Potassium, the most abundant intracellular cation, is an essential nutrient that is naturally present in many foods and available as a dietary supplement. Potassium is present in all body tissues and is required for normal cell function because of its role in maintaining intracellular fluid volume and transmembrane electrochemical gradients [1,2]. Potassium has a strong relationship with sodium, the main regulator of extracellular fluid volume, including plasma volume.
The total amount of potassium in the adult body is about 45 millimole (mmol)/kg body weight (about 140 g for a 175 pound adult; 1 mmol = 1 milliequivalent [mEq] or 39.1 mg potassium) . Most potassium resides intracellularly, and a small amount is in extracellular fluid [2-4]. The intracellular concentration of potassium is about 30 times higher than the extracellular concentration, and this difference forms a transmembrane electrochemical gradient that is maintained via the sodium-potassium (Na+/K+) ATPase transporter . In addition to maintaining cellular tonicity, this gradient is required for proper nerve transmission, muscle contraction, and kidney function.
Potassium is absorbed via passive diffusion, primarily in the small intestine [2,4,5]. About 90% of ingested potassium is absorbed and used to maintain its normal intracellular and extracellular concentrations [3,5]. Potassium is excreted primarily in the urine, some is excreted in the stool, and a very small amount is lost in sweat. The kidneys control potassium excretion in response to changes in dietary intakes, and potassium excretion increases rapidly in healthy people after potassium consumption, unless body stores are depleted [2,6]. The kidneys can adapt to variable potassium intakes in healthy individuals, but a minimum of 5 mmol (about 195 mg) potassium is excreted daily in urine . This, combined with other obligatory losses, suggests that potassium balance cannot be achieved with intakes less than about 400–800 mg/day.
Normal serum concentrations of potassium range from about 3.6 to 5.0 mmol/L and are regulated by a variety of mechanisms [3,7]. Diarrhea, vomiting, kidney disease, use of certain medications, and other conditions that alter potassium excretion or cause transcellular potassium shifts can cause hypokalemia (serum levels below 3.6 mmol/L) or hyperkalemia (serum levels above 5.0 mmol/L) [3,5,7,8]. Otherwise, in healthy individuals with normal kidney function, abnormally low or high blood levels of potassium are rare.
Assessing potassium status is not routinely done in clinical practice, and it is difficult to do because most potassium in the body is inside cells. Although blood potassium levels can provide some indication of potassium status, they often correlate poorly with tissue potassium stores [3,9,10]. Other methods to measure potassium status include collecting balance data (measuring net potassium retention and loss); measuring the total amount of potassium or the total amount of exchangeable potassium in the body; and conducting tissue analyses (e.g., muscle biopsies), but all have limitations .
Sources of Potassium
Potassium is found in a wide variety of plant and animal foods and in beverages. Many fruits and vegetables are excellent sources, as are some legumes (e.g., soybeans) and potatoes. Meats, poultry, fish, milk, yogurt, and nuts also contain potassium [3,5]. Among starchy foods, whole-wheat flour and brown rice are much higher in potassium than their refined counterparts, white wheat flour and white rice .
Milk, coffee, tea, other nonalcoholic beverages, and potatoes are the top sources of potassium in the diets of U.S. adults . Among children in the United States, milk, fruit juice, potatoes, and fruit are the top sources .
It is estimated that the body absorbs about 85%–90% of dietary potassium [1,2]. The forms of potassium in fruits and vegetables include potassium phosphate, sulfate, citrate, and others, but not potassium chloride (the form used in salt substitutes and some dietary supplements; see supplements section below) .
Selected food sources of potassium are listed in Table 2.
|Table 2: Selected Food Sources of Potassium |
|Apricots, dried, ½ cup||1,101||31|
|Lentils, cooked, 1 cup||731||21|
|Prunes, dried, ½ cup||699||20|
|Squash, acorn, mashed, 1 cup||644||18|
|Raisins, ½ cup||618||18|
|Potato, baked, flesh only, 1 medium||610||17|
|Kidney beans, canned, 1 cup||607||17|
|Orange juice, 1 cup||496||14|
|Soybeans, mature seeds, boiled, ½ cup||443||13|
|Banana, 1 medium||422||12|
|Milk, 1%, 1 cup||366||10|
|Spinach, raw, 2 cups||334||10|
|Chicken breast, boneless, grilled, 3 ounces||332||9|
|Yogurt, fruit variety, nonfat, 6 ounces||330||9|
|Salmon, Atlantic, farmed, cooked, 3 ounces||326||9|
|Beef, top sirloin, grilled, 3 ounces||315||9|
|Molasses, 1 tablespoon||308||9|
|Tomato, raw, 1 medium||292||8|
|Soymilk, 1 cup||287||8|
|Yogurt, Greek, plain, nonfat, 6 ounces||240||7|
|Broccoli, cooked, chopped, ½ cup||229||7|
|Cantaloupe, cubed, ½ cup||214||6|
|Turkey breast, roasted, 3 ounces||212||6|
|Asparagus, cooked, ½ cup||202||6|
|Apple, with skin, 1 medium||195||6|
|Cashew nuts, 1 ounce||187||5|
|Rice, brown, medium-grain, cooked, 1 cup||154||4|
|Tuna, light, canned in water, drained, 3 ounces||153||4|
|Coffee, brewed, 1 cup||116||3|
|Lettuce, iceberg, shredded, 1 cup||102||3|
|Peanut butter, 1 tablespoon||90||3|
|Tea, black, brewed, 1 cup||88||3|
|Flaxseed, whole, 1 tablespoon||84||2|
|Bread, whole-wheat, 1 slice||81||2|
|Egg, 1 large||69||2|
|Rice, white, medium-grain, cooked, 1 cup||54||2|
|Bread, white, 1 slice||37||1|
|Cheese, mozzarella, part skim, 1½ ounces||36||1|
|Oil (olive, corn, canola, or soybean), 1 tablespoon||0||0|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for potassium used as the basis for the values in Table 2 is 3,500 mg for adults and children aged 4 and older, but the DV will increase to 4,700 mg when the updated Nutrition and Supplement Facts labels are implemented [15,16]. The updated labels and DVs must appear on food products and dietary supplements beginning in January 2020, but they can be used now . Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture’s Nutrient Database website  lists the nutrient content of many foods and provides a comprehensive list of foods containing potassium ordered by food name and by nutrient content. The 2015–2020 Dietary Guidelines for Americans provides a list of foods with at least 5% of the DV for potassium per serving.
In dietary supplements, potassium is often present as potassium chloride, but many other forms—including potassium citrate, phosphate, aspartate, bicarbonate, and gluconate—are also used . The Supplement Facts panel on a dietary supplement label declares the amount of elementalpotassium in the product, not the weight of the entire potassium-containing compound. Some dietary supplements contain potassium iodide in microgram amounts, but this ingredient serves as a form of the mineral iodine, not potassium.
Not all multivitamin/mineral supplements contain potassium, but those that do typically provide about 80 mg potassium . Potassium-only supplements are also available, and most contain up to 99 mg potassium. Information on many dietary supplements that contain potassium is available in the Dietary Supplement Label Database from the National Institutes of Health, which contains label information from tens of thousands of dietary supplement products on the market.
Many dietary supplement manufacturers and distributors limit the amount of potassium in their products to 99 mg (which is only about 3% of the DV) because of two concerns related to potassium-containing drugs. First, the FDA has ruled that some oral drug products that contain potassium chloride and provide more than 99 mg potassium are not safe because they have been associated with small-bowel lesions . Second, the FDA requires some potassium salts containing more than 99 mg potassium per tablet to be labeled with a warning about the reports of small-bowel lesions [20,21]. In accordance with a ruling by Congress, the FDA may not limit the amount of any nutrient, including potassium, in a dietary supplement, except for safety-related reasons . However, the FDA has not issued a ruling about whether dietary supplements containing more than 99 mg potassium must carry a warning label [15,21].
Only a few studies have examined how well the various forms of potassium in dietary supplements are absorbed. A 2016 dose-response trial found that humans absorb about 94% of potassium gluconate in supplements, and this absorption rate is similar to that of potassium from potatoes . According to an older study, liquid forms of potassium chloride (used as drugs to treat conditions such as digitalis intoxication or arrhythmias due to hypokalemia) are absorbed within a few hours . Enteric coated tablet forms of potassium chloride (designed to prevent dissolution in the stomach but allow it in the small intestine) are not absorbed as rapidly as liquid forms .
Many salt substitutes contain potassium chloride as a replacement for some or all of the sodium chloride in salt. The potassium content of these products varies widely, from about 440 mg to 2,800 mg potassium per teaspoon . Some people, such as those with kidney disease or who are taking certain medications, should consult their healthcare provider before taking salt substitutes because of the risk of hyperkalemia posed by the high levels of potassium in these products.