union and the US have not set a RDA for the general population.

Inhibitors/stimulators:

The following components have been found to inhibit the absorption of potassium:

Anti-inflammatory agents– Potassium absorption is hindered by the anti-inflammatory agents colchicine and salicylazosulfapyridine.

Laxatives– Potassium absorption is hindered by laxatives such as phenolphthalein, cascara sagrada, and bisacodyl.

Various antimicrobial agents– Potassium absorption is hindered by various antimicrobial agents such as tetracycline and neomycin.

Functions in the Body

Maintenance of membrane potential

Potassium is the principal positively charged ion (cation) in the fluid inside of cells, while sodium is the principal cation in the fluid outside of cells. Potassium concentrations are about 30 times higher inside than outside cells, while sodium concentrations are more than 10 times lower inside than outside cells. The concentration differences between potassium and sodium across cell membranes create an electrochemical gradient known as the membrane potential. Ion pumps in the cell membrane, especially the sodium potassium-ATPase pumps, maintain a cell's membrane potential. These pumps use ATP (energy) to pump sodium out of the cell in exchange for potassium. Their activity has been estimated to account for 20%-40% of the resting energy expenditure in a typical adult. The large proportion of energy dedicated to maintaining sodium/potassium concentration gradients emphasizes the importance of this function in sustaining life. Tight control of cell membrane potential is critical for nerve impulse transmission, muscle contraction, and heart function.

Cofactor for enzymes

A limited number of enzymes require the presence of potassium for their activity. The activation of sodium, potassium-ATPase requires the presence of sodium and potassium. The presence of potassium is also required for the activity of pyruvate kinase, an important enzyme in carbohydrate metabolism.

Blood buffer system

Potassium is an essential constituent of several blood buffer systems. Potassium complexes bind ionically with the sulphate group of sulphuric acid, thereby reducing the acidity of the system by forming a potassium sulfate salt. Potassium has a similar action in base buffer systems with the conversion of the strong base potassium hydroxide into the relatively neutral water molecule.

Muscle contraction

After the transmission of a nerve impulse, during which sodium ions are shifted across the nerve's synaptic membrane, potassium and sodium are exchanged by the previously mentioned "pump" mechanism (so as to restore the original sodium concentration on the external side of the membrane). This "pumping" of sodium outside is essential to prepare for subsequent nerve transmission. Potassium acts to relax muscle contraction in opposition to calcium, which induces contraction

Deficiency

An abnormally low plasma potassium concentration is referred to as hypokalemia. Hypokalemia is most commonly a result of excessive loss of potassium, e.g., from prolonged vomiting, the use of some diuretics, some forms of kidney disease, or disturbances of metabolism. The symptoms of hypokalemia are related to alterations in membrane potential and cellular metabolism.

Low dietary intakes of potassium do not generally result in hypokalemia. However, recent research indicates that insufficient dietary potassium may increase the risk of a number of chronic diseases.

Toxicity

Abnormally elevated serum potassium concentrations are referred to as hyperkalemia. Hyperkalemia occurs when potassium intake exceeds the capacity of the kidneys to eliminate it. Acute or chronic renal (kidney) failure, the use of potassium-sparing diuretics, and insufficient aldosterone secretion (hypoaldosteronism) may result in the accumulation of excess potassium due to decreased urinary potassium excretion. Oral doses greater than 18 grams taken at one time in individuals not accustomed to high intakes may lead to severe hyperkalemia, even in those with normal kidney function.

Hyperkalemia may also result from a shift of intracellular potassium into the circulation, which may occur with the rupture of red blood cells (hemolysis) or tissue damage (e.g., trauma or severe burns). Symptoms of hyperkalemia may include tingling of the hands and feet, muscular weakness, and temporary paralysis. The most serious complication of hyperkalemia is the development of an abnormal heart rhythm (cardiac arrhythmia), which can lead to cardiac arrest

Regulation

The movement of potassium into extracellular fluid from muscle cells is an important part of the contraction mechanism of muscle tissue.

Potassium is pumped into the cell by active transport systems, which concomitantly pump sodium out of the cell. The preferential segregation of sodium and potassium across the cell's biological membrane is important in maintaining osmotic balance, the electrochemical gradient of membranes, and the regulation of extracellular fluid volume. This mechanism of ion pumping is also instrumental in the restoration of potassium/sodium gradient after the ionic transmission of nerve impulses.

Potassium is principally found within cellular fluids and its counterpart, sodium, is mostly found within the extracellular fluids. The segregation of these two ions occurs by means of an adenosine triphosphate (ATP) driven "pump." The pump consists of two proteins within the cellular membrane which, upon energy release from ATP, transport three sodium molecules to the outside of the cell membrane, while simultaneously bringing in two potassium molecules.

A similar pumping mechanism is used in the transport of glucose from the intestine into the bloodstream. High sodium concentrations in the intestinal fluids tend to promote the movement of sodium across the mucosal cells of the intestine. As sodium is moved across the cells, glucose is concomitantly moved into the cells. The concentration of glucose within the cells builds up until it begins to diffuse into the bloodstream. The "pump" mechanism pumps the sodium into the blood in exchange for potassium, thereby eliminating sodium buildup within the cell.

Potassium is absorbed readily in the small intestine; excess potassium is excreted through the urine. Aldosterone hormone tends to promote potassium excretion in substitution for sodium absorption. This is done by activation of the renal "pump" proteins, which simultaneously exchange potassium for sodium across the biological membrane.