Selenium as a dietary supplement
Selenium is a trace mineral recognized as essential for health and commonly promoted as a dietary supplement for its potential health benefits. It plays a crucial role in the body's antioxidant defense system, particularly in the production of glutathione peroxidase, which protects cells from damage caused by free radicals. While selenium is proposed primarily for cancer prevention, it has also been suggested for various conditions such as acne, anxiety, asthma, heart disease, and male infertility, among others.
The recommended daily intake varies by age and reproductive status, with most adults needing about 55 micrograms. Food sources rich in selenium include animal products, whole grains, and certain nuts, particularly Brazil nuts. Research indicates that selenium supplementation may offer some cancer protective effects, especially in individuals with low baseline selenium levels, but results are mixed and not universally conclusive. Furthermore, while some studies indicate potential benefits for immune function and psychological well-being, there is limited evidence supporting selenium for many other health claims.
Safety considerations are important, as excessive intake can lead to toxicity, and certain medications can affect selenium absorption. Overall, while selenium supplements may be beneficial for specific populations, individuals should approach supplementation cautiously and ideally consult with a healthcare professional.
Selenium as a dietary supplement
DEFINITION: Natural substance essential for health and promoted as a dietary supplement for specific health benefits.
- PRINCIPAL PROPOSED USE: Cancer prevention
- OTHER PROPOSED USES: Acne, anxiety, asthma, cataracts, cervical dysplasia, depression, diabetic neuropathy, fibromyalgia, general well-being, gout, heart disease prevention, human immunodeficiency virus support, male infertility, multiple sclerosis, osteoarthritis, psoriasis, rheumatoid arthritis, ulcers
Overview
Selenium is a trace mineral that the body uses 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. There is some preliminary evidence that selenium supplements might provide some protection against some types of cancer among people living in the United States, but this evidence is far from definitive.
Requirements and Sources
The official US and Canadian recommendations for daily intake (in micrograms) of selenium are as follows: infants to six months of age (15) and seven to twelve months of age (20), children one to three years of age (20) and four to eight years of age (30), children nine to thirteen years of age (40), children and adults aged fourteen years and older (55), pregnant females (60), and nursing females (70).
The 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. However, most people in the US and Canada are believed to consume more than enough selenium.
Foods containing significant and reliable amounts of selenium include animal products, such as meat, seafood, and dairy foods, and whole grains and vegetables grown in selenium-rich soils. These include wheat germ, nuts (particularly Brazil nuts), oats, whole-wheat bread, bran, brewer's yeast, 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. In addition, medications that reduce stomach acid, such as proton pump inhibitors or H2 blockers, may reduce the body’s absorption of selenium.
Therapeutic Dosages
In controlled trials of selenium, typical dosages were 100 to 200 micrograms (mcg) daily. The two general types of selenium supplements available to consumers are organic and inorganic forms. These two terms have a very specific chemical meaning and have nothing to do with “organic” foods. In chemistry, “organic” means that 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 are organic forms of selenium, such as selenomethionine (selenium bound to methionine, an essential amino acid) or high-selenium yeast (which contains selenomethionine). However, other research on both animals and humans suggests that selenite supplements are about as good as organic forms of selenium. These contradictory results suggest that any differences in absorption, if they exist, are relatively minor.
Therapeutic Uses
Preliminary studies hint that supplemental selenium may help prevent some forms of cancer. However, this evidence cannot be taken as reliable.
Selenium is required for a well-functioning immune system. Based on this, selenium has been suggested as a treatment for human immunodeficiency virus (HIV) infection. Early studies showed little to no benefit. In a large trial reported in 2007, selenium supplements called Selenomax reduced viral load. However, this study had numerous flaws in its statistical methods. For reasons that are not clear, another study found that selenium supplements decreased symptoms of psychological anxiety in HIV-positive persons undergoing highly active retroviral therapy. Research indicates that selenium levels decrease in the body as HIV progresses, but the relationship has shown contradictory evidence concerning benefit with supplementation. Additionally, some supplements may interfere with the medication used in HIV treatment.
One study of healthy people in the United Kingdom (where marginally low selenium intake is common) found that the use of selenium supplements improved general immune function, as measured by response to poliovirus immunization. A preliminary double-blind trial suggests that selenium supplements may improve fertility in males who are selenium deficient. Weak evidence suggests that selenium might be helpful for diabetic neuropathy.
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 that it is helpful for these conditions.
A small study among nursing home residents found that low levels of the mineral selenium were associated with depression. Moreover, eight weeks of supplementation tended to improve the mood of the most seriously depressed persons with low selenium levels. The same was not true of the two other nutrients investigated, folate and vitamin C.
Evidence regarding the use of selenium for preventing heart disease is mixed. Research generally indicates that heart disease, heart failure, and atherosclerosis risks increase with low selenium levels, but taking selenium supplements has not been shown to be effective in treating or preventing heart-related illnesses. Selenium supplements may help reduce cholesterol in some people, which may improve heart health, but these supplements also have a high risk of interacting with many popular cholesterol medications.
A study of 197 people with moderately severe asthma failed to find the benefit of selenium (100 mcg daily). Low selenium levels have been associated with increased likelihood of developing certain kinds of rheumatoid arthritis. However, selenium supplements do not appear to help rheumatoid arthritis once it has developed. Despite hopes to the contrary, it does not appear that selenium supplements can help prevent type 2 diabetes; rather, they might increase the risk of developing the disease.
A review of sixty-nine studies totaling more than 350,000 people concluded that consuming foods high in selenium was associated with lower breast, skin, lung, colon, and prostate cancers.
Scientific Evidence
Somewhat inconsistent evidence suggests that selenium supplements may help prevent cancer, but evidence from observational studies indicates that a low intake of selenium is tied to an increased risk of cancer. However, such studies are notoriously unreliable. Only double-blind trials can determine whether selenium supplements can help prevent cancer.
The most important double-blind study on selenium and cancer was conducted by researchers at the University of Arizona Cancer Center. 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 a 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 the risk of certain forms of skin cancer. 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 to allow all participants to take selenium.
When all the results were tabulated, it became clear that the selenium-treated group developed almost 66 percent fewer prostate cancers, 50 percent fewer colorectal cancers, and about 40 percent fewer lung cancers as compared with the placebo group. (All these results were statistically significant.) Selenium-treated persons also experienced a statistically significant (17 percent) decrease in overall mortality, a greater than 50 percent decrease in lung cancer deaths, and nearly a 50 percent 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. It has not been reported whether this limitation of benefit to low-selenium participants was true of the other forms of cancer too.
While this evidence is promising, it has one major flaw. The laws of statistics reveal that when researchers start to deviate from the question that their research was designed to answer, the results may not be trustworthy.
Combining the results of twelve 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. This difference cannot be explained without more research. Also, 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. In addition, animal trials have found anticancer benefits.
However, one study published in the early twenty-first century reported negative results in transplant recipients. People who undergo organ transplants are at particularly high risk of skin cancer linked to the human papillomavirus. In this double-blind study, 184 organ transplant recipients were given either placebo or selenium at a dose of 200 milligrams daily. The results in 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 (in micrograms) of various nutrients based on estimations of what should be safe for virtually all healthy persons. These tolerable upper intake levels are, thus, conservative guidelines. For selenium, they have been set as follows: infants to six months of age (45) and seven to twelve months of age (60), children one to three years of age (90) and four to eight years of age (150), children nine to thirteen years of age (280), children and adults aged fourteen years and older (400), pregnant and nursing females (400). Note that these dosages apply to combined dietary and supplemental intake of selenium. When deciding how much selenium is safe to take, one should note that most adults already receive about 100 mcg of selenium in their daily diet.
Maximum safe doses of selenium for persons with severe liver or kidney disease have not been established. The safety of long-term use of selenium supplements is not well-documented, and some studies imply long-term use, particularly in amounts over 200 mcgs, may increase the risk of diabetes and high cholesterol. Other potential risks include skin rashes, brittle fingernails, fatigue, and irritability.
Highly excessive selenium intake, beginning at about 900 mcg daily, can cause selenium toxicity. Signs include depression, nervousness, emotional instability, nausea, vomiting, and, in some cases, loss of hair and fingernails.
Important Interactions
Persons who are taking medications that reduce stomach acid, such as H2 blockers or proton pump inhibitors, may need extra selenium.
Individuals undergoing chemotherapy treatment taking the drug cisplatin may acquire a selenium deficiency. Selenium supplements decrease the side effects of the chemotherapy drugs like cisplatin, doxorubicin, and bleomycin, but they may also decrease their effectiveness.
Many other drugs interact with selenium supplements or the body’s selenium levels, including cholesterol medications, oral contraceptives, corticosteroids, blood thinners, barbiturates, gold salts, and aspirin.
Bibliography
Dreno, B., et al. “Effect of Selenium Intake on the Prevention of Cutaneous Epithelial Lesions in Organ Transplant Recipients.” European Journal of Dermatology, vol. 17, 2007, pp. 140-45.
Gosney, M. A., et al. “Effect of Micronutrient Supplementation on Mood in Nursing Home Residents.” Gerontology, vol. 54, 2008, pp. 292-99.
Hurwitz, B. E., et al. “Suppression of Human Immunodeficiency Virus Type 1 Viral Load with Selenium Supplementation.” Archives of Internal Medicine, vol. 167, 2007, pp. 148-54.
Navas-Acien, A., J. Bleys, and E. Guallar. “Selenium Intake and Cardiovascular Risk: What Is New?” Current Opinion in Lipidology, vol. 19, 2008, pp. 43-49.
"Selenium." National Institutes of Health, 15 Apr. 2024, ods.od.nih.gov/factsheets/Selenium-HealthProfessional. Accessed 20 Sept. 2024.
"Selenium." Harvard School of Public Health, Mar. 2023, www.hsph.harvard.edu/nutritionsource/selenium. Accessed 20 Sept. 2024.
Shimada, Briana K., et al. “The Impact of Selenium Deficiency on Cardiovascular Function.” International Journal of Molecular Sciences, vol. 22, no. 19, Oct. 2021. doi.org/10.3390/ijms221910713. Accessed 22 Nov. 2024.
Vinceti, M., et al. "Selenium for Preventing Cancer." Cochrane Database of Systematic Reviews, vol. 1, 2018. doi:10.1002/14651858.CD005195.pub4.