Potassium

Potassium (kalium: atomic weight 39.098) is an alkali metal with valence I. Two of the three naturally occurring isotopes. 39 (93.22%) and 41 (6.77%). are stable: isotope 40 (0.012%) is radioactive.

Abbreviation

ADH antidiuretic hormone Nutritional summary

Function: Potassium is the main cationie osmolyte within cells. The element plays a major role in body electricity (maintenance of cellular polarity, neuronal signaling, heart impulse transmission, and muscle contraction), nutrient and metabolite transport, and enzyme activation.

Food sources: Fruits and vegetables tend to contain large amounts of potassium, whereas animal-derived foods and grains provide smaller amounts.

Requirements: Healthy adults should consume at least 1600mg potassium per day. Increased losses, often due to use of certain diuretics or laxatives, have to be balanced with higher intakes. Inadequate intakes lead to low plasma potassium concentration (hypokalemia) with an increased risk of heart arrhythmia, muscle weakness, paralysis, alkalosis (increased blood pll). and eventually death. Higher than minimal (but not excessive) intakes arc likely to be beneficial by lowering the risk of hypertension. Excessive intake: When potassium intakes exceed the amount that can be removed (about ISg/day), the rising plasma concentration (hyperkalemia) can cause muscle weakness, interference with electric conductance in the heart (arrhythmia), and eventually death due to cardiac arrest. The risk of hyperkalemia is particularly high in patients with renal failure.

Dietary sources

Both plant and animal-derived foods are among the best sources. Potassium-rich foods of piatlt origin include avocados (6Jmg,'g), spinach (raw 5.6g). bananas (4 mg/g), oats (3.5mgg), and rye Hour (3.4mg g). Relatively potassium-rich animal products are halibut (5.8mg/g), tuna (5.7mgg), mackerel (4.1 mg/g). and salmon <3.Smg'g). Slightly less is available in meats, such as pork (3.2 mg/g), beef (3.0 mg/g), and chicken (2.4mg,gl. The potassium content is relatively low in wheat flour < 1.5mg.'g), eggs (1.2 mg/g). and cheeses (around I mgg). and very low in rice (0,4 mgg). Orange juice (2.0 mg g) and milk (l.5mg g) are important sources because ofthe large consumed volumes. Losses with leaching during preparation or cooking of foods can be significant due to the high solubility of potassium salts in water. Boiled and drained spinach, for example, contains 17% less potassium than the same amount of raw spinach. The dillcr-ence between raw and cooked kale is nearly 50%.

Median intakes in the United States have been estimated at 3.2 g per day (Green etai, 2002),

Digestion and absorption

Small, but significant, amounts of potassium are secreted with saliva (15 54mmol day), gastric (4-12 mmol day), and pancreatic (5 15 mo! day) juice. Nearly all ingested and

Small Intesline: Enterocyte

I cr

Small Intesline: Enterocyte

I cr

Capillary endothelium

Brush border membrane

Figure 11.4 Intestinal absorption of potassium m

Capillary lomen

Brush border membrane

Capillary endothelium

Figure 11.4 Intestinal absorption of potassium secreted potassium is absorbed from the small intestine. Net secretion occurs in the distal colon.

Significant amounts of ingested and secreted potassium follow the osmolar gradient along the paracellular route. lo overcome the electric potential difference of 5 mV from lumen to basolateral fluid space, the potassium concentration in the lumen contents has to be at least 2mmol,l above the concentration m blood (Younoszai. 1989).

A hydrogen/potassium-exchanging ATPase ( EC3.6.3.10 ) at the luminal side ofthe distal colonic epithelium works in conjunction with potassium channels to recycle and conserve potassium. This exchanger is distinct from the one responsible for acid production in the stomach. The export of potassium into the basolateral space is similarly balanced by active reuptake via the sodium/potassium-exchangmg ATPase (IX'3.6.3.9). These potassium-recycling systems are essential for the efficient absorption of sodium and other cations.

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