Selenium is a trace mineral usually found in seafood, kidney, liver, wheatgerm, bran, tuna, onions, tomatoes, broccoli, garlic and brown rice. Brazil nuts are an especially rich source. The Recommended Daily Amount (RDA) in the UK for both males and females is 55 mcg.
Selenium functions primarily as a component of the antioxidant enzyme glutathione peroxidase which works with vitamin E in preventing free radical damage to the cells. Higher selenium status or selenium supplementation has been shown to have antiviral effects, it is essential for successful male and female reproduction and it has been shown to help reduce the risk of autoimmune thyroid disease.
Low selenium intake is associated with an increased risk of cancer, heart disease and low immune function. It has also been associated with inflammatory diseases and cognitive decline. Severe selenium deficiency is associated with a serious heart disorder known as Keshan disease, especially prevalent in china where selenium levels in the soil is very low. Severe deficiency is also associated with an arthritic condition known as Kashin-Beck disease, again particularly prevalent in china. Symptoms can include muscle weakness and heart disturbances.
Cancer-The antioxidant properties and immune system support mechanisms of selenium have led scientists to explore the potential role of selenium in the prevention of cancer. In a Cochrane review of selenium and cancer prevention studies, compared with the lowest category of selenium intake, the highest intake category had a 31% lower cancer risk and 45% lower cancer mortality risk as well as a 33% lower risk of bladder cancer and, in men, 22% lower risk of prostate cancer (1). A meta-analysis of 20 epidemiologic studies showed a potential inverse association between toenail, serum, and plasma selenium levels and prostate cancer risk (2). Furthermore, analysis of a trial involving 1,312 U.S. adults with a history of cancers of the skin found that over a period of 6 years taking 200 mcg/day selenium was associated with a 52% to 65% lower risk of prostate cancer (3).
Auto-immune Diseases-The thyroid gland has a higher concentration of selenium than any other organ in the body and therefore selenium plays an important role in thyroid function. A randomized trial compared the effects of 200 mcg/day selenium with1,200 mg/day anti-inflammatory agent, or placebo for 6 months in 159 patients with mild Graves’ orbitopathy, an auto-immune eye condition. Those receiving the selenium reported a higher quality of life compared to the placebo group but not the anti-inflammatory group (4).
Male infertility– Selenium contributes to normal spermatogenesis. This just means it is required for the formation of healthy sperm in the male reproductive system. A deficiency in selenium may therefore be a factor in male infertility and a supplement would be a sensible measure to protect against such a deficiency.
Healthy skin, hair and nails– Selenium contributes to the maintenance of normal hair and to the maintenance of normal nails.This suggests a deficiency may lead to a deterioration in the quality of hair and nails, and many people have been known to use selenium supplements to help prevent falling hair and dandruff, and for brittle nails. For the skin, selenium contributes to the protection of cells, from oxidative stress. This means it acts as an ‘antioxidant’. This action has long been linked with protection form the effects of ‘free radicals’ – potentially damaging substances that are linked with the signs of premature ageing and degradation of skin collagen.
The human body requires only a small amount of selenium. Dosages as low as 900mcg per day over prolonged periods of time can produce signs of selenium toxicity which can include depression, nervousness, emotional instability, nausea and vomiting. The Tolerable Upper Intake Level for Selenium is 300ug (mcg) per day for adults in the UK.