Selenium Supplementation and Its Effect

Posted by Ben White on سبتمبر 04, 2017

Original of this article was published on ZRT Laboratory Blog. Last reviewed: May 2026.

While optimal selenium levels are imperative for proper thyroid function, abnormally high selenium intake can be toxic — causing gastrointestinal disturbances, changes in nails and hair, weakness, convulsions and decreased cognitive function. So it’s important to understand the ways in which we take in selenium, both in the diet and as supplements.

A balance of nutrients is required by our bodies to maintain good health, and selenium (Se) is an important one that often gets overlooked. This essential element is a required component of the selenoproteins, which include those needed to convert thyroid hormone from the inactive to the active form, and several important antioxidants.

When choosing a selenium supplement, the various forms available can be confusing. The most common forms found in the diet and as supplements are summarised below.

Organic Forms of Selenium

Selenomethionine (as L-selenomethionine; sometimes called selenium monomethionine) is formed in plants when selenium substitutes for sulfur during synthesis of the amino acid methionine. It is the predominant form of selenium in Brazil nuts, cereals, legumes, and fish.
Selenocysteine, a selenium-containing amino acid, is incorporated into 25 known selenoproteins during protein synthesis. These selenoproteins are vital to human health, and include the antioxidants glutathione peroxidase and selenoprotein P, the thioredoxin reductases, and the deiodinases involved in thyroid hormone activation.
Methylselenocysteine (also known as Se-methyl-selenocysteine) and its derivative gamma-glutamyl Se-methylselenocysteine are metabolic products of selenocysteine that are the predominant form of selenium in plants of the allium (onions, garlic) and brassica (cruciferous vegetables such as broccoli and cabbage) families.

Inorganic Selenium

Selenite is used in supplements and is the most common form of selenium used in pet foods.
Selenate is the predominant form of selenium in shellfish and vegetables (other than the allium and brassica families) grown in seleniferous soil.

Selenium-Enriched Yeast

The supplement known as selenium-enriched yeast is yeast grown in a medium containing inorganic selenium, usually sodium selenite. The selenium becomes organically bound to the yeast, mostly being incorporated into the yeast proteins. The supplement therefore consists predominantly of selenomethionine (between 60 and 80%), but also has small amounts of selenocysteine, methylselenocysteine and gamma-glutamyl Se-methylselenocysteine. Reputable manufacturers ensure that the majority (at least 90%) of the selenite is organically bound to the yeast; otherwise, the product may simply contain a mixture of inorganic selenite and yeast.

What Form Is Best?

Selenium in any form — whether from food or supplements, organic or inorganic — is utilised by the body for selenoprotein synthesis after first being metabolised to hydrogen selenide, the cellular storage form of selenium. Surplus selenium is converted to methylated metabolites and excreted via the urine and in the breath. Excessive buildup of hydrogen selenide can lead to its oxidation, resulting in the production of reactive oxygen species (ROS) and oxidative toxic effects in the body.

All forms of selenium are well absorbed, but absorption of selenomethionine is the best. It uses the same active transport mechanism as for methionine, one of the 9 essential amino acids that can only be obtained from the diet. Some supplements include vitamin E, and although it has been claimed that this facilitates selenium absorption, there is no evidence that this is the case; vitamin E is included because its antioxidant effects are synergistic with selenium.

If you are concerned about selenium toxicity or deficiency, our Heavy Metals & Mineral Balance Test uses ICP-MS analysis of blood spot and dried urine to measure selenium alongside other essential and toxic elements — giving a comprehensive picture of your mineral status.

Selenium Species with Anti-Cancer Effects

The anticancer effects of selenium differ depending on which form is used. An observed association between low selenium status and an increased risk of prostate cancer led to studies of selenium supplementation. Supplementation with selenium-enriched yeast in the Nutritional Prevention of Cancer (NPC) trial reduced the incidence of prostate cancer by 52–65%, but the Selenium and Vitamin E Cancer Prevention Trial (SELECT) used selenomethionine supplements and was discontinued after 5.5 years when no benefit was seen. The small amounts of methylselenocysteine and gamma-glutamyl Se-methylselenocysteine in selenium-enriched yeast are believed to be responsible for the observed anticancer effects in the NPC trial.

The pro-oxidant properties of selenium species such as selenite also contribute to ROS-mediated apoptosis. Cancer cells usually exist under mildly oxidative conditions and are more susceptible to oxidative stress than normal cells, and so research is ongoing in the development of anticancer drugs based on selenite.

For those wanting to assess selenium alongside thyroid function — given selenium’s critical role in thyroid hormone activation — our Thyroid, Heavy Metals & Essential Elements Test combines thyroid markers with an essential elements panel including selenium, iodine and other key minerals in a single at-home test.

How Much Selenium Do We Need?

When selenium levels are optimised there has been an observed decrease in the risks of prostate, lung, colorectal, and bladder cancer. However, excessive selenium supplementation in selenium-replete populations has been found to increase the risk of type 2 diabetes. Additional selenium supplements may therefore be detrimental to people with already adequate or high dietary selenium intake.

The current recommended dietary allowance (RDA) in adults is 55 micrograms/day. Urine testing while maintaining the patient’s usual diet and supplementation can help determine whether selenium status is adequate, too low or too high. Our ICPMS Toxic and Essential Elements Profile (Thyroid Essential) measures urinary selenium and other essential elements via ICP-MS analysis — the most accurate method for assessing selenium status and guiding safe supplementation decisions.

Original of this article was published on ZRT Laboratory Blog.

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