Selenium

Selenium is a trace element that is naturally present in many foods, added to others, and available as a dietary supplement. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection.

Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine). Both forms can be good dietary sources of selenium. Soils contain inorganic selenites and selenates that plants accumulate and convert to organic forms, mostly selenocysteine and selenomethionine and their methylated derivatives.

Able 1 lists the current rdas for selenium in mcg. For infants from birth to 12 months, the FNB established an AI for selenium that is equivalent to the mean intake of selenium in healthy, breastfed infants.

Table 1: Recommended Dietary Allowances (rdas) for Selenium
Age	Male	Female	Pregnancy	Lactation
Birth to 6 months	15 mcg*	15 mcg*
7–12 months	20 mcg*	20 mcg*
1–3 years	20 mcg	20 mcg
4–8 years	30 mcg	30 mcg
9–13 years	40 mcg	40 mcg
14–18 years	55 mcg	55 mcg	60 mcg	70 mcg
19–50 years	55 mcg	55 mcg	60 mcg	70 mcg
51+ years	55 mcg	55 mcg

*Adequate Intake (AI)

Selenium and Health

This section focuses on four diseases and disorders in which selenium might play a role: cancer, cardiovascular disease, cognitive decline, and thyroid disease.

Cancer

Because of its effects on DNA repair, apoptosis, and the endocrine and immune systems as well as other mechanisms, including its antioxidant properties, selenium might play a role in the prevention of cancer.

Epidemiological studies have suggested an inverse association between selenium status and the risk of colorectal, prostate, lung, bladder, skin, esophageal, and gastric cancers In a Cochrane review of selenium and cancer prevention studies, compared with the lowest category of selenium intake, the highest intake category had a 31% lower cancer risk and 45% lower cancer mortality risk as well as a 33% lower risk of bladder cancer and, in men, 22% lower risk of prostate cancer. The authors found no association between selenium intake and risk of breast cancer. A meta-analysis of 20 epidemiologic studies showed a potential inverse association between toenail, serum, and plasma selenium levels and prostate cancer risk

Randomized controlled trials of selenium supplementation for cancer prevention have yielded conflicting results. The authors of a Cochrane review concluded, based on nine randomized clinical trials, that selenium might help prevent gastrointestinal cancers but noted that these results need to be confirmed in more appropriately designed randomized clinical trials A secondary analysis of the double-blind, randomized, controlled Nutritional Prevention of Cancer Trial in 1,312 U.S. adults with a history of basal cell or squamous cell carcinomas of the skin found that 200 mcg/day selenium as high-selenium baker’s yeast for 6 years was associated with a 52% to 65% lower risk of prostate cancer This effect was strongest in men in the lowest tertile of selenium concentrations who had a baseline prostate-specific antigen (PSA) level of 4 ng/ml or lower. The Selenium and Vitamin E Cancer Prevention Trial (SELECT), a randomized, controlled trial in 35,533 men aged 50 years or older from the United States, Canada, and Puerto Rico, was discontinued after 5.5 years when analyses showed no association between supplementation with 200 mcg/day selenium with or without 400 international units (IU)/day vitamin E and prostate cancer risk An additional 1.5 years of follow-up data on participants after they stopped taking the study supplements confirmed the lack of a significant association between selenium supplementation and prostate cancer risk

In 2003, the FDA allowed a qualified health claim on foods and dietary supplements containing selenium to state that while “some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer… FDA has determined that this evidence is limited and not conclusive” More research is needed to confirm the relationship between selenium concentrations and cancer risk and to determine whether selenium supplements can help prevent any form of cancer.

Cardiovascular disease

Selenoproteins help prevent the oxidative modification of lipids, reducing inflammation and preventing platelets from aggregating For these reasons, experts have suggested that selenium supplements could reduce the risk of cardiovascular disease or deaths associated with cardiovascular disease.

The epidemiological data on the role of selenium in cardiovascular disease have yielded conflicting conclusions. Some observational studies have found an inverse association between serum selenium concentrations and risk of hypertension or coronary heart disease. A meta-analysis of 25 observational studies found that people with lower selenium concentrations had a higher risk of coronary heart disease However, other observational studies failed to find statistically significant links between selenium concentrations and risk of heart disease or cardiac death, or they found that higher selenium concentrations are associated with an increased risk of cardiovascular disease

Several clinical trials have examined whether selenium supplementation reduces the risk of cardiovascular disease. In one randomized, placebo-controlled study, for example, 474 healthy adults aged 60 to 74 years with a mean baseline plasma selenium concentration of 9.12 mcg/dl were supplemented with 100, 200, or 300 mcg selenium per day or placebo for 6 months. The supplements lowered levels of total plasma cholesterol and non–high-density-lipoprotein (HDL) plasma cholesterol (total cholesterol levels minus HDL levels) compared with the placebo group, whereas the 300 mcg/day dose significantly increased HDL levels. Other trials have provided evidence that selenium supplementation (200 mcg/day) or supplementation with a multivitamin/multimineral pill containing selenium (100 mcg/day) does not reduce the risk of cardiovascular disease or cardiac death. A review of trials of selenium-only supplementation for the primary prevention of cardiovascular disease found no statistically significant effects of selenium on fatal and nonfatal cardiovascular events. Almost all of the subjects in these clinical trials were well-nourished male adults in the United States.

The limited clinical-trial evidence to date does not support the use of selenium supplements for preventing heart disease, particularly in healthy people who already obtain sufficient selenium from food. Additional clinical trials are needed to better understand the contributions of selenium from food and dietary supplements to cardiovascular health.

Cognitive decline

Serum selenium concentrations decline with age. Marginal or deficient selenium concentrations might be associated with age-related declines in brain function, possibly due to decreases in selenium’s antioxidant activity.

The results of observational studies are mixed. In two large studies, participants with lower plasma selenium levels at baseline were more likely to experience cognitive decline over time, although whether the participants in these studies were selenium deficient is not clear . An analysis of NHANES data on 4,809 elderly people in the United States found no association between serum selenium levels (which ranged from lower than 11.3 to higher than 13.5 mcg/dl) and memory test scores.

Researchers have evaluated whether taking an antioxidant supplement containing selenium reduces the risk of cognitive impairment in elderly people. An analysis of data from the Supplementation en Vitamins et Minéraux Antioxydants (SU.VI.MAX) study on 4,447 participants aged 45 to 60 years in France found that, compared with placebo, daily supplementation with 120 mg ascorbic acid, 30 mg vitamin E, 6 mg beta-carotene, 100 mcg selenium, and 20 mg zinc for 8 years was associated with higher episodic memory and semantic fluency test scores 6 years after the study ended. However, selenium’s independent contribution to the observed effects in this study cannot be determined. The authors of a systematic review that included nine placebo-controlled studies concluded that the available clinical evidence is insufficient to determine whether selenium supplements can prevent Alzheimer’s disease.

More evidence is required to determine whether selenium supplements might help prevent or treat cognitive decline in elderly people.

Thyroid disease

Selenium concentration is higher in the thyroid gland than in any other organ in the body, and, like iodine, selenium has important functions in thyroid hormone synthesis and metabolism.

Epidemiological evidence supporting a relationship between selenium levels and thyroid gland function includes an analysis of data on 1,900 participants in the SU.VI.MAX study indicating an inverse relationship between serum selenium concentrations and thyroid volume, risk of goiter, and risk of thyroid tissue damage in people with mild iodine deficiency. However, these results were statistically significant only in women. A cross-sectional study in 805 adults with mild iodine deficiency in Denmark also found a significant inverse association between serum selenium concentration and thyroid volume in women.

Randomized, controlled trials of selenium supplementation in patients with thyroid disease have had varied results. In one randomized, double-blind, placebo-controlled trial, 100, 200, or 300 mcg/day selenium for 6 months in 368 healthy adults aged 60 to 74 years had no effect on thyroid function, even though plasma selenium levels increased significantly. Another randomized, double-blind, placebo-controlled trial compared the effects of 200 mcg/day selenium (as sodium selenite), 1,200 mg/day pentoxifylline (an antiinflammatory agent), or placebo for 6 months in 159 patients with mild Graves’ orbitopathy. Compared with patients treated with placebo, those treated with selenium but not pentoxifylline reported a higher quality of life. Furthermore, ophthalmic outcomes improved in 61% of patients in the selenium group compared with 36% of those in the placebo group, and only 7% of the selenium group had mild progression of the disease, compared with 26% of those in the placebo group.

Women with thyroid peroxidase antibodies tend to develop hypothyroxinemia while they are pregnant and thyroid dysfunction and hypothyroidism after giving birth. The authors of a Cochrane review of hypothyroidism interventions during pregnancy concluded, based on a trial that administered supplements containing 200 mcg selenium as selenomethionine daily to 151 pregnant women with thyroid peroxidase antibodies , that selenomethionine supplementation in this population is a promising strategy, especially for reducing postpartum thyroiditis . However, the authors called for large randomized clinical trials to provide high-quality evidence of this effect.

Additional research is needed to determine whether selenium supplements can help prevent or treat thyroid disease.

Health Risks from Excessive Selenium

Chronically high intakes of the organic and inorganic forms of selenium have similar effects. Early indicators of excess intake are a garlic odor in the breath and a metallic taste in the mouth. The most common clinical signs of chronically high selenium intakes, or selenosis, are hair and nail loss or brittleness. Other symptoms include lesions of the skin and nervous system, nausea, diarrhea, skin rashes, mottled teeth, fatigue, irritability, and nervous system abnormalities.

As discussed earlier, Brazil nuts contain very high amounts of selenium (68–91 mcg per nut) and could cause selenium toxicity if consumed regularly. Acute selenium toxicity has resulted from the ingestion of misformulated over-the-counter products containing very large amounts of selenium. In 2008, for example, 201 people experienced severe adverse reactions from taking a liquid dietary supplement containing 200 times the labeled amount. Acute selenium toxicity can cause severe gastrointestinal and neurological symptoms, acute respiratory distress syndrome, myocardial infarction, hair loss, muscle tenderness, tremors, lightheadedness, facial flushing, kidney failure, cardiac failure, and, in rare cases, death.

The FNB has established us for selenium from food and supplements based on the amounts of selenium that are associated with hair and nail brittleness and loss (see Table 3).

Table 3: Tolerable Upper Intake Levels (uls) for Selenium *
Age	Male	Female	Pregnancy	Lactation
Birth to 6 months	45 mcg	45 mcg
7–12 months	60 mcg	60 mcg
1–3 years	90 mcg	90 mcg
4–8 years	150 mcg	150 mcg
9–13 years	280 mcg	280 mcg
14–18 years	400 mcg	400 mcg	400 mcg	400 mcg
19+ years	400 mcg	400 mcg	400 mcg	400 mcg

*Breast milk, formula, and food should be the only sources of selenium for infants.

Interactions with Medications

Selenium can interact with certain medications, and some medications can have an adverse effect on selenium levels. One example is provided below. Individuals taking this and other medications on a regular basis should discuss their selenium status with their healthcare providers.

Cisplatin

Cisplatin, an inorganic platinum chemotherapy agent, is used to treat ovarian, bladder, lung, and other cancers. Cisplatin can reduce selenium levels in hair and serum but whether these reductions have a clinically significant impact is not known. Some small studies have shown that selenium supplementation can reduce cisplatin’s toxicity but the authors of a Cochrane review concluded that the evidence that selenium supplementation alleviates the side effects of chemotherapy is insufficient.

Selenium and Healthful Diets

The federal government’s 2015-2020 Dietary Guidelines for Americans notes that “Nutritional needs should be met primarily from foods. … Foods in nutrient-dense forms contain essential vitamins and minerals and also dietary fiber and other naturally occurring substances that may have positive health effects. In some cases, fortified foods and dietary supplements may be useful in providing one or more nutrients that otherwise may be consumed in less-than-recommended amounts.”

For more information about building a healthy diet, refer to the Dietary Guidelines for Americans and the U.S. Department of Agriculture’s my plate.

The Dietary Guidelines for Americans describes a healthy eating pattern as one that:

• Includes a variety of vegetables, fruits, whole grains, fat-free or low-fat milk and milk products, and oils.

Many whole grains and dairy products, including milk and yogurt, are good sources of selenium. Some ready-to-eat breakfast cereals are fortified with selenium, and some fruits and vegetables contain selenium.

• Includes a variety of protein foods, including seafood, lean meats and poultry, eggs, legumes (beans and peas), nuts, seeds, and soy products.

Pork, beef, turkey, chicken, fish, shellfish, and eggs contain high amounts of selenium. Some beans and nuts, especially Brazil nuts, contain selenium.

• Limits saturated and trans fats, added sugars, and sodium.
• Stays within your daily calorie needs.