The role of selenium in nutrition – A review

Abstract

The role of selenium has been changed over the last decade. The element that was previously considered to be toxic turned out to be present in the human body in amounts of 10–15 mg, and almost every cell of our body contains it. Selenium contributes to growth, supports healthy muscle activity, reproductive organs, reduces the toxicity of certain elements such as mercury, supports the immune system, and even delays the spread of certain viruses (influenza, Ebola, HIV). Selenium-deficient areas of Europe could be a risk for their populations. The recommended daily intake (RDA) of selenium is 55 µg/day, while WHO and FAO have set up the daily tolerable dose at 400 µg/day. We must count with the harmful effects of selenium overdose, but it is almost impossible to introduce this amount into our body solely with food. Our selenium sources can be refilled with food supplements or selenium-enriched functional foods. In the review article, we report about the role of selenium in the environment, selenium-enriched plants, selenium-enriched yeast, the role of selenium in animal feed and in the human body, the opportunities of selenium restoration, selenium-enriched animal products, and the selenium content of milk.

Full text

Acta Univ. Sapientiae, Alimentaria, 11 (2018) 128–144
DOI: 10.2478/ausal-2018-0008
The role of selenium in nutrition –
A review
R. Juh´aszn´e T´oth
e-mail: tothre@agr.unideb.hu
J. Csap´o
e-mail: csapo.janos@gmail.hu
University of Debrecen, Faculty of Agricultural and Food Sciences and
Environmental Management, Institute of Food Technology,
HU-4032 Debrecen, B¨osz¨orm´enyi ´ut 138.
Abstract. The role of selenium has been changed over the last decade.
The element that was previously considered to be toxic turned out to be
present in the human body in amounts of 10–15 mg, and almost every
cell of our body contains it. Selenium contributes to growth, supports
healthy muscle activity, reproductive organs, reduces the toxicity of cer-
tain elements such as mercury, supports the immune system, and even
delays the spread of certain viruses (influenza, Ebola, HIV). Selenium-
deficient areas of Europe could be a risk for their populations. The rec-
ommended daily intake (RDA) of selenium is 55 µg/day, while WHO and
FAO have set up the daily tolerable dose at 400 µg/day. We must count
with the harmful effects of selenium overdose, but it is almost impossible
to introduce this amount into our body solely with food. Our selenium
sources can be refilled with food supplements or selenium-enriched func-
tional foods. In the review article, we report about the role of selenium
in the environment, selenium-enriched plants, selenium-enriched yeast,
the role of selenium in animal feed and in the human body, the oppor-
tunities of selenium restoration, selenium-enriched animal products, and
the selenium content of milk.
Keywords and phrases: selenium, selenium supplementations, seleno-enzymes, seleno-
amino acids, selenocysteine, selenomethionine, nutrition, enriched animal products, func-
tional foods, milk
128

The role of selenium in nutrition – A review 129
1 Introduction
The role of selenium has been considerably appreciated thanks to recent years’
research. Selenium was discovered in 1817 by Berzelius and Gahn (Sz´eles et
al., 2007). In the 1930s, it was considered that selenium is a toxic heavy metal,
which in higher doses leads to the destruction of the living organism (Vernie,
1984). In 1943, its carcinogenic effect has been described (Nelson et al., 1943).
Some years later, Clayton and Baumann (1949) found out that selenium sup-
plementation decreases the number of tumorous cases. The essential role of
selenium was first published in 1957, when rat experiments proved that sele-
nium added to food prevented the necrosis of the liver (Schwarz &Foltz, 1957).
From 1966, we can read about the anti-cancer effects (Shamberger &Rudolph,
1966). The activity of selenium-dependent enzyme proteins was studied in
1973 (Turner &Stadtman, 1973). Glycine reductase and glutathione perox-
ide is found in bacteria and mammals (Rotruck et al., 1973). In 1976, the
chemical characterization of the selenoprotein component of glycine reductase
was studied, and selenocysteine as the organoselenium moiety was identified
(Cone et al., 1976).
Researchers have found that if so many selenium forms are known, no se-
lenium content of the organism can be deduced from the selenium yield. Ac-
cording to Thomassen and Nieboer (1995), we must count with toxicity, acces-
sibility, and the study of absorption in the organism. The oxidation state and
the complex training with other substances must be investigated, and thus the
distribution and volume of the selenic alterations can be determined by means
of speciation analysis (Ebdon et al., 2001).
2 Selenium in the environment
Selenium is rarely found in its elemental form in the environment (Craig, 1986).
Soil, water, and all living organisms contain –2 (selenide), +4 (selenite), and
+6 (selenate) oxidation status (G´omez-Ariza et al., 1998), but these forms
depend on the environmental effects (Skinner, 1999). Selenates and selenites
are water-soluble, so they occur most often in these forms in water (G´omez-
Ariza et al., 1999).
Besides the inorganic alterations, there are so many forms of organic bind-
ings in which selenium is present as selenide (McSheehy et al., 2000; Michalke
et al., 2001). They are mostly seleno-amino acids or their derivatives. Foods
of plant origin include selenomethionine and those of animal origin contain
selenomethionine and selenocysteine. Selenomethionine is essential to humans

130 R. Juh´aszn´e T´oth, J. Csap´o
and animals, but they can produce selenocysteine from selenomethionine in
the organism (Beilstein &Whanger, 1986).
3 The role of selenium in the human body
Selenium plays a significant role in many physiological processes in a direct
or indirect way. Our foods contain selenomethionine and selenite. Selenite
is reacting in our organism with the thiols in the effect of glutathione, and
then it forms H2Se. Selenocysteine is formed from selenomethionine through
various processes. Selenocysteine decomposes into hydrogen selenide as the
result of the β-lyase enzyme. Approximately 90% of selenocysteine integrates
into the proteins of our organism (Mitchell et al., 1976). The amount of these
selenoproteins decreases when the diet is incomplete. Most of them have been
identified in the 20th century, such as the iodotyrosine deiodinase, which is
responsible for the activation of thyroid hormones (Allan et al., 1999), or
selenoprotein P (Ungv´ari, 2015).
With the average selenium intake, the excess is excreted in the urine as a
seleno-amino sugar in large quantities, it is excreted through the respiratory
processes as dimethyl-selenide, while the urine may excrete trimethylselenium
ions (Suzuki &Ogra, 2002; Kobayashi et al., 2002; Bendhal &Gammelgaard,
2004).
The most important role of selenium is its antioxidant effect. This is ex-
pressed by the interaction with various enzymes (Awashti et al., 1975; Moln´ar,
2013; Rig´o, 2002). It plays a key role in the function of the glutathione perox-
idase enzyme, which responds to hydrogen peroxide and other harmful lipids
and phospholipid hydroxides to prevent harmful free radicals, inhibit DNA
damage and the development of metabolic active carcinogens (Karag et al.,
1998). The amount is determined by the amount of selenium and reduced
glutathione in our body (Meister &Anderson, 1983). Selenium is incorpo-
rated into the enzyme as selenocysteine, where the sulphur is located. In the
body’s antioxidant defence system, the biochemical property provides the im-
portance of selenium to reduce the amount of sulphur more easily (Cser &
Sziklai-L´aszl´o, 1998).
The iodotyrosine deiodinase enzyme assists in the production and function
of T3and thyroxin hormones, wherefore selenium is essential for growth and
normal thyroid function (Wilson et al., 1992; Holben &Smith, 1999). Thio-
redoxin reductase regulates cell growth (Mustacich &Powis, 2000), while sele-
noprotein N is responsible for normal muscle development (Zhang et al., 2012).

The role of selenium in nutrition – A review 131
The antioxidant effect of selenium can prevent the oxidation of LDL choles-
terol (Gey, 1998), reduce inflammation, strengthen the immune system, help
protect the body against oxidative stress, and thereby indirectly reduce HIV
virulence (Dworkin, 1994; Stone et al., 1997; Weeks et al., 2012). Selenium
reduces some toxic elements, such as mercury toxicity, via inhibiting their
absorption by forming insoluble compounds (Feroci et al., 2005).
In the case of people on a healthy diet, the risk of selenium deficiency is
small, but in selenium-deficient areas, such as Hungary or Germany (Gondi et
al., 1992), there may be a health risk of persistent selenium nutrition (Ellis &
Salt, 2003).
Selenium deficiency affects about half a billion people annually (Combs,
2001). Many diseases may develop and exacerbate, for example, depression
(Finley &Penland, 1998), cardiovascular disasters, tumour disorders, thyroid
dysfunction, or spread of viruses (influenza, HIV, Ebola) (Tam´as, 2000); it
weakens the viability of sperms (Reilly, 1998), but some studies also write
about the selenium’s capability of delaying ageing (Bankhofer, 1988).
Selenium deficiency may cause Keshan disease, which has been discovered
in China – it mainly affects children and causes cardiopulmonary dysfunction,
leading to myocardial infarction. The Kashin–Beck syndrome (degenerative
joint disease) can also be linked to selenium deficiency (Burke &Opeskin,
2002). Recent studies have shown that inadequate selenium supply can be
associated with Down’s syndrome and the development of infant cretinism
(Ani et al., 2007, Chanoine, 2003).
The normal human body (60–70 kg) contains 10 to 15 mg of selenium. Al-
most every cell of our body contains it, but most of the selenium accumulates
in the kidneys, liver, spleen, pancreas, and testes. According to Codex Al-
imentarius Hungaricus (152/2009 (XI. 12 )FVM ), the recommended daily
intake (RDA) of adults is 55 µg/day. According to the Institute of Medicine,
Food and Nutrition Board (2000), the maximum limit of Se is 400 µg/day
over which negative selenium effects are expected (Arthur, 1991). The first
symptoms of selenium poisoning are metallic mouth taste, garlic smell breath,
in chronic cases, hair loss, the loss of nails, skin rashes, discolouration of the
teeth, and ultimately neurological disorders. Acute selenium toxicity only
rarely causes death, and the lethal dose of selenium is 5–10 mg/kg (Olson,
1986). This quantity cannot be taken in with food (Ungv´ari, 2015).

132 R. Juh´aszn´e T´oth, J. Csap´o
Table 1: Comparison of Recommended Sun Intake (RDA) and Maximum
Tolerable Volume (UL) by age
Age RDA UL
(year) (µg Se/day) (µg Se/day)
1–3 20 90
4–8 30 150
9–13 40 280
14–18 55 400
19– 55 400
Source: Institute of Medicine, 2000
4 Opportunities of selenium restoration
The selenium content of foods is highly varied. The South American Brazilian
walnut (Bertholletia excelsa) has the highest selenium content, more than 100
µg per piece (Chang, 1995). The richest selenium source among our foods is
animal organs (animal meat) and seafood. Since the content of selenium in
foods consumed during a daily meal is not significant, increasing the amount of
food cannot increase selenium intake. Our selenium needs can be covered with
dietary supplements and foods enriched with selenium. Dietary supplements
can be used to satisfy our needs of nutrients and physiological substances since
the 1980s. Nowadays, excellent products are available in capsules or tablets for
this purpose (Horacsek et al., 2006). Selenium-encapsulated food supplements
have already been produced at the University of Debrecen (Eszenyi et al.,
2011). These products mainly contain selenium, selenate, selenomethionine,
or selenium-enriched yeast. Selenium occurs naturally or in near natural form
in selenium-enriched functional foods (Csap´o et al., 2016). When preparing
foods with this technology, the plant or animal is supplied with selenium as a
nutritional supplement that undergoes several transformations and reaches its
natural form. During the transformations, the oxidation state of the selenium
may change, and so it is important to track what form the plant or animal
product contains. The cancer-preventing effect of selenium-enriched garlic has
been reported (Clement &Lisk, 1995), but selenium-enriched bread, pastry,
eggs, and margarine had already been on the market by then in Hungary.

The role of selenium in nutrition – A review 133
5 Selenium-enriched plants
Plants can transform selenite into organic selenium form by fertilization or
spraying. It is safe for animals and humans because the chances of overdose
with selenium consumed with plant foods are low (Terry et al., 2000). Even
in selenium-rich soils, the selenium content of plants does not reach 10 mg/kg
in dry matter. Most plants contain only 1–2 mg/kg of Se, but there are
some that can accumulate a larger amount. Plants belonging to the family
of Brassicaceae and Fabaceae can produce up to several thousand milligrams
of selenium in a kilogram of dry matter (Ellis &Salt, 2003). This can be
explained by the fact that the plants mainly synthesize methylselenocysteine,
which is stored for a long time (Brown &Arthur, 2001), but it is not incor-
porated into plant tissues; they can also be used for the purification of toxic
soils (Ba˜nuelos et al., 2011). In the case of normal Se content, the element is
incorporated into the plant proteins as selenocysteine and selenomethionine.
Selenium-enriched garlic and green onion, chive and broccoli contain selenium
in the form of methylselenocysteines. Wheat, maize, rice, and soybean sele-
nium forms (Beilstein et al., 1991; Tam´as &Csap´o, 2015) were investigated,
and it was found that selenium is mostly present as selenomethionine in these
plants.
In the case of plants, fertilizers or foliar fertilizers with different selenium
contents may be used to increase the selenium content. In the case of animals,
inorganic selenium formulations can be used to increase the selenium content
of animal tissues, but this is better if the feed contains organically bound
selenium such as selenium-enriched fodder (Hidiroglou &Jenkins, 1975).
6 The role of selenium in animal feed
How selenium deficiency does affect the function of animal organisms? Florian
and his colleagues (2010) found that lesion of the colon has occurred in mice
raised with selenium-poor feed. In the case of animals, selenium shortage
can lead to muscular dystrophy anaemia, growth disorders, infertility, heart
diseases, and increasing taint of diseases (Dredge, 2005). As for cattle, the
shortage of selenium negatively influences the production of milk, the risk of
udder inflammation increases, and fertility decreases.
The supplement of the minerals can be done by salt lick in addition to forage.
Before the appearance of salt lick, grinded salt was given to the animals.
Since 1920, there are salt licks that are manufactured especially for animals.
The researchers noticed that animals’ needs of minerals are more efficiently

134 R. Juh´aszn´e T´oth, J. Csap´o
satisfied by blocks rather than grinded salt added to their forage (Sampson,
1923). Moreover, the NaCl makes animals drink more water, which promotes
the milk production and the health of the livestock. On the market, many salt
licks of different ingredients are available. In the USA, the colours of the salt
licks indicate the different types in the following way:
– The white only contains NaCl.
– The yellow contains Sulphur.
– The red has iron and iodine added to it.
– The blue contains cobalt and iodine.
– The brown contains cobalt, iodine, copper, molybdenum, magnesium,
and potassium.
– The black one contains the ingredients of the brown and selenium (Keyes,
2012).
It is customary in Hungary that cattle farms and bull nurseries have 60
mg/kg of selenium-containing salt blocks for animals. By adding a small
amount of selenium (and partially vitamin E), lambs can be protected from
white muscular disease (WMD) and pigs can be treated with vitamin E de-
ficiency (VESD syndrome). Its effect is similar to the human mechanism
through selenium-containing enzymes. In the case of selenium deficiency, the
absence of thyroid hormones T3 and T4 in animals may also occur, which may
also affect the animal weight benefits as well.
7 Selenium-enriched animal products
By feeding with selenium feed additives, we could produce selenium-enriched
meat, eggs, or cow’s milk (Csap´o &Albert, 2018). As most of our foods contain
little selenium, it would be necessary to develop functional products in order to
increase the Hungarian population’s selenium intake. The production of foods
with an increased selenium level is relatively complicated. One of the forms of
selenium is added to animal feed, whereafter it undergoes transformation in
the body until it finally reaches a natural form. Since the oxidation state of
selenium changes during the transformation, it is necessary to monitor what
form of selenium is present in animal products.
As for pigs, feed supplementation uses organic and inorganic selenium-
containing substances to increase the amount of Se in meat. In piglets and
sows, the selenium content of the offal and the muscles increases with the ad-
dition of an appropriate quantity and quality of selenium (Surai, 2006). In

The role of selenium in nutrition – A review 135
laying hens, they act in the same way so as to increase the selenium content
of the egg, whereas in cattle this is a common method of selenium-enriching
meat and milk.
Selenium supplementation increases the activity of selenium-containing en-
zymes such as glutathione peroxidase. Selenomethionine is incorporated into
the body’s protein, thereby providing the body’s selenium supply.
Despite the decrease in milk consumption, it is also a main source of se-
lenium, and so it is advisable to increase its selenium content. Knowing the
selenium concentration, the health status of the stock and the udder is measur-
able. The addition of selenium to the feed of dairy cows allows the production
of selenium-enriched milk.
8 Selenium-enriched yeast
The most commonly marketed selenium source is selenium yeast (Schrauzer,
2000). Selenium-enriched yeast is produced by fermenting Saccharomyces cere-
visiae in high sodium selenite, sodium selenate, and selenomethionine medium.
Yeast cells are destroyed by heat treatment, then spray dried, and finally
checked for the organic and inorganic selenium content of the product. Se-
lenomethionine can integrate into the body proteins and serve as a selenium
source (Thomson, 2004 a,b). The total selenium content can be up to 3,000
mg/kg, which is found as selenomethionine in yeast proteins (Polatajko et
al., 2004), while selenocysteine is only present in small amounts (Kotrebai et
al., 2000). The inorganic selenium content of selenium yeast is also useful
in the formation of proteins but does not become a selenium reserve (Varo
et al., 1988). In some European countries, selenized yeast has already been
authorized as a feed additive to produce functional food. The absorption of
selenium is influenced using yeast strain, the technology of production, and
the selenium form (Fox et al., 2005). They also found that selenium-enriched
yeast is more efficiently incorporated into the body than the inorganic form,
and so it is available for a longer period.
In 1993, a research group studied the use of selenium yeast and selenome-
thionine in breast-feeding and non-breast-feeding mothers. It was found that
the levels of selenium in blood increased because of treatments. Each of the
groups receiving selenomethionine increased the blood plasma selenium level;
however, this was only observed in non-lactating mothers consuming selenized
yeast. The amount of selenium in the milk of selenium-treated mothers in-
creased (Mcguire et al., 1993).

136 R. Juh´aszn´e T´oth, J. Csap´o
From the above examples, you can see that the addition of selenium yeast
activates the selenium enzymes in the body and selenomethionine builds on
the protein of the body to provide continuous selenium replacement, and it
is slowly excreted. The absorption of selenized yeast is slower than that of
selenomethionine because the former has to be dissected before the valuable
substances can be released. Slower absorption is also caused by the presence
of inorganic salts in addition to organic selenium formulations.
9 Milk as selenium source
In Hungary, according to the latest figures of the Central Statistical Office,
milk consumption per capita was 161.0 litres in 2015. This shows a decreasing
tendency compared to previous years; nevertheless, milk is a selenium source
for humans as part of our basic diet. The average selenium content of milk
is 25 µg/l – milk and dairy products amount to 6–10% of the daily selenium
intake (Csap´o &Csap´on´e, 2002). Selenium supplementation in cattle feed
provides an opportunity to increase the Se content of milk. Se supplementation
can be accomplished by feeding with selenium-enriched plants or inorganic
sodium selenite or by addition of organic selenomethionine, selenocysteine, or
selenium-enriched yeast. However, when selecting the additive, it is important
not to ignore that selenium in the rumen of ruminant animals may be reduced
to insoluble selenide or elemental selenium. Hydrogen selenide is released as
rumen and intestinal gases, and the elemental selenium leaves the faeces. It is
practical to favour the organic form because its absorption is more satisfying
than that of inorganic form (Bokori et al., 2003). Several scientists carried
out experiments to measure the change of milk selenium level in selenium
supplementation. Surai (2006) published that organic and inorganic selenium
treatments increase plasma selenium level.
In 2010, Australian researchers investigated the elimination of selenium in
cattle. They found that 66% of selenium intake left the body with urine,
faeces, and gases, 17% of which were excreted in the milk, and 17% were
incorporated into animal tissues (Walker et al., 2010). An Italian research
team published their results in 2010 and 2011, in which the effects of sodium
selenite and selenized yeast were investigated on selenium status and milk
selenium content. The incorporation of SeCys and SeMet, or selenium yeast,
is more effective than inorganic forms and reduces heat stress in dairy cattle
(Calamari et al., 2010, 2011).

The role of selenium in nutrition – A review 137
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