Selenium is a trace mineral that our bodies use to produce glutathione peroxidase . Glutathione peroxidase is part of the body's ]]>antioxidant]]> defense system; it works with vitamin E to protect cell membranes from damage caused by dangerous, naturally occurring substances known as free radicals.

China has very low rates of colon cancer, presumably because of the nation's low-fat diet. However, in some parts of China where the soil is depleted of selenium, the incidence of various types of cancer is much higher than in the rest of the country. This fact has given rise to a theory that selenium deficiency is a common cause of cancer, and that selenium supplements can reduce this risk.

As we will see, there is some preliminary evidence that selenium supplements might provide some protection against some types of cancer among people living in the US, but this evidence is far from definitive.



The official US and Canadian recommendations for daily intake of selenium are as follows:

  • Infants
    • 0-6 months: 15 mcg
    • 7-12 months: 20 mcg
  • Children
    • 1-3 years: 20 mcg
    • 4-8 years: 30 mcg
    • 9-13 years: 40 mcg
  • Males and Females
    • 14 years and older: 55 mcg
  • Pregnant Women : 60 mcg
  • Nursing Women : 70 mcg

Selenium content of food varies depending on the selenium content of the soil in which it was grown. Studies suggest that many people in certain developed countries, including New Zealand, Belgium, and Scandinavia, do not get enough selenium in their diets. 3-6]]> However, most individuals in the US and Canada are believed to consume more than enough selenium. ]]>46]]>

Foods containing significant and reliable amounts of selenium include animal products like meat, seafood, and dairy foods, as well as whole grains and vegetables grown in selenium-rich soils. These include wheat germ, nuts (particularly Brazil nuts), oats, whole-wheat bread, bran, red Swiss chard, brown rice, turnips, garlic, barley, and orange juice.

Certain digestive conditions, such as ]]>Crohn’s disease]]> , short-bowel syndrome, and ]]>ulcerative colitis]]> may impair selenium absorption. ]]>47]]> In addition, medications that reduce stomach acid such as ]]>proton pump inhibitors]]> or ]]> H 2 blockers ]]> may reduce absorption of selenium. ]]>12]]>


Therapeutic Dosages

In controlled trials of selenium, typical dosages were 100 mcg to 200 mcg daily.

The two general types of selenium supplements are available to consumers are organic and inorganic forms. These terms have a very specific chemical meaning and have nothing to do with "organic" foods. In chemistry, organic means a substance's chemical structure includes carbon. Inorganic chemicals have no carbon atoms.

The inorganic form of selenium, selenite, is essentially selenium atoms bound to oxygen. Some research suggests that selenite is harder for the body to absorb than organic forms of selenium, such as selenomethionine (selenium bound to methionine, an essential amino acid) or high-selenium yeast (which contains selenomethionine). 13,14]]> However, other research on both animals and humans suggests that selenite supplements are about as good as organic forms of selenium. ]]>15,16]]> These contradictory results suggest that any differences in absorption, if they exist at all, are relatively minor.


Therapeutic Uses

Preliminary studies hint that supplemental selenium may help prevent some forms of cancer]]> ; ]]>17-22]]> however, this evidence has a long way to go before it can be taken as reliable. ( ]]>See below]]> .)

Selenium is required for a well-functioning immune system. ]]>24]]> Based on this, selenium has been suggested as a treatment for people with ]]>HIV]]> . Early studies showed little to no benefit. ]]>25-32,62]]> A large trial reported in 2007 reported that use of selenium supplements reduced viral load. ]]>62]]> However, this study suffered from numerous flaws in its statistical methods. For reasons that are not clear, another study found that selenium supplements decreased symptoms of psychological anxiety in patients undergoing highly active retroviral therapy (HAART). ]]>56]]>

One study of healthy people in the UK (where marginally low selenium intake is common) found that use of selenium supplements improved general immune function, as measured by response to poliovirus immunization. ]]>57]]>

A preliminary double-blind trial suggests that selenium supplements may improve ]]>fertility in males]]> who are selenium deficient. ]]>54]]> Weak evidence suggests that selenium might be helpful for ]]>diabetic neuropathy]]> . ]]>23]]>

Selenium has also been recommended for many other conditions, including ]]>acne]]> , ]]>anxiety]]> , ]]>cataracts]]> , ]]>cervical dysplasia]]> , ]]>fibromyalgia]]> , ]]>gout]]> , ]]>multiple sclerosis]]> , ]]>osteoarthritis]]> , ]]>psoriasis]]> , and ]]>ulcers]]> , but there is no real evidence as yet that it is actually helpful.

A small study among nursing home residents found that low levels of the mineral selenium was associated with ]]>depression]]> . Moreover, 8 weeks of supplementation tended to improve the mood of the most seriously depressed patients with low selenium levels. ]]>67]]> The same was not true of the two other nutrients investigated, folate and vitamin C.

Current evidence regarding use of selenium for ]]>preventing heart disease]]> is more negative than positive. ]]>48-53]]>

A large (about 500 participants) double-blind, placebo-controlled study failed to find that use of selenium supplements at 100 mcg, 200 mcg, or 300 mcg daily improved mood or ]]>general well-being]]> . ]]>59]]>

A study of 197 people with moderately severe ]]>asthma]]> failed to find benefit with selenium (100 mcg daily). ]]>61]]>

Low selenium levels have been associated with increased likelihood of developing certain kinds of ]]>rheumatoid arthritis]]> . ]]>34]]> However, selenium supplements don't appear to help rheumatoid arthritis once it has developed. ]]>35,36,44,45]]>

Despite hopes to the contrary, it does not appear that selenium supplements can help prevent type 2 ]]>diabetes]]> , but rather might increase the risk of developing the disease. ]]>64]]>


What Is the Scientific Evidence for Selenium?

Somewhat inconsistent evidence suggests that selenium supplements may help prevent cancer.

Evidence from observational]]> studies indicates that low intake of selenium is tied to increased risk of cancer. ]]>39,40]]> However, such studies are notoriously unreliable as guidelines to therapy. Only ]]>double-blind]]> trials can truly determine whether selenium supplements can help prevent cancer. (For information on why this is so, see ]]>Why Does This Database Rely on Double-blind Studies?]]> )

The most important double-blind study on selenium and cancer was conducted by researchers at the University of Arizona Cancer Center. ]]>37]]> In this trial, which began in 1983, 1,312 people were divided into two groups. One group received 200 mcg of yeast-based selenium daily; the other received placebo. Participants were not deficient in selenium, although their selenium levels fell toward the bottom of the normal range. The researchers were trying to determine whether selenium could lower the incidence of skin cancers.

As it happened, no benefits for skin cancer were seen. (In fact, careful analysis of the data suggests that selenium supplements actually marginally increased risk of certain forms of skin cancer. ]]>58]]> ) However, researchers saw dramatic declines in the incidence of several other cancers in the selenium group. For ethical reasons, researchers felt compelled to stop the study after several years and allow all participants to take selenium.

When all the results were tabulated, it became clear that the selenium-treated group developed almost 66% fewer prostate cancers, 50% fewer colorectal cancers, and about 40% fewer lung cancers as compared with the placebo group. (All these results were statistically significant.) Selenium-treated subjects also experienced a statistically significant (17%) decrease in overall mortality, a greater than 50% decrease in lung cancer deaths, and nearly a 50% decrease in total cancer deaths. A subsequent close look at the data showed that only study participants who were relatively low in selenium to begin with experienced protection from lung cancer or colon cancer; people with average or above average levels of selenium did not benefit significantly. ]]>55, 60]]> It has not yet been reported whether this limitation of benefit to low-selenium participants was true of the other forms of cancer as well.

While this evidence is promising, it has one major flaw. The laws of statistics tell us that when researchers start to deviate from the question their research was designed to answer, the results may not be trustworthy. Currently, other studies are underway in an attempt to validate the findings accidentally discovered in this trial.

Interestingly, combining the results of 12 recent placebo-controlled trials investigating the association between ]]>antioxidant]]> supplementation and cancer, researchers found that men who took selenium experienced an overall reduction in the incidence of cancer. No similar effect, however, was observed in women. ]]>65]]> This difference cannot be explained without more research. In addition, selenium supplementation appeared to modestly lower cancer mortality in both men and women.

Other evidence for the possible anticancer benefits of selenium comes from large-scale Chinese studies showing that giving selenium supplements to people who live in selenium-deficient areas reduces the incidence of cancer. ]]>38]]> In addition, animal trials have found anticancer benefits. ]]>41,42]]>

However, one study published in 2007 reported negative results in transplant patients. ]]>63]]> People who undergo organ transplants are at particularly high risk of skin cancer linked to the human papilloma virus (HPV). In this double-blind study, 184 organ transplant recipients were given either placebo or selenium at a dose of 200 mg daily. The results over two years failed to show benefit; both the placebo and the selenium group developed precancerous and cancerous lesions at the same rate.


Safety Issues

The US Institute of Medicine issues guidelines for the maximum total daily intake of various nutrients, based on estimations of what should be safe for virtually all healthy individuals. These tolerable upper intake levels (ULs) are, thus, conservative guidelines. For selenium, they have been set as follows: 43]]>

  • Infants
    • 0-6 months: 45 mcg
    • 7-12 months: 60 mcg
  • Children
    • 1-3 years: 90 mcg
    • 4-8 years: 150 mcg
    • 9-13 years: 280 mcg
  • Males and Females
    • 14 years and older: 400 mcg
  • Pregnant or Nursing Women : 400 mcg

Note that these dosages apply to combined dietary and supplemental intake of selenium. When deciding how much selenium it’s safe to take, keep in mind that most adults already receive about 100 mcg of selenium in the daily diet.

Maximum safe doses of selenium for individuals with severe liver or kidney disease have not been established. There is some evidence that supplementing selenium over the long-term in areas where selenium is already adequate in the diet may increase the risk of diabetes and perhaps hypercholesterolemia. ]]>66]]>

Highly excessive selenium intake, beginning at about 900 mcg daily, can cause selenium toxicity. ]]>46]]> Signs include depression, nervousness, emotional instability, nausea, vomiting, and in some cases loss of hair and fingernails.


Interactions You Should Know About

If you are taking medications that reduce stomach acid, such as H 2 blockers ]]> or ]]>proton pump inhibitors]]> , you may need extra selenium.