Selenium is an essential trace element which is so important for human health that its incorporation into proteins – as selenocysteine, the 21st amino acid – is under direct genetic control.
Selenium health benefits
Selenium, in the form of selenocysteine, forms an integral part of at least 25 human proteins, including the selenoenzymes – powerful antioxidants which protect tissues from metabolic damage. Selenium enhances the action of a liver enzyme (P450) involved in detoxifying cancer-causing chemicals, is involved in the repair of damaged genes and the interconversion and deactivation of thyroid hormones.
Selenium is needed for normal cell growth and immunity. It protects against a range of degenerative diseases such as hardening and furring up of the arteries, emphysema, liver cirrhosis, cataracts, arthritis, stroke and heart attack.
Within the EU, the authorised nutrition and health claims for selenium include:
- Selenium contributes to normal sperm formation
- Selenium contributes to the maintenance of normal hair
- Selenium contributes to the maintenance of normal nails
- Selenium contributes to the normal function of the immune system
- Selenium contributes to the normal thyroid function
- Selenium contributes to the protection of cells from oxidative stress.
Dietary sources of selenium
The best food source of selenium is Brazil nuts. Selenium is also found in fish, poultry, meats (especially game), wholegrains such as wheat, mushrooms, onions, garlic, broccoli and cabbage.
Selenium and immunity
Selenium is needed for antibody synthesis and also stimulates the production and activity of T lymphocytes and natural killer cells which fight viral and bacterial infections. The production of antibodies, increases up to thirty fold if supplements of selenium and vitamin E are taken together.
People taking 100mcg/day selenium supplements have a significantly better immune response when immunised with live polio virus vaccine, and cleared the virus from their system more quickly, than those taking placebo.
Lack of selenium reduces the activity of T-lymphocytes and decreases antibody production. This reduced immunity makes it more likely that a viral infection will survive long enough within the body for mutation to a more virulent genotype to occur. Lack of selenium also affects the way respiratory epithelial cells respond to influenza virus exposure. As a result, lack of selenium increases the risk of more severe influenza infections.
Selenium and influenza
Lack of selenium is now recognised as a driving force for viral mutations, which may explain why so many new, pathogenic influenza viruses emerge from Asia, where selenium intakes are among the lowest in the world. Symptoms of influenza are also more severe in selenium deficient hosts and lung pathology persists for longer.
Influenza viruses recovered from selenium-deficient hosts consistently show changes in the genes coding for viral matrix proteins. In one study, influenza viruses isolated from three different hosts showed identical mutations in 29 different nucleotide positions, seven of which would result in the insertion of a different amino acid from the original. The changed amino acid sequences in the viral proteins are believed to increase the virulence of the influenza virus by allowing more rapid uncoating of viral RNA from its associated proteins so it can start replicating more rapidly.
Selenium and cancer
Selenium enzymes have powerful antioxidant actions that help to protect against cancer and trigger the programmed cell death (apoptosis) of abnormal cells. In parts of the world where soil selenium levels are low, the incidence of cancer increases by two to six fold. Those with the lowest selenium intakes have the highest risk of developing leukaemia or cancers of the colon, rectum, breast, ovary, pancreas, prostate gland, bladder, skin and lungs. These risks are even higher if intakes of vitamin E and vitamin A are also low. Selenium has been shown to prevent the growth of cancer cells in the laboratory, and some evidence suggests it is involved in triggering programmed cell death (apoptosis) of abnormal cells.
An early clinical trial involving over 1300 people with a previous history of cancer of the skin found that those taking 200mcg selenium supplements per day had a 52% lower risk of cancer death compared with those taking placebo. While there were no effects on skin cancer, the incidence of prostate cancer fell by 63%, colorectal cancer by 58%, lung cancer by 46%. The blinded phase of this trial was therefore stopped early as it was considered unethical to withhold selenium from the placebo group.
A recent Cochrane analysis of 63 studies involving over a million people confirmed that people with the highest selenium intake had a 31% lower incidence of cancer and a 40% reduced risk of cancer mortality. The most pronounced decrease was for cancers of the stomach, bladder and prostate gland.
However, this is not necessarily a causal link, and some studies have not found any association between selenium intakes and cancer. There is not yet any convincing evidence that selenium supplements can prevent cancer in humans.
Selenium and thyroid health
The thyroid gland has the highest concentration of selenium in the body, where it protects cells against the free radicals generated by thyroid metabolism. If selenium is in short supply, thyroid cells become damaged and are replaced by scar tissue (fibrosis). Enzymes containing selenium (thyroxine deiodinases) also regulate the production of active tri-iodothyronine hormone. Low selenium levels increase the risk of thyroid cancer and of developing an enlarged multinodular thyroid goiter and other thyroid diseases. When selenium and iodine intakes are both low, there is a high risk of a severely underactive thyroid (myxoedema).
Selenium supplementation can enhance the effect of antithyroid drugs in patients with recurrent Graves’ disease.
Selenium and brain health
Selenium plays a vital antioxidant role within the brain, protecting tissues from oxidative damage. Research involving over 1,110 males found that those with the lowest selenium levels were almost four times more likely to die from a stroke than those with the highest levels. Selenium may also protect against dementia and have a beneficial role in Alzheimer’s treatment.
Selenium and fertility
Selenium is important for both male and female fertility. Low levels of selenium have been linked with miscarriage and pre-eclampsia during pregnancy. Taking selenium supplements can increase sperm motility and improve the chance of conception for some men with reduced sperm quality.
Selenium and rheumatoid arthritis
Rheumatoid arthritis is a chronic inflammatory disease that has been associated with low levels of selenium. An early study found that the highest selenium levels appear to have a greater than 80% lower risk of developing RA than those with low levels. Data from 14 studies involving 716 people found that, overall, those with rheumatoid arthritis had blood selenium levels that were a third lower than those without rheumatoid arthritis, although this was skewed by an unusually large effect in Asia, where people with rheumatoid arthritis had selenium levels that were more than threefold lower than those without RA.
It’s possible that these findings show effect rather than cause, however, as the increased inflammation present in rheumatoid arthritis may increase selenium requirements. Taking selenium supplements may reduce disease activity, but has not been found to affect the number of painful and swollen joints.
The mineral content of food depends on the soils in which they are grown or reared. During the last ice age, selenium was leached from the soil in many parts of Europe, including the UK. When the UK started sourcing wheat from Europe rather than America and Canada, average selenium intakes fell dramatically from 60mcg to 34mcg per day between 1978 and 1994. As a result, UK blood selenium concentrations also fell by around 50% between 1974 and 1991 as intakes were only half our estimated requirements.
Symptoms of selenium deficiency
Selenium deficiency is associated with symptoms and signs such as age spots, pale finger nail beds, increased susceptibility to infection, premature wrinkling of skin, poor growth, subfertility, arthritis, high blood pressure, coronary heart disease, cataracts, pancreatitis, muscle weakness, hypothyroidism and an increased risk of some cancers.
Selenium is important for healthy muscle fibres, including those found in the heart. In parts of China, selenium intakes are low enough to cause a form of heart failure (Keshan Disease) and an unpleasant, deforming type of arthritis known as Kashin-Beck disease. These risks seem to be even higher if intakes of the antioxidant vitamins A, C and E are also low.
The EU tolerable upper intake level is 300mcg per day from all sources, including food and supplements.
Supplements typically supply 50mcg to 200mcg selenium daily.
Supplements that supply selenium already incorporated into the amino acid, selenocysteine (from selenium-enriched yeasts) or selenomethionine have greater bioavailability than those containing selenium salts such as selenium selenite.
Selenium-enriched yeast supplements are notoriously ‘smelly’, but look out for low-odour products which, thanks to new coating technology, are more pleasant to take.
Intakes above 800mcg selenium per day cause toxicity, leading to a garlic odour on the breath (from dimethyl selenide), fragile or black fingernails, a metallic taste in the mouth, dizziness, nausea and hair loss.