Selenium is an element that is essential in the human diet in small amounts, but toxic at high levels. All animals require selenium, since we evolved from the sea where it is abundant. Plants do not appear to require selenium for survival, but incorporate it into their structures. Edible plants contain differing amounts of selenium, as the element is found in varying amounts in rocks and soil throughout the world. The element is found in higher amounts where there were inland seas.
Thyroid hormone: While iodine is essential for the synthesis of thyroid hormone, a selenium dependent enzyme must be present to activate this hormone, which in turn regulates normal growth, development and metabolism.
Sperm development: a recent study from Vanderbilt University showed that a protein containing selenium is essential for normal sperm development and male fertility.
Viral Infection: selenium deficiency appears to enhance the virulence of some viral infections, by inducing mutations in the expression of some viral genes. In a region of China where soils are very low in selenium, young women and children develop a disease of the heart muscle; this seems to be worsened by a change in a common virus that affects the heart. Selenium supplementation can protect against this, but cannot reverse the damage to the heart muscle.
Cancer – a mixed picture: Chronic infection with viral hepatitis B or C increases the risk of liver cancer; selenium supplements given in China in an area with a high incidence of hepatitis B and liver cancer reduced the incidence of this cancer. Similar findings were seen in Taiwan. A study in Finland showed that low selenium status in creased the risk of lung cancer: the association was more pronounced in smokers. On the other hand, a prospective study of more than 60,000 female nurses in the US found no association between selenium levels and total cancer risk. The data on prostate cancer are confusing – several studies have shown lower selenium levels in men with prostate cancer, but a recent large study from the National Cancer Institute, following 295,344 men who were cancer free on enrollment, found that excessive multivitamin use (more than 7 times per week) increased the risk of fatal prostate cancer compared with never users. The positive association with prostate cancer and excessive multivitamin use was strongest in men with a family history of prostate cancer or in men who took supplemental selenium, zinc, or beta-carotene. Further studies are needed to understand whether selenium levels in the body are linked to prostate cancer, and several are underway. Selenium supplements (200 micrograms daily) increased the risk of squamous cell skin cancer in a study from Memorial Sloan-Kettering, and did not alter the incidence of basal cell skin cancers.
HIV/AIDS: Declining selenium levels are found in HIV infected people and are markers of disease progression and severity. A randomized control trial of HIV positive men and women from the University of Miami found that selenium supplements (200 micrograms a day) taken for 2 years significantly decreased the rates of hospitalization. A further study showed a decreased viral load and more CD4 cells with the same dose of selenium.. Proteins containing selenium negatively affect the replication of the AIDS virus. Many AIDS patients in the US take selenium supplements, and it is also used in some African countries/
Who needs selenium supplements? Based on the information currently available, it is wise for most people to get this trace element in their diet, and not from supplements. Supplemental selenium should be considered by people with HIV infections, and by those with severe gastrointestinal problems such as Crohn’s disease or surgical removal of part of the stomach or intestines.
Food Sources of Selenium: Organ meats, shellfish and other healthy (non-endangered) fish are the important sources of selenium in our diets. Whole grains are also important sources (avoid white flour and white rice), as are sunflower seeds, and other nuts. Whole grains grown in Nebraska and the Dakotas have high levels (but who knows the origin of the wheat in our daily bread?) Brazil nuts grown in areas of Brazil with selenium-rich soil may provide 50 to 100 mcg in one nut, which is an excellent daily amount for adults. However, Brazil nuts grown on selenium-poor soil may provide 10 times less – another ‘who knows?’ conundrum. Studies from New Zealand have shown that the selenium in Brazil nuts improves serum selenium levels as effectively as supplements. My final conclusion from all this is that adults can benefit from a daily Brazil nut! You can give a few Brazil nuts a week to children over age 9. Good news - it seems that the cultivation of Brazil nuts will improve conditions in the rainforests of Brazil; the nut is not suitable for growth in plantations and can only grow in the rainforest itself.
Sadja Greenwood, MD back issues onsadjascolumns.blogspot.com