Potassium is the most abundant positively charged ion inside cells. Approximately ninety percent of potassium is located in the intracellular fluid, with about 10 percent found in extracellular fluid, and only 1 percent present in blood plasma. Similar to sodium, potassium levels in the bloodstream are strictly regulated. The hormone aldosterone primarily controls potassium levels, but other hormones (such as insulin) also play a role. When potassium levels in the blood increase, the adrenal glands release aldosterone. The aldosterone acts on the collecting ducts of the kidneys, stimulating an increase in the number of sodium-potassium pumps. Sodium is then reabsorbed and more potassium is excreted. Because potassium is required for maintaining sodium levels and hence fluid balance, about 200 milligrams of potassium are lost from the body every day.

Functions of Potassium in the Body

Nerve impulse involves not only sodium but also potassium. A nerve impulse moves along a nerve via the movement of sodium ions into the cell. To end the impulse, potassium ions rush out of the nerve cell, thereby decreasing the positive charge inside the nerve cell. This diminishes the stimulus. To restore the original concentrations of ions between the intracellular and extracellular fluid, the sodium-potassium pump transfers sodium ions outside the cell in exchange for potassium ions inside the cell. Once the ion concentrations are restored, a nerve cell is ready to receive the next impulse. Similarly, in muscle cells, potassium is involved in restoring the normal membrane potential and ending muscle contraction. Potassium also is involved in protein synthesis, energy metabolism, and platelet function, and acts as a buffer in blood, playing a role in acid-base balance.

Imbalances of Potassium

Insufficient potassium levels in the body (hypokalemia) can be caused by a low dietary potassium intake or high sodium intake, but more commonly, it results from medications (mainly diuretics) that increase water excretion. The signs and symptoms of hypokalemia are related to the functions of potassium in nerve cells and consequently skeletal and smooth-muscle contraction. The signs and symptoms include muscle weakness and cramps, respiratory distress, and constipation. Severe potassium depletion can cause the heart to have abnormal contractions and can even be fatal. High levels of potassium in the blood, or hyperkalemia, also affect the heart. It is a silent condition as it often displays no signs or symptoms. Extremely high levels of potassium in the blood disrupt the electrical impulses that stimulate the heart and can cause the heart to stop. Hyperkalemia is usually the result of kidney dysfunction.

Needs and Dietary Sources of Potassium

The  AI for potassium is based on the levels associated with a decrease in blood pressure, a reduction in salt sensitivity, and a minimal risk of kidney stones. For adult males and females above the age of nineteen, the adequate intake of potassium is 4,700 grams per day. The AIs for other age groups are listed in Table 10.7 “Adequate intakes for potassium”.

Food Sources of Potassium

Fruits and vegetables that contain high amounts of potassium are spinach, lettuce, broccoli, peas, tomatoes, potatoes, bananas, apples and apricots. Whole grains and seeds, certain fish (such as salmon, cod, and flounder), and meats are also high in potassium. The “Dietary Approaches to Stop Hypertension” (DASH) diet emphasizes potassium-rich foods and will be discussed in greater detail in the next section.

Bioavailability

Greater than 90 percent of dietary potassium is absorbed in the small intestine. Although highly bioavailable, potassium is a very soluble mineral and is easily lost during the cooking and processing of foods. Fresh and frozen foods are better sources of potassium than canned.