Selenium is an essential mineral found in certain foods. Brazil nuts are the highest natural source of selenium, with a single nut providing around 95 mcg of this mineral. Other good sources include tuna, cod, shrimp, salmon, eggs and turkey. For those who are vegetarian or vegan, mushrooms can be an excellent source of selenium to supplement their diet. A varied diet should provide enough selenium for most people; however supplements may be necessary if you have specific dietary restrictions or health concerns related to low levels of this nutrient.