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Selenium in Human Nutrition

WHAT IS SELENIUM?

Selenium is a micronutrient required in very small amount to maintain health in humans and animals. As the body cannot produce it on its own, it has to be supplied by diet and thus belongs to the family of essential trace elements.

The element itself is found in the soil, from where it is incorporated into plants. It can be found in form of inorganic selenium salts (sodium selenite and selenate) or in organic compounds called selenols, among which the selenium-containing amino acids selenomethionine and selenocysteine, which can be incorporated into proteins. Proteins in which selenocysteine has been incorporated are known as selenoproteins.

In humans, selenocysteine, also called the 21st amino acid, can be incorporated into proteins following a complex regulated machinery, whereas selenomethionine, on the other hand, can be incorporated into any protein with the same mechanism as methionine, by which the body mistakes selenomethionine for methionine. 

Nevertheless, all dietary sources of selenium, including selenomethionine, contribute to selenoprotein formation through metabolization pathways. 

There are about 25 selenoproteins in our body, many of which are enzymes that act to protect the body against oxidative damage (for a review see (1)).

WHAT DOES SELENIUM DO IN YOUR BODY?

Selenium is mostly known for its potent antioxidant properties. Indeed, it is a required oligoelement for the synthesis and function of about 20-40 enzymes, among which most of them help prevent cellular damage from natural by-products of oxygen metabolism, called reactive oxygen species (ROS) or free radicals (2, 3). Excessive amounts of ROS can cause deleterious effects, such as damages to DNA and oxidation of cellular proteins, which may contribute to the development of chronic diseases including cancer, heart disease and neurodegenerative diseases (4). However, not all selenoproteins have an antioxidative role.  Some are important in thyroid function, in particular in hormone production (5). Selenium is also essential for the proper function of the immune system and is known to have antiviral properties (6, 7). Effects on inflammatory responses are among the other key activities identified for selenoproteins (8).

WHICH FOODS PROVIDE SELENIUM?

Selenium content in food varies greatly since it depends on the local soil selenium composition from which plants are grown or animals are raised.

Most regions of the United States and Canada have selenium-rich soils, whereas Finland, New Zealand and parts of China and, to a lesser extent, the United Kingdom and Europe in general are the poorest regions of the globe.

Interestingly, dietary selenium intake by European populations has fallen around 50% over the last three decades. Intake is now estimated to be in a sub-optimal range, which has been raising some health concerns lately (9, 10). This is most likely related to the increased use of European Union wheat for bread flour over traditional North American varieties.

Interestingly, Finland as introduced an enrichment program of agricultural fertilizers with sodium selenite since 1984. As a result, the dietary selenium intake has tripled in this country while the incidence of heart disease and of certain cancers has decreased. However, this remains a correlation; a direct proof that this decrease is due to increased selenium intake has yet to be provided.

All consideration about soils taken apart, award winner dietary source for selenium are Brazil nuts (11). Other good sources of selenium are fish (in particular tuna fish), eggs and poultry. Some cheese types like Emmental, Roquefort, Feta and Camembert also contain some selenium.

Selenium in foods such as bread, cereals, seafood and those mentioned above is predominantly found as amino acid derivatives selenomethionine and selenocysteine (12). These organically bound forms show better bioavailability than inorganic forms like selenate, which makes up to 50% of selenium content in some plants, including the leaves of beets, cabbage, and onions (13).

CONSEQUENCES OF DEFICIENCIES AND EXCESS, FINDING THE OPTIMAL DOSE

Selenium deficiency can occur in:

•    People living in regions where soil concentration of selenium is low.

•    People who rely on total parenteral nutrition as their sole source of nutrition (14).

•    People with severe gastrointestinal disorders such as Crohn’s disease, that result in decreased selenium absorption (15, 16).

In humans, selenium deficiency can result in Keshan disease, a juvenile cardiomyopathy found in the Chinese Keshan region, where the intake can be particularly low (<15 μg/d) (17, 18). There is also evidence for selenium deficiency being involved in impaired immune function, and increased incidence of cancer, cardiovascular and other degenerative diseases, as well as overall mortality (10, 19).

Furthermore, a subclinical deficiency – that is to say, in which elusive clinical signs prove difficult to pin down diagnosis – is suspected to be associated with various diseases including cardiovascular and inflammatory disorders, asthma, weakened immunity, cancer, cataracts, etc.

Dietary recommendations for selenium are currently difficult to make. Required selenium intake varies among populations and has changed over time, constantly being re-assessed. They were at first exclusively based on the selenium level required to optimize the activity of one antioxidative selenium-containing enzyme (glutathione peroxidase). However, subsequent recognition of a wide range of other selenoproteins with equal importance in human health has significantly changed the thinking (20).

The human body can tolerate quite high levels of selenium without adverse effects on health. However, selenium intoxication, termed selenosis, appears upon prolonged high intake of around 1 mg selenium/day. Symptoms include gastrointestinal upset, hair loss, nausea, irritability, fatigue, and mild nerve damage.

Since selenium deficiency is globally rare and taking into account the danger of overdose, it is better to look for natural dietary sources better than excessive complementation on an everyday basis.

CURRENT ISSUES AND CONTROVERSIES ABOUT SELENIUM

Whether selenium can help protect against cancer is a long debated story. Whereas some observational studies have indicated a protective role for selenium (21-23), others could not find any correlation (24). More studies are needed to investigate the topic. Whereas European Food Safety Authority (EFSA) does not mention any claim relating selenium and cancer, the US Food and Drug Administration (FDA) has allowed a series of qualified health claims associating selenium and cancer reduction. However, these so-called qualified claims, although supported by scientific evidence, do not meet the significant scientific agreement standard. As a result, the product label may state, “May reduce the risk of certain cancers” but must also include the disclaimer, “The FDA has determined that this evidence is limited and not conclusive.”

This example shows the limitations of observational studies as they were conducted so far and points out the urge for new research strategies, as discussed by Prof J. Kaput (see our article on the subject). In particular, advances in nutrigenetics and nutrigenomics should provide a more personalized, genotype-based approach, in order to optimize selenium intakes at an individual level (20).

Furthermore, selenium doesn’t act independently but rather in conjunction with a wide range of other micronutrients including zinc, folate, vitamins D, E, B6, and B12, which are also involved in the regulation of immunity and oxidative stress. It is thus necessary to take into account the full picture that encompasses complex interactions between genes, gene products, and environmental factors.

To read more about nutrigenetics and nutrigenomics perspectives in this field, consult our article on the subject.


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