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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.

Most selenium is in the form of selenomethionine in animal and human tissues, where it can be incorporated nonspecifically with the amino acid methionine in body proteins. Skeletal muscle is the major site of selenium storage, accounting for approximately 28% to 46% of the total selenium pool. Both selenocysteine and selenite are reduced to generate hydrogen selenide, which in turn is converted to selenophosphate for selenoprotein biosynthesis.

The most commonly used measures of selenium status are plasma and serum selenium concentrations. Concentrations in blood and urine reflect recent selenium intake. Analyses of hair or nail selenium content can be used to monitor longer-term intakes over months or years. Quantification of one or more selenoproteins (such as glutathione peroxidase and selenoprotein P) is also used as a functional measure of selenium status. Plasma or serum selenium concentrations of 8 micrograms (mcg)/dL or higher in healthy people typically meet needs for selenoprotein synthesis.

Recommended Intakes

Intake recommendations for selenium and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences). DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and sex, include:

• Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.
• Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.
• Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.
• Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

Table 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.

*Adequate Intake (AI)

Sources of Selenium

Food

Brazil nuts, seafoods, and organ meats are the richest food sources of selenium. Other sources include muscle meats, cereals and other grains, and dairy products. The amount of selenium in drinking water is not nutritionally significant in most geographic regions. The major food sources of selenium in the American diet are breads, grains, meat, poultry, fish, and eggs.

The amount of selenium in a given type of plant-based food depends on the amount of selenium in the soil and several other factors, such as soil pH, amount of organic matter in the soil, and whether the selenium is in a form that is amenable to plant uptake. As a result, selenium concentrations in plant-based foods vary widely by geographic location. For example, according to the U.S. Department of Agriculture Food Composition Database, Brazil nuts have 544 mcg selenium/ounce, but values from other analyses vary widely.

The selenium content of soil affects the amounts of selenium in the plants that animals eat, so the quantities of selenium in animal products also vary. However, selenium concentration in soil has a smaller effect on selenium levels in animal products than in plant-based foods because animals maintain predictable tissue concentrations of selenium through homeostatic mechanisms. Furthermore, formulated livestock feeds generally contain the same levels of selenium.

Dietary Supplements

Selenium is available in multivitamin/multimineral supplements and as a stand-alone supplement, often in the forms of selenomethionine or of selenium-enriched yeast (grown in a high-selenium medium) or as sodium selenite or sodium selenate. The human body absorbs more than 90% of selenomethionine but only about 50% of selenium from selenite.

Few studies have compared the relative absorption and bioavailability of different forms of selenium. In one investigation, 10 groups of selenium-replete subjects were randomly assigned to receive a placebo or either 200 or 600 mcg/day selenium as selenomethionine, sodium selenite, or high-selenium yeast (in which an estimated 75% of selenium was in the form of selenomethionine) for 16 weeks. Selenium bioavailability, based on urinary excretion, was greatest for selenomethionine and lowest for selenite. However, supplementation with any of these forms only affected plasma selenium levels and not glutathione peroxidase activity or selenoprotein P concentration, confirming that study participants were selenium replete before they began taking selenium supplements.

Selenium Intakes and Status

Most Americans consume adequate amounts of selenium. According to an analysis of data from the 2009–2010 National Health and Nutrition Examination Survey (NHANES), the average daily selenium intake in Americans aged 2 years and older from foods is 108.5 mcg and from both foods and supplements is 120.8 mcg. Adult men have higher daily intakes (134 mcg from foods and 151 mcg from foods and supplements) than adult women (93 mcg from foods and 108 mcg from foods and supplements). In the United States, 18% to 19% of adults and children use a dietary supplement containing selenium.

According to an analysis of NHANES data from 2003–2004, the mean serum selenium concentration in U.S. adults aged 40 years or older is 13.67 mcg/dL. Men have slightly higher serum selenium levels than women, and whites have higher levels than African Americans.

Selenium intakes and serum concentrations in the United States and Canada vary somewhat by region because of differences in the amounts of selenium in soil and in local foods consumed. For example, concentrations are higher in residents of the Midwestern and Western United States than in the South and Northeast. The extensive transport of food typically allows people living in low-selenium areas to obtain sufficient amounts of selenium.

Selenium Deficiency

Selenium deficiency produces biochemical changes that might predispose people who experience additional stresses to develop certain illnesses. For example, selenium deficiency in combination with a second stress (possibly a viral infection) leads to Keshan disease, a cardiomyopathy that occurred in parts of China prior to a government-sponsored selenium supplementation program that began in the 1970s. Before the Chinese government supplementation program, adults in the Keshan disease areas had average selenium intakes of no more than 11 mcg/day; intakes of at least 20 mcg/day protect adults from Keshan disease.

Selenium deficiency is also associated with male infertility and might play a role in Kashin-Beck disease, a type of osteoarthritis that occurs in certain low-selenium areas of China, Tibet, and Siberia. Selenium deficiency could exacerbate iodine deficiency, potentially increasing the risk of cretinism in infants.

Groups at Risk of Selenium Inadequacy

Selenium deficiency is very rare in the United States and Canada, and selenium deficiency in isolation rarely causes overt illness. The following groups are among those most likely to have inadequate intakes of selenium.

People living in selenium-deficient regions

Selenium intakes in North America, even in low-selenium regions, are well above the RDA. However, people in some other countries whose diet consists primarily of vegetables grown in low-selenium areas are at risk of deficiency. The lowest selenium intakes in the world are in certain parts of China, where large proportions of the population have a primarily vegetarian diet and soil selenium levels are very low. Average selenium intakes are also low in some European countries, especially among populations consuming vegan diets. Although intakes in New Zealand were low in the past, they rose after the country increased its importation of high-selenium wheat.

People undergoing kidney dialysis

Selenium levels are significantly lower in patients undergoing long-term hemodialysis than in healthy individuals. Hemodialysis removes some selenium from the blood. In addition, hemodialysis patients are at risk of low dietary selenium intakes due to anorexia resulting from uremia and dietary restrictions. Although selenium supplementation increases blood levels in hemodialysis patients, more evidence is needed to determine whether supplements have beneficial clinical effects in these individuals.

People living with HIV

Selenium levels are often low in people living with HIV, possibly because of inadequate intakes (especially in developing countries), excessive losses due to diarrhea, and malabsorption. Observational studies have found an association between lower selenium concentrations in people with HIV and an increased risk of cardiomyopathy, death, and, in pregnant women, HIV transmission to offspring and early death of offspring. Some randomized clinical trials of selenium supplementation in adults with HIV have found that selenium supplementation can reduce the risk of hospitalization and prevent increases of HIV-1 viral load; preventing HIV-1 viral load progression can lead to increases in numbers of CD4 cells, a type of white blood cell that fights infection.

Selenium and Health

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 Supplémentation en Vitamines 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 ULs 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).

*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.”

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.

Selenium Dosage in Studies

Many studies have attempted to determine the 'Just Right' selenium dosage, and while there are no clear cut answers as to the exact optimal dose for optimal health, the studies that HAVE been done give us some clues.

200 mcg (micrograms) is the most frequently used dose in studies that obtained beneficial results. Most of these studies were done over relatively short periods of time, but some were done over long periods of time, so it seems that 200 mcg per day is a relatively safe dosage to take for relatively long periods of time.

One study that analyzed many other studies on selenium found that dosages of about 300 mcg per day of selenium began to show ill effects on the thyroid gland as well as hair and nail weakening and loss.

So from this study, we can determine that 300 mcg per day of selenium is likely to be unnecessary and may cause harm over the long term. So, taking between about 100 mcg to 200 mcg seems to be the 'just right' selenium dosage that is beneficial for health, but that does not cause problems in MOST people. Again, this will be dependent upon how much you ALREADY get in your diet, your specific metabolism and other factors that science may not even know about. *

Selenium Overdose is a real phenomenon that has been documented in many areas around the world that have high selenium content in their water and their soil. It has also been documented in people taking large doses of selenium supplements for long periods of time. So, it is important that if you do choose to supplement with selenium that you be aware of selenium overdose symptoms and the appropriate and toxic dosages.

It is interesting that few people are aware of selenium toxicity - even though it is relatively EASY to have a selenium overdose and it occurs relatively frequently; yet it seems that just about EVERYONE is concerned about an Overdose on Vitamin D even though it is extremely DIFFICULT and there are very few instances of it- and NO instances of it from natural sources.

How We Know about the Symptoms of Selenium Toxicity

There are many areas that have very high naturally occurring selenium levels. And animals that graze are particularly susceptible to selenium toxicity. But humans have been affected too- mainly in areas with high selenium levels in the WATER, as most humans don't eat the large amount of selenium containing foods that would be required in order to become toxic from foods grown in high selenium areas. But selenium overdoses in nature can and do happen.

Selenium toxicity also can and does occur from taking selenium supplements in too high a dose for too long. Because too LOW selenium levels can contribute to health problems like:

• • Selenium Deficiency Heart Failure
• • A Weakened Immune System
• • Complications of Influenza
• • Some Cancers
• • Thyroid Problems

So, there are possibly many good reasons to take selenium supplements. But it's extremely important that the Selenium Dosage you choose - IF you choose to supplement at all - is 'Just Right'. In other words, it needs to be high enough for you to receive any health benefits that it may provide, but not too much to cause selenium toxicity.

Signs and Symptoms of Selenium Overdose

So, there are many signs and symptoms of Selenium Overdose- some occur very early and some occur much later

• Early Signs: Some of the early signs of Selenium Toxicity are endocrine hormone disturbances- particularly thyroid issues.

  Poor hair and nail growth are early signs that are probably the easiest to notice. Interestingly, thyroid problems are also a Sign of Selenium Deficiency as well!

• Later signs: One of the later signs of selenium toxicity are fatigue- particularly because of thyroid and adrenal gland dysfunction. *