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

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0027 A study evaluating the nutrient density of snacks

and meals, and the effect of eating frequency on mean

daily nutrient intake in female students (aged 17–26)

has shown that as the eating frequency increased, the

number of snacks and the number of different snack

items in the diet increased, while the number of meals

remained constant. Snacks had lower nutrient

densities than meals for nonstarch polysaccharide,

minerals, and vitamins, except vitamin C.

0028 Mango bars fortified with desiccated coconut

powder or soyprotein concentrate are quite accept-

able initially, but storage beyond 90 days lowers their

rating, compared to plain mango.

0029 A low-calorie, low-cholesterol, shelf-stable expan-

ded snack product can be prepared from raw or

mechanically separated, comminuted meat mixed

with pregelatinized flour and processed through

high-temperature, short-time screw-type cooker

extruder. A gun-puffed, extruded snack food, low in

fat and high in carbohydrates, prepared by incorpor-

ating fruit and vegetable extracts with oat flour,

starch, and sugars, is low in fat and high in calories.

0030 Substantial efforts have been made to reduce the

‘snacking habit’ of various populations, but snack-

food consumption continues to increase. Conse-

quently, there is a tendency to increase the production

of more nutritious snacks, including those made from

corn. Composite flours using corn, chickpea, soya

bean meal, and methionine can be formulated, ex-

truded into snack foods, and fried. These products

have higher protein-efficiency ratio values than the

controls, while the color, flavor, texture, and overall

acceptability remain similar. These products area good

alternative to commercially available snack foods.

See also: Adolescents; Calcium: Physiology; Children:

Nutritional Requirements; Dietary Fiber: Physiological

Effects; Effects of Fiber on Absorption; Flour: Dietary

Importance; Food Fortification; Iron: Physiology;

Magnesium; Protein: Quality; Sodium: Physiology;

Vitamin B

: Properties and Determination

Further Reading

Anon (1993) CMC smooths production of extruded snacks,

cereals. Prepared Foods 162(10): 81.

Anon (1996) Healthy starch innovations for snacks and

cereals. Food Tech Europe 3(2): 46.

Bednarcyk NE (1987) Nutrient contribution of cookies and

crackers. Cereal Foods World 38(5): 367.

Chauhan SK, Joshi VK and Lal BB (1993) Apricot–soy fruit

bar: a new protein enriched product. Journal of Food

Science and Technology 30(6): 457–458.

Conninghum FE, Neumann PE and Huber GR (1993)

Extruded Meat-based Snack Food and Method for

Preparing Same. US Patent.

Delroy B (1985) The role of snack foods. Food Technology

in Australia 37(4): 154–158.

Henshaw FO and Ihedioha CN (1992) Shelf life studies on

some Nigerian indigenous snackfoods. Nahrung 36(4):

405–407.

Jack FR, O’Neill J, Piacentini MG and Schroeder MJA

(1997) Perception of fruit as a snack: a comparison

with manufactured snack foods. Food Quality and

Preference 8(3): 175–182.

Madonna MD (1983) Pretzels as low calorie snacks. Cereal

Foods World 28(5): 297.

Matz SA (1976) Nutritional Supplementation. Snack Food

Technology. Westport, CT: AVI.

Mir MA and Nirankar N (1993) Storage changes in forti-

fied mango bars. Journal of Food Science and Technol-

ogy 30(4): 279–282.

Mitchell V and Boustani P (1990) Cereal bars. Nutrition

and Food Science 127: 20–21.

Morgan KJ (1983) The role of snacking in American diet.

Cereal Foods World 28(5): 305.

Olusola-Omueti and Morton ID (1996) Development by

extrusion of soyabari snack sticks; a nutritionally im-

proved soya–maize product based on the Nigerian snack

(Kokoro). International Journal of Food Science and

Nutrition 47(1): 5.

Ranhotra GS (1985) Nutritional profile of corn and flour

tortillas. Cereal Foods World 30(10): 703.

Ray EE (1994) Method of Preparing Snack Food Products.

US Patent.

Ruxton CHS, Kirk TR and Belton NR (1996) The contri-

bution of specific dietary patterns to energy intakes in

7–8 year old Scottish school children. Journal of Human

Nutrition and Dietetics 9(1): 23–31.

Stevens MPA, van Zuilichem DJ and de Waart J (1991)

Tapsea-vegetable: Development of a new snack. Voe-

dingsmiddelentechnologie 24(2): 12–14.

Tettweiler P (1990) Snacks – Food, Fun and Fashion. Dra-

gocco Report 3, pp. 79–104. Holzminden, Germany:

DRA-GOCCO.

Warren AB, Hnat DL and Michnowski J (1983) Protein

fortification of cookies, crackers, and snack bars: uses

and needs. Cereal Foods World 28(8): 441.

Webb DR, Harrison GG, Min JL and Mei HH (1997)

Estimated consumption and eating frequency of olestra

from savoury snacks using menu sensus data. Journal of

Nutrition 127: 1547S–1554S.

Whybrow S and Kirk TR (1997) Nutrient intake and

snacking frequency in female students. Journal of

Human Nutrition and Dietetics 10(4): 237–244.

Zammer CM (1995) Gun-puffed vegetable snacks:

a new way to eat your veggies. Food Technology

49 (10): 64.

SNACK FOODS/Dietary Importance 5337

SODIUM

Contents

Properties and Determination

Physiology

Properties and Determination

R Moreno Rojas and M A Amaro Lo

pez, University

of Co

rdoba, Co

rdoba, Spain

Background

0001 Sodium, prepared for the first time by the British

chemist Humphry Davy in 1807, is the eleventh

element of the periodic table (atomic number 11).

Its symbol is Na, and it is included in group 1 (or

IA), thus corresponding to the alkali metals. It is an

extremely soft silvery metallic element, so much

so that it can be cut with a knife (hardness of 0.4).

It is a very reactive element that tends to be used

extensively in the chemical industry. It rusts very

quickly when exposed to air and reacts violently

with water, forming sodium hydroxide and hydro-

gen, and producing a yellowish flame. It has a fusion

point of 98



C, a boiling point of 883



C, an atomic

weight of 22.98977, and a relative density of

0.97 g cm

3

0002 Sodium has an electronic structure of [Ne] 3s

and consequently, it always acts with valence 1. It

has only one external electron, in orbital 3s, and

so the degree of attraction that it suffers with re-

spect to the nucleus rivals the repulsion of the lower

orbitals, making it very easy for this electron to dis-

appear, forming the Na

cation, its most common

state.

0003 Two per cent of the earth’s crust comprises sodium,

making it the sixth most abundant element in nature,

where it is always found in a combined state. Sodium

is currently manufactured by electrolytic procedures

on melted sodium chloride. Its combining facility

yields numerous compounds; with oxygen it forms

O, Na

, and even NaO

, an important whiting

and rusting agent. However, the most abundant form

and one of the main sources for man is in combin-

ation with chlorine in the form of sodium chloride

(NaCl), which is usually extracted from the sea, salt

lakes, or in mineral form as rock salt. Sodium can also

be found in the form of sodium carbonate (Na

sodium bicarbonate (NaHCO

), and sodium sulfate

(Na

). Also worth mentioning is sodium hydrox-

ide, known as caustic soda, which is used throughout

industry, from the manufacture of soap to rayon,

paper, rubber, oilskin, in petrol refineries or in the

textile industry. Sodium fluoride (NaF) is used as an

antiseptic, as a poison for rats and cockroaches, and

in the manufacture of ceramics. Nitrate from Chile

(sodium nitrate) is used as a fertilizer. Sodium thio-

sulfate (Na

5H

O) is used in photography as a

fixing agent and is even used in the nuclear power

industry as a cooling agent.

0004All these specific uses of sodium are only a few

examples, since this cation is one of the most common

companions of different molecules, both in nature

and when created by man for different purposes.

0005In terms of the nutritional characteristics of

sodium, we could say that 90% of sodium provided

in the diet is obtained from the sodium chloride that

we consume. The rest of the sodium that reaches

our organism may originate from other sources such

as bicarbonate or sodium glutamate. Of the total

sodium intake, it has been shown that only 10%

originates naturally from foods, around 15% from

salt added to cooking or to season food once cooked,

and the remaining 75% from the processing and

manufacturing stages of foods.

0006Given the physiological need to supply the human

body with sodium (see Sodium: Physiology) in order

to maintain extracellular osmotic pressure, and as a

result of the obligatory losses through urine and

sweat, we need to consume sodium on a regular

basis, just like most animals. In ancient times, this

meant a dependence on salt deposits, and a huge

value was placed on what is now common salt, so

much so that this gave rise to the Latin word salar-

ium, corresponding to the payment in the form of salt

made to soldiers in ancient Rome, and it was also

used as money in Tibet and Ethiopia, as a tax in

Asian countries, and as a ritual element in Greek,

Roman, Hebrew, and Christian religions.

0007This physiological need for sodium may explain

why our language contains specific receptors corres-

ponding to the taste of salt and our appetite for this

taste in food. For this reason, and because of the

tradition of preserving certain foods in salt, which is

5338 SODIUM/Properties and Determination

today probably unnecessary, although culturally and

gastronomically unnoticeable, we have reached a

situation in which salt is consumed, and with it

sodium, in developed societies. Consumption is esti-

mated to be normally more than 6 g per day, the

minimum amount required for the control of blood

pressure in hypertense individuals. (See Hyperten-

sion: Hypertension and Diet.)

Preparation of Samples for Sodium

Analysis

0008 The determination of sodium in foods may be achieved

using a variety of sample preparations, ranging from

noninvasive methods, such as those used for deter-

mining sodium by selective electrodes, to complete

incineration of the sample and subsequent solubiliza-

tion of the ash in acid solutions. Each analytical

technique requires specific preparation of the sample,

since the instrumental requirements in each are very

different.

0009 In any case, and since sodium is usually found in

soluble form in foods, the simplest method of prepar-

ing samples involves washing and then analyzing the

sodium from the solution sample; for this purpose,

the food often only has to be crushed, solvent added,

homogenized, and filtered or centrifuged. Solvents to

be used may be simple water or slightly acidic or

alkaline solutions.

0010 Some methods even propose the direct use of food

(particularly liquids) in the detection equipment.

Although these may yield good analytical results,

other food components such as fats, sugars, or pro-

teins can also deteriorate or stain the instruments.

0011 Given the stable character of sodium, the most

aggressive techniques usually yield very good results

in terms of the recovery of analyses; hence, the use

of incineration at around 500



C overnight and

subsequent recovery in weak acid solutions can be

used as a standard sample preparation method for a

wide range of foods, modifying temperature and time

according to the food in order to obtain soft ash,

evidence of good interaction.

0012 Other methods for destroying organic matter, such

as acid hydrolysis, can also be used without fear of

undermining the detection of sodium, and a wide

range of acids may be used in variable concentrations,

without significantly affecting results in terms of the

determination of sodium content.

0013 The use of methods for destroying organic matter

by incineration or by acid hydrolysis is particularly

recommended, because of the possibility of obtaining

solutions in which other cations of interest for food

can be analyzed, such as K, Ca, Fe, Cu, Zn, etc.

Methods of Analysis

0014Although sodium may be determined by means of a

wide variety of instrumental techniques, in line with

the applications recommended in scientific publica-

tions and the most suitable instruments, the technique

used most often is atomic spectroscopy, either by

emission or by absorption (Table 1).

0015This technique is based on the atomic properties of

the elements, and more specifically on their capacity

to react in response to specific stimuli, modifying

their orbital distribution, for which purpose they

absorb light at a specific wavelength for each element,

passing on to a state of higher energy, or lower energy

if passing from the previous stage, emitting light at

the same wavelength. In the case of sodium, the rec-

ommended specific lengths are 589.6 and 589.0 nm.

0016In order to achieve the passage of electrons to a

state of higher energy, it may be necessary to increase

the temperature, usually by means of a flame, or the

incidence of a beam of light of a specific wavelength

(Table 2). The latter technique enables the determin-

ation of the analytical concentration by determining

the difference between the energy supplied to the

sample and that which remains after crossing – this

is genetically known as atomic absorption spectros-

copy. In contrast, if the energy of the element is in-

creased by another method (flame), what is usually

measured in order to determine the concentration of

the analyte is the emission of characteristic wave light

when returning to the state of lower energy; this

technique is known as atomic emission spectroscopy.

Emission Spectroscopy

0017Atomic spectrophotometers and emission flame pho-

tometers can be used to determine sodium content. In

both cases, a sample in a liquid state is introduced into

a fuel carrier (acetylene or propane) and combined

with atomized or nebulized N

air or oxygen. This

dispersion is led to a lighter where the fuel combusts.

The heat of the flame prompts an increase in the

energy of the sodium atoms, which, upon returning

tbl0001Table 1 Standard conditions for sodium analysis by atomic

absorption spectrometry

Spectrometer setting Potassium

Wavelength/slit (nm) 589.0/0.2

Nebulizer Spoiler

Oxidant Air

Fuel C

Flame condition Oxidizing

Optimum concentration range in solution (mgml

1

) 1–20

Interferences Ionization

SODIUM/Properties and Determination 5339

to their fundamental state, emit a light beam of char-

acteristic wavelength that is quantified by a detection

system. The amount of light emitted is proportional

to the concentration of the analyte in the sample. In

addition to the option of atomic absorption, in which

a hollow cathode lamp must be used in order to

generate the specific lengthwave, most atomic spec-

trophotometers enable emission spectroscopy to be

performed, an option that requires a lamp and pro-

vides good results.

0018 With this technique, the optimum gain is adjusted

to the highest standard of sodium used, and an

attempt is made to keep the lighter free of sodium

impurities emitted by the actual samples and stand-

ards analyzed, since these impurities may introduce

undesirable emissions or obstruct the passage of

fluids through the lighter; when the flow decreases,

the signal obtained will also decrease. Avoiding

samples with high concentrations of sodium (such as

undiluted sea waters, cheese sera, etc.) is considered

good practice.

0019 A further potential problem lies in small fluctu-

ations that may occur in the flame or in the carried

mixture; these may lead to inaccurate and variable

results. To correct this problem, some devices are

equipped with a second detector that enables the

emission of a second element (usually lithium) to be

estimated, with a different wavelength to that of

sodium. This second element must be added to solu-

tions of both samples and patterns in a stable and

known concentration. When there is a variation

caused by the flame or the flow, both detectors must

reflect this simultaneously, distinguishing this from

variations in the analyte.

0020 Another precautionary step that must be observed

when working in atomic emission with sodium

involves adjustment of the calibration line, since, in

most cases, this is not linear; instead, it displays a

decrease in the slope when concentrations increase.

This is due to the fact that the area of linearity

between the analytical concentration and atomic

emission is exceeded. The simplest solution involves

diluting the samples until the standards that include

them present a straight adjustment; however, it is

feasible to make a logarithmic adjustment between

the emission and the concentration of the type

E ¼a*ln[Na] þb, or by means of a simple logarith-

mic transformation of the concentrations prior to

the calculation of the regression line, and the subse-

quent retrotransformation when applying the linear

equation in order to calculate concentrations. This

type of adjustment is usually extremely useful when

the sample values are high but not very different from

one another, the results of the equation being valid,

provided that all samples are located between the

minimum and maximum values of the patterns used.

As a result of this restriction, calculations such as

those corresponding to the detection or concentration

limit cannot be performed reliably on the standards

used, although the high concentrations used guaran-

tee that the readings can never be confused with the

fluctuation of the baseline of the device.

Atomic Absorption Methods

0021Although the basis of the technique is the same as that

for atomic emission, and in many cases, the same

equipment is used for both analyses, it is true that

this technique is favored by fewer analysts, owing in

part to the fact that for normal concentrations of

sodium in foods, calculations may be performed for

emissions with little need to perform dilutions, and

because of the added cost of using a hollow cathode

lamp for analysis, without this being justified by the

analytical improvements achieved. However, the

existence of multielement lamps provides researchers

with the possibility of using this technique with

their samples at no further cost. This technique

tends to be recommended more than the emission

technique when the sample concentrations present

very low values, and lower detection limits may be

obtained.

Ion-selective Electrodes

0022Together with the optical techniques of absorption

and atomic emission, this is one of the most common

tbl0002 Table 2 Accuracy and precision of sodium analysis by flame emission

Accuracy Certified (g kg

1

)Found(gkg

1

) IC (95%)

Rec (RSD)

Citrus leaves SRM-1572 0.16 + 0.02 0.17 + 0.03 0.11–0.24 108 (18)

Nonfat milk powder NIST-1549 4.97 + 0.10 5.17 + 0.14 4.90–5.46 104 (2.8)

Precision Relative standard deviation

Plant food 1.15

Dairy food 1.2

Intervals of confidence (95%).

Recovery percentages and relative standard deviation.

5340 SODIUM/Properties and Determination

techniques used for analyzing sodium content in

foods; one of the reasons is the low cost of the instru-

ments required, their ease of use, speed, noninvasive

nature, and even the possibility of using them in

food production lines without causing any inter-

ruptions.

0023 The technique is based on the capacity to enable

the passage of ions through a glass membrane, each

type of membrane being specific to an ion or small

group of ions. In the case of sodium, the probe is

capable of detecting this activity once the membrane

has been crossed; this activity is proportional to

concentration. Possibilities of interference with other

ions exist, i.e., specificity is not as high as with spectro-

morphometric methods, and the formation of com-

plexes with other substances reduces sodium activity

and therefore its capacity for determination. However,

its usefulness becomes clear when the expected levels

of sodium are high, as is the case with salted products,

such as ham, cheese, etc., in which controlling salt

penetration may be recommended. (See Potassium:

Properties and Determination.)

Inductively Coupled Plasma Atomic Emission

Spectrometry (ICPAES)

0024 ICPAES enables samples with a high degree of

variability and minimum interferences to be used,

and these can be prepared either by wet oxidation

or by dry ashing. A wide range of concentrations

can be used without the need for dilution or concen-

tration, and many of the mineral elements can be

quantified, thus reducing analysis time. These advan-

tages, together with the high speed and excellent

instrument stability, make the ICPAES method highly

attractive in view of its analytical applications.

Other Techniques

0025 A large number of other analytical techniques have

been described that may be used to determine sodium

content in foods, ranging from classical gravimetric

methods, now virtually obsolete, to techniques that

involve the use of instruments that are expensive and

sophisticated, this factor substantially limiting their

use. The most noteworthy techniques described in the

Further Reading list include the following: nuclear

magnetic resonance (NMR) spectroscopy, ion chroma-

tography, capillary ion electrophoresis analysis, elec-

tron-probe microanalysis, helium glow photometry,

ion-microprobe analysis, laser probe analysis, ion-

scattering spectrometry, auger spectroscopy, impact

radiation analysis, enzymatic determination, micro-

wave-induced plasma atomic emission spectrometry

and a-particle backscattering analysis. (See Potassium:

Properties and Determination.)

0026Of all these less common methods, NMR spectros-

copy been used increasingly in recent years for deter-

mining sodium content, mainly in living tissue or

biological samples. Therefore, it can be used for

foods, particularly for establishing sodium distribu-

tion in such samples. In this technique, Na

has a

nuclear magnetic moment of 3/2, which allows it to

undergo nuclear magnetic resonance;

Na is ex-

tremely easy to detect in biological samples. For

more information on this technique (see Spectros-

copy: Nuclear Magnetic Resonance.) Nevertheless,

this technique must be used with caution, since com-

parative studies indicate discrepancies in terms of

results obtained in connection with the quantification

of sodium when using this technique, which may only

detect 60% of sodium.

0027In contrast, other methods are merely variations on

the major analytical techniques, such as emission

spectrometry or selective electrodes, are normally

used in microsamples such as blood serum or other

body products, and are rarely used in foods.

See also: Hypertension: Hypertension and Diet;

Potassium: Properties and Determination; Sodium:

Physiology; Spectroscopy: Nuclear Magnetic

Resonance

Further Reading

Ammon D (1986) Ion Selective Microelectrodes. New

York: Springer.

Benton Jones J Jr. (1984) Developments in the measurement

of trace metal constituents in foods. In: Gilbert J (ed.)

Analysis of Food Contaminants, pp. 157–202, Barking,

UK: Elsevier.

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 fluorescence (TXRF). Zeitschrift fu

r Lebensmitte-

lunterersuchung und -forschung 190(4): 331–335.

Intersalt Cooperative Research Group (1988) Intersalt:

an international study of electrolyte excretion and

blood pressure. Results for 24 hour urinary sodium

and potassium excretion. British Medical Journal 297:

319–328.

Kane MR, Fregly MJ and Berbard RQ (eds) (1980) Bio-

logical and Behavioral Aspects of Salt Intake. New

York: Academic Press.

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.

SODIUM/Properties and Determination 5341

Physiology

A R Michell, University of London, Hatfield,

Hertfordshire, UK

This article is reproduced from Encyclopaedia of Food Science,

Physiological, Clinical and Nutritional

Importance of Sodium

0001 Despite the fact that the body contains more calcium

and potassium, sodium is arguably the most import-

ant cation because it dictates the volume of extracel-

lular fluid (ECF) and its concentration affects osmotic

concentration of both ECF and intracellular fluid

(ICF). Abnormalities of ECF sodium concentration

cause movement of water into or out of cells, thus

altering the osmotic concentration of ICF in parallel

and causing swelling or shrinkage of cells. The main

impact of this is on the brain because its cells are

rigidly enclosed by the cranium.

0002 Sodium depletion is mainly caused by enteric, renal,

or adrenal disease, and sodium retention is caused by

renal disease; healthy kidneys are well able to excrete

excess dietary salt. However, chronic ingestion of

excess salt, whether or not it increases ECF volume,

is a predisposing or exacerbating factor in hyperten-

sion. Until the 1980s, knowledge of the regulation of

body sodium mainly concerned defenses against de-

pletion, whereas the last two decades have seen a

rapid growth in knowledge of the mechanisms which

excrete excess sodium. This seems appropriate since

most species, especially humans, dogs, and laboratory

rats, are exposed to dietary sodium intakes well above

their nutritional requirement. (See Renal Function

and Disorders: Kidney: Structure and Function.)

0003 The nutritional requirement is a reflection of

obligatory losses (maintenance) and the needs of

growth, pregnancy, and lactation. Abnormal losses

owing to disease, or in animals such as humans and

horses which sweat extensively, raise the requirement.

The impact of equine sweating is different from that

in humans. Human sweat always contains sodium at

concentrations well below plasma levels (and when

aldosterone secretion is raised, levels of sweat sodium

fall very low); horse sweat is hypertonic but this helps

to offset the osmotic effect of the increased respira-

tory water loss during exertion, i.e., it may be a

defense against hypernatremia, rather than a poten-

tial cause of sodium depletion.

0004 Consideration of the physiology of sodium thus

includes its distribution in the body, regulation of

total content and concentration, causes of and re-

sponses to depletion or excess, and their nutritional

implications.

Distribution

0005Sodium behaves physiologically as a cation, i.e., a

positively charged ion; its distribution and effects

are fairly independent of the negative ions (anions)

which originally accompanied its ingestion, though

they may affect its absorption and excretion. Most

sodium is in ECF (Table 1), kept there by the sodium

pump, an enzyme system, (Na

-K

)-ATPase, which

uses substantial amounts of energy (adenosine tri-

phosphate; ATP) in maintaining a low intracellular

sodium concentration and a high intracellular potas-

sium (K

) concentration. Sodium transport is a cen-

tral issue in the physiology of sodium for a number of

reasons:

0006It helps to maintain the ionic environment of ICF

and the volume of ECF.

0007It prevents cell swelling (the Na

efflux exceeds

the K

influx).

0008It establishes gradients which, in various tissues,

allow transport of other cations in exchange, other

anions in parallel or organic solutes – these are

often cotransported with sodium down concentra-

tion gradients which are secondary to the low

sodium environment created by the pump.

0009It establishes the membrane voltages on which

excitability and secretory activities frequently

depend.

0010The energy expenditure of the pump is a substan-

tial portion of total metabolic activity and contrib-

utes to thermogenesis.

0011Sodium transport is not only a key factor in the

retention and loss of sodium in the kidney, gut,

salivary, and sweat glands but also influences the

excretion or retention of many other solutes. Thus,

for example, diuretics intended to promote sodium

excretion may also cause unintentional losses of

potassium and magnesium. Similarly, when renal

tbl0001Table 1 Summary of sodium (Na) distribution and requirements

TypicalplasmaNa concentration (mmoll

1

) 145 (130^160)

TypicalbodyNa content (mmolkg

1

) 50^55

Typicalproportion (%) of total Na

Intracellular fluid 10

Extracellular fluid 50

Bone 40

Maintenance requirement in mammals (mmol kg

1

day

1

)

Sheep 01

Cattle and goats 03^07

Pigs 06

Rats 06

Dogs 02^05

Humans ?<06

1 mmol ¼ 23 mg Na

, 58.5 mg NaCl.

5342 SODIUM/Physiology

sodium excretion increases appropriately in re-

sponse to ingestion of excess salt, there may also

be unwanted losses of calcium and in postmeno-

pausal women these may contribute to loss of

bone mineral. (See Electrolytes: Analysis; Thermo-

genesis.)

Bone also contains substantial quantities of sodium

but, as yet, its significance is unknown since it

does not appear to be mobilized during sodium deple-

tion. Gut fluids contain considerable amounts of

sodium, mostly secretory rather than dietary, and

mostly reabsorbed in more distal regions of the

intestine.

Extracellular Sodium

0012 Of the extracellular fluid, most is in interstitial fluid

(ISF) in the tissue spaces, providing the transport

medium between capillaries and cells. The sodium

concentration in plasma is slightly above that in ISF

because plasma contains more proteins, notably albu-

min, which do not readily escape into ISF across the

capillary membranes, and the effect of their negative

charges is to hold more positively charged ions,

notably sodium, in circulation (Gibbs–Donnan equi-

librium).

0013 The main effects of excess ECF volume are seen as

expanded ISF, visible clinically as edema (or ascites,

when fluid accumulates in the abdomen rather than

the tissue spaces). Mild edema is merely a cosmetic

problem in itself but pulmonary and cerebral edema,

or severe ascites, are potentially serious forms. Edema

can result from excess ingestion or retention of

sodium (overall expansion of ECF) or leakage from

plasma to ISF, with plasma volume continuously re-

plenished by renal sodium retention. Such maldistri-

bution of ECF occurs if plasma albumin is very low

(renal leakage, hepatic impairment, or severe malnu-

trition), or with excessive capillary blood pressure

(venous blockage, inactivity, heart failure, or arterio-

lar dilation, e.g., from heat or allergy), capillary

damage, or lymphatic blockage. Lymphatic blockage

prevents the removal of proteins which have leaked

into ISF. Accumulation of protein in ISF undermines

the osmotic gradient which normally favors uptake of

water at the venous end of the capillary, where the

pressure is lower. Since edema involves the expansion

of a larger compartment (ISF) from a smaller one

(plasma), it is only possible as long as the latter is

replenished; hence the kidney, while seldom the pri-

mary cause of edema, is always the enabling cause;

the use of diuretics is therefore appropriate in the

treatment of nonrenal as well as renal causes of

edema.

0014The main effect of inadequate ECF volume is to

reduce plasma volume and thus to compromise

cardiovascular function, in extreme cases by causing

circulatory shock.

Regulation of ECF Sodium

0015In a mature, nonpregnant, nonlactating, healthy

animal, sodium excretion matches sodium intake

and is often used to estimate it, although this is not

reliable, especially when intake is low. Dietary

sodium is readily available, i.e., readily absorbed;

thus the traditional view of sodium regulation em-

phasizes renal regulation of urinary Na

loss. This

oversimplifies the more subtle interplay seen, for

example, in herbivorous animals, where salt appetite

may contribute to regulation by intensifying during

sodium depletion. Moreover, in many herbivores the

feces, rather than urine, may be the major route of

sodium excretion and the gut may therefore be an

important regulator of sodium balance. Indeed, it is

interesting that sodium transport mechanisms in the

small intestine show considerable similarities to those

of the proximal part of the renal tubules (e.g., linked

transport of Na

, glucose, and amino acids) whereas

the colon, like the distal nephron, responds to the

salt-retaining (and potassium-shedding) hormone of

the adrenal cortex, aldosterone.

0016Provided that the adrenal gland is healthy, urinary

and fecal sodium loss can be reduced virtually to zero.

Sweat loss can also be very low, although with severe

exertion in hot climates the volume of sweat may

exceed the ability of aldosterone to reduce its sodium

concentration and net loss of sodium can occur.

Aldosterone also reduces salivary sodium (and raises

]). (See Potassium: Physiology.)

0017There are two components to the regulation of ECF

sodium: the total amount of sodium retained, and its

concentration. The former is regulated by mechan-

isms which directly affect sodium, whereas the latter

is essentially regulated via water balance. Thus what-

ever sodium is retained in ECF is ‘clothed’ with the

appropriate amount of water to maintain the normal

plasma sodium concentration within narrow limits;

deviations of less than 1% (hard to measure in the

laboratory) trigger corrective responses. Thus a raised

plasma sodium concentration (e.g., after water loss)

stimulates both thirst and renal water conservation;

antidiuretic hormone (ADH) from the posterior pitu-

itary reduces urine output through its effect on the

renal collecting ducts. Even one of these mechanisms

can defend body water; thus diabetes insipidus (inad-

equate production or effect of ADH) does not cause

severe dehydration but polydipsia (increased fluid

intake; thirst is a sensation).

SODIUM/Physiology 5343

0018 Excess salt intake does not raise plasma sodium

concentration (hypernatremia) if water is available

and the patient can drink; the excess sodium is

diluted. The resulting increase in ECF volume then

stimulates increased sodium excretion. Sodium also

enables ECF to hold water against the osmotic ‘pull’

of the solutes in ICF and sodium thus functions as the

‘osmotic skeleton’ of ECF; it is the main determinant

of its volume.

0019 Plasma sodium concentration is therefore only

indirectly related to sodium balance. When ECF

volume, notably circulating volume, is severely re-

duced, this stimulus, rather than Na

concentration,

becomes the main drive for thirst and ADH secretion.

Until ECF volume is restored, water is retained (to

protect ECF volume) even though this undermines the

protection of ECF Na

concentration and, as a result,

plasma sodium falls. Thus, during sodium depletion,

contraction of ECF volume precedes significant

reductions of plasma Na

and is therefore a poor

index of sodium status.

Sodium-Retaining Hormones

0020 Sodium depletion, by reducing plasma volume and

renal perfusion, stimulates the production of renin

(from the kidneys) which generates angiotensin in

circulation. This hormone is a vasoconstrictor (so

protects blood pressure), stimulates thirst (so helps

to restore ECF volume) and, above all, stimulates

sodium retention both directly (renally) and indirectly

(by stimulating adrenal secretion of aldosterone); it

thus reduces the sodium concentration of urine, feces,

saliva, and sweat, but not milk. (See Hormones:

Adrenal Hormones.)

0021 Indices of aldosterone secretion (reduced sodium

or potassium concentration in urine, feces, etc.) are

often taken as evidence of sodium depletion or inad-

equate sodium intake, but the following points apply:

0022 Aldosterone secretion is also stimulated directly

by hyperkalemia (elevated plasma K

) and pro-

motes potassium excretion.

0023 Such interpretations involve a subjective judgment

concerning adequate or excessive sodium intake.

Because physiologists and clinicians were trad-

itionally more concerned with sodium depletion

as well as its consequences and the defenses

against it, elevated aldosterone secretion was read-

ily seen as a warning signal. However, if sodium

intakes associated with increased aldosterone have

no other harmful effects, and especially if excess

sodium intakes cause concern, low levels of aldos-

terone secretion might equally indicate excessive

salt intake.

While sodium reabsorption in the distal nephron,

influenced by aldosterone, is particularly important

because it can produce sodium-free urine and

promote potassium loss, the great majority of renal

sodium reabsorption occurs elsewhere: about 25% in

the loop of Henle and most in the proximal tubule.

The loop is also a main site of magnesium reabsorp-

tion, hence the tendency for loop diuretics to cause

hypomagnesemia.

0024The factors controlling proximal reabsorption are

incompletely understood but their effect is clear:

proximal reabsorption of sodium increases or de-

creases according to the need to enhance or diminish

plasma volume. Since the fluid in the proximal tubule

is similar to plasma, having been formed from it by

glomerular filtration, it has the ideal composition for

this purpose.

Natriuretic Hormones

0025Excretion of excess sodium involves not only suppres-

sion of salt-retention mechanisms but also activation

of sodium-shedding (natriuretic) mechanisms. Two

types of hormones are involved: atrial natriuretic pep-

tide (ANP), produced by the cardiac atria when they

are overstretched (reduction of ECF volume being an

appropriate response to cardiac overload), and active

sodium transport inhibitors (ASTIs), probably pro-

duced within the brain. These were probably the ori-

ginal molecules associated with the receptors binding

cardiac glycoside drugs and are therefore also called

endogenous digitalis-like inhibitors (EDLIs); their

exact identity remains uncertain. ANP has various

effects which essentially oppose those of the salt reten-

tion induced by aldosterone: it increases sodium excre-

tion, lowers arterial pressure, and promotes movement

of ECF towards the interstitial compartment.

0026Other hormones (e.g., sex steroids, parathyroid

hormone, calcitonin, thyroid hormone, prolactin)

affect renal sodium retention or loss but are not

thought to regulate it. (See Hormones: Thyroid Hor-

mones; Pituitary Hormones.)

Adequate, Inadequate, and Excess

Sodium

0027It is unlikely that adult daily maintenance require-

ment exceeds 0.6 mmol per kg body weight and

could well be below this in many mammals. New-

born, growing, pregnant, or lactating animals have

increased requirements. The appropriate sodium

intake for humans remains controversial, with some

cultures managing on less than 1 mmol day

1

, while

western intakes may be in the range 200–300 mmol

day

1

– more where processed foods are heavily

5344 SODIUM/Physiology

consumed. There has been insufficient awareness

among physicians and human nutritionists of just

how high such intakes are, compared with require-

ment in other animals. Granted that humans are

bipeds with a stressful lifestyle quite different from

those of animals, there is no real evidence that human

obligatory losses or sodium requirements are signifi-

cantly greater. Rather, there is an ingrained tradition

of regarding sodium intake as a benign pleasure, in-

volving a harmless and healthy dietary constituent.

The main warnings against this view come from the

fact that hypertension is virtually unknown in low-

salt cultures and that they do not even have an age-

related rise in normal blood pressure. Moreover, there

are numerous studies which, when rigorously ana-

lyzed, indicate that human arterial pressure and salt

intake are positively correlated – sufficiently to antici-

pate large reductions in the prevalence of hyperten-

sion in response to manageable reductions in dietary

sodium. Unfortunately, such reductions are handi-

capped by inadequate food labeling and the fact that

most sodium is added by the processor rather than the

consumer.

0028 Because obligatory losses of sodium are so low,

dietary sodium depletion is hard to induce and

sodium deficiency usually results from losses caused

by renal, adrenal, or enteric disease; renal disease may

cause either retention or loss of sodium. Globally,

both in humans and animals, the most common

cause of sodium deficits is acute diarrhea. Fortu-

nately, sufficient gut usually remains unaffected for

uptake of sodium and water to be stimulated by

suitably formulated oral rehydration solutions.

These essentially restore ECF volume (and acid–base

balance), allowing natural defenses to overcome the

underlying cause of the diarrhea. Despite some

species variations, such solutions usually work best

if their glucose:sodium ratio (in mmol l

1

) is close to

unity and they are virtually isotonic (i.e., they have a

similar osmotic concentration to ECF; hypertonic

solutions draw water into the gut). The function of

glucose in these solutions is to promote sodium

uptake; its nutritional contribution is trivial. Anions

such as citrate, acetate, propionate, bicarbonate, and

amino acids (e.g., glycine and alanine) may further

enhance the uptake of sodium and therefore water.

These sodium cotransport mechanisms are very

similar to those of the proximal renal tubule.

0029Sodium is thus central to the management of two

of the most widespread clinical problems: hyperten-

sion (in humans) and diarrhea. Indeed, the World

Health Organization (WHO) regards the discovery

of oral rehydration, which depends on restoration of

enteric sodium uptake, as the main life-saving devel-

opment in twentieth-century medicine. This powerful

clinical application rests on a simple physiological

observation concerning an elementary but vital diet-

ary constituent. (See Hypertension: Physiology;

Hypertension and Diet; Nutrition in the Diabetic

Hypertensive.)

See also: Electrolytes: Analysis; Hormones: Adrenal

Hormones; Thyroid Hormones; Pituitary Hormones;

Hypertension: Physiology; Hypertension and Diet;

Nutrition in the Diabetic Hypertensive; Potassium:

Physiology; Renal Function and Disorders: Kidney:

Structure and Function; Thermogenesis