Vitamin C And Selenium

Contents

  • Summary
  • Function
    • Selenoproteins
    • Nutrient interactions
  • Deficiency
    • Individuals at risk
    • Keshan disease
    • Kashin-Beck disease
  • The RDA
  • Disease Prevention
    • Cancer
    • Cardiovascular disease
  • Disease Treatment
    • Immune dysfunction
    • Infectious diseases
    • Autoimmune thyroid diseases
  • Sources
    • Food
    • Supplements
    • Selenium-enriched foods
  • Safety
    • Toxicity
    • Type 2 diabetes
    • Drug interactions
  • LPI Recommendation
  • Authors and Reviewers
  • References

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Summary

  • Selenium exerts various biological functions mainly as part of the amino acid, selenocysteine, which is found in at least 25 selenocysteine-containing proteins (selenoproteins). (More information)
  • Five glutathione peroxidases, three thioredoxin reductases, three iodothyronine deiodinases, and one methionine sulfoxide reductase B1 are among the best characterized selenoproteins with known functions. (More information)
  • Impaired antioxidant protection in selenium-deficient individuals may affect physiological responses to stress. Keshan cardiomyopathy and Kashin-Beck osteoarthropathy are diseases occurring specifically in areas of selenium deficiency in Asia. (More information)
  • The current recommended dietary allowance (RDA) set by the US Institute of Medicine is 55 μg/day for adolescents and adults of all ages. (More information)
  • Early observational studies have found either null or inverse associations between selenium exposure and risk of site-specific cancers. However, current evidence from intervention trials in selenium-replete participants does not support a protective effect of selenium supplementation against cancer. (More information)
  • Preliminary evidence from randomized controlled trials suggests that selenium supplementation may prevent viral load progression and increase immune cell count in HIV-positive patients. (More information)
  • The levels and chemical forms of selenium in plant-based food vary according to the composition and selenium content of soil in which the plants are grown. Selenium-rich food sources include Brazil nuts, grains, seafood, organ meats, poultry, and dairy products. (More information)
  • The tolerable upper intake level (UL) for selenium is 400 μg/day for adolescents and adults and includes both selenium obtained from food, which averages about 100 μg/day for adults in the US, and selenium from supplements. (More information)
  • Because some evidence suggests that high serum selenium concentrations may have adverse effects on glycemic control, individuals with high selenium status and/or those at risk for type 2 diabetes mellitus should avoid taking selenium supplements. (More information)

Selenium is a trace element that is essential in small amounts, but like all essential elements, selenium can be toxic at high levels. Unlike plants, most animals — including humans — require selenium for the appropriate functioning of a number of selenium-dependent enzymes known as selenoproteins. During protein synthesis (translation), the amino acid selenocysteine is incorporated into elongating proteins at very specific locations in the amino acid sequence in order to form functional selenoproteins. Although higher plants do not appear to require selenium for survival, they can incorporate it non-specifically into sulfur-containing molecules when the mineral is present in the soil (1). Of note, in animals, the amino acid selenomethionine can be nonspecifically incorporated into proteins in place of methionine (2). However, only selenocysteine-containing proteins are regarded as selenoproteins (Figure 1).

Figure 1. Cysteine, Methionine, and Selenium-containing Analogs. Chemical structures of cysteine, selenocysteine, methionine, and selenomethionine.

Function

Selenoproteins

Twenty-five genes coding for selenoproteins have been identified in humans (3). The insertion of selenocysteine into selenoproteins during translation is directed by the presence of a selenocysteine-insertion sequence (SECIS) within selenoprotein mRNAs. Briefly, the recognition of SECIS by the translational machinery results in the recruitment of specific translational factors that decode in-frame UGA codons by inserting selenocysteine into elongating selenoproteins (4).

Research is gradually uncovering the metabolic functions of all human selenoproteins, including splicing variants (3). Some of the selenoproteins with an identified function include:

Glutathione peroxidases

Five selenium-containing glutathione peroxidases (GPx1-4 and GPx6) have been identified: GPx1 (cytosolic GPx), GPx2 (epithelial cell-specific GPx expressed in intestinal lining and lungs), GPx3 (highly expressed in thyroid gland and kidneys), GPx4 (phospholipid-hydroperoxide GPx; PHGPx), and GPx6 (expressed in the olfactory epithelium) (4). GPx isoenzymes are all antioxidant enzymes that reduce potentially damaging reactive oxygen species (ROS), such as hydrogen peroxide and lipid hydroperoxides, to harmless products like water and alcohols by coupling their reduction with the oxidation of glutathione (Figure 2). Spermatogenesis and male fertility are highly dependent on GPx4 and selenoprotein P (SEPP1; see below). In the testes, GPx4 reduces phospholipid hydroperoxides, hence protecting immature spermatozoa cells against oxidative stress. GPx4 is also a major structural protein of the capsule embedding mature sperm mitochondrial helix involved in sperm motility. SEPP1 is essential for selenium supply to the testes, and animal models lacking the SEPP1 gene are infertile due to poor selenium tissue bioavailability, defective GPx4 synthesis, and impaired sperm maturation (5).

Figure 2. Selenoproteins in Thiol-based Antioxidant Systems: the thioredoxin antioxidant system and glutathione antioxidant system.

Thioredoxin reductases

In mammals, three selenocysteine-containing thioredoxin reductase (TrxR) isoenzymes have been identified in the thioredoxin system: cytosolic TrxR1, mitochondrial TrxR3, and testes-specific thioredoxin glutathione reductase TGR. TrxRs are homodimeric enzymes, and each monomer contains FAD- and NADPH-binding domains and a selenocysteine-containing catalytic site. TrxRs catalyze the reduction of a wide range of substrates, including thioredoxin and protein disulfide isomerase (PDI) (see Figure 2 above). TrxRs also serve as electron donors for the regeneration of small antioxidants, possibly recycling ascorbic acid (vitamin C), α-lipoic acid, α-tocopherol (vitamin E), and coenzyme Q10 from their oxidized forms (6). The maintenance of thioredoxin in a reduced form by TrxRs is important for regulating cell growth and survival. The protein thioredoxin, together with TrxR1 (or TrxR3), NADPH, and FAD, constitute the thioredoxin antioxidant system involved in the reduction of antioxidant enzymes (e.g., peroxiredoxins, methionine sulfoxide reductases, and ribonucleotide reductase) and of many oxidation/reduction (redox)-sensitive signaling proteins (7). TrxR1 is one of the most investigated selenoproteins and regarded as one of the major antioxidant enzymes and redox regulators in mammalian cells.

Iodothyronine deiodinases (thyroid hormone deiodinases)

The thyroid gland releases very small amounts of biologically active thyroid hormone (triiodothyronine or T3) and larger amounts of an inactive form of thyroid hormone (T3 precursor: thyroxine or T4) into the circulation. Most of the biologically active T3 in the circulation and inside cells is generated by the removal of one iodine atom from T4 in a reaction catalyzed by selenium-dependent iodothyronine deiodinase enzymes. Two different selenium-dependent iodothyronine deiodinases (DIOs type 1 and 2) can deiodinate T4, thus increasing circulating T3, while a third iodothyronine deiodinase (DIO type 3) can convert both T3 and T4 to inactive metabolites (Figure 3) (8). Of note, inactivation of the genes encoding DIOs in rodent models has revealed a role for DIO type 1 in iodine homeostasis and the importance of DIOs type 2 and 3 in the maturation of auditory and visual systems during fetal development (8). Thus the importance of selenium in normal development, growth, and metabolism is not limited to its role in the regulation of thyroid gland function.

Figure 3. Deiodination of Thyroid Hormones. Iodothyronine deiodinases (DIOs) are selenoenzymes that catalyze the deiodination (removal of iodine) from iodothyronines. Specifically, DIO1 and DIO2 catalyze the deiodination of thyroxine (T4) that generates the biologically active triiodothyronine (T3). DIO3 inactivates T3 and T4 by removing iodine atoms from the inner ring.

Selenoprotein P

Selenoprotein P (SEPP1) is predominantly produced by the liver, a major storage site for selenium, and secreted in the plasma. The full-length glycoprotein contains a selenium-rich domain with nine selenocysteine residues, as well as a thioredoxin-like catalytic site with one selenocysteine residue. SEPP1 constitutes the major form of selenium transport to peripheral tissues (9). SEPP1 also functions as an antioxidant that protects cells from oxidative damage by enabling full activity of thioredoxin reductases and glutathione peroxidases through adequate supply of selenium to extrahepatic tissues (see Glutathione peroxidases). SEPP1 appears to be especially critical for selenium homeostasis in the brain and testes where apolipoprotein E receptor 2 (apoER2) facilitates the uptake of SEPP1. Megalin is another SEPP1-specific lipoprotein receptor that helps limit urinary selenium loss through SEPP1 re-uptake by the kidneys (10). Moreover, SEPP1 has been recently implicated in the regulation of glucose metabolism and insulin sensitivity (11).

Selenoprotein W

Selenoprotein W (SEPW or SelW) exists in different isoforms (homologues) and is highly conserved across species. In humans, SEPW is expressed in numerous tissues, with highest levels found in skeletal muscle and heart (12). SEPW contains a selenocysteine residue and a cysteine residue that binds to a glutathione molecule, suggesting a role in redox regulation (13). The expression of SEPW is correlated with selenium status and appears to be sensitive to low-selenium supply (14, 15). SEPW expression in the brain has been found to confer protection against oxidative stress-induced neuronal cell death (16). SEPW also appears to be a negative regulator for 14-3-3 proteins. Indeed, 14-3-3 inhibition by SEPW in breast cancer cells was found to increase cell proliferation and cell survival through increasing resistance to genotoxic stress (17). In skeletal muscle cells, SEPW was shown to reduce the binding of 14-3-3 to TAZ, allowing TAZ translocation to the nucleus and subsequent activation of muscle cell differentiation genes (18). Finally, SEPW was found to prevent the degradation of the epidermal growth factor receptor (EGFR) in breast and prostate epithelial cells in culture. EGFR is constitutively activated in many tumors, and evidence of a role for SEPW in EGFR activation and signaling may help shed light on the relationship between selenium status and cancer risk (19).

Selenophosphate synthetase 2

There is no free pool of the amino acid selenocysteine in cells such that selenocysteine synthesis takes place on a specialized tRNA during the translation of selenoprotein mRNAs. The reaction is catalyzed by pyridoxal 5'-phosphate (PLP)-dependent L-seryl-tRNASec selenium transferase and uses selenophosphate (monoselenium phosphate) as the selenium donor (Figure 4) (20). Selenophosphate synthetase 2 is a selenoenzyme that catalyzes the ATP-dependent synthesis of selenophosphate from hydrogen selenide (Figure 4) (3).

Figure 4. Synthesis of Selenocysteine. See text for description.

Methionine-R-sulfoxide reductase B1 (formerly selenoprotein R)

The methionine sulfoxide reduction system is involved in the protection against oxidative stress and is especially critical for the regeneration of proteins damaged by reactive oxygen species (ROS). Indeed, ROS can oxidize methionine residues (methionine sulfoxides) within proteins and potentially impair their activities. In humans, two stereospecific families of methionine sulfoxide reductases (MsrA and MsrB) are encoded by a single MSRA gene and three MSRB genes (MSRB1-3). MsrA catalyzes the reduction of the S-form of methionine sulfoxide; the R-form of methionine sulfoxide is reduced by MsrB1, 2, or 3. Only MsrB1 has been characterized as a selenoprotein with one selenocysteine residue in its catalytic site. MsrB1 appears to be involved in the redox regulation of certain proteins. In macrophages, reorganization of the actin cytoskeleton necessary for chemotaxis and phagocytosis requires MsrB1-dependent reduction of methionine-R-sulfoxide residues within actin (21). Studies using MSR gene inactivation in mice have also shown that methionine sulfoxide reduction is implicated in the regulation of the methionine cycle (reviewed in 22). Both the thioredoxin (Trx) and GSH-dependent glutaredoxin (Grx) antioxidant systems have been found to reduce methionine sulfoxide reductases in vitro and/or in vivo (see Figure 2 above) (22).

15 kDa selenoprotein

15 kDa selenoprotein (selenoprotein 15; SEP15) is highly expressed in several tissues, including prostate, kidney, testes, liver, and brain (23). Although its function is not known, SEP15 was found to interact with the endoplasmic reticulum UDP-glucose:glycoprotein glucosyltransferase (UGGT), an enzyme involved in the quality control of glycoprotein folding (24, 25). Because SEP15 has a thioredoxin-like catalytic site, SEP15 is thought to either regulate UGGT activity or the redox state of UGGT substrates (26). Mice lacking a functional SEP15 were found to develop nuclear cataract (lens opacification) at a very early age suggesting that SEP15 may be critical to the quality control system of protein folding in the lens (27). SEP15 may also be implicated in anticancer mechanisms (reviewed in 28).

Selenoprotein S

The mammalian selenoprotein S (known as SEPS1, SelS, or VCP-interacting membrane selenoprotein [VIMP]) is an endoplasmic reticulum (ER) membrane protein. SEPS1 is involved in the cellular response to ER stress (ER-associated degradation; ERAD) activated by the detection of misfolded proteins. SEPS1 contributes to the removal and transfer (retrotranslocation) of misfolded proteins from the ER lumen to the cytosol where proteins are tagged with ubiquitin before being degraded. A polymorphism or variation in the sequence within an ER-response element located in the SEPS1 promoter was found to result in reduced SEPS1 promoter activity and gene expression (29). The polymorphism corresponding to the substitution of a guanine (G) by an adenine (A) at nucleotide -105 (-105G>A) has been associated with increased plasma levels of pro-inflammatory cytokines. In addition, a case-control study recently reported that the A allele was more prevalent in individuals affected by Hashimoto thyroiditis (HT) — a T-cell-mediated autoimmune disease resulting in the destruction of thyroid cells — than in healthy controls (30). Other associations between SEPS1 polymorphism (including -105G>A) and susceptibility to various conditions, such as preeclampsia, coronary artery disease, or gastrointestinal cancers, strongly suggest a role for this selenoprotein in the regulation of inflammatory and immune responses (31-34).

Other less well-characterized selenoproteins, which are also localized in the ER lumen and/or membrane, include selenoproteins K, M, N, and T (35).

Nutrient interactions

Antioxidant nutrients

The importance of selenium to biological systems, and specifically to the cellular redox (pro-oxidant/antioxidant) balance, is derived from its presence as selenocysteine in the catalytic site of selenoproteins (see Function). Other minerals that are critical components of antioxidant enzymes include copper (as superoxide dismutase), zinc (as superoxide dismutase), and iron (as catalase). Selenium acts in synergy with the antioxidant vitamins, vitamin C (ascorbic acid) and vitamin E (α-tocopherol), by regenerating them from their oxidized forms and promoting maximal antioxidant protection (36-38).

Iodine

While iodine is an essential component of thyroid hormones, the selenium-containing iodothyronine deiodinases (DIOs) are enzymes required for the conversion of thyroxine (T4) to the biologically active thyroid hormone, triiodothyronine (T3) (see Function). DIO1 activity may also be involved in regulating iodine homeostasis (39). The selenoenzymes, glutathione peroxidases, also play a critical role in thyroid function because they catalyze the degradation of peroxides generated during thyroid hormone synthesis (8). The epidemiology of coexisting iodine and selenium deficiencies in central Africa, but not in China, has been linked to the prevalence of myxedematous cretinism, a severe form of congenital hypothyroidism accompanied by mental and physical retardation. Selenium deficiency may be only one of several undetermined factors that might exacerbate the detrimental effects of iodine deficiency (40). Interestingly, selenium deficiency in rodents was found to have little impact on DIO activities as it appears that selenium is being supplied in priority for adequate synthesis of DIOs at the expense of other selenoenzymes (8).

Deficiency

Insufficient selenium intake may negatively affect the activity of several selenium-responsive enzymes, including glutathione peroxidases (GPx1 and GPx3), iodothyronine deiodinases, selenoprotein W, and methionine-R-sulfoxide reductase B1 (MsrB1). Even when severe, isolated selenium deficiency does not usually result in obvious clinical illness. Yet, compared to subjects with adequate selenium status, selenium-deficient individuals might be more susceptible to additional physiological stresses (41). Prolonged selenium deficiency may likely contribute to Keshan and Kashin-Beck diseases (see below).

Individuals at increased risk of selenium deficiency

Selenium deficiency has been reported in chronically ill patients who were receiving total parenteral nutrition (TPN) without added selenium for prolonged periods of time. Muscular weakness, muscle wasting, and cardiomyopathy (inflammation and damage to the heart muscle) have been observed in these patients. Nowadays, TPN solutions are routinely supplemented with selenium. The risk of selenium deficiency may be increased following bariatric surgery or in severe gastrointestinal conditions, such as Crohn's disease. Some specialized medical diets like those used to treat certain metabolic disorders, including phenylketonuria, homocystinuria, and maple syrup urine disease, need to be supplemented with selenium to ensure optimum selenium status in patients (42).

Keshan disease

Keshan disease is a fatal form of dilated cardiomyopathy that was first described in young women and children in a selenium-deficient region in China. The acute form of the disease is characterized by the sudden onset of cardiac insufficiency, while the chronic form results in moderate-to-severe heart enlargement with varying degrees of cardiac insufficiency (43). The incidence of Keshan disease is closely associated with very low dietary intakes of selenium and poor selenium nutritional status. Selenium supplementation (in the form of sodium selenite; Na2SeO3) was found to protect people from developing Keshan disease but could not reverse heart muscle damage once it had occurred (43). A recent case-control study reported that selenium-responsive glutathione peroxidase 1 (GPx1) activity was significantly lower in Keshan patients compared to healthy individuals. Interestingly, a specific GPX1 polymorphism resulting in a proline-to-leucine transition at position 198 (Pro198Leu) is associated with a reduction in GPx1 activity and found to be more prevalent in Keshan patients. This GPX1 polymorphism might confer a greater susceptibility to Keshan disease in carriers with low selenium nutritional status (44).

While selenium deficiency is a major etiological factor of Keshan disease, the seasonal and annual variation in disease occurrence suggested that other factors, especially an infectious agent, might be involved in addition to selenium deficiency (45). Coxsackie virus B3 is one virus type that has been isolated from Keshan patients, and animal studies have shown that this virus was capable of causing an inflammation of the heart (myocarditis) in selenium-deficient mice. Studies in mice also indicated that oxidative stress induced by selenium deficiency could result in changes in the viral genome, such as to convert a relatively harmless strain of coxsackie virus B3 into a myocarditis-causing strain (43). Although not proven in Keshan disease, it is possible that selenium deficiency may increase the virulence of viruses with the potential to invade and damage the heart muscle (46).

Kashin-Beck disease

Kashin-Beck disease (KBD) is another endemic condition that affects an estimated 2.5 million people in Tibet, northern and central China, North Korea, and southeastern Siberia (47). KBD is characterized by the degeneration of articular cartilage between joints (osteoarthritis) that can result in joint deformities and dwarfism in the most severe forms of the disease. The disease affects children as young as two years old. As with Keshan disease, KBD is prevalent in selenium-deficient provinces and thus generally affects people with very low selenium intakes (47). Recent studies have suggested that increased susceptibility to KBD in selenium-deficient populations might result from a reduced antioxidant protection associated with polymorphisms in GPX genes (48, 49). Yet, the etiology appears to be multifactorial, as a number of other causative factors have been suggested for KBD, including fungal toxins in grain, iodine deficiency, and contaminated drinking water (43).

Recent meta-analyses of a few small trials and prospective cohort studies have indicated that improving selenium nutritional status in children living in endemic areas may help reduce KBD incidence (50). Also, there is limited evidence to suggest that selenium supplementation could be useful in the treatment of patients with KBD. A meta-analysis of 10 randomized controlled trials reported a significant increase in the repairing rate of bone lesions in KBD children supplemented with sodium selenite for at least one year (51). Larger trials of higher quality are needed to assess whether selenium supplementation could result in disease remission.

The Recommended Dietary Allowance (RDA)

The dietary reference intakes (DRIs) for selenium were last revised in 2000 by the Food and Nutrition Board (FNB) of the US Institute of Medicine. The most recent RDA is based on the estimated average requirement (EAR) needed to maximize antioxidant enzyme glutathione peroxidase (GPx) activity in plasma (52).

Table 1. Recommended Dietary Allowance (RDA) for Selenium
Life Stage Age Males (μg/day) Females (μg/day)
Infants 0-6 months 15 (AI) 15 (AI)
Infants 7-12 months 20 (AI) 20 (AI)
Children 1-3 years 20 20
Children 4-8 years 30 30
Children 9-13 years 40 40
Adolescents 14-18 years 55 55
Adults 19 years and older 55 55
Pregnancy all ages - 60
Breast-feeding all ages - 70

Of note, the third National Health and Nutrition Examination Survey (NHANES III) reported that over 99% of the US participants had serum selenium concentrations consistent with selenium requirements being met (52), suggesting selenium supplementation is not needed for Americans.

Disease Prevention

Cancer

Animal studies

There has been considerable research on the effect of selenium supplementation on the incidence of cancer in animals. More than two-thirds of over 100 published studies in 20 different animal models of spontaneous, viral, and chemically induced cancers found that selenium supplementation (to at least adequate intake levels) significantly reduces tumor incidence, especially in comparison to selenium-deficient diets (53). Evidence of cancer-inhibiting effects of selenium has provided a strong rationale for investigating potential associations between selenium intake and cancer risk in humans.

Observational studies

Most of the early epidemiological evidence from case-control and nested case-control studies suggested either null or inverse associations between selenium exposure and risk of site-specific cancers (54). Markers of selenium exposure include toenail and blood selenium content, as well as plasma glutathione peroxidase (GPx) activity. However, it is not clear whether they adequately reflect selenium exposure from dietary and supplemental sources (see Sources) or selenium distribution in tissues and organs that may be affected by cancer. In the Danish Prospective Diet, Cancer, and Health prospective study that followed over 57,000 men and women for 14 years, the risk of rectal cancer was found to be 42% higher in current smokers compared to nonsmokers. No difference between smokers and nonsmokers regarding supplemental and dietary intakes of antioxidant micronutrients, including selenium, was found to contribute to the association of smoking and rectal cancer (55). Yet, because studies have consistently reported lower blood selenium concentrations and GPx activities in smokers compared to nonsmokers (reviewed in 56), estimation of selenium intakes might not be a reliable marker of selenium exposure in this population. Also, the chemical forms of selenium found in food are varied (see Sources) and may have very different biological and toxicological effects (57, 58).

A recent Cochrane review included 55 completed observational studies — mostly with a nested case-control design — published over three decades (54). A meta-analysis of 16 of these observational studies, including over 144,000 individuals, reported that higher versus lower selenium status was associated with a 31% lower risk of cancer at any site and a 40% lower risk of cancer-related mortality. A significantly lower risk was reported for bladder cancer (5 studies) and prostate cancer (17 studies); however, higher selenium status was not inversely associated with risks of breast cancer (8 studies), lung cancer (12 studies), colorectal cancer (5 studies), and gastric cancer (5 studies) (54). Another meta-analysis of 16 observational studies reported an inverse relationship between breast cancer and serum selenium concentrations (59). Gender differences in cancer susceptibility have been reported in some studies, although consistent evidence of different effects in men and women appears to be lacking.

Single nucleotide variations (polymorphisms) in the sequence of genes can modify gene expression level and the stability and activity of the synthesized proteins. For example, a proline-to-leucine transition caused by a specific polymorphism in the GPX1 gene (rs1050450 C>T) is associated with reduced GPx1 enzymatic activity. The description of several polymorphisms in genes encoding selenoproteins has led to evaluation of possible associations with selenium status and cancer incidence. Notably, certain polymorphisms in genes coding for selenoproteins have been associated with increased risks of gastric and colorectal cancers (reviewed in 60). Additionally, a number of studies have investigated the effect of selenoprotein polymorphisms on the relationship between selenium status and prostate cancer risk. A nested case-control study within the EPIC-Heidelberg cohort has combined genotyping for several selenoprotein variants with markers of selenium status (61). Briefly, the study showed that a GPX1 gene polymorphism (rs1050450 C>T) affected the association between selenium concentrations and prostate cancer risk. Specifically, selenium concentrations were found to be inversely associated with prostate cancer risk only among carriers of the GPX1 T allele. Additional variants in selenoprotein genes may mitigate the effect of selenium status on the risk of prostate cancer (62, 63). In another nested case-control study within the Physicians' Health Study (PHS), individuals in the highest versus lowest quartile of selenium concentrations were found to have a reduced risk of prostate cancer-related mortality except if carrying a specific variant in 15kDa selenoprotein gene (SEP15 rs561104 G>A) (64). More research is needed to further unravel the mechanisms underlying the influence of gene-diet interactions on the risk of developing cancer.

Intervention trials

Community-based studies: A very early intervention study of selenium supplementation was undertaken in China among a general population of 130,471 individuals living in a high-risk area for viral hepatitis B infection and liver cancer. The trial provided table salt enriched with sodium selenite to the population of one township (20,847 people) using four other townships as controls. During an eight-year follow-up period, the average incidence of liver cancer was reduced by 35% in the selenium-supplemented population, while no reduction was found in the control populations. In a clinical trial in the same region, 226 individuals with evidence of chronic hepatitis B infection were supplemented daily with either 200 μg of selenium in the form of a selenium-enriched yeast tablet or a placebo yeast tablet. During the 4-year follow-up period, 7 out of 113 individuals on the placebo developed primary liver cancer, while none of the 113 subjects supplemented with selenium developed liver cancer (65).

Randomized controlled trials: The double-blind, placebo-controlled Nutritional Prevention of Cancer (NPC) study in 1,312 older adults with a history of nonmelanoma skin cancer found that supplementation with 200 μg/day of selenium-enriched yeast (selenized yeast) for an average of 7.4 years resulted in a 52% decrease in prostate cancer incidence in men (reviewed in 66). The protective effect of selenium supplementation was greatest in men with lower baseline plasma selenium and prostate-specific antigen (PSA) levels. A reduced incidence in lung, colorectal, and total cancer was also associated with supplementation with 200 μg/day (67) but not with 400 μg/day of selenium-enriched yeast (68). Besides, selenium supplementation increased the risk of one type of skin cancer (squamous cell carcinoma) by 25%. A larger randomized, placebo-controlled intervention trial (the SELECT study) in more than 35,000 middle-aged and selenium-replete men, randomized to receive selenium (in the form of selenomethionine, 200 μg/day) and/or vitamin E supplementation, was halted because of concerns regarding an increased risk of type 2 diabetes with selenium and increased risk of prostate cancer with vitamin E (69, 70). In addition, the supplementation of selenium, alone or together with vitamin E, did not show any benefits regarding the risk of prostate, lung, or colorectal cancers after 5.5 years of follow-up (71, 72). Outcomes from other smaller trials (reviewed in 66) also suggested either a lack of an effect or the possibility of an increased risk of cancer. The lack of a beneficial effect of selenium supplementation was supported in a recent meta-analysis of randomized controlled trials (54).

Cardiovascular disease

Low activity levels of the selenoenzymes, glutathione peroxidases (GPx), have been reported in oxidative stress-related diseases, including cardiovascular disease (CVD) (73). Theoretically, the maintenance of an optimal selenium status has the potential to protect against oxidative stress (including lipid peroxidation) and could eventually prevent chronic inflammation and cardiovascular disorders. Yet, analyses of cross-sectional data from 13,887 US adults included in the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994) failed to show any significant associations between serum selenium concentrations and mortality from CVD, coronary artery/heart disease (CAD), or stroke (74). In addition, while individuals with renal insufficiency are at higher risk of developing CAD compared to those with normal kidney function, that risk was not found to be greater with low rather than normal selenium concentrations in serum (≤98 ng/mL vs. >98 ng/mL) (75).

A cross-sectional study based on NHANES 2003-2004 data from 2,638 participants ages 40 years and over found that the risk of high blood pressure (hypertension), a major contributing factor for CVD, was 73% higher in individuals in the upper versus lowest quintile of serum selenium concentrations (≥150 ng/mL vs. <122 ng/mL) (76). Yet, a recent systematic review of the literature failed to find enough evidence to support any relationship between serum selenium concentrations and hypertension (77). Besides, a few observational studies have also reported associations between normal-to-high selenium status and elevated serum lipid levels in selenium-replete populations, speculating that selenium might interfere with lipid metabolism and adversely affect cardiovascular health (78, 79). At present, randomized controlled trials have not provided consistent results regarding the effect of selenium supplementation on lipid levels nor have they demonstrated any additional cardiovascular benefits of selenium in individuals with suboptimal or optimal selenium intakes (80).

Disease Treatment

Immune dysfunction

Selenium deficiency has been associated with impaired immunity and chronic inflammation (81). A considerable amount of research conducted in cell culture and animal models indicates that selenium plays essential roles in regulating the migration, proliferation, differentiation, activation, and optimal function of immune cells, thus influencing innate immunity, B-cell dependent antibody production, and T-cell immunity (reviewed in 82). Recent evidence on the role of selenium and selenoproteins in the production of lipid mediators (called eicosanoids) involved in inflammatory responses suggests that selenium supplementation might mitigate dysfunctional inflammatory responses that contribute to the pathogenesis of many chronic health conditions (83). At present, randomized controlled trials are needed to evaluate the potential benefits of selenium supplementation in inflammatory disorders, such as asthma (84) and inflammatory bowel disease (85).

Infectious diseases

HIV/AIDS

In areas of widespread malnutrition, deficiencies in micronutrients (including selenium) are common in individuals infected by the human immunodeficiency virus (HIV) that causes acquired immunodeficiency syndrome (AIDS). Before antiretroviral therapy (ART) became the standard for HIV treatment, observational studies had consistently reported associations between low serum selenium concentrations and HIV infection in well-nourished subjects (86). Poor selenium status has been linked to increased risks of dilated cardiomyopathy and mortality in HIV-infected children and adults, as well as mother-to-child HIV transmission and perinatal mortality (reviewed in 87. Early laboratory studies have suggested that HIV might disrupt normal antioxidant defenses in infected T-cells by reducing the levels of selenoproteins, i.e., thioredoxine reductases and glutathione peroxidases (87). Interestingly, a recent cross-sectional study found that HIV-seropositive individuals receiving ART for more than two years had undetectable plasma viral loads, higher CD4 lymphocyte T-cell counts, and adequate serum selenium concentrations compared to ART-naïve subjects (88). Because the antioxidant activity of selenoproteins may interfere with viral replication in HIV-infected immune cells (89,90), it has been suggested that selenium supplementation might serve as a potential adjunct to ART for HIV patients.

A few trials of selenium supplementation in HIV-infected individuals have been conducted. A randomized, double-blind, placebo-controlled trial in 186 HIV-positive adults initially found that selenium supplementation at 200 μg/day for two years significantly decreased the rate of hospital admissions (91). Another randomized, double-blind, placebo-controlled trial in 174 HIV-positive individuals reported that 200 μg/day of selenium supplementation (in the form of selenium-enriched yeast) for nine months increased serum selenium concentrations, improved CD4 lymphocyte T-cell count, and prevented any progression of the HIV viral load (92). In a third double-blind trial in Tanzania, 913 pregnant women between 12 and 27 weeks' gestation were randomized to receive 200 μg/day of selenium (as selenomethionine) or a placebo until six months after birth. Selenium supplementation had no effect on maternal CD4, CD8, and CD3 T-cell counts and on HIV viral load, but it significantly decreased the risk of acute or persistent diarrhea (93, 94). In addition, the risk of death between six weeks and six months postpartum was significantly reduced in infants of mothers supplemented with selenium compared to placebo (94).

A recent four-armed trial in Botswana randomized 878 HIV-positive adults at an early stage of the infection to receive either a placebo treatment, multivitamins (vitamins B, C, and E), 200 μg/day of selenium, or both multivitamins and selenium for 24 months (95). Unlike selenium alone, supplementation with multivitamins (with or without selenium) reduced the risk of immune decline by significantly increasing the time before ART initiation became necessary (i.e., when CD4 T-cell count fell below 251 cells/mcL) compared to placebo. In the study, a combined outcome of (1) CD4 T-cell count falling below 251 cells/mcL; (2) occurrence of AIDS-defining conditions; and (3) AIDS-related death — whichever happened first — was used to evaluate disease progression in the different arms of treatment. Compared to placebo, there was a longer period of time from randomization to the date of the composite outcome in individuals supplemented with multivitamins plus selenium, but not in those who received multivitamins or selenium alone (95). More research is needed to replicate these preliminary results, especially in settings and communities where malnutrition hastens the progression of HIV infection and access to antiretroviral therapy may be limited.

Sepsis

The systemic inflammatory response syndrome (SIRS) results from a systemic inflammatory response that can be due to an infection (sepsis) (96). Severe sepsis and septic shock — defined as persistent sepsis-induced low blood pressure — are associated with elevated mortality rates in critically ill patients (96, 97). Because systemic inflammatory responses involve excessive oxidative stress, it has been suggested that providing antioxidant nutrients like selenium may improve the outcome of critically ill patients in intensive care units. Two recent meta-analyses of randomized controlled trials found that intravenous selenium supplementation (as sodium selenite) in critically ill patients with SIRS, sepsis, or septic shock resulted in significantly reducing the risk of mortality by 17% to 27% (98, 99). More trials are needed to identify the appropriate schedule of selenium administration (in terms of dose, route, and treatment duration) and to assess additional outcomes (e.g., infectious complications and length of hospital stay).

Autoimmune thyroid diseases

Hashimoto thyroiditis (HT; chronic autoimmune thyroiditis) is an autoimmune disease characterized by T-cell infiltration in the thyroid gland and circulating autoantibodies (predominantly raised against thyroid peroxidase), causing prolonged inflammation, tissue damage, and hypothyroidism (8). While the function of the thyroid gland of healthy individuals is usually protected from variations in selenium supply, selenium deficiency and genetic polymorphisms affecting the activity of selenoproteins might be potential contributing factors to autoimmune thyroid diseases. A recent systematic review (100) identified four randomized controlled trials that evaluated the effect of selenium supplementation as an adjunct treatment to T4 replacement therapy (levothyroxine) in HT patients (101-104). While three out of four studies suggested a reduction in levels of circulating autoantibodies, none of them provided information on whether selenium may improve mood- and health-related symptoms to allow for a decreased dosage of levothyroxine. Another randomized controlled trial found that selenium supplementation improved the well being of patients affected by another autoimmune thyroid disease leading to hyperthyroidism (Graves disease) (105). The results of two ongoing, randomized, placebo-controlled trials — the CATALYST in HT patients and the GRASS trial in patients with Graves disease — may provide insight into an effect of selenium on thyroid-specific quality-of-life criteria and inform clinical decision making (106, 107).

Sources

Food sources

The richest food sources of selenium are organ meats and seafood, followed by muscle meats. Drinking water is not considered to be a significant source of selenium in North America. However, in areas, where high levels of selenium in soil contribute to the selenium content of the water, higher levels of selenium may be found in wells used for drinking water (108). In general, there is wide variation in the selenium content of plants and grains, especially because some plants, including garlic, Brazil nuts, and multiple Brassica species, tend to accumulate selenium ('selenium accumulators'), while other assimilate selenium to a lesser extent ('non-accumulators'). The assimilation of selenium by plants also depends on soil selenium content. Brazil nuts grown in areas of Brazil with selenium-rich soil may provide more than 100 μg of selenium in one nut, while those grown in selenium-poor soil provide 10 times less (109). In the US, grains are a good source of selenium, but fruit and vegetables tend to be relatively poor in selenium.

Various chemical forms (species) of selenium are found in selenium accumulators, including selenate (inorganic selenium), selenomethionine, selenocysteine, selenium-methyl-selenocysteine, and γ-glutamyl-selenium-methyl-selenocysteine. Although the two latter compounds are predominant in plants of the Allium and Brassicaceae families (which include garlic, onion, and broccoli), wheat, other grains (including Brazil nuts), and soy are rich in selenomethionine and contain smaller amounts of selenocysteine and selenate. Less is known about selenium species and distribution in dietary sources of animal origin. Animal nutrition and growth conditions certainly contribute to the selenium species formed and their relative quantities, and it is assumed that the metabolic pathway of dietary selenium in animals is similar to that in humans. Selenocysteine is predominantly formed in animals fed inorganic selenium, while selenomethionine is derived from dietary sources of plant origin (reviewed in 110).

In the US, the national survey NHANES III reported mean dietary intakes ranging between 100.5 μg/day and 158.5 μg/day in adults ages 19-50 years (52). Table 2 lists some good food sources of selenium and their selenium content in micrograms (μg). For more information on the selenium content of specific foods, search USDA's FoodData Central.

Table 2. Some Common Food Sources of Selenium
Food Serving Selenium (μg)
Brazil nuts (from selenium-rich soil) 1 ounce (6 kernels) 543.5*
Tuna (yellowfin, cooked, dry heat) 3 ounces 92.0
Oysters (Pacific, raw) 3 ounces 65.4
Clams (mixed, cooked, steamed) 3 ounces 54.4
Halibut (Atlantic and Pacific, cooked, dry heat) 3 ounces 47.1
Shrimp (cooked, steamed) 3 ounces 42.1
Salmon (Chinook, cooked, dry heat) 3 ounces 39.8
Noodles (egg, cooked, enriched) 1 cup 38.2
Crab (queen, cooked, steamed) 3 ounces 37.7
Pork (lean, tenderloin, cooked, roasted) 3 ounces 32.5
Beef (lean, plate steak, cooked, grilled) 3 ounces 30.6
Chicken (light-meat, cooked, roasted) 3 ounces 25.8
Rice (brown, long-grain, cooked) 1 cup 19.1
Sunflowers seed kernels (dried) ¼ cup 18.6
Whole-wheat bread 2 slices 16.4
Milk (fat free or skim) 8 fluid ounces (1 cup) 7.6
*Above the tolerable upper intake level (UL) of 400 μg/day.

Supplements

Selenium supplements are available in several forms. Sodium selenite and sodium selenate are inorganic forms of selenium. Sodium selenate is almost completely absorbed, but a significant amount is excreted in the urine before it can be incorporated into proteins. Sodium selenite is only about 50% absorbed but is better retained than selenate once it is absorbed. Selenomethionine is about 90% absorbed (52); however, only about 34% may then actually be converted to free selenomethionine (111). Selenomethionine and selenium-enriched yeast, which mainly supply selenomethionine, are also available as supplements. The consumer should be aware that some forms of selenium-enriched yeast on the market contain yeast plus mainly inorganic forms of selenium.

Although both inorganic and organic forms of selenium can be metabolized to selenocysteine by the body and incorporated into selenoenzymes, they may not equally contribute to the maintenance of an adequate selenium status. In intervention trials, supplementation with an organic form of selenium (selenomethionine) more effectively increased blood selenium concentrations compared to supplementation with inorganic forms (i.e., sodium selenite and sodium selenate) (110). Yet, inorganic forms may increase plasma glutathione peroxidase (GPx) activity more effectively than organic forms (reviewed in 112). It has also been suggested that the incorporation of selenomethionine in place of methionine into tissue proteins may ensure that selenium is available upon protein turnover (110).

Selenium-enriched foods

Selenium-enriched foods have been of interest to scientists, especially because of the suggestion that some of the chemical forms of selenium produced by plants might be more potent modifiers of cancer risk than those currently available in supplements. Although there is currently no evidence of long-term health benefits associated with the consumption of selenium-enriched foods, results from animal studies and intervention trials suggest that protein-based sources of selenium are more effective at increasing GPx activity than selenium-enriched yeast and selenomethionine (112). Food fortification may also represent a cost-effective strategy to improve selenium nutritional status in populations at risk of inadequacy (113).

Safety

Toxicity

Although selenium is required for health, high doses of selenium can be toxic. Acute and fatal toxicities have occurred with accidental or suicidal ingestion of gram quantities of selenium. Clinically significant selenium toxicity was reported in 13 individuals after taking supplements that contained 27.3 mg (27,300 μg) per tablet due to a manufacturing error. Chronic selenium toxicity (selenosis) may occur with smaller doses of selenium over long periods of time. The most frequently reported symptoms of selenosis are hair and nail brittleness and loss. Other symptoms may include gastrointestinal disturbances, skin rashes, a garlic breath odor, fatigue, irritability, and neurologic disorders. In an area of China with a high prevalence of selenosis, toxic effects occurred with increasing frequency when blood selenium concentrations reached a level corresponding to an intake of 850 μg/day.

The Food and Nutrition Board (FNB) of the US Institute of Medicine set the tolerable upper intake level (UL) for selenium at 400 μg/day in adults based on the prevention of hair and nail brittleness and loss and early signs of chronic selenium toxicity (52). The UL of 400 μg/day for adults includes both selenium obtained from food and selenium from supplements (Table 3).

Table 3. Tolerable Upper Intake Level (UL) for Selenium
Age Group UL (μg/day)
Infants 0-6 months 45
Infants 6-12 months 60
Children 1-3 years 90
Children 4-8 years 150
Children 9-13 years 280
Adolescents 14-18 years 400
Adults 19 years and older 400

Do selenium supplements increase the risk for type 2 diabetes mellitus?

A few studies have examined the relationship between selenium status and type 2 diabetes mellitus. In the cross-sectional analysis of NHANES III (1988-1994) data from 8,876 adult participants, the highest versus lowest quintile of serum selenium concentrations (≥137 ng/mL vs. <111 ng/mL) was associated with an increased risk of type 2 diabetes (114). Data analyses from 917 participants (≥40 years of age) of NHANES 2003-2004 also indicated an increased prevalence of type 2 diabetes in the highest versus lowest quartile of serum selenium concentrations (≥147 ngm/L vs. <124 ng/mL). Individuals in the highest versus lowest quartile of serum selenium concentrations also had higher levels of plasma glucose and glycated hemoglobin, suggestive of poor glycemic control (115). The randomized, double-blind, placebo-controlled study in 1,312 participants in the Nutritional Prevention of Cancer (NPC) trial found that selenium supplementation (200 μg/day; mean follow-up of 7.7 years) significantly increased the risk for type 2 diabetes in participants in the highest tertile of baseline plasma selenium concentrations (116). In addition, in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), more cases of type 2 diabetes were found in the selenium group (200 μg/day; median follow-up of 5.5 years) than in the placebo group, but this was only a trend and not statistically significant (72).

At present, the mechanisms behind these observations are not well understood. An increase in insulin sensitivity has been reported in individuals with congenital (inborn) deficiency of most selenoproteins (117). Results from several animal studies also indicated that selenium supplementation and selenoproteins may interfere with insulin action and glucose homeostasis (reviewed in 118). On the other hand, recent studies have found that impaired glucose metabolism in patients with type 2 diabetes may affect SEPP1 expression and selenium homeostasis (11, 119, 120). While more research is needed to fully understand the interplay between carbohydrate metabolism and selenium homeostasis, the use of selenium supplements is considered unlikely to increase the risk of type 2 diabetes in healthy individuals but should be discouraged in those with high selenium status and/or at increased risk for developing type 2 diabetes (118).

Drug interactions

At present, few interactions between selenium and medications have been reported (121). For example, the anticonvulsant drug valproic acid and the chemotherapeutic agent cisplatin may lower circulating selenium concentrations in treated subjects (122, 123). Also, supplemental sodium selenite was found to reduce the toxicity of the antibiotic nitrofurantoin and the herbicide paraquat in animal studies (124).

Antioxidant supplements and statins

A three-year randomized controlled trial in 160 patients with documented coronary artery disease (CAD) and low high-density-lipoprotein (HDL) levels found that a combination of simvastatin (Zocor) and niacin increased HDL2 subfraction levels, inhibited the progression of coronary artery stenosis (narrowing), and decreased the frequency of cardiovascular events (125). Surprisingly, when an antioxidant combination (1,000 mg of vitamin C, 800 IU of vitamin E (d-α-tocopherol), 100 μg of selenium, and 25 mg of β-carotene daily) was taken with the simvastatin-niacin combination, the protective effects were diminished. However, the individual contribution of selenium cannot be established, and other studies have reported that antioxidant vitamins alone could interfere with the action of HDL-raising drugs, including statins (126).

Linus Pauling Institute Recommendation

The average American diet is estimated to provide about 100 μg/day of selenium, an amount that is well above the current RDA (55 μg/day) and appears sufficient to optimize plasma and cellular glutathione peroxidase (GPx) activity. A recent 10-week randomized controlled study in healthy British adults (ages, 50-64 years) estimated that about 105 μg/day of total selenium intake was required to maximize the plasma concentrations of selenoprotein P (SEPP1), another useful biomarker of selenium status (127). However, a similar trial in an American cohort with higher baseline plasma selenium concentrations found no effect of selenium supplementation on SEPP1 concentrations (128). While the amount of selenium in multivitamin/mineral (MVM) supplements varies considerably, MVM supplements rarely provide more than the Daily Value (DV) of 70 μg. Eating a varied diet and taking a daily MVM supplement should provide sufficient selenium for most people in the US and help improve selenium status in populations with lower selenium intakes outside the US.

Men

At present, the effect of selenium supplementation on cancer risk is not clear enough to support a general recommendation for an extra selenium supplement, especially in men with serum selenium concentrations consistent with adequate selenium intakes. The SELECT trial found that 200 μg/day of selenium did not reduce the risk of prostate cancer (72), refuting the results of the NPC trial (see Cancer). Another recent multicenter, randomized, double-blind, placebo-controlled trial (the Negative Biopsy Trial) in 699 men at high risk of prostate cancer found no effect of either 200 μg/day or 400 μg/day of selenium on prostate cancer risk during a mean follow-up of 36 months (129). In addition, because current evidence suggests a U-shaped relationship between selenium status and prostate cancer risk (130), men should avoid taking supplemental selenium that would exceed 200 μg/day.

Women

At present, there is no clinical evidence showing that selenium supplementation above recommended levels decreases the risk of breast cancer although some, but not all, observational studies have found an inverse relationship between selenium status and breast cancer in women (59).

Older adults (>50 years)

Aging has not been associated with significant changes in the requirement for selenium. A five-year, randomized, double-blind, placebo-controlled trial in healthy Danish older people (ages at inclusion, 60-74 years) found that selenium supplementation (100-300 μg/day) had little-to-no impact on circulating levels of antioxidant enzymes, including GPx (131).

Authors and Reviewers

Originally written in 2001 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in October 2003 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in November 2007 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in November 2014 by:
Barbara Delage, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in June 2015 by:
Petra A. Tsuji, Ph.D., M.P.H.
Assistant Professor
Department of Biological Sciences
Towson University

Copyright 2001-2021  Linus Pauling Institute

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Vitamin C And Selenium

Source: https://lpi.oregonstate.edu/mic/minerals/selenium