About Herbs

Bottom Line: Selenium may help prevent certain cancers in some people, but the effect is not universal. Selenium does not appear to be able to treat heart disease or rheumatoid arthritis, and it is not an effective treatment for cancer.

Selenium is an element found normally obtained in the diet from foods such as Brazil nuts, seafood, meats, cereals and grains. Selenium is an essential part of the antioxidant enzyme glutathione-peroxidase, which protects cells from damage and DNA from mutations. For this reason, it has been studied for the prevention of diseases that are caused by or aggravated by this type of cellular damage, including cancer, heart disease, and rheumatoid arthritis. Selenium is also necessary for proper function of the immune system, but it is not known whether higher-than-normal levels of selenium can stimulate the immune system. Long-term use of selenium may increase the risk of certain types of skin cancer

Cancer prevention
Patients with a history of either basal cell or squamous cell carcinoma were enrolled in a clinical trial to see if selenium could prevent new skin cancers. 1312 patients were randomly assigned to take either 200 micrograms of selenium or a placebo pill every day. After ten years of following these patients, the people taking selenium developed just as many cases of skin cancer as the group taking the placebo pill. However, after following this group for another three years, the researchers happened to notice that patients taking selenium did have a lower overall risk of cancer and a lower risk of prostate cancer than patients who took a placebo pill, but results were inconclusive for lung or colorectal cancers. However, these beneficial effects were restricted to men, former smokers, and people who had lower blood levels of selenium before the study started. This study shows that taking this dose of selenium every day is relatively safe.

Selenocysteine, selenomethionine, selenate, selenite

Selenium is an essential trace element required by the glutathione-peroxidase pathway ⁽¹⁾. It acts as an antioxidant, and regulates thyroid hormone action and the reduction of vitamin C. Selenium is used to treat and prevent cancer, boost the immune system, and for cardiovascular and rheumatic disease ⁽²⁾. Bioavailability is dependent on organic versus inorganic supplements, ranging from 50% to nearly 100%. A recent analysis of five brands of commercially available selenium found that almost all contained up to 19% less than the labeled dosage ⁽⁴⁾. Methylation is the primary route of metabolism with a majority eliminated via the kidneys ⁽²⁾.
Clinical studies have evaluated the role of selenium in cancer prevention with intriguing results. SELECT (Selenium and Vitamin E Cancer Prevention Trial), conducted by the Southwest Oncology Group, is enrolling 32,400 men to study the effects of supplementation on the risk of prostate cancer ⁽⁸⁾; enrollment began in July 2001 and final results are expected in 2013 ⁽¹¹⁾. Selenium is effective at reducing therapy-related lymphedema ^{(12) (14)}. According to a recent study, selenium supplementation over a period of 3 months resulted in significant decrease in hair loss, abdominal pain, and loss of apetite in ovarian cancer patients undergoing chemotherapy. However, it is not clear from the study if selenium interferes with the action of chemotherapeutic drugs ⁽¹⁵⁾. Recent data from four clinical trials suggest that selenium may prevent gastrointestinal cancer ⁽¹⁶⁾ ; a meta-analysis of 16 studies showed that it may afford protection against lung cancer ⁽¹⁷⁾.
Findings from another clinical trial suggest effectiveness of selenium supplementation in reducing HIV-I viral load in infected individuals ⁽¹⁹⁾. Although some studies point to effectiveness of selenium in improving glucose metabolism, it was not found useful in prevention of type 2 diabetes; moreover, it may increase the risk for the disease ⁽²⁰⁾. Adverse events from selenium are usually gastric in nature, although chronic selenosis can occur with doses greater than 1000 microgram/day. This toxicity is characterized by muscle weakness, fatigue, peripheral neuropathy, skin rash, nail and hair changes, irritability, and possibly hepatic necrosis. Long-term use of selenium may increase the risk of certain types of skin cancer ⁽¹³⁾. Daily recommended intake is 55 micrograms and can be obtained from seafood, meat, and fortified grain products. The tolerable limit is 400 micrograms, although all studies to date have been conducted with a maximum of 200-microgram doses ⁽²⁾.

Brazil nuts, seafood, muscle and organ meats, cereals and grains
^{(1) (2)}

Selenium is an essential structural element of the antioxidant enzyme glutathione-peroxidase that takes part in a system to convert aggressive oxidation products and intracellular free radicals into less reactive or neutral components ⁽³⁾. Other biological functions of selenium include regulation of thyroid hormone action and regulation of the reduction status of vitamin C ⁽²⁾. 

Absorption:
Absorption of selenium is efficient and not regulated. More than 90 percent of selenomethionine, the major dietary form of the element, is absorbed by the same mechanism as methionine itself. Selenocysteine appears to be absorbed very well also. Of the inorganic forms, selenate is almost completely absorbed, but with a significant fraction lost in the urine before incorporation into tissue. Selenite has a more variable absorption probably related to interactions with substances in the gut lumen, but it is better retained, once absorbed, than selenate. Absorption of selenite is generally greater than 50 percent. Selenate and selenite are not major dietary constituents, but are commonly used to fortify foods and as selenium supplements.
Distribution:
Selenomethionine enters the methionine pool in the body and shares the fate of methionine until catabolized by the transsulfuration pathway ultimately leading to the reduced form.
Metabolism / Excretion:
Ingested selenocysteine, selenate, and selenite are all apparently metabolized by methylation to selenide. The selenide can be metabolized to selenophosphate, the precursor of selenocysteine in selenoproteins. The mechanism that regulates production of excretory metabolites has yet to be elucidated. The excretory metabolites appear in the urine primarily.
⁽²⁾

A recent analysis of five commercially available brands revealed actual content variability to be between 81% and 123% of the stated dose. One brand varied by 54% between different lots of the same supplement.
⁽⁴⁾

Supplement Interactions: High doses of selenium may decrease vitamin C absorption.
⁽⁵⁾

Clark LC, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. JAMA 1996;276:1957-63.
A prospective, randomized evaluation of skin cancer prevention for patients with a history of either basal cell or squamous cell carcinoma. 1312 patients were randomized to receive either selenium 200 mcg or a placebo daily and return to the clinic every 6 months for follow-up evaluation. Enrollment began in 1983 and was completed December 1993. The only adverse event involved gastrointestinal disturbances that lead to the withdrawal of consent in 21 selenium patients and 14 placebo. No chronic selenosis was noted. A total of 8271 person-years of follow-up was accumulated and indicated no significant difference between treatment groups on the recurrence of squamous or basal cell carcinomas. The authors did note that there was a statistically significant reduced relative risk of carcinoma incidence (lung, colorectal, prostate) for patients receiving selenium, however additional studies are need to validate.

Duffield-Lillico AJ, et al. Selenium supplementation and secondary prevention of nonmelanoma skin cancer in a randomized trial. J Natl Cancer Inst. 2003 Oct 1;95
In a followup analysis of the above trial, researchers summarized the entire blinded treatment period through January 1996 consisting of 9904 person-years. At this stage of followup patients with a history of nonmelanoma skin cancer were seen to have an increase in the risk of squamous cell carcinoma and total nonmelanoma skin cancer when supplemented with 200 mcg selenium daily. Furthermore, nonmelanoma skin cancer patients who had elevated baseline plasma selenium concentrations seemed to gain no protection against other cancers by selenium supplementation.