Selenium is a trace element that acts as an antioxidant, protecting the polyunsaturated fats in cell membranes from damage by free radicals. Selenium is found in foods from both plants and animals. Selenium is found in foods from both plants and animals. The richest sources of selenium are fish (tuna fish is an excellent source) and brazil nuts. Mushrooms are a good source but the selenium content of plant foods depends very much on the soil in which the crop is grown. Bread was the most important contributor to the selenium content of the diet, as it was made from hard North American wheat that was rich in the trace element. However, in the last 15 years or so the North American wheat has been replaced softer, home-grown wheat that has a lower content of selenium, and hence bread has become a much less important source. In most diets meat and dairy products are now the main sources, and it is possible that vegetarians (especially vegans) may have low intakes.
Some epidemiological studies have suggested that low intakes of selenium may be associated with higher incidence of cancers, especially of the gut and respiratory tract, but as yet there is no confirmation of this. The potential role for selenium in the prevention of heart disease is also unclear at present, and there is no proven benefit for selenium in treating arthritis or hair and nail problems.
Selenium is toxic if taken in excess. Symptoms include hair loss, nail changes skin lesions, diarrhea, nausea, tiredness, and peripheral nerve damage. There is a very narrow margin between the safe and toxic levels of selenium. Doses of between 50 and 100 ug are safe, but the intake from all sources should not exceed 450 ug per day. It is suggested that the maximum dose from supplements should not exceed 200 ug per day. Selenium is often sold in combination with the antioxidant vitamins A, C, and E.