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

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Vanhoe H (1993) A review of the capabilities of ICP-MS for

trace element analysis in body fluids and tissues. Journal

of Trace Elements and Electrolytes in Health and

Disease 7: 131.

Wolfe WR, Holden JM and Schubert A (1992) Selenium

content of selected foods important for improved assess-

ment of dietary intake. Journal of Food Composition

and Analysis 5: 2.

Physiology

C D Thomson, University of Otago, Dunedin,

New Zealand

Introduction

0001 Selenium first attracted attention in the 1930s as a

toxic trace element that caused ‘alkali disease’ in

livestock consuming high-selenium plants. In 1957,

selenium was shown to be essential for animals when

traces of this element prevented liver necrosis in vita-

min E-deficient rats, and later to prevent a variety of

economically important diseases such as white muscle

disease in cattle and sheep, hepatosis dietetica in

swine, and exudative diathesis in poultry. The dem-

onstration in 1973 of a biochemical function for

selenium as a constituent of the enzyme glutathione

peroxidase helped to explain the interrelationship

between selenium and vitamin E. The importance of

selenium in human nutrition was highlighted in

reports in 1979 of the selenium responsive condition,

Keshan disease in China, and selenium deficiency in a

patient on total parenteral nutrition in New Zealand.

Considerable research during the last two decades

has provided information on the molecular biology,

metabolism, and importance of selenium in human

nutrition, leading to a much greater understanding

of the functions of selenium and the establishment

of recommended dietary intakes.

Metabolism of Selenium

0002 Comparatively little is known about the forms of

selenium in foods. The major form in cereals and

other plants is selenomethionine, while the most

likely form in animal foods is selenocysteine, as this

is the active form in functional selenoproteins in the

mammalian body. Inorganic forms are often used

in experimental diets and as supplements, but the

extent of occurrence of inorganic or other forms

of selenium in foods is not clear. The metabolism of

selenium, including absorption, transport, distribu-

tion, excretion, retention, and transformation to

the active form, is very much dependent on the

chemical form and amount of selenium ingested,

and on interacting dietary factors. There is consider-

able species variation in many aspects of selenium

metabolism.

Absorption

0003Selenium is absorbed mainly from the duodenum.

Selenomethionine and methionine share the same

active transport mechanism, but little is known

about the transport of selenocysteine. Absorption of

inorganic forms of selenium such as selenite and

selenate is via a passive mechanism.

0004While the absorption of selenium is generally high

in humans, probably about 80% from food selenium,

selenomethionine appears to be better absorbed than

selenite. Absorption of selenium is unaffected by sel-

enium status, and there appears to be no homeostatic

regulation of absorption. Selenium is better absorbed

from a high-protein diet, and the absorption of sele-

nomethionine may be influenced by the methionine

content of the diet.

Bioavailability

0005As well as absorption, utilization of a nutrient may

also include transformation to a biochemically active

form, which, for selenium, is assessed from changes in

tissue glutathione peroxidase. Few studies have been

made in humans of the bioavailability of selenium,

but animal studies show a wide variation in the avail-

ability from different foods. In rats, the bioavailabil-

ity from mushrooms, tuna, wheat, beef kidney, and

Brazil nuts is 5, 57, 83, 97, and 124%, respectively, in

comparison with sodium selenite. In humans, the

bioavailability of selenium from fish is low in com-

parison with wheat. Human studies also show differ-

ences among various forms of selenium such as

selenate, wheat, and yeast, but this also depends

upon the criterion of measurement used for availabil-

ity, indicating the need to consider several variables,

including short-term changes in glutathione peroxid-

ase activity, long-term retention of tissue selenium,

and metabolic conversion to biologically active

forms.

Transport

0006Little is known about the transport of selenium in the

body, although it appears to be transported bound to

plasma proteins. A selenium-containing protein called

selenoprotein P, isolated from rat and human plasma,

SELENIUM/Physiology 5117

has been suggested as a transport protein, and plasma

also contains extracellular glutathione peroxidase,

but lower-molecular-weight forms of selenium are

more likely to act as transport proteins.

Metabolism and Distribution

0007 An outline of selenium metabolism is shown in

Figure 1. In animal tissues, selenium occurs in associ-

ation with protein, and is present in two main com-

partments or forms. The first is selenocysteine, which

is present as the active form of selenium in selenopro-

teins, including the selenoenzyme glutathione per-

oxidase. The second is selenomethionine, which is

incorporated in place of methionine in a variety of

proteins, unregulated by the selenium status of the

animal.

0008 Selenium levels in tissue are influenced by dietary

selenium intake; this is reflected in the wide variation

in blood selenium levels of residents of countries with

differing soil selenium levels (Figure 2). The total

amount of selenium in the body ranges from 3 to

20 mg, depending on intake of selenium. Retention

of selenium is also profoundly influenced by the form

administered, with selenomethionine being much

more effective in raising blood selenium levels than

sodium selenite or selenate. Selenomethonine appears

to follow the same metabolic pathway as methionine

and is incorporated nonspecifically into protein in its

place. This contributes to tissue selenium, where it

may in fact accumulate, but has no physiological

function and is not available for synthesis of func-

tional forms of selenium until it is catabolized. Selen-

ate is reduced to selenite, then selenide, and in this

oxidation state (2) is introduced into selenocysteine,

the active form of selenoproteins. The selenium from

inorganic selenium, or from catabolism of seleno-

methionine or selenocysteine, is incorporated into

selenocysteine, which enters regulated selenium

metabolism and is then incorporated into seleno-

proteins. Selenium intake from selenate, selenite, or

selenocysteine, unlike that from selenomethionine,

will be excreted in urine if in excess.

Incorporation of Selenium into Selenoproteins

0009All functional mammalian selenoproteins contain

selenocysteine at the active site. Selenium is incorpor-

ated by replacement of oxygen into serine to form

selenocysteine, while serine is attached to a unique

tRNA (transfer ribonucleic acid). Selenocysteine is

encoded by a unique stop codon UGA on the mRNA

specific for the selenoprotein. Regulation of seleno-

protein synthesis appears to be via individual mRNAs

(messenger RNAs) at the transcriptional or posttran-

scriptional levels in response to selenium availability,

as well factors such as the chemical form of selenium

and oxygen exposure. However, there is differential

depression of selenoprotein synthesis in response to

an inadequate selenium supply with preservation of

the metabolically most important proteins. This has

been termed the ‘hierarchy’ of importance of func-

tional selenoproteins.

Excretion

0010Urine is the principal route of excretion of selenium,

followed by feces, which mainly contain unabsorbed

selenium. Homeostasis of selenium is achieved by

regulation of its excretion. Daily urinary excretion

of selenium is closely associated with plasma selen-

ium and dietary selenium intake and has been used as

an indicator of selenium status. Balance studies show

Selenomethionine

in methionine pool

Selenium

metabolism

Excretory

metabolites in

urine, breath

Transport

form

Selenocysteine,

inorganic, other forms

Selenomethionine

Dietary selenium Tissue selenium

Selenomethionine in

tissue proteins

Selenocysteine in

glutahione peroxidase,

other selenoproteins

fig0001 Figure 1 Outline of selenium metabolism.

5118 SELENIUM/Physiology

that over a wide range of intakes, urinary excretion

accounts for 50–60% of the total amount excreted.

Measurement of the plasma renal clearance of selen-

ium, which expresses its rate of excretion in the urine

in terms of the amount contained in a unit volume of

plasma, shows that the kidneys of residents of the

low-selenium country, New Zealand, excrete selen-

ium more sparingly than those of North Americans,

and Chinese from low-selenium areas have an even

lower renal clearance. This indicates a possible

adaptation to low selenium status.

0011 Trimethylselenonium ion, one of several urinary

metabolites, was once thought to be a detoxification

product of selenium and appears to be a minor me-

tabolite in humans. Another methylated metabolite,

methylselenol, may constitute a larger proportion of

urinary selenium. Small losses of selenium occur

through the skin or hair or, at high intakes, in expired

air as volatile dimethylselenide.

Excretion of Selenium in Milk

0012 Most of the selenium in human milk is protein-

bound; at least nine selenium-containing proteins

have been detected in milk with glutathione peroxid-

ase accounting for 15–30% of total selenium.

Role of Selenium in the Body

Selenoproteins

0013Selenium exerts its biological effect through several

selenoproteins, of which there may be upwards of 30

in mammalian systems. Identification and character-

ization of many of these proteins in recent years have

resulted in major advances in our understanding

of the function of selenium. The following seleno-

proteins have been purified and studied.

0014glutathione peroxidases;

0015cytosolic, cellular

0016gastrointestinal

0017plasma

0018phospholipid hydroperoxide

0019selenoprotein P;

0020iodothyronine 5

-deiodinases;

0021sperm capsule selenoprotein;

0022selenoprotein-W;

0023thioredoxin reductase;

0024selenophosphate synthetase;

002558-, 56-, and 14-kDa Se-binding protein.

0026Glutathione peroxidases The first of these sele-

noproteins to be characterized was glutathione

3000

800

350

300

250

200

150

100

Venezuela

USA

Australia

Finland

New Zealand

China

Auckland

122 Otago

104

116

210

Number of subjects

Whole blood selenium (ng ml

−1

)

fig0002 Figure 2 Blood selenium concentrations of healthy adults in Venezuela, the USA, Australia, Finland, New Zealand, and China (low-

and high-selenium areas). (To convert nanograms of Se per milliliter to micromoles per liter, multiply by 0.0127.)

SELENIUM/Physiology 5119

peroxidase, which consists of four identical subunits,

each containing one selenocysteine at the active site.

Activity of this enzyme can be reduced to less than

1% in tissues of selenium-deficient animals. Glu-

tathione peroxidase present in cells (including eryth-

rocytes), plasma, and the gastrointestinal tract may

function in vivo to remove hydrogen peroxide,

thereby preventing the initiation of peroxidation of

membranes and oxidative damage. However, the sig-

nificance of this function in the body is uncertain, and

it seems likely that the oxidant defense role for selen-

ium is exerted more through other selenoproteins.

Glutathione peroxidase may have more specific

functions in arachidonic acid metabolism in plate-

lets, microbiocidal activity in leukocytes, and the

immune response mechanism or perhaps as a storage

protein.

0027 Another selenium-containing enzyme, phospho-

lipid hydroperoxide glutathione peroxidase, is differ-

ent from the classic glutathione peroxidase in that it

can metabolize fatty acid hydroperoxides that are

esterified in phospholipids in cell membranes. This

enzyme can inhibit microsomal lipid peroxidation

and is most likely the basis of the selenium/vitamin

E interaction in the pathogenesis of several deficiency

diseases in animals.

0028 Selenoprotein P A plasma protein designated sele-

noprotein-P has been purified and characterized from

rat and human plasma and is also associated with

endothelial cells. Selenoprotein-P is a glycoprotein

containing selenium as selenocysteine, and its concen-

tration in rat plasma falls to less than 10% of the

control in selenium deficiency. Its function is un-

known, but it may have an extracellular oxidant de-

fense role because its presence correlates with

selenium protection of selenium-deficient rats against

diquat-induced lipid peroxidation and liver necrosis.

A transport role has also been suggested for this

protein.

0029 Iodothyronine deiodinases The discovery that type I

iodothyronine 5

-deiodinase and, more recently, type

II and type III deiodinases are selenoproteins indicates

a role for selenium in the metabolism of thyroid hor-

mones that are essential for growth and development.

This enzyme catalyzes the conversion of thyroxine

) to its active metabolite triiodothyronine (T

)in

the liver and kidney, and selenium deficiency results

in an increase in levels of plasma T

and a corres-

ponding decrease in levels of more active T

. Selen-

ium is preferentially supplied to this enzyme rather

than glutathione peroxidase when the selenium supply

is inadequate. The interactions of selenium and iodine

deficiencies have implications for both human and

animal health and livestock production. In humans,

selenium deficiency can either exacerbate or amelior-

ate the effects of concurrent iodine deficiency as has

been observed in the development of iodine deficiency

disorders in Zaire.

0030Thioredoxin reductase A recent addition to the

list of selenoproteins is thioredoxin reductase, an

NADPH-dependent flavoenzyme that reduces the

disulfide of thioredoxin. This enzyme regenerates

ascorbic acid from dehydroascorbic acid. The activity

of thioredoxin reductase declines in selenium defi-

ciency, and there is evidence that selenium is present

in the active site as selenocysteine.

0031Selenoprotein W Selenoprotein W is a selenoprotein

found in muscle and other tissues. Its concentration

decreases during selenium deficiency, and it may be

involved in the development of muscular degener-

ation in selenium-deficient sheep.

0032Other selenoproteins Several other selenium-

containing enzymes have been identified in

microorganisms, and other selenoproteins have

been found in animal tissues, suggesting further

functions for selenium. These include a specific sele-

noprotein in prostate tissue, a mitochondrial seleno-

protein in sperm, and a 14-kDa and 56/58-kDa

selenium-binding protein. One of two selenopho-

sphate synthetases recently identified contains seleno-

cysteine, and it is possible that this enzyme may be

involved in the regulation of selenium homeostasis.

Selenium and Host Defense Towards Viruses

0033The seasonal nature of Keshan disease suggested

the likely involvement of an infectious agent, which

led to recent work on host response to myocarditic

and amyocarditic strains of Coxsackie virus B

mice. Selenium deficiency potentiated cardiotoxi-

city of myocarditic strains, but in addition, the amyo-

carditic strain caused heart lesions in selenium

deficient mice. This indicated that this specific nutri-

ent deficiency allowed a benign virus to become viru-

lent, apparently as a result of a change in the viral

genome. The observation that selenium deficiency

may accelerate viral evolution may help to clarify

the etiology of Keshan disease and may also be

applicable to other RNA viruses. Vitamin E defi-

ciency had effects similar to those observed with

selenium deficiency, which suggests the involvement

of antioxidant properties of the two nutrients. This is

the first reported instance of an influence of host

nutritional status on the genetic composition of the

pathogen.

5120 SELENIUM/Physiology

Selenium and Immune Function

0034 Adequate selenium is essential for a variety of aspects

of the immune function. Selenium is necessary for the

development of the acquired immune system, and has

a role to play in the defense system of animals against

bacteria and other infections. The recent finding of a

role for selenium in cell-mediated immunity to Cox-

sackie virus in mice indicates a protective effect of

selenium against viral infection. The mechanisms for

the involvement of selenium in the immune system

are likely to be related to its antioxidant function

through one of the antioxidant selenoproteins

glutathione peroxidase, phospholipid glutathione

peroxidase, or selenoprotein-P.

Deficiency

0035 Interaction between selenium and vitamin E is ob-

served in the etiology of many deficiency diseases in

animals and pure selenium deficiency is in fact rare.

Thus, selenium deficiency may only occur when low

selenium status is linked with an additional stress

such as chemical exposure, increased oxidant stress

due to vitamin E deficiency, exercise, or increased

dietary intake of polyunsaturated fatty acids. Al-

though residents in some low-selenium areas have

low blood selenium, glutathione peroxidase activity,

and Selenoprotein P levels, there is little evidence that

these are suboptimal or have resulted in any notice-

able oxidative damage or changes in other oxidative

defense mechanisms. Moreover, people have not

shown noticeably improved health when glutathione

peroxidase activity is saturated by selenium supple-

mentation. However, an exception to this general rule

may be found in the problem of Keshan disease in

China. Whether any of the newer functions of selen-

ium are suboptimal in persons with low selenium

status is being investigated.

Selenium-responsive Diseases in Humans

0036 Keshan disease, an endemic cardiomyopathy occur-

ring in low-selenium areas of China, was reported in

1979 to be responsive to supplementation with

sodium selenite. The principal pathological finding is

multifocal necrosis of the myocardium that causes

cardiac enlargement, congestive heart failure, cardio-

genic shock, and death. The disease is associated with

low selenium intake and low blood and hair levels,

and affects mainly children and women of child-

bearing age. It is probably the only case of naturally

occurring selenium deficiency. Because some features

of Keshan disease (e.g., seasonal variation) cannot be

explained solely on the basis of a very low selenium

status, Chinese researchers have suggested that other

factors may be involved, such as a virus, mineral im-

balance, or environmental toxins. Recent research on

Coxsackie virus B-induced myocarditis in selenium-

deficient mice supports a possible viral involvement in

Keshan disease. Another disease that has been associ-

ated with poor selenium status in China and Russia is

Kashin–Beck disease, an endemic osteoarthritis that

occurs during preadolescent or adolescent years.

Again, other etiologic factors such as contamination

of grain by fungi and high concentrations of organic

matter (e.g., fulvic acid) may be contributing factors.

The main features of the disease are shortened stature

and joint deformation, resulting from multiple focal

necroses in the growth plate of tubular bones.

0037Selenium deficiency has been associated with long-

term intravenous nutrition, because of the low levels

of selenium in the fluids. Clinical symptoms of

cardiomyopathy, muscle pain, and muscular weak-

ness are responsive to selenium supplementation,

but are not seen in all patients with extremely low

selenium status, indicating that there may be other

interacting factors. Furthermore, children on very

low selenium synthetic diets for inborn errors of me-

tabolism such as phenylketonuria do not develop

selenium-deficiency syndromes.

0038It is not clear whether syndromes in animals such as

the selenium-responsive muscular dystrophy observed

in sheep are related to human diseases. Although per-

sistent anecdotal reports from New Zealand farmers

in low-selenium areas indicate their conviction that

selenium relieves the farmers’ muscular aches and

pains, double-blind trials have failed to give a clear-

cut answer.

Selenium and Cancer

0039Several lines of scientific enquiry suggest that an in-

creased risk of cancer occurs as a result of low levels

of selenium in the diet. An association between selen-

ium and cancer was first proposed 30 years ago when

it was observed that regional cancer mortality rates

in the USA correlated with selenium exposure, as

reflected by concentrations in plants. Evidence for

the role of selenium as an anticarcinogenic agent

comes from in vitro and animal studies that suggest

that selenium is protective against tumorigenesis at

high levels of intake and from case-controlled pro-

spective studies in human subjects. However, epi-

demiological evidence linking a low selenium status

with an increased incidence of cancer is conflicting.

Few intervention studies in humans have been carried

out. In Linxian China, a significant effect of a com-

bination of vitamin E, b-carotene and selenium was

observed on mortality, particularly from esophageal

cancer. Recently a 10-year controlled clinical trial to

test the efficacy of selenium in preventing skin cancer

SELENIUM/Physiology 5121

was completed in the USA. There was no effect of

daily supplementation with 200 mg of selenium on the

primary endpoint skin cancer, but there was a statis-

tically significant reduction in several additional end-

points such as total cancer mortality (50%), and

incidence of cancer of the prostate (63%), lung

(46%), colorectal (58%), and total cancer. Further

studies are needed to confirm these observations.

Selenium and Cardiovascular Disease

0040 Similarly, dietary deficiency of selenium has been

implicated in the etiology of cardiovascular diseases,

but the evidence at present is less convincing than for

cancer. Although a large case-control study in Finland

suggested that selenium was an independent risk

factor for myocardial infarction in a low-selenium

population, evaluation of prospective epidemio-

logical studies has failed to provide sufficient evi-

dence to implicate selenium deficiency in most

aspects of cardiovascular disease. While some investi-

gations have observed a relationship between low

serum-selenium levels and risk of coronary disease,

others have not. However, selenium may have some

role in protection against thrombosis and low-density

lipoprotein oxidation, in particular in individuals

such as smokers at risk from increased oxidant stress.

Further evidence must come from controlled inter-

vention trials to clarify any possible role for selenium

in atherosclerotic disease.

Toxicity

0041 The margin between an adequate and toxic intake of

selenium is quite narrow. Overexposure or selenosis

may occur from consuming high-selenium foods

grown in seleniferous areas in Venezuela and some

areas of China. People following long-term liberal

megadosing can also attain an undesirably high sel-

enium status. The most common sign of poisoning is

loss of hair and nails, but lesions of the skin, nervous

system, and teeth may also be involved. Garlic odor

on the breath is an indication of excessive selenium

exposure resulting from expiration of dimethylsele-

nide. Sensitive biochemical techniques are lacking for

selenium toxicity, which is at present diagnosed from

hair loss and nail changes. Some effects of selenium

toxicity are seen in individuals with dietary intakes as

low as 900 mg, and the maximum safe dietary intake

has been suggested as 400 mg per day.

Assessment of Selenium Status

0042 Blood selenium concentration is generally considered

a useful measure of both selenium status and intake,

but other tissues are often assessed as well. Plasma

selenium reflects short-term status and erythrocyte

selenium long-term status, but blood selenium con-

centrations are influenced by the chemical form of

selenium ingested as a result of the different patterns

of absorption and retention outlined previously. Toe-

nails are often used, but selenium-containing sham-

poos restrict the use of hair. Urinary excretion can

also be used to assess selenium status, and total

dietary intake is estimated as twice the daily urinary

excretion.

0043The close relationship between blood or red-cell

glutathione peroxidase activity and selenium con-

centrations (Figure 3) is useful for assessment in

people with a relatively low status, but not once

the saturating activity of the enzyme is reached at

blood selenium concentrations above 100 mgl

1

(1.27 mmol l

1

). More recently, mesurement of sele-

noprotein P has been used to assess selenium status,

and there is the potential for measurement of other

enzymes as functional markers. However, their use is

limited at present by the lack of simple assay tech-

niques. Furthermore, the conclusions drawn from

measurement of one selenoprotein may not apply to

all biological functions of selenium because of the

differences in responses of tissues and these proteins

to deficient, adequate, or high levels of selenium in

terms of their ‘hierarchy’ of importance. Therefore,

there may be no single indicator of functional selen-

ium status, but rather a series of markers that apply to

specific problems associated with suboptimal selen-

ium status. The situation is further complicated by the

large number of interacting factors, including, pro-

tein, methionine, polyunsaturated fatty acids and

other oxidant stressors, vitamin E, other trace elem-

ents, and heavy metals such as mercury, cadmium,

and lead.

Dietary Intake

0044Food is the major source of selenium; drinking water

contributes little. Dietary intake varies with the

geographic source of foods and eating habits of the

people. Plant food concentrations reflect selenium

content of soils and its availability for uptake, as

plants do not require selenium for growth; cereals

and grains grown in soils poor or rich in selenium

may vary over 100-fold in selenium content. Animal

foods vary less. Fish and organ meats are the richest

sources followed by muscle meats, cereals, grains,

and dairy products, with fruits and vegetables mostly

poor sources. Average daily dietary intakes vary con-

siderably, depending on the levels of selenium in soils

(Table 1), ranging from 10–20 mg of selenium in low

soil-selenium areas of China where Keshan disease is

endemic, 30–60 mg in New Zealand, and up to 200 mg

5122 SELENIUM/Physiology

in seleniferous areas in Venezuela. In 1985, selenium

was added to fertilizers in Finland to increase selen-

ium intake throughout the population, and the daily

intake rose from 40 mg to close to 100 mg per day,

resulting in an increase in serum selenium in healthy

individuals from 65–70 mgl

1

in 1985 to 120 mgl

1

1989–1991. In New Zealand, intakes are increasing

due to increasing importation of Australian wheat,

and selenium status is rising accordingly. However,

this increase is greater than expected from the

increase in wheat selenium, indicating that other

dietary factors are involved. These differing intakes

in various countries are reflected in the wide range of

blood selenium concentrations found in residents of

these countries (Figure 2).

Requirements and Recommended Dietary

Allowances (RDAs)

0045Many countries have proposed recommended dietary

intakes based upon estimates of requirements from

Chinese intakes for endemic and nonendemic Keshan

disease areas, as well as intakes at which maximal

levels of plasma glutathione peroxidase activity oc-

curred. The recommended dietary intake for Austra-

lia was the first set in the world in 1986 and may have

been cautiously on the high side at 85 and 70 mg

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

Selenium concentration (µg Se per milliliter of erythrocytes)

Glutathione peroxidase activity (units per gram of Hb)

fig0003 Figure 3 Relationship between selenium concentration of erythrocytes and glutathione peroxidase activities for New Zealand

residents: Otago patients (s): Otago blood donors (m) and overseas subjects (h). (To convert nanograms of Se per milliliter to

micromoles per liter, multiply by 0.0127.) From Rea HM, Thomson CD, Campbell DR and Robinson MF (1979) Relation between

erythrocyte selenium concentrations and glutathione peroxidase (EC 1.11.1.9) activities in New Zealand residents and visitors to New

Zealand. British Journal of Nutrition 42: 201–208, with permission.

tbl0001 Table 1 Daily dietary intakes of selenium and whole blood

selenium concentrations

Country Seleniumintake

(mgperday)

Blood selenium

(mgl

1

)

China (Keshan area) 9–11 10–20

New Zealand (before 1990) 28–32 50–100

(post 1990) 30–60 60–120

Finland (1985) 30 50–100

Finland (1989–1990) 100 120

UK 60 80–320

USA 62–216 150–400

Venezuela (Caracas) 218 350

China (seleniferous area) 5000 3200

From Thomson CD (2002) Selenium. In: Mann JI and Truswell AS (eds)

Essentials in Human Nutrition, 2nd edn, pp. 172–181. Oxford: Oxford

University Press, with permission.

SELENIUM/Physiology 5123

selenium for Australian men and women. Others,

including the recommended intakes of the USA, the

UK, and other European countries, are summarized in

Table 2. Each set of recommended intakes can be met

from habitual diets in each country. Whether optimal

health depends upon saturation of glutathione perox-

idase activity has yet to be resolved, leading to dis-

agreement regarding the use of this approach for

assessing selenium requirements. The US RDAs are

based on the desirability of the full activity of glu-

tathione peroxidase, whereas a WHO group con-

cluded that only two-thirds of the maximal activity

was needed, based on the observation that abnormal-

ities in metabolism of hydrogen peroxide in blood

cells is apparent only when enzyme activity falls to

one-quarter or less of normal. This has led to a wide

range of dietary recommendations, with the WHO

recommendations approaching intakes in countries

with a naturally low selenium status such as New

Zealand. In the future, several of the newly dis-

covered selenoproteins might be used as endpoints

for determining selenium requirements. However,

the maximal activity of some of these proteins occurs

at dietary intakes of selenium less than those needed

for maximal glutathione peroxidase activity. This

would no doubt lead to dietary recommendations

lower than current values.

See also: Bioavailability of Nutrients; Cadmium:

Toxicology; Cancer: Diet in Cancer Prevention; Coronary

Heart Disease: Prevention; Dietary Requirements of

Adults; Mercury: Toxicology; Tocopherols: Physiology

Further Reading

Arthur JR, Beckett GJ and Mitchell JH (1999) The inter-

actions between selenium and iodine deficiencies in man

and animals. Nutrition Research Reviews 12: 55–73.

Barceloux DG (1999) Selenium. Clinical Toxicology 37:

145–172.

Beck MA (1998) The influence of antioxidants on viral

infection. Nutrition Reviews 56: S140–S145.

Burk RF and Levander OA (1999) Selenium. In: Shils ME,

Olson JA, Shike M and Ross CA (eds) Modern Nutrition

in Health and Disease, 9th edn, pp. 265–276. Baltimore,

MD: Williams & Wilkins.

Dreosti I (1990) Selenium. In: Truswell A, Dreosti I, English

R, Ritihauser I and Palmer N (eds) Recommended Nu-

trient Intakes Australian Papers. Sydney: Australian Pro-

fessional Publications.

Levander OA and Burk RF (1996) Selenium. In: Ziegler EE

and Filer LJ (eds) Present Knowledge in Nutrition, 7th

edn, pp. 320–328. Washington, DC: ILSI Press.

Reilly C (1996) Selenium in Food and Health. London:

Blackie Academic & Professional.

Robinson MF (1989) Selenium in human nutrition in New

Zealand. Nutrition Reviews 47: 99–107.

Thomson CD (1998) Selenium speciation in human body

fluids. Analyst 123: 827–831.

World Health Organization (1996) Trace Elements in

Human Nutrition and Health, pp. 105–122. Geneva:

WHO.

tbl0002 Table 2 Recommended intakes of selenium for adults (mg/day)

Australia

(1990)

RDI

USA &

Canada

(2000)

RDA

UK (1991) Germany, Austria,

Switzerland (2000)

RNI

WHO

(1996)

LNRI RNI

Men 85 55 40 75 30–70 40

Women 70 55 40 60 30–70 30

World Health Organization (1996) Trace Elements in Human Nutrition and

Health, pp. 105–122. Geneva: WHO.

RDI, recommended dietary intake; RDA, recommended dietary allowance;

RNI, reference nutrient intake; LNRI, lower reference nutrient intake; NR,

normative requirement estimate.

5124 SELENIUM/Physiology

SENSORY EVALUATION

Contents

Sensory Characteristics of Human Foods

Food Acceptability and Sensory Evaluation

Practical Considerations

Sensory Difference Testing

Sensory Rating and Scoring Methods

Descriptive Analysis

Appearance

Texture

Aroma

Taste

Sensory Characteristics of

Human Foods

R York, Canadian Food Inspection Agency, Winnipeg,

Manitoba, Canada

M Vaisey-Genser, The University of Manitoba,

Winnipeg, Manitoba, Canada

Background

0001 Satisfaction with the sensory features of foods and

beverages is a major determinant of the consumer’s

nutritional status. If something does not taste ‘good,’

it is not likely to become part of the food habit.

However, the colloquial term ‘good’ is a reflection

of an integrated response to the several food features

that are considered in detail in following sections.

The present discussion is an introduction to sensory

evaluation that focuses on its ongoing evolution as a

discipline.

Definitions

0002 Early work in sensory evaluation was referred to as

organoleptic testing, a term that was discarded in the

1960s because it failed to recognize the complexity of

the sensory impression. In 1975, the US Institute of

Food Technologists defined the subject as a scientific

discipline used to evoke, measure, analyze, and inter-

pret reactions to those characteristics of foods and

materials as they are perceived by the senses of

sight, smell, taste, touch, and hearing. This definition

has since been extended to include the purpose of

providing information for decision-making with eco-

nomic consequences. As the field increases in sophis-

tication, the term ‘sensory science’ is becoming the

descriptor of choice rather than sensory evaluation.

The sequence and the strategy for sensory science

measurements is illustrated in Figure 1.

Justification

0003Sensory measurements provide a unique resource

of information for the evaluation of foods. Humans

are sensitive and complex instruments capable of

measuring and integrating simple and complex

stimuli, repeating measurements after a brief resting

stage, and differentiating relevant from irrelevant

information. Accordingly, most physical tests for

‘ideal’ product characteristics are based on human

definition. The results are reliable and repeatable as

long as the test conditions are controlled.

History

0004Among sensory features, aroma seems to have cap-

tured major interest in early times, perhaps reflecting

the importance of spices for foods and of other aro-

matics used in perfumes and oils. In 320 bce (Before

the Common Era), a treatise on odor was put forth

by Theophrastus in Greece. That was followed by the

occasional classic on the nature of smell capped by

Linnaeus, odor classification, which appeared in

1756. Over time, the ‘expert taster’ became relied

upon within specific commodities, most notably

teas, wines, and other alcoholic beverages.

0005Sensory evaluation as we know it today stems from

the attention to methodology that began in the latter

part of the nineteenth century. This included methods

as simple as the score card to monitor commodity

quality and as sophisticated as the Weber–Fechner

Law, which related the sensory response quantita-

tively with incremental changes in a physical stimu-

lus. This spawned the discipline of psychophysics, a

SENSORY EVALUATION/Sensory Characteristics of Human Foods 5125

hybrid of psychology and physiology. The late nine-

teenth century saw the introduction of statistical

methods for handling sensory data.

0006 There were two sensory landmarks in the twentieth

century: the development of statistically sturdy meas-

urement methods and attention to the selection,

training, and performance monitoring of sensory

panelists. In the early 1930s, rating scales were de-

veloped to measure the intensity of specific food qual-

ities such as the tenderness of meat, and concurrently,

paired comparison and rank order methods were

introduced. The early 1950s was marked by the intro-

duction of the flavor profile system for the qualitative

analysis of foods by the Arthur D. Little Company

(ADL) in Massachusetts. ADL introduced training of

selected members from an industry in the systematic

‘descriptive analysis’ of their products.

0007 The mid-1950s saw further advances in statistical

methods for both design and analysis as well as the

launch in the USA of forced-choice methods such as

duo–trio and the triangle test, which originated in the

USA and Sweden, respectively. These tools are still

widely used by industry for sensory testing in their

quality control laboratories. A singular contribution

from this era was the nine-point hedonic scale for

measuring how much a food was liked or disliked.

Developed by the food acceptance branch of the US

Quartermaster Food and Container Institute in Chi-

cago, this tool has been used far beyond its original

purpose, which was to assist the development of

more acceptable rations for the military. It remains

a worldwide standard tool for food-preference

measurement.

0008 The momentum in sensory evaluation continued

throughout the 1960s marked by three major events:

(1) the 1961 formation of Committee E-18 within the

American Society for Testing of Materials (ASTM),

(2) the publication in 1963 of the Texture Profile

Method by scientists at General Foods Corporation

led by Alina Szczesniak, and (3) the 1965 Academic

Press publication of Principles of Sensory Evaluation

of Food, written by Amerine, Pangborn, and Roessler

at the University of California in Davis, a book that

promptly became a standard text. Further steps for-

ward were the 1968 opening of the Monell Chemical

Senses Center in Philadelphia to concentrate on basic

research of the senses under the leadership of physi-

ologist Morley Kare, and the 1969 introduction of the

Journal of Texture Studies.

0009In the early 1970s, the International Organization

for Standardization/Organization Internationale de

Normalisation (ISO) formed a subcommittee, TC34/

SC12, for Sensory Evaluation with the aim of having

internationally agreed upon standards for sensory

methodologies. This group currently includes repre-

sentatives from 17 countries as members and 30

countries as observers.

0010During the 1970s and 1980s, newer methods

of data handling erupted with the maturing of the

computer age. Multivariate analyses became the

norm, facilitating methods such as response surface

and principal-component analyses. Communication

worldwide was sparked by international symposia

such as the Mediterranean Odor Conference in

Cannes in 1970 followed by the later conferences

in Switzerland, Sweden, and the UK. New journals

such as 1974s Chemical Senses and Flavor (now

Chemical Senses), and later, in 1986, the Journal of

Sensory Studies emerged followed by Food Quality

and Preference in 1988. The 1990s saw the formation

of the Sensometrics Society for the development of the

special methodologies and statistical methods, which

were needed by the fields of sensory and consumer

science.

Take action

Analyze and

interpret

data from

these numbers

Measure

perceptions

with

sensory

analysis test

methods

Numbers guide

product decisions

Perceptions become

meaningful numbers

Decisions have

economic

consequences

Human perceptions

about products

fig0001 Figure 1 Sequence and strategy of sensory science measurements.

5126 SENSORY EVALUATION/Sensory Characteristics of Human Foods