Selenium is an essential nutrient which is present in grains, nuts, and meat. Brazilian nuts are considered the most concentrated source of selenium. Low soil levels can cause deficiency states in some parts of Europe and China. Selenium is required for the functioning of both glutathione peroxidase (a natural antioxidant) and thyroid selenoproteins. Notably, selenium is also required for sperm motility proteins and is related to the prevention of thrombogenesis. The symptoms of deficiency include a higher cancer risk, a greater risk to develop atheroma, cardiomyopathy, and arthritis. There is no agreement as for the most reliable method to assess selenium status, and serum levels are considered a poor guide.