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Selenium Benefits, Side Effects & Safety Information

Published: 13th May 2018. Last updated: 21th July 2019.

Shaun Ward MSc ANutr

Staff Writer


Introduction

Selenium is an essential mineral for health and is present within at least 25 proteins, named selenoproteins.

Many of these selenoproteins have important metabolic roles such as for reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection.

Selenium is naturally present in many foods, added to others by fortification methods, and available as a dietary supplement.

There are 2 forms of selenium, inorganic and organic. Inorganic forms are found in soil and are present within plant foods, whereas animal foods are mainly constituted of organic selenium. Both forms can be good dietary sources of selenium.

The Potential Benefits of Selenium

Below is a full list of the main health benefits Selenium gives consumers:

It's An Antioxidant

Selenium is as an important component within antioxidant enzymes (selenoenzymes) such as glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinases.

Selenoenzymes are found to have strong antioxidant activity and play a significant role in protecting cells against oxidative damage from reactive oxygen species and reactive nitrogen species [1].

Although the exact benefits of these antioxidant capabilities are not known, it is speculated that they have a crucial role for most of selenium’s effects on other health outcomes i.e cancer, heart disease, thyroid function.

It Boosts Immune System

The immune system represents the processes of the body that provide resistance to infection and toxins.

Studies have demonstrated that inadequate selenium intakes can impair immune functions, resulting in a reduced immune response to destroy antigens - proteins on the surface of viruses, fungi, or bacteria [2].

Selenium deficiency has the ability to turn harmless viruses in hosts into virulent pathogens, increasing the risk of illness and disease.

Selenium also regulates the activity of enzymes such as glutathione peroxidase which are responsible for protecting cell walls from oxidative damage. When a deficiency is clinically measured, cell walls are induced to significantly more amounts of damage and toxicity.

If vitamin E is taken alongside selenium, a synergistic effect can occur to further maintain the integrity of cell walls.

It Improves Thyroid Function

The thyroid gland is responsible for secreting hormones that regulate growth and development through the regulation of metabolic processes.

Selenium concentration is higher in the thyroid gland than in any other organ in the body. This is because the proteins that activate thyroid hormones are selenium-containing enzymes.

Epidemiological evidence supports the relationship between selenium levels and thyroid gland function [3].

The importance of selenium for normal immune system also has a carry-over effect for preventing autoimmune thyroiditis – inflammation of the thyroid – by reducing the number of antibodies directed at the thyroid gland.

Supplementing with selenium has shown to significantly lower inflammatory activity of the thyroid if it is elevated [4].

It May Prevent Cardiovascular Disease

The increased production of free radicals such as reactive oxygen species can exert physiological stress, and if these compounds are not regulated, they can damage proteins and DNA within cells.

The damage caused by free radicals is closely linked to cardiovascular disease, as reactive oxygen species can cause LDL cholesterol to be oxidized, which initiates the atherogenic processes that cause the disease [5].

Specifically, a 50% increase in blood selenium levels is associated with a ~25% reduction in coronary artery disease [6].

It is hypothesized that the presence of selenium increases the concentration of antioxidant selenoenzymes and selenoproteins which may help to reduce oxidation of LDL and thereby decrease the incidence of heart diseases [7] [8].

In addition, adequate selenium levels may upregulate LDL receptors that can effectively remove LDL from the blood, potentially avoided high amounts of cholesterol in the blood [9].

It May Prevent Cancer Formation

A large scientific review found that those with high selenium levels have improved defense systems against certain types of certain cancer - breast, lung, colon and prostate [10].

This is backed by other studies that show that groups with the highest intakes of selenium have a ~30% lower risk of getting cancer, and a ~45% lower risk of cancer mortality [11].

In addition, supplementation may be effective at reducing cancer risk, as a study found that supplementing with selenium-enriched yeast reduced overall cancer morbidity rates by 50% [12].

As selenium has functions for DNA repair, immune systems, and antioxidant properties, there are many ways that it might prevent cancer. Current data suggests the main chemopreventative role of selenium is due to its antioxidant defense systems that reduce oxidative stress and limit DNA damage [13].

Other possible anticancer mechanisms of selenium include the initiation of programmed cell death systems for malfunctioned cells [14].

It May Prevent Neurodegeneration

Naturally, selenium concentrations decline with age. However, if there are excessive deteriorations in selenium concentrations then age-related declines in brain function can be accelerated [15]. This is possibly due to large decreases in the antioxidant activity in the brain.

Given this, selenium supplementation in elderly individuals has been associated with higher episodic memory and semantic fluency test scores [16].

Sources and Bioavailability

As the selenium content of food is heavily dependent on the selenium concentration of soil, differences in selenium content vary widely between geographical regions. This is the case not just for plant-based sources, as animals also need to also be consuming good-quality sources of selenium to increase their levels.

This being said, the best sources of selenium are:

  • Brazil nuts (the number 1 source!)
  • Tuna
  • Halibut
  • Sardines
  • Shrimp
  • Mushrooms
  • Beans
  • Sunflower seeds
  • Meat
  • Liver
  • Eggs
  • Brown rice
  • Oats

Although it is speculated that water is a good source of selenium, modern-day filtering of regular drinking water is not nutritionally significant in most geographic regions.

The bioavailability of selenium in plant and animal foods is high, typically over 90%.

Bioavailability of selenium supplements, in the form of selenate and selenite, are also comparatively high and can aid individuals with a selenium deficiency from inadequate dietary intake.

Selenium Deficiency

Selenium deficiency is prevalent in over 500 million people across the world, mainly due to an insufficient dietary intake.

The may indicate that people are not making correct food choices, however in many geographical regions, the quality of soil is poor because of rainfall, evaporation, and pH levels. Consequently, the selenium content of soil appears to be getting worse.

Historically, this deficiency has caused serious health effects such as Keshan disease – a weakening of the heart so that it is unable to pump effectively. More recent evidence also links this deficiency to other serious diseases such as cancer and heart disease.

Symptoms of selenium deficiency include:

  • Infertility
  • Fatigue
  • Muscle weakness
  • Hair loss
  • Weakened immune system

Groups most at risk of selenium deficiency are:

  • People living in selenium-deficient regions: China, New Zealand, and some European countries the worst soil content of selenium.
  • People undergoing kidney dialysis: Hemodialysis can remove large amounts of selenium from the blood.
  • People living with HIV: This disease can impair absorption and increase selenium losses from diarrhea.

Selenium Safety and Side Effects Information

Although selenium is necessary for health, excess selenium consumption can cause side effects. In rare circumstances, very high selenium intakes can be toxic and potentially fatal.

Therefore, people should be aware that there is a small window from selenium being therapeutic to toxic. This is an even more important consideration for those choosing to supplement selenium, as certain brands may contain selenium levels far above the recommended daily intakes.

For most people, the recommended amount of selenium is 55mcg per day. The tolerable upper limit of selenium is 400mcg per day.

Common signs of toxicity from selenium include:

  • Hair loss
  • Dizziness
  • Nausea
  • Vomiting
  • Facial flushing
  • Tremors
  • Muscle soreness
  • Fatigue
  • Irritabilities

More severe cases of selenium toxicity, although extremely rare, are:

  • Serious intestinal issues
  • Heart attack
  • Kidney failure
  • Death

An early indicator of excess selenium intake, yet not quite deemed toxic, is a garlic odor in the breath and a metallic taste in the mouth (so be aware of any strange criticisms from your partner!).

Conclusion

Selenium is an essential mineral for health and is involved with reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection.

Selenium is a strong antioxidant and may improve the immune system and thyroid functioning. It may also help protect against cardiovascular diseases and cancer.

Selenium deficiency is highly prevalent in areas where soil quality is poor. Signs of selenium deficiency are infertility, fatigue, muscle weakness, hair loss, and a weakened immune system.

However, the upper tolerable limit is low, and people should look at for signs of toxicity such as facial flushing, dizziness, vomiting and tremors.

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