Selenium is an essential trace mineral that occurs naturally in the soil and is absorbed by plants and crops, from where it enters the human food chain. New Zealand soils have long been low in selenium which in turn affects its content in foods and which transfers to a low serum (blood) level in the general New Zealand population. While an increase in the selenium content of our food supply – largely due to imported selenium-enriched foods, and supplemental animal feeds – has improved our collective selenium status, it is still worthwhile ensuring that we are consuming adequate amounts.
Why do we need selenium?
We use selenium to make approximately 25 different types of selenoproteins; complexes made up of the mineral selenium and the amino acid cysteine ( to form selenocysteine). There are about 25 different selenocysteine-containing selenoproteins in human tissues.
A key action of selenoproteins is to act as antioxidants – the best known of these is glutathione peroxidise. Glutathione peroxidise is a major cellular antioxidant, and also helps extend the life of other antioxidants such as polyphenols, and vitamins C and E.
Selenium also plays an important role in the control of thyroid hormone metabolism, healthy thyroid function, and immune functions by supporting our response to infections. A study published in 2000 called “The importance of selenium to human health”, found that higher selenium levels helped slow down the replication of the HIV virus. The study actually showed that AIDS patients with low levels of selenium were 20 times more likely to die from an AIDS-related illness than those patients with healthy levels of the mineral. Additionally the study found that there was a relationship between selenium levels and early miscarriage, male infertility, mood problems, cardiovascular disease and arthritis. Another study showed that increased levels of selenium may reduce skin cancer by up to 60%. This is of particular interest here in New Zealand where we have a high incidence of skin cancers due to our sun exposure. Recent studies have also pointed to the benefits of selenoproteins for our eyes, with evidence suggesting that selenium supplementation may have a role in preventing cataract formation and age-related macular degeneration.
Selenium is readily available in the following foods:
Brazil nuts – these are the best source of selenium and just eating few of these daily will keep your levels topped up.
Seafood’s including crab, lobster, tuna
Kidney and liver
Onions, garlic, broccoli, tomatoes and mushrooms – remembering that food content will be governed by soil quantities of selenium
How much selenium do we need?
In New Zealand a safe upper limit for selenium intake is set at of 400 micrograms daily, however most supplements contain a maximum of 150mcg daily as most people will be getting selenium from various dietary sources. While 400mcg is considered a safe intake that will not produce toxicity in the majority of the population, there is evidence that selenium supplementation may be not be recommended for those people who already have an adequate selenium status. Studies have linked elevated selenium levels to type II diabetes, higher levels of total cholesterol, and triglycerides and peripheral vascular disease. Evidence suggests that smokers may require a higher intake of selenium, however always check with a health professional if you want to take a higher dose of selenium than is recommended in any dietary supplement.