Potassium in Human Nutrition and Health

Introduction

To maintain life and health, humans must consume and absorb through the intestinal tract adequate' amounts of the chemical element, potassium, — chemical symbol, K — in its ionic form (K⁺), consumed usually in the form of potassium salts of organic acids in food (e.g., potassium citrate, the potassium salt of citric acid), among which foods non-grain plant foods (vegetables and fruits) provide the richest supply of potassium salts of organic acids per calorie of food item consumed.

In regard to what constitutes an 'adequate' amount of dietary potassium, in 2004-2006, the Institute of Medicine Division of the National Academy of Science (IOM) ^[1] and its Food and Nutrition Board (FNB) ^[2] recommended that adult humans consume 4700 milligrams (mg) of potassium per day, which, calculated from the atomic weight of potassium (39.1 mg per mmol), corresponds to 120 millimoles (mmol) potassium per day: 4700 mg/39.1 mg/mmol=120 mmol. That recommended intake of potassium substantially exceeds estimates from recent surveys of average intakes of potassium by the general population, raising the possibility that a persisting state of suboptimal body potassium content, and rate of body throughput of potassium, prevails in the general population. ^[3] [4] Subsequent sections will consider arguments from some researchers that the IOM and FNB recommendations fall short of optimal. Subsequent sections will also discuss potassium intake recommendations for children and special groups.

Preliminary Considerations

Potassium ranks as the most abundant inorganic cation (positive ion) inside animal cells (intracellular), and as such contributes critically in numerous important ways to the optimal functioning of cells and therefore to optimal functioning of the organ systems and individuals composed of those cells. Among other metabolic functions, potassium plays a role in the synthesis of proteins and in the biochemical transformations required for carbohydrate metabolism.

By influencing the electrical potential difference across the cell membrane, the ratio of the concentrations of potassium in intracellular fluid (ICF) to that in the cells' surrounding extracellular fluid (ECF) has important effects on the rate of transmission of electrical activity (as ion current pulses) along nerve fibers and skeletal and smooth muscle cells, which, among other things, affects the degree of contraction of the smooth muscles of arteries and arterioles (vascular tone).^[5] Inasmuch as extracellular potassium varies in the 3-6 mmol/L range, while intracellular potassium concentrations average about 145 mmol/L, small changes in extracellular potassium concentration have a greater effect on the ICF-to-ECF potassium concentration ratio than similar small changes in intracellular potassium concentration. Subsequent sections discuss the implication of changes in the ICF-to-ECF potassium concentration ratio in human physiology.

In healthy persons whose daily consumption of potassium does not vary greatly, the amount of potassium consumed equals the amount excreted, by the kidney and gastrointestinal tract predominantly. Physiologists refer to that equality as zero net external body potassium balance. Of the major electrolytes, potassium has the highest ratio of potassium consumed to the amount of potassium in the extracellular compartment, a characteristic that presents a challenge in maintaining extracellular potassium concentrations within a set narrow range in the face of variations in daily potassium intake, a challenge met by efficient excretion and somewhat less efficient conservation, mediated by homeostatic mechanisms not completely elucidated.^[6]

Disturbances relating to body potassium deficiency may result from:

• prolonged inadequate consumption of potassium-containing foods;
• inappropriately large rates of excretion of potassium in urine;
• inappropriately large excretion rates of potassium in feces.

Disturbances relating to body potassium excess may result from:

• drugs and kidney diseases that impair the kidney’s ability to excrete potassium in urine;
• deficiency of hormones that act to promote kidney and gastrointestinal excretion of potassium.

Requirements for Potassium Consumption by Humans

Humans must habitually and regularly consume ionic potassium because the body does not build stores of it to serve as readily available reserves during prolonged periods of dietary potassium deprivation or abnormally increased external losses through excretion by the kidneys or gastrointestinal tract. The kidneys continue to excrete some amount of potassium in the urine even when potassium intake ceases for weeks to months.

Potassium-rich foods include most non-cereal-grain plant sources of food: leafy green vegetables (e.g., spinach, lettuces, chard, broccoli, kale), vine fruits (e.g., squash, tomatoes, cucumbers, etc.), root vegetables (e.g., carrots, rutabagas), and tree fruits (see below).

The Institute of Medicine of the National Academies of Science^[1] and its Food and Nutrition Board^[2] recommend as "Adequate Intake" (AI) of potassium, in mmol/day, as 77 and 97 for children ages 1-3 and 4-8 years, respectively, and as 115 and 120 for children 9-13 and 14-18 years, respectively. For adult men and women, ages 19 to >70 years, they recommend an AI of potassium as 120 mmol/day, and the same amount for pregnant women as young as 14 years, increasing to 130 mmol/day for lactating women.^[7] The Institue of Medicine of the National Academies of Science claims:

— The AIs for potassium are based on a level of dietary intake that should maintain lower blood pressure levels, reduce the adverse effects of sodium chloride intake on blood pressure, reduce the risk of recurrent kidney stones, and possibly decrease bone loss. — ^[1]

The claim reflects concerns about inadequate potassium consumption in:

• contributing to hypertension (abnormally high arterial blood pressure), at least in part through its effects to constrict the small arteries (arterioles) that deliver blood to muscles and other organs, and to promote extracellular fluid volume expansion through renal retention of sodium chloride;
• mitigating the effect of dietary sodium chloride ('salt') in contributing to hypertension, to kidney stone formation, and to osteoporosis (soft, fracture-prone bones).

   Actual consumption of potassium by Americans, 2003-2004 and 2005-2006

The U.S. Department of Agriculture released in 2008 the results of the National Health and Nutrition Examination Survey (NHANES) data for 2005-2006, giving the average values for the consumption of various nutrients, including potassium. ^[4] The table below shows the average values for potassium consumption by Americans (2005-2006) with the recommended amounts given in the preceding section.

Because the Food and Nutrition Board asserts that eating ordinary foods imposes no danger of consuming excess potassium except in disease states characterized potassium non-tolerance, most Americans could likely achieve "adequate intakes" or more by doubling their potassium intake.

Note that average consumption of potassium by adults falls well below recommended amounts, with American women consuming half the recommended amount on average, and American men about two-thirds. Of note, those findings do not indicate improvement in achieving "adequate intakes" over the findings reported for years 2003-2004.^{[3] [8]}

Children and adolescents showed a similar range of insufficiencies of potassium consumption in the two reports. ^[3] [4]

Without increasing total energy (calorie) intake, Americans could increase potassium consumption to achieve "adequate intake" by reducing intake of potassium-poor foods and increasing intake of potassium-rich foods.^[9]

Potassium Content of Foods

To fully understand the biological effects of dietary potassium ions (cations) requires understanding the nature and effects of the negatively charged ions (anions) accompanying potassium in foods, balancing potassium’s positive charge, maintaining electroneutrality. In natural diets not subjected to commercial processing that includes addition of potassium salts—typically potassium chloride—the anions accompanying potassium in foods comprise mostly a variety of organic anions (e.g., citrate, fumarate), in amounts sufficient to nearly balance the positive charge of the potassium ions (i.e., in near chemical equivalent amounts).^[10] Following absorption of the ingested potassium charge-neutralizing organic anions by the gastrointestinal tract, the body converts a large fraction of them to bicarbonate (an acid-neutralizing substance, or base), as an end-product of metabolism.^[11] Thus, diets with differing amounts of potassium exert potassium-induced biological effects associated with, and often interacting with, the effects of the differing amounts of the acid-neutralizing base, bicarbonate, as generated by the body from the potassium-accompanying organic anions.

Physiologists often cannot dissect out the specific effects of the co-ions potassium and bicarbonate, when, for example, a person increases their dietary intake of potassium-rich foods, which typically contain bicarbonate-generating organic anions in abundance (see the table below and its accompanying text).

The table below shows the potassium content of the major food groups, indicating the relation of potassium content to the net acid (or bicarbonate) load supplied to the body by each food group (see comments following table). [NB: Because potassium ions have a single charge (univalent), 1 millimole (mmol) of potassium equals 1 milliequivalent (meq) of potassium.]

The table shows variously positive (+) and negative (–) values of net acid load to the body, representing acid-producing and bicarbonate-producing chemical equivalents, respectively, in the units specified. Values of net acid load: calculated first for individual food items then averaged per food group, as per the nutrient compositional values reported in:^[12]. Acid load calculations: as described in:^[13]

Note that net acid-producing foods tend to have much higher ratios of protein-to-potassium than do net bicarbonate-producing foods (regression of net acid load against the ratio, Protein-to-Potassium, gave a statistically correlation coefficient: r=0.48, p=0.05). Note the relatively low values of protein and potassium in the cereal grain group, of which whole grains comprised six of the seven items in the group.

The table reveals a number of important aspects of food potassium:

• Per unit energy content (kilocalories, abbrev: kcal), non-cereal-grain plant foods provide the most abundant source of potassium. In other words, to increase consumption of potassium, one needs ingest fewer kilocalories consuming non-cereal-grain plant foods than any other major food group.
• The top five plant-food sources of potassium (meq/kcal): root vegetables (celeriac, rutabaga, turnips, carrots, parsnips, sweet potato, potato, yams, onions); vegetable fruit (aka vine fruit) (tomatoes, zucchini, eggplant, cucumbers); leafy greens (spinach, lettuce, kale, chard); stalks (celery, broccoli stalks); mushrooms (not strictly a plant).
• The food sources richest in potassium also supply bicarbonate to the body, as indicated by negative values of "net acid load". A negative value of "net acid load" corresponds to a positive value of "net base load".
• The potassium content in acid-producing foods (animal-source foods and cereal grains) average about one-fifth that of plant-source foods. Thus animal source foods in general do not contain sufficient quantities to yield on metabolism the amounts of bicarbonate required to neutralize the acid generated from protein metabolism and other substances they contain.
• The ratio of protein-to-potassium in plant-source foods falls short of the ratio in animal-source foods by a factor of 10.
• Legumes provide moderate amounts of potassium with little or no acid or bicarbonate load.

The table below shows the content of potassium in selected food items, expressed in mmol of potassium per 100 kcal of food item, an expression that enables consumers planning to increase their potassium (and base) intake to choose food items for potassium consumption while remaining cognizant of the calorie cost of the food item. Note the arbitrary selection of food items in the table.

The Paleolithic Perspective on the 'Optimal' Dietary Requirement of Humans

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References and Notes Cited in Text as Superscripts

1. ↑ ^{1.0 1.1 1.2} Otten JJ, Hellwig JP, Meyers LD (editors) 2006) Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. National Academies Press. Pages 370-379. ISBN 0-309-65646-X
   • Excerpt: Another dietary measure to lower blood pressure is to consume a diet rich in potassium. A potassium-rich diet also blunts the effects of salt on blood pressure, may reduce the risk of developing kidney stones, and possibly decrease bone loss with age. The recommended intake of potassium for adolescents and adults is 4,700 mg/day. Recommended intakes for potassium for children 1 to 3 years of age is 3,000 mg/day, 4 to 8 years of age is 3,800 mg/day, and 9 to 13 years of age is 4,500 mg/day. Potassium should come from food sources. Potassium-rich fruits and vegetables include leafy green vegetables, fruit from vines, and root vegetables. Although meat, milk, and cereal products contain potassium, the form of potassium in these foods is not as readily available for absorption.
2. ↑ ^{2.0 2.1} Panel on Dietary Reference Intakes for Electrolytes and Water. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Food and Nutrition Board. Institute of Medicine of The National Academies (2004) Dietary Reference Intakes For Water, Potassium, Sodium, Chloride, and Sulfate “Potassium” pp. 186-268. The National Academies Press, Washington, D.C.
3. ↑ ^{3.0 3.1 3.2} What We Eat in America, NHANES 2003-2004, Tables. 1. Nutrient Intakes: Mean Amounts Consumed per Individual, One Day, 2003-2004 (Downloadable PDF File)
4. ↑ ^{4.0 4.1 4.2} Nutrient Intakes: Mean Amounts Consumed per Individual, One Day, 2005-2006. U.S. Department of Agriculture, Agricultural Research Services, Fast Facts, Reports/Articles, and Tables (2005-2006).
5. ↑ Moczydlowski EG. (2009) Electrophysiology of the Cell Membrane. In: Boron WF, Boulpaep EL (editors), Medical Physiology, 2nd ed. Saunders/Elsevier: Philadelphia. ISBN 9781416031154.
6. ↑ Youn JH, McDonough AA. Recent Advances in Understanding Integrative Control of Potassium Homeostasis. Annu Rev Physiol. 2008 Aug 29. Abstract
   • Abstract: The potassium homeostatic system is very tightly regulated. Recent studies have shed light on the sensing and molecular mechanisms responsible for this tight control. In addition to classic feedback regulation mediated by a rise in extracellular fluid (ECF) [K+], there is evidence for a feedforward mechanism: Dietary K+ intake is sensed in the gut, and an unidentified gut factor is activated to stimulate renal K+ excretion. This pathway may explain renal and extrarenal responses to altered K+ intake that occur independently of changes in ECF [K+]. Mechanisms for conserving ECF K+ during fasting or K+ deprivation have been described: Kidney NADPH oxidase activation initiates a cascade that provokes the retraction of K+ channels from the cell membrane, and muscle becomes resistant to insulin stimulation of cellular K+ uptake. How these mechanisms are triggered by K+ deprivation remains unclear. Cellular AMP kinase–dependent protein kinase activity provokes the acute transfer of K+ from the ECF to the ICF, which may be important in exercise or ischemia. These recent advances may shed light on the beneficial effects of a high-K+ diet for the cardiovascular system.
7. ↑ Note: The Food and Nutrition Board of the U.S. Institute of Medicine sets values for a parameter called "Adequate Intake" (AI) for nutrients when insufficient data precludes setting a "Recommended Dietary Intake" (RDA) for a nutrient. It bases AI values on the amount of the nutrient a group (or groups) of apparently healthy people consume, as determined by observation or experiment. Of particular note, determination of AI values requires judgments of state of health, in particular in reference to the nutrient’s currently known health effects.
8. ↑ Kimmons J. Gillespie C, Seymour J, Serdula M, Blanck HM. (2009) Fruit and Vegetable Intake Among Adolescents and Adults in the United States: Percentage: Meeting Individualized Recommendations. Medscape J. Med. 11(1):26. Full Text.

   • Abstract: Context: Fruit and vegetable intake is an important part of a healthy diet and is associated with numerous positive health outcomes. MyPyramid provides recommendations for fruit and vegetable consumption based on individual calorie requirements as determined by an individual's age, sex, and physical activity level. Objectives: To determine (1) median fruit and vegetable consumption from all dietary sources among adolescent and adult consumers and the percentage of adolescents and adults meeting individual recommended intake levels based on caloric requirements and (2) consumption levels among various demographic groups, intake levels from subtypes of fruits and vegetables, and primary contributors to fruit and vegetable intake. Design: Analysis of 2-day, 24-hour recall data from the 2003-2004 National Health and Nutrition Examination Survey (NHANES), a continuous, nationally representative, cross-sectional survey. Results: This study included dietary contributions of fruits and vegetables from all dietary sources. Fewer than 1 in 10 Americans meet their calorie-specific MyPyramid fruit or vegetable recommendations. Higher intake was not observed in subgroups with higher recommendations for fruit and vegetable consumption based on caloric requirements. The primary contributors to total fruit intake were whole fruits among adults and fruit juices among adolescents. The largest single contributor to overall fruit intake was orange juice. Potatoes dominated vegetable consumption, particularly among adolescents, in whom fried potatoes increased the median vegetable intake from 0.72 cup to 1.21 cups per day. Dark green and orange vegetables and legumes accounted for a small portion of vegetable intake, and few people met the recommendations. Conclusions: Few American adolescents or adults reported consuming the recommended amounts of fruits or vegetables. Increasing consumption will probably require multifaceted approaches that augment educational campaigns with policy and environmental strategies aimed at the food system at large, from farm to plate, including schools, worksites, and retail establishments. Increasing America's fruit and vegetable consumption is an important public health strategy for weight management and reduction of risk for chronic disease.

9. ↑ Americans still refusing to eat their veggies: Less than one-third of adults eat recommended daily servings, survey shows.
10. ↑ Note: Inorganic phosphates account for a smaller fraction of the potassium charge-neutralizing anions than do organic anions.
11. ↑ Note:  If one consumes citric acid, for example, the body metabolizes it to carbon dioxide and water. If one consumes potassium citrate, the body metabolizes it to potassium bicarbonate.
12. ↑ Souci SW, Fachmann W, Kraut H. (2000) Food Composition and Nutrition Tables. Stuttgart, Germany: Medpharm GmbH Scientific Publishers
13. ↑ Sebastian A, Frassetto LA, Sellmeyer DE, Merriam RL, Morris RC, Jr. (2002) Estimation of the net acid load of the diet of ancestral preagricultural Homo sapiens and their hominid ancestors. American Journal Clinical Nutrition 76:1308-16

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