General text about the lack of minerals can be found here.
Selenium is found in the body only in trace amount, but it is a quite significant element as it serves as important antioxidant by eliminating oxygen radicals produced during body metabolism and so reducing the threat of oxidative stress. Larger amount of selenium is found in fish, offal, eggs and nuts.
Significant selenium deficiency is very rare, because normally we consume enough selenium in the diet. Increased threat of selenium deficiency may occur in people with malabsorption of nutrients, chronic alcoholics and the elderly.
The symptoms of selenium deficiency are complex and they are mainly based on its antioxidant effect. It is assumed that the lack of selenium accelerates the process of atherosclerosis and this leads to more frequent emergence of diseases of the cardiovascular system, including myocardial infarction and ischemic stroke. Selenium has also probably also protective effect against cancers that may be more frequent in patients with selenium deficiency. In addition, the lack of selenium may cause weakened immunity.
Selenium can be administered in dietary supplements. The required daily dose in an average adult is about 50-100 micrograms of selenium. Doses above 800-1000 micrograms daily are clearly harmful and they may cause digestive problems. Persons with excessive intake of selenium also suffer from brittle nails.