Selenium is a vital nutrient required for optimal bodily functioning. In particular it: supports thyroid health, aids in detoxification, boosts immunity and encourages healthy reproductive processes.
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The current recommended daily intake is between 50-70mcg - depending on age and gender - though kids, teenagers and pregnant women will have slightly different requirements. However, the upper limit is 400mcg and may cause side effects.
While present in trace amounts in many foods, selenium is abundantly found in seafood or easily satisfied with 1-2 brazil nuts daily.