Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

chlorine, thus explaining why it is common for both

alkali metals to appear in the form of their correspond-

ing chloride salts. Given this relationship with cellular

liquids, potassium, sodium, and chlorine are also

known as electrolytes (ions with greater proportions

in the composition of organic fluids).

0005 The K

ion is the main intracellular cation, with an

approximate [K

] of 5.6 gl

1

in cellular fluids, ap-

proximately 30 times more concentrated than in

plasma or interstitial liquid (0.15–0.20 gl

1

). Its

high intracellular concentration is regulated by the

cell membrane through the sodium–potassium pump.

The intracellular K

regulates the catalytic action of

numerous enzymes, through the attachment of the

cation to active locations with negative charges of

the enzymatic proteins, modifying the conformation

of the molecule and its activity, as well as participat-

ing in cellular division processes.

0006 The small percentage of extracellular potassium

(2% of body potassium) is of great physiological

importance, since it is a critical determinant of neuro-

muscular excitability (nervous impulse and contrac-

tion of bone muscles). At the cellular membrane level,

the transport and permeability of energy-dependent

potassium, with simultaneous excretion of sodium

linked to the Na/K enzyme ATPase, is essential for

generating the potentials of membranes required for

the proper functioning of nervous and muscular cells.

It also helps to maintain the acid–base equilibrium

and blood pressure.

0007 Homeostasis of potassium is still the subject of

research, although it is known that 90% of dietetic

potassium is absorbed in the small intestine and that

body potassium (1.6–2 g per kilogram of body

weight) is regulated by renal glomerular filtration

and tubular secretion, potassium being lost on a

daily basis through urine, gastrointestinal secretions

(ileum and colon) and, to a lesser degree, sweat. Pro-

vided that renal function is normal, it is practically

impossible to reach an excessive level of potassium

with normal dietary intake, since the kidney is capable

of excreting more potassium than it can filter. (See

Potassium: Physiology; Sodium: Physiology.)

0008 The effect of potassium on blood pressure has been

discussed in recent reports on metaanalysis; these

have confirmed that increases in doses of potassium

from 60 to 80 mmol per day (2.3–3.1 g per day) may

prompt a decrease of 4 mmHg in systolic blood pres-

sure and possibly reduce the number of deaths related

to high blood pressure by 25%.

0009 The use of potassium chloride salts as substitutes

for sodium chloride, in individuals in whom the

intake of sodium is restricted because of problems

relating to hypertension, is a questionable alternative,

since cardiac arrythmias have been reported in

association with the excessive intake of salt substitutes

containing potassium chloride. In this connection,

hyperkalemia is recommended in healthy individuals

for intakes exceeding 17.5 g per day (acute toxicity

limit), highly unlikely in normal diets, and certain

risks for individuals with renal dysfunctions not

detected with intakes of potassium exceeding 5.9 g

per day (safest maximum dose). Cardiovascular or

neuromuscular complications arising from situations

of hypo- and hyperkalemias are resolved favorably by

correcting the plasmatic potassium levels. (See Hyper-

tension: Hypertension and Diet.)

0010The mean potassium intake in Western populations

ranges between 1.6–5.9 g per day and the required

dietary intakes of this are met without any problem,

thanks to the ubiquitous presence of potassium in

both vegetable- and animal-based foods. The Scien-

tific Committee for Food of the European Commu-

nity recognizes that the Minimum Recommended

Intake of potassium is 1600 mg d

1

, and the Refer-

ence Intake for the population 3100 g d

1

Analysis of Potassium

0011Potassium can be analyzed using a number of differ-

ent methods, although many of these are not com-

monly used for routine analysis of potassium in food

and biological samples. In the case of potassium,

there has been a shift from the old gravimetric

methods, based on the precipitation of potassium

using chloroplatinate or tetraphenylboron, to the

spectroscopic methods, mainly flame emission and

flame absorbance spectroscopy and which, according

to the scientific community, are the most commonly

used.

0012However, there are other techniques for analyzing

potassium that are not as ‘popular’ as those men-

tioned above, and equally applicable to other mineral

elements. These include selective electrodes, nuclear

magnetic resonance spectroscopy, X-ray analysis,

helium glow photometry, inductively coupled plasma

optical emission, inductively coupled plasma atomic

fluorescence, ion-scattering spectrometry, and other

methods. Another procedure can be used – namely,

radioactive dilution of potassium isotopes – when

determining both potassium content and its distribu-

tion in extra- and intracellular compartments.

0013Emission and absorbance spectrometry have been

the most widely used techniques for analyzing trace

elements in biological and food samples. Their wide-

spread use is justified by their analytical specificity,

good detection limits, excellent accuracy, and rela-

tively low cost. Both techniques are based on energy

modifications of the electronic orbital structure of the

atoms of mineral elements in response to certain

POTASSIUM/Properties and Determination 4645

stimuli (flame emission spectrometry and incidence of

a beam of light of specific wavelength for absorption).

(See Sodium: Properties and Determination.)

0014 The determination of potassium by spectroscopy

requires a stage of preparation of samples for analysis

that entails the drying, grinding, and destruction of

the organic matter of the sample. There are two alter-

natives for destroying the organic matter of biological

and food samples, both with different variants: wet

oxidation and dry ashing. Both techniques yield rea-

sonably comparable results, and it is often the analyst

who decides on which technique to employ. (See Cad-

mium: Properties and Determination; Spectroscopy:

Atomic Emission and Absorption.)

Flame Emission Spectroscopy

0015 The determination of potassium in biological and

food samples, as in the case of sodium, may be per-

formed using flame photometers and atomic absorp-

tion spectrophotometers (with the option of working

in emission conditions). A specific wavelength of

766.5 nm is used in order to determine potassium,

and the detector is adjusted to the response given to

the pattern of greatest concentration used. Efforts

must be made to avoid possible interference problems

owing to the matrix characteristics of the sample in

flame emission photometry; this can be achieved by

separating the element from the object of study, elim-

inating the ions responsible for the interferences, or

using a compensatory technique. For this purpose,

recommended practice for correcting interferences

includes the addition of lithium (as an internal stand-

ard) both to the samples and to the standards. It is

important to obtain a suitable calibration curve,

either linear by diluting the samples, or by logarith-

mic adjustment between emission values and potas-

sium concentration. (See Sodium: Properties and

Determination.)

Flame Absorption Spectroscopy

0016 The determination of potassium using an atomic ab-

sorption spectrophotometer requires a light source

(wavelength for K ¼766.5 nm) and an atomization

source (flame). For the light source, arc discharge

lamps may be used for K, although hollow cathodes

are preferred. Standard working conditions for ana-

lyzing potassium by atomic absorption spectroscopy

are listed in Table 1.

0017 Comparing flame emission photometry and atomic

absorption spectroscopy, the detection limits for po-

tassium (mgml

1

) are 0.0005 and 0.005, respectively.

Using an atomic absorption spectrophotometer, the

accuracy and precision of potassium analysis by

flame emission can be obtained (Table 2).

0018Table 3 shows a series of official methods for de-

termining potassium, based on flame emission and

absorption spectrophotometry.

Inductively Coupled Plasma-atomic Emission

Spectrometry (ICP-AES)

0019There are numerous references to the use of this

powerful technique in multielement analysis by emis-

sion spectroscopy. ICP-AES enables samples with a

high degree of variability and minimum interferences

to be used. Samples can be prepared by either wet

oxidation or dry ashing, and a wide range of concen-

trations can be used without the need to dilute or

concentrate the sample. Also, many of the other min-

eral elements can be quantified, thus reducing analy-

sis time. These advantages, together with the high

speed and excellent instrument stability, make ICP-

AES highly attractive.

0020For potassium determined by ICP-AES in plant

tissues, a solution detection limit of 0.06 (mgml

1

)

and a sample detection limit of 2.0 (mgg

1

, based on

a 2-g sample diluted to 50 ml) are recommended.

There is also an AOAC Official Method (984.27)

for the determination of potassium in infant formula

by means of ICP-AES, which recommends the

following ICP emission spectrometer operating par-

ameters for potassium; wavelength (nm) ¼766.5, no

background correction and low standard ¼0 and

high standard ¼ 200 (mgml

1

). The AOAC also rec-

ommends an Official Method (985.01) for the analy-

sis of potassium in plants and pet foods by ICP

spectroscopy.

Potassium-selective Electrodes

0021Potassium-selective electrodes have developed from

the first ion-selective glass electrodes, with little ion

selectivity, to potassium-selective electrodes based

on valinomycin, a highly specific neutral carrier

compound for analyzing potassium, to potassium-

sensitive electrodes with ion-exchange compounds

dissolved in organic solvents that are not very specific

tbl0001Table 1 Standard conditions for potassium analysis by atomic

absorption spectrometry

Spectrometers setting Potassium

Wavelength/slit (nm) 766.5/0.2

Nebulizer Spoiler

Oxidant Air

Fuel C

Flame condition Oxidizing

Optimum concentration range in solution (mgml

1

) 1–10

Detection limit (mgml

1

) 0.005

Sensitivity 1% absorption (mgml

1

) 0.05

Interferences Ionization

4646 POTASSIUM/Properties and Determination

because of significant interference induced by the

presence of sodium and other cations in organic solu-

tions. Ion-exchange electrodes have faster response

times than those of organic solvent solution mem-

branes of valinomycin, although the latter show

fewer interferences from other ions. However, neither

type of electrode is effective in organic solvents. Fur-

thermore, electrodes measure ion activity and there-

fore do not detect ions linked to other molecules or

potassium that is not in solution.

0022 Ion-selective electrodes are also incorporated into

gasometric equipment for measuring electrolytes (Na,

K, Cl, and ionized Ca), in oxygenation, ventilation,

basic-acidic state, and electrolytic metabolism of pa-

tients with respiratory problems. In this context, it is

useful to quantify the exchange of gases and electro-

lytes through cellular membranes; for this purpose,

the so-called anion gap is calculated, which inter-

relates with sodium (anion gap Na), potassium

(anion gap K), bicarbonate, and chlorine. This par-

ameter provides information on electrolytic alter-

ations and the presence of toxins in blood, and also

helps to control quality in laboratories by interrelat-

ing informed values (in normal individuals, not very

high values).

0023 Some developments in the field of potassium-

selective electrodes have focused on the appearance

of novel potassium-selective valinomycins in the

development of ion-selective sensors based on new

technologies and on the combination of these ion-

selective electrodes with other analytical methodolo-

gies, as shown in Table 4.

Nuclear Magnetic Resonance (NMR)

0024NMR is used to study the relationship between metab-

olism and function in living systems as simple as cells in

culture and as complex as human subjects (e.g., the

effect of temperature on the sodium/potassium pump

in red blood cells). As a noninvasive, nondestructive

spectroscopic technique, NMR offers a powerful ap-

proach to the study of ion balance in intact biological

systems. The aims are to validate new NMR methods,

to elucidate tissue-specific mechanisms, and to draw

conclusions with respect to basic concepts and design in

tissue metabolism and function.

0025The possibility of determining potassium by NMR

is due, on the one hand, to the relatively high concen-

trations of the cation in cellular tissue and, on the

other hand, to the natural abundance of

K nuclide

(NMR sensitivity is about 2000 times less than that of

one hydrogen atom) and the short longitudinal relax-

ation time. The concentrations of intracellular and

extracellular potassium in tissues may be quantified

by applying impermeable cell-membrane chemical

shift reagents.

0026However, this quantification by NMR measurement

depends on the NMR invisibility of the potassium

actually present, in accordance with other techniques.

Several previous studies have shown that only approxi-

mately 20% of cardiac intracellular potassium is visible

with current NMR techniques. This NMR sensitivity

of potassium (about 60%) represents a serious obstacle

to its proper quantification in biological tissues. (See

Spectroscopy: Nuclear Magnetic Resonance.)

tbl0003 Table 3 Official methods recognized for potassium determination by flame emission and absorption spectrophotometry

Officialmethods

Sodium and Potassium in Seafood Flame Photometric Method, AOAC Official Method 969.23

Potassium in Fruits and Fruit Products Rapid Flame Photometric Method, AOAC Official Method 965.30

Potassium and Sodium in Wines Flame Spectrophotometric Method, AOAC Official Method 963.13

Potassium in Distilled Liquors Flame Photometric Method, AOAC Official Method 963.08

Potassium in Beer Atomic Absorption Spectrophotometric Method, AOAC Official Method 987.02

Potassium in Water Atomic Absorption Spectrophotometric Method, AOAC Official Method 973.53

Potassium in Infant Formula, Enteral Products and

Pet Foods

Atomic Absorption Spectrophotometric Method, AOAC Official Method 985.35

tbl0002 Table 2 Accuracy and precision of the potassium analysis by flame emission

Accuracy Certified (g kg

1

)Found(gkg

1

)IC(95%)

rec (RSD)

Citrus leaves SRM-1572 18.2 + 0.6 17.7 + 0.5 16.8–18.7 97 (3)

Nonfat milk powder 16.9 + 0.30 16.3 + 0.41 15.5–17.1 96 (5)

NIST-1549

Precision RSD

Plant food 1.35

Dairy food 2.55

Intervals of confidence (95%).

Recovery percentages and relative standard deviation (RSD).

POTASSIUM/Properties and Determination 4647

0027 A study of factors affecting

K NMR detectability

in rat thigh muscle showed that the signal may be

substantially higher (up to 100% of total tissue po-

tassium) than values previously reported of around

40%, these signals presenting two superimposed

components – one broad and one narrow – and in-

volving improvements in spectral parameters (signal-

to-noise ratio and baseline roll), together with

computer simulations of spectra that enable a spec-

trum quality with a major effect on the amount of

signal detected, largely owing to the loss of detect-

ability of the broad signal component.

0028 Biological and biochemical cellular research in

connection with the quantification of ions in intracel-

lular and extracellular compartments, transport and

ionic metabolism at the level of different cells

and biological tissues, are based on the use of

NMR spectroscopy, together with

Na and

NMR studies.

0029 Proper ion equilibrium between intra- and extracel-

lular compartments is required for normal physio-

logical function. Conversely, alterations in membrane

ion transport occur in numerous pathological states. By

introducing an anionic paramagnetic shift reagent into

the medium, NMR signals of intra- and extracellular

and K

can be resolved, enabling ion transport

processes to be studied by NMR. Unfortunately, rare

NMR active nuclides that are isotopes of the 100%

naturally abundant

and

are not available

for tracer kinetic studies of Na

and K

transport.

However, Cs is a biologically active analog of K

,and

the 100% naturally abundant NMR active

133

nu-

clide can be employed to examine K

transport. Other

studies have shown the potential of

K NMR as a

useful tool in the study of protein–cation interactions

and the binding of K

to double-helical DNA.

X-ray Analysis

0030X-ray analysis is another nondestructive, noninva-

sive, in-vivo technique for determining mineral elem-

ents – including potassium – in foods and biological

samples. X-ray analysis is based on bombarding a

small area of the powdered sample with high-energy

X-rays, measuring the Ka line of the element using

one of several different detectors according to the

wavelength of the emitted radiation. The limitations

of this technique are related to scope, in terms of

detected elements and their concentration range.

X-ray analysis is combined with the use of an elec-

tronic microscope for quantifying and locating the

mineral element in the sample, even at the subnuclear

compartmental level. Another limitation of the X-ray

tbl0004 Table 4 Analytical methodologies with ion-selective electrodes for potassium analysis

Investigation Reference

A potassium-ion selective electrode with valinomycin-based

poly(vinyl chloride) membrane and a poly(vinyl ferrocene)

solid contact

Hauser PC, Chiang DWL. and Wright GA (1995) Analytica Chimica

Acta 302(2–3): 241–248.

Miniaturized ion-selective sensor chip for potassium

measurement in a biomedical application

Uhlig A, Dietrich F, Schnakenberg U, Hintsche R and Lindner E

(1996) Sensors and Actuators B: Chemical Abstract Export

34(1–3): 252–257.

Synthesis of novel potassium selective valinomycins Dawson JR, Dory YL, Mellor JM and McAleer JF et al. (1996)

Tetrahedron 52(4): 1361–1378.

Optical sensors for sodium, potassium, and ammonium ions

based on lipophilic fluorescein anionic dye and neutral

carriers

Wang E, Zhu L, Ma L and Patel H (1997) Analytica Chimica Acta

357: 85–90.

Recording of neuronal network properties with near-infrared

dark-field microscopy and microelectrodes

Holthoff K and Witte OW (1997) Electrochimica Acta 42(20–22):

3241–3246.

Flow injection analysis of potassium using an all-solid-state

potassium-selective electrode as a detector

Komaba S, Arakawa J, Seyama M, Osaka T, Satoh I and

Nakamura S (1998) Ta la n t a 46: 1293–1297.

Transferability of results obtained for sodium, potassium and

chloride ions with different analyzers

Rodriguez-Garcia J, Sogo T, Otero S and Paz JM (1998) Clinica

Chimica Acta 275: 151–162.

Plasticizer-free all-solid-state potassium-selective electrode

based on poly(3-octylthiophene) and valinomycin

Bobacka J, Ivaska A and Lewenstam A (1999) Analytica Chimica

Acta 385: 195–202.

Simultaneous detection of monovalent anions and cations using

all solid-state contact PVC membrane anion and cation-

selective electrodes as detectors in single column ion

chromatography

Isildak I and Asan A (1999) Ta l a n t a 48: 967–978.

Novel sensors for potassium, calcium, and magnesium ions

based on a silicon transducer as a light-addressable

potentiometric sensor

Seki A, Motoya K, Watanabe S and Kubo I (1999) Analytica Chimica

Acta 382: 131–136.

Reference ranges of electrolyte and anion gap on the Beckman

E4A, Beckman Synchron CX5, Nova CRT, and Nova Stat Profile

Ultra

Lolekha PH, Vanavanan S, Teerakarnjana N and

Chaichanajarernkul U (2001) Clinica Chimica Acta 307: 87–93.

4648 POTASSIUM/Properties and Determination

technique relates to the complexity of the sample

preparation process for electronic microscopy.

0031 The bibliography describes different X-ray analysis

techniques for measuring potassium in food and bio-

logical samples, as indicated in Table 5.

0032 X-ray fluorescence (XRF) can be successfully used

for the qualitative and quantitative elemental analysis

of various agricultural products. Its simplicity, high

throughput and automation possibilities make it

useful for screening large numbers of samples. The

K content of tea samples has been determined by XRF

analysis (an uncommon method for mineral analysis

in food), and the findings have been compared with

the results obtained using atomic emission tech-

niques, with the conclusion that the XRF system can

be used effectively for quantitative analysis of the

K content of tea samples.

0033 The methods for preparing the specimens of liquid

media of the organism and XRF analysis are simple

and fast, entail no disintegration of the sample, and

allow measurements of elements at sigma 1 ¼0.02 at

concentrations of 12 mgml

1

to be obtained. The

method is preferable and promising for several basic

elements (P, S, Cl, K, Ca), which are difficult to meas-

ure using other methods.

0034 The use of total reflection X-ray fluorescence an-

alysis (TXRF) in life sciences is considered a powerful

analytical tool for simultaneous multielement deter-

mination. TXRF is basically an energy-dispersive

technique, with the sample being excitated in total

reflection geometry. This technique only requires

minute samples with simple preparation and involves

a large dynamic measuring scale. Simultaneous detec-

tion of almost all chemical elements and lower limits

of detection are achievable in optimized excitation

conditions. The preferred sample types are aqueous

and acidic solutions, particularly samples digested

with HNO

. Special preparation techniques are re-

quired for solids and other samples.

0035The application of the proton-induced X-ray-emis-

sion (PIXE) method has enabled us to determine not

only the concentrations of elemental composition,

including K, but also their localization in different

artery-wall regions. Furthermore, the usefulness of

the micro-PIXE method for studying biomedical ma-

terials has also been considered.

Radioactive Isotopes

0036The dilution of radioactive isotopes of potassium is

applied to establish the concentration and distribu-

tion of the amounts of potassium in the different

sample compartments. The dilution of radioactive

isotopes enables the volumes of the organic tissue

compartments to be determined. The differences in

potassium concentrations between the extra- and

intracellular spaces may be studied by diluting the

radioactive isotopes of potassium.

0037For this purpose, it is important to take into ac-

count that animal and plant tissues present different

potassium compartments in which the ion is accumu-

lated, and this conditions the radioactive isotope

dilution process. In animal tissue, three cellular com-

partments (plasma þextracellular fluid, cells, and

spaces where ions are closely fixed or form com-

plexes) are described, whereas in plant tissue, only

two spaces (cytoplasm and vacuoles) must be con-

sidered, since the extracellular space is small in size.

This compartmental distribution conditions the pro-

cess of radioactive isotope dilution.

0038In animal tissue, the method involves incorporating

a known amount of the tracer isotope of potassium

into the extracellular compartment and monitoring

the changes in plasma isotope concentration until

equilibrium is reached with all the compartments.

This entails estimating the distribution of potassium

(1) in extracellular fluid, using tracer polymers (inulin

or polyethylene glycols) that cannot penetrate cell

membranes, (2) in cellular volume, by subtracting

the extracellular volume from the total distribution

space (determined using labeled water, tritiated or

deuterated), and (3) in the space where the ion is

strongly fixed or forms complexes (this is calculated

by subtracting the amount of extracellular potassium

þcellular potassium from the amount of potassium

isotope injected).

0039In plant tissue, the dilution of radioactive isotopes of

potassium presents limitations owing to the difficulty

involved in terms of separating vacuolar volume and

cytoplasmic volume. For this reason, more appropriate

methods than radioactive dilution methods have been

tbl0005 Table 5 X-ray techniques for potassium analysis in food and

biological samples

Samples X-ray analysis

Meat and meat products X-ray fluorescent analysis

Serum and peripheral blood cells

Fresh green tea, black tea, and

tea residue

Leaves of lettuce Electron microprobe X-ray

analysis

Walls of the mosaic virus-infected

wheat leaf cells

Energy-dispersive X-ray

analysis

Perisperm tissues of seeds

Cancerous and normal tissues Total reflection X-ray

fluorescence analysis

In vegetable foodstuffs and their

respective cell fractions

Cancerous breast tissue

Human brains Proton-induced X-ray

emission analysis

Human atherosclerotic artery wall

POTASSIUM/Properties and Determination 4649

developed for determining cytoplasmic and vacuolar

potassium concentrations, namely ion-selective micro-

electrodes or X-ray microprobe analysis.

See also: Cadmium: Properties and Determination;

Hypertension: Hypertension and Diet; Potassium:

Physiology; Sodium: Properties and Determination;

Physiology; Spectroscopy: Atomic Emission and

Absorption; Nuclear Magnetic Resonance

Further Reading

Ammon D (1986) Ion Selective Microelectrodes. New

York: Springer-Verlag.

Benton Jones J, Jr. (1984) Developments in the measure-

ment of trace metal constituents in foods. In: Gilbert J

(ed.) Analysis of Food Contaminants, pp. 157–202,

Barking, UK: Elsevier.

Cappuccio FP and MacGregor GA (1991) Does potassium

supplementation lower blood pressure? A meta-analysis

of published trials. Journal of Hypertension 9: 465–473.

Commission of the European Communities (1993) Nutrient

and Energy Intakes for the European Community.

Reports of the Scientific Committee for Food, Thirty-

first Series, pp. 174–178.

Gunther K and von Bohlen A (1990) Simultaneous multi-

element determination in vegetable foodstuffs and their

respective cell fractions by total-reflection X-ray fluor-

escence (TXRF). Zeitschrift fu

r Lebensmitteluntersu-

chung und -forschung 190(4): 331–335.

Intersalt Cooperative Research Group (1988) Intersalt: a

international study of electrolyte excretion and blood

pressure. Results for 24 hour urinary sodium and potas-

sium excretion. British Medical Journal 297: 319–328.

Luft FC (1990) Sodium, chloride and potassium. In: Brown

M (ed.) Present Knowledge in Nutrition, 6th edn, pp.

233–240. Washington, DC: International Life Sciences

Institute Nutrition Foundation.

Springer C (1987) Measurement of metal cation compart-

mentalization in tissue by high-resolution metal cation

NMR. Annual Review of Biophysics and Biochemistry

375–399.

Physiology

M P Navarro, Unidad de Nutricio

n, Estacio

Experimental del Zaidin, CSIS Granada, Spain

M P Vaquero, Ciudad Universitaria, Madrid, Spain

Introduction

0001 Potassium is the most abundant intracellular cation in

the human body and plays an important role in a

variety of cell functions. This review summarizes the

main aspects of potassium physiology and nutrition,

including absorption, transport, distribution, storage,

and excretion, as well as homeostatic mechanisms of

K balance.

Role in the Body

0002Potassium is essential for the muscular, cardiovascu-

lar, nervous, endocrine, respiratory, digestive, and

renal systems. Cell metabolism depends on the main-

tenance of a high intracellular K

concentration.

Many of the body functions of potassium are due to

its ionic character: it generates gradients of concen-

tration, potential, and pressure. Potassium is the pre-

dominant osmotically active species inside the cell.

Together with other ions such as sodium and chloride,

which are characteristic of the extracellular fluid,

potassium determines osmolarity and plays a major

role in the distribution of fluids inside and outside the

cell and hence in the maintenance of cellular volume.

In addition, potassium participates in the regulation

of the acid–base balance and is involved in cellular

growth and division, energy transduction, glyco-

genesis, protein synthesis, hormone secretion, etc.

Consequently, its deficiency causes growth retard-

ation, with a pronounced decrease in circulating

levels of growth hormone and somatomedin C and

inhibition of protein synthesis.

0003Cell excitability and muscle contraction depend on

potassium. The transmembrane electrical potential is

determined by the ratio of the intracellular to extra-

cellular potassium and sodium concentrations, in par-

ticular that of potassium. Differences in K

and Na

concentrations across cell membranes are maintained

by the specific permeability to each of these ions and

by K

/Na

-ATPase activity (K

/Na

-pump). Thus,

is critical for the excitability of nerve and muscle

cells.

0004During vigorous exercise, potassium is released

from muscle cells, leading to an increase in extracel-

lular potassium concentrations which facilitates on-

going muscle contraction and induces vasodilatation,

increasing local blood flow. However, liberation of

potassium also leads to muscular fatigue. Training

reduces the exercise-induced rise in plasma K

con-

centration and also increases the total activity of Na-

K pumps in muscle. The potassium internal balance

helps to delay the onset of fatigue during exercise and

to restore homeostasis during recovery.

0005Potassium can be defined as a cardioprotector

nutrient. It acts directly on the heart, regulating its

mechanical and electrical properties. Epidemiological

data suggest that potassium intake and blood pressure

are inversely correlated. The greatest hypotensive

4650 POTASSIUM/Physiology

effect of potassium occurs in hypertensive patients

and in subjects with a high sodium intake. The mech-

anisms involved include: enhancing natriuresis,

baroreflex sensitivity, direct vasodilatation, cate-

cholaminergic functions, improvement of glucose

tolerance, and effects on the central nervous and the

renin–angiotensin–aldosterone systems.

0006 Potassium also protects against stroke, which is the

third leading cause of death worldwide (after coron-

ary artery disease and cancer). This electrolyte may

decrease ischemic as well as hemorrhagic stroke risk

through its effect on blood pressure on one hand, and

by inhibiting the formation of free radicals at the

endothelial cell level, thus affecting vasomotion, on

the other hand. Moreover, potassium inhibits the pro-

liferation of smooth muscle cells, platelet aggrega-

tion, and arterial thrombosis. Potassium may also

prevent the death of cerebellar neurons.

0007 Interrelationships exist between potassium and

other nutrients. Potassium and sodium are strongly

metabolically interrelated, principally due to K

-ATPase. This enzyme also provides the driving

force for the transport of other solutes, such as amino

acids, phosphate, vitamins, and glucose. Potassium

depletion, which is more intense if there is a simultan-

eous excess of sodium, enhances urinary loss of

calcium. This interaction may have adverse effects

on bone and blood pressure.

Requirements and Daily Intakes

0008 To date, no recommended potassium intake has been

unanimously established. The UK report on dietary

reference values established a reference nutrient

intake of 3500 mg day

1

for adults. Lower amounts

were recommended for infants (mg day

1

): 800 and

850, 0–3 months and 4–6 months, respectively; 700,

7–12 months; 800, 1–3 years; 1100, 4–6 years; 2000,

7–10 years; and 3100, 11–14 years of age. No specific

levels were recommended during pregnancy, lacta-

tion, or for the elderly. The Nutrition Working

Group of the European International Life Sciences

Institute (ILSI) suggested a higher recommended

daily intake (3900 mg). This recommendation was

aimed at promoting a potassium intake similar to

that of sodium, in molar terms, i.e., an intake of

100 mmol day

1

of each cation (2300 mg and

3900 mg for Na and K, respectively). However,

more recently a Na:K ratio below 1 (1600 mg and

3500 mg for Na and K, respectively) has been recom-

mended to prevent hypertension, stroke, and cardio-

vascular disease.

0009 During exercise potassium needs may be increased

owing to higher losses in sweat, especially in hot

climates and in unaccustomed individuals. These

needs should be satisfied by eating food rich in potas-

sium and exceptionally by potassium supplements.

0010Published values of nutrient consumption demon-

strate that in some populations, sodium intake is

higher than recommended, while potassium intake

should increase. In the UK, the 1991 report of the

National Food Survey Committee showed that the

average sodium intake was 170% of the reference

intake, while that of potassium was 80% of the refer-

ence value. In Spain, data from the same year revealed

that the estimated potassium intake was satisfactory

but that of sodium was 140% above the reference

value.

0011Potassium is ubiquitous in all kinds of food, but the

best sources are vegetables and fruits because they

combine a high potassium content with low sodium

concentration (Table 1). Potassium is an essential

element of all forms of life, whereas sodium and

tbl0001Table 1 Potassium and sodium content of various foods

(mg per 100 g of edible portion)

Potassium Sodium

Vegetables

Potatoes 320 11

Cabbage 275–310 7–12

Carrots 235 60–90

Spinach 500–633 65–140

Legumes

Red kidney beans 1370 18

Soya beans 1730 5

Lentils 940 12

Fruits

Banana 400 1

Melon 100–310 5–32

Avocado pear 400–600 2–4

Orange 195 2–3

Apple 120 2–3

Dried fruits

Raisins 1020 60

Figs 970 62

Almonds 780 14

Walnuts 450 7

Cereals

White bread 100 460–540

Brown bread 175–240 540–636

Rice 110 6

Meat and fish

Beef, veal, lamb 230–360 52–110

Chicken 320 81

Bacon 350 1860

Herring 320 120

Halibut 410 60

Tuna 400 47

Mussels 320 290

Milk

Whole cows’ milk 140 55

Various

Chocolate 300 11

Tomato ketchup 590 1120

POTASSIUM/Physiology 4651

chlorine are essential for animals but not for many

plants. Meat and fish contain important amounts of

potassium because they are highly cellular tissues, but

they also contain large amounts of sodium chloride,

due to the extracellular fluid present in animal food.

Raw foods are preferable to processed ones because

during food preparation salt is usually added to

enhance flavor and retard bacterial growth (e.g.,

bread, bacon, tomato ketchup, and cookies). Potas-

sium salts, however, are not palatable and are not

added to food during processing.

0012 Legumes represent a good source of potassium

because they provide at least 1 g potassium per

100 g portion and very little sodium. Dried fruits,

for example, apricots, figs, and prunes, should also

be mentioned, since they often exceed 0.5 and even

1 g potassium per 100 g portion. Nuts such as pista-

chios and almonds contain a considerable quantity of

potassium, but the addition of salt during roasting

upsets the ideal sodium-to-potassium ratio normally

present in these foods. Bananas are known to be one

of the fruits richest in potassium, together with coco-

nuts, melons, avocados, and kiwi fruit. Milk is the

largest dietary source of potassium for infants.

0013 All these data should be considered, taking into

account the amount of each food item consumed.

For example, a ration of legumes is approximately

80 g while those of meat and milk can easily reach

200 g. Although cereals are low in potassium, they

account for almost 75% of potassium ingestion in

western diets. There is a wide range of potassium

intake by western populations (1560–5850 mg). It is

higher in vegetarians and people who consume more

food from plant than animal sources. The Mediterra-

nean diet provides a mean daily intake of potassium

equivalent to the recommended value (3500 mg).

0014 Although thermal treatments have no effect on

dietary potassium, during processes such as boiling,

soaking, or canning it can be reduced as a result of

leaching, particularly if food is to be cut into pieces

and the liquid is not consumed.

0015 In order to protect against hypertension and stroke,

except in clinical hypokalemia, potassium from good

dietary sources should be encouraged rather than

potassium from supplements, because such supple-

ments may have severe adverse effects. Fruits and

vegetables also contain magnesium and fiber which

seem to have protective effects.

Absorption

0016 Normal subjects absorb about 90% of ingested po-

tassium in the intestinal tract. The net absorption

is the difference between fluxes from lumen to

blood and from blood to lumen. In the human small

intestine, K

permeability is high and potassium ab-

sorption is carried out across the epithelium of duo-

denum, jejunum, and ileum by passive mechanisms in

response to electrochemical gradients and solvent

drag. In the proximal small bowel K

is concentrated

through the absorption of water, providing a driving

force for the movement of this cation across the intes-

tinal mucosa, preferably through the tight junctions

between enterocytes. Duodenum and jejunum absorb

this ion even more rapidly than water. Indeed, shortly

after a meal, the K

concentration [K

] in jejunum

rapidly reaches plasma levels. In the ileum, the trans-

epithelial electrical potential difference strongly influ-

ences its movement. There is no evidence of active

potassium absorption in the small intestine, but the

existence of an apical membrane H

-ATPase

could suggest active K

transport.

0017In the colon K

may be secreted or absorbed in

response to variations in potassium status: net secre-

tion occurs when the luminal concentration is less

than 20–25 mmol l

1

, while net K

absorption takes

place when levels are above 25 mmol l

1

. There are

two mechanisms for K

secretion; the major mechan-

ism involves potential-dependent, passive flux,

mostly via the tight union, or by facilitated transcel-

lular diffusion. The second is an active serosa-to-

mucosa secretory mechanism in the proximal and

distal colon which is the result of uptake across the

basolateral membrane mediated by Na

-ATPase

and Na

-K

-Cl



cotransport and movement across

the apical membrane through potassium channels.

Active K

absorption also occurs in the distal colon,

in which the K

uptake appears to be an exchange for

across the apical membrane. This process is ener-

gized by adenosine triphosphatases (ATPases) located

in that membrane. Passive potassium absorption may

also take place in the proximal colon.

Bioavailability

0018The availability of dietary potassium is high because

potassium salts are entirely soluble and few dietary

factors modify its digestive utilization. Olive oil favors

potassium uptake, while some fibers and certain ion

exchange resins may decrease K

absorption.

0019Physiological status also affects the nutritive util-

ization of potassium. Infants absorb a greater propor-

tion of salts than adults. This may be due to the

increased permeability of the immature small intes-

tinal epithelium of newborns, and especially to a

higher activity of the K

-absorptive pumps in the

colonic apical membrane, while K

-secreting chan-

nels are less relevant. During the second half of preg-

nancy, fecal potassium decreases. Urinary K

output

increases in parallel with Na

retention; nevertheless,

4652 POTASSIUM/Physiology

enough potassium is retained to cover gestational

needs. Glomerular filtration rates fall with age, ac-

companied by limitations of K

secretion and Na

conservation. Many drugs used in the elderly alter

potassium homeostasis.

Transport

0020 Potassium, being an electrolyte, is transported mainly

under ionic form in the body fluids. Intracellular

potassium concentration is 110–160 mmol l

1

of cells

while plasma concentration averages 4–5mmoll

1

of which only 10–20% is bound to proteins.

0021 Figure 1 schematically illustrates the intracellular

and paracellular mechanisms of potassium transport.

Different potassium transport mechanisms can oper-

ate simultaneously. They are usually interrelated and

are also linked to other ion transport systems. Each

name, except the last one, corresponds to an entire

family of transporters.

0022 Na

-ATPase activity is responsible for the

maintenance of the extra- and intracellular Na

and

concentrations against electrochemical gradients.

This ATPase is found in the plasma membrane of

virtually all animal cells and represents an enormous

metabolic energy cost which increases the entropy of

the system. It is a carrier protein that pumps 2K

and 3Na

out of the cell during each cycle of con-

formational changes driven by adenosine triphos-

phate (ATP) hydrolysis. This process is electrogenic,

meaning that one net positive charge is removed from

the cell every pump cycle. Many hormones and vari-

ous neurotransmitters can modulate K

/Na

-ATPase

activity.

0023 Another active K

transport is controlled by the

-ATPase which ejects H

in exchange for K

This pump has an important role in some gastro-

intestinal cells and in the renal tubules.

0024 Other potassium transport systems are driven by

the force of ion gradients. The free energy gained

during the movement of an inorganic ion down an

electrochemical gradient (sum of the concentration

gradient and the electrical potential difference) is

used as the driving force to pump other solutes

against their electrochemical gradient. Thus, the car-

rier protein acts as a coupled transporter and electro-

neutrality is maintained. Cotransport depends on

ATP only indirectly. Various Na

-K

-Cl



cotranspor-

ters, which carry 1Na

,1K

and 2Cl



inside the

cell, have been identified in salivary glands, gastro-

intestinal tract, and renal tubules. The K

-Cl



cotransporter, related to the former ones, plays an

important role in erythrocytic volume. The first

cotransport mediates K

influx while the second

mediates efflux.

ATP

3Na

2Cl

−

-ATPase

-K

-Cl cotransport

channels

Paracellular diffusion

Electrochemical gradients for Na

and Cl

−

Electrochemical gradient for K

fig0001Figure 1 Potassium transport via intracellular and paracellular

pathways. ATP, adenosine triphosphate.

POTASSIUM/Physiology 4653

0025 Passive transport of K

occurs via intracellular and

paracellular pathways. The intracellular mechanism

involves K

channels, which are pores in the plasma

membrane made by a specific protein. Channels have

‘gates’ which open and close in response to specific

stimuli: voltage, stretch, ATP, Ca

2þ

, hormones,

neurotransmitters, etc. Various stimuli sometimes

act together on a channel. K

channels exhibit great

diversity and participate in many cell functions.

Distribution and Storage

0026 Body potassium concentration is 45–55 mmol kg

1

body weight, thus a 70-kg adult man contains ap-

proximately 135 g potassium. Women contain 35–

40 mmol kg

1

of body weight, and during childhood

and in the elderly, body potassium concentration is

also lower than that for young men. These differences

are due to the lower muscle mass in these groups.

Almost 98% of the potassium in the body is inside

the cells, making it the major intracellular cation.

More than 95% of total potassium is exchangeable.

0027 Total potassium is an index of lean body mass

because potassium is only present in the fat-free com-

partments of the body. It can be measured with K

natural isotope present as a small fraction of the total

potassium, which emits g-rays and can be detected by

a sensitive whole-body counter, although other iso-

topes and other techniques that are not based on total

potassium are also available.

0028 Studies of body composition in healthy humans

have demonstrated that the total body potassium/

total body nitrogen ratio is constant. However, in

chronic protein-energy malnutrition this ratio can

decrease owing to the reduction of muscle proteins

and loss of intracellular potassium. The cellular ex-

change of sodium and potassium is altered, leading to

potassium loss and increased intracellular sodium

and water. In contrast, exercise and training increase

total lean body mass, thereby causing a rise in potas-

sium body content. Increase in muscular mass is only

possible if proteins, as well as potassium, phosphorus

and magnesium, which are the main intracellular

elements, are present in the diet in adequate amounts.

0029 In the extracellular fluid (ECF) an increases of potas-

sium equivalent to 1% of total body potassium may

double the concentration of potassium in plasma,

resulting in muscle hypopolarization. However, if

such an increase is stored in the cells, only minor intra-

cellular changes would result, with little change to the

potassium concentration difference across the cell

membranes. To accomplish this buffer function, the

major reservoir of potassium is the muscle, followed

by the liver and erythrocytes, although each cell pos-

sesses the capacity for accumulating potassium.

Excretion

0030About 90% of daily potassium intake is excreted

in the urine and 10% in the stool, although this

last percentage can be somewhat higher in cases of

diarrhea. When dietary potassium is severely re-

stricted, its fecal loss decreases to approximately

3.5 mmol day

1

. This presumably represents obliga-

tory potassium losses related to K

digestive secre-

tions (salivary, gastric, biliary, and pancreatic), cell

desquamation, and mucus secretion. Potassium con-

centration in sweat is about 10 mmol l

1

, so loss

through perspiration is small, provided that the

climate and exercise conditions are not extreme.

Renal Potassium Handling

0031The kidney plays the main role in potassium excre-

tion. Renal potassium excretion shows a circadian

rhythm, characterized by peak output during the

subject’s activity.

0032K

is freely filtered by the glomerulus. Usually,

urinary potassium excretion is 5–15% of the amount

filtered, which indicates the existence of tubular po-

tassium reabsorption. The renal tubules are capable

of reabsorbing and secreting potassium in response to

various stimuli.

0033Proximal tubule The proximal tubule is responsible

for the reabsorption of approximately 60% of previ-

ously filtered potassium. Solvent drag and diffusion

have been proposed as the major driving forces of this

process, and K

is reabsorbed largely via the paracel-

lular pathway. However, some observations suggest

participation of a transcellular route of potassium re-

absorption by an active mechanism. The fractional

rates of K

reabsorption are similar to those of sodium

and water along the proximal tubule and changes in

fluid and potassium transport are closely coupled.

0034In this region the basolateral K

channels have

higher conductance than the apical ones, and this is

essential to maintain a negative intracellular potential

and hence promote reabsorption of positively

charged carriers (Na

-glucose cotransport, etc.)

0035Loop of Henle The concentration of K

increases as

the filtrate passes through the loop of Henle, where

permeability for potassium is very high. At the end of

the descending limb the amount of K

present usually

exceeds that of the glomerular filtrate. There is a net

passive secretory entry of potassium into the prox-

imal straight tubule and the descending thin limb of

Henle, which arises from reabsorption in the collect-

ing tubule and partly in the ascending limb. These

pathways of K

transport constitute potassium

recycling in the renal medulla.

4654 POTASSIUM/Physiology