Human metabolism and renal excretion of selenium compounds after oral ingestion of sodium selenate dependent on trimethylselenium ion (TMSe) status

Relative bioavailability of selenium in rice using a rat model and its application to human health risk assessment

Deficiencies of selenium (Se), a necessary microelement for humans, can be remedied by appropriately supplying Se-enriched rice. However, overconsumption of Se-enriched rice poses a potential risk. To accurately assess Se human health risks associated with Se-enriched rice consumption, we developed a rat in vivo model to systematically explore the relative bioavailability of Se (Se-RBA) from Se-enriched rice from a wide geographic range. Se concentrations were in the range of 0.06 ± 0.05 to 0.15 ± 0.15 mg kg-1, averaging 0.12 ± 0.11 mg kg-1, in 196 rice samples from 21 Chinese provinces, and selenomethionine (SeMet) was the dominant Se fraction (58.0-96.5%). The Se-RBA of Se-enriched rice calculated from urine ranged from 34.86% to 102.29%, averaging 62.27% (n = 12), and was positively correlated with the proportion of SeMet in rice (p < 0.05, R2 = 0.51). Furthermore, the Se intake calculated based on the Se-RBA indicated that the Se intake of consumers of Se-enriched rice was far less than the tolerable upper intake level. Thus, the limits established by law assume overestimates of the actual nutritional value of the Se content in Se-enriched rice, and it is important to consider Se bioavailability. The current study offers suggestions for future research and provides methods to reduce the uncertainty in estimating the health risks associated with Se intake from rice.

Selenium intake and multiple health-related outcomes: an umbrella review of meta-analyses

Selenium is an essential trace metalloid element that is associated with fundamental importance to human health. Our umbrella review aimed to evaluate the quality of evidence, validity, and biases in the relationship between selenium intake and health-related outcomes according to published systematic reviews with pooled data and meta-analyses. Selenium intake is associated with a decreased risk of digestive system cancers, all-cause mortality, depression, and Keshan disease, when in children reduce the risk of Kashin-Beck disease. Additionally, selenium supplementation can improve sperm quality, polycystic ovary syndrome, autoimmune thyroid disease, cardiovascular disease, and infective outcomes. Selenium supplementation also has relationship with a decreased concentration of serum lipids including total cholesterol and very low-density lipoprotein cholesterol. However, no evidence has shown that selenium is associated with better outcomes among patients in intensive care units. Furthermore, selenium intake may be related with a higher risk of type 2 diabetes and non-melanoma skin cancers. Moreover, most of included studies are evaluated as low quality according to our evidence assessment. Based on our study findings and the limited advantages of selenium intake, it is not recommended to receive extra supplementary selenium for general populations, and selenium supplementation should not be continued in patients whose selenium-deficient status has been corrected.

Human serum albumin-bound selenium (Se-HSA) in serum and its correlation with other selenium species

INTRODUCTION

Selenium (Se) is a trace element with different toxicological and nutritional properties according to its chemical forms. Among the wide range of selenium species, human serum albumin-bound selenium (Se-HSA) has still uncertain composition in terms of organic or inorganic selenium species. This study aimed at investigating the relation between Se-HSA levels with total selenium and the specific organic and inorganic selenium species.

METHODS

We determined levels of total selenium and selenium species in serum of participants enrolled in two populations of the Emilia-Romagna region, in Northern Italy. Anion exchange chromatography coupled with inductively coupled plasma dynamic reaction cell mass spectrometry was used as quantification method. Correlations between Se-HSA and the other selenium compounds were analyzed using linear regression and restricted cubic spline regression models, adjusted for potential confounders.

RESULTS

The first cohort comprised 50 participants (men/women: 26/24) with median (interquartile range, IQR) age 50 (55-62) years, while the second was composed of 104 participants (M/W: 50/54), median (IQR) age 48 (44-53) years. Median (IQR) levels of total selenium were 118.5 (109-136) µg/L and 116.5 (106-128) µg/L, respectively, while Se-HSA was 25.5 µg/L (16.2-51.5) and 1.1 (0.03-3.1) µg/L, respectively. In both populations, Se-HSA was positively associated with inorganic selenium species. Conversely, Se-HSA was inversely associated with organic selenium, especially with selenoprotein P-bound-Se (Se-SELENOP) and less strongly with selenomethionine-bound-Se (Se-Met), while the relation was null or even positive with other organic species. Evaluation of non-linear trends showed a substantially positive association with inorganic selenium, particularly selenite, until a concentration of 30 µg/L, above which a plateau was reached. The association with Se-SELENOP was inverse and strong until 100 µg/L, while it was almost null at higher levels.

CONCLUSIONS

Our findings seem to indicate that Se-HSA incorporates more selenium when circulating levels of inorganic compounds are higher, thus supporting its mainly inorganic nature, particularly at high circulating levels of selenite.

A time-course investigation of the human urinary excretion of the hydrogen sulfide biomarker trimethylsulfonium

Hydrogen sulfide is a toxic gas but also recognized as an endogenously produced metabolite in humans playing key roles. We previously identified trimethylsulfonium, which can be a methylation product of hydrogen sulfide but the stability in the production of trimethylsulfonium has not been investigated. In the present work, the intra- and inter-individual variability in the excretion of trimethylsulfonium over 2 months in a group of healthy volunteers was investigated. Urinary levels of trimethylsulfonium (mean: 56 nM, 95% CI: 48-68 nM) were > 100-fold lower than the conventional hydrogen sulfide biomarker thiosulfate (13 µM, 12-15 µM) and the precursor for endogenous hydrogen sulfide production cystine (47 µM, 44-50 µM). There was no correlation between urinary trimethylsulfonium and thiosulfate. Higher intra-individual variability in the excretion of trimethylsulfonium (generally 2-8 fold) than that for cystine (generally 2-3 fold) was found. Trimethylsulfonium displayed significant inter-individual variability with two concentration clusters at 117 nM (97-141) and 27 nM (22-34). In conclusion, the observed inter- and intra-individual variability must be considered when using urinary trimethylsulfonium as a biomarker.

Associations of selenium exposure with blood lipids: Exploring mediating DNA methylation sites in general Chinese urban non-smokers

Selenium (Se) is widely distributed in the total environment and people are commonly exposed to Se, while the potential effects and mechanisms of Se exposure on blood lipids have not been well established. This study aimed to assess the associations of urinary Se (SeU) with blood lipids and explore the potential mediating DNA methylation sites. We included 2844 non-smoke participants from the second follow-up (2017-2018) of the Wuhan-Zhuhai cohort (WHZH) in this study. SeU and blood lipids [i.e., total cholesterol (TC), triglycerides (TG), low-density lipoprotein cholesterol (LDL), and high-density lipoprotein cholesterol (HDL)] for all participants were determined. The associations of SeU with blood lipids were analyzed by generalized linear models. Then, we conducted the blood lipids related epigenome-wide association studies (EWAS) among 221 never smokers, and the mediation analysis was conducted to explore the potential mediating cytosine-phosphoguanine (CpG) sites in the above associations. In this study, the SeU concentration of the participants in this study was 1.40 (0.94, 2.08) μg/mmol Cr. The SeU was positively associated with TC and LDL, and not associated with TG and HDL. We found 131, 3, and 1 new CpG sites related to TC, HDL, and LDL, respectively. Mediation analyses found that the methylation of cg06964030 (within MIR1306) and cg15824094 (within PLCH2) significantly mediated the positive association between SeU and TC. In conclusion, high levels of Se exposure were associated with increased TC and LDL among non-smokers, and the methylation of MIR1306 and PLCH2 partly mediated Se-associated TC increase. These findings provide new insights into the effects and mechanisms of Se exposure on lipids metabolism and highlight the importance of controlling Se exposure and intake for preventing high blood lipids.

Bioconversion of inorganic selenium to less toxic selenium forms by microbes: A review

In recent years, microbial conversion of inorganic selenium into an efficient and low-toxic form of selenium has attracted much attention. With the improvement of scientific awareness and the continuous progress of nanotechnology, selenium nanoparticles can not only play the unique functions of organic selenium and inorganic selenium but also have higher safety, absorption and biological activity than other selenium forms. Therefore, the focus of attention has gradually shifted beyond the level of selenium enrichment in yeast to the combination of biosynthetic selenium nanoparticles (BioSeNPs). This paper primarily reviews inorganic selenium and its conversion to less toxic organic selenium and BioSeNPs by microbes. The synthesis method and potential mechanism of organic selenium and BioSeNPs are also introduced, which provide a basis for the production of specific forms of selenium. The methods to characterize selenium in different forms are discussed to understand the morphology, size and other characteristics of selenium. In general, to obtain safer and higher selenium content products, it is necessary to develop yeast resources with higher selenium conversion and accumulation.

Scientific opinion on the tolerable upper intake level for selenium

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the tolerable upper intake level (UL) for selenium. Systematic reviews of the literature were conducted to identify evidence regarding excess selenium intake and clinical effects and potential biomarkers of effect, risk of chronic diseases and impaired neuropsychological development in humans. Alopecia, as an early observable feature and a well-established adverse effect of excess selenium exposure, is selected as the critical endpoint on which to base a UL for selenium. A lowest-observed-adverse-effect-level (LOAEL) of 330 μg/day is identified from a large randomised controlled trial in humans (the Selenium and Vitamin E Cancer Prevention Trial (SELECT)), to which an uncertainty factor of 1.3 is applied. A UL of 255 μg/day is established for adult men and women (including pregnant and lactating women). ULs for children are derived from the UL for adults using allometric scaling (body weight0.75). Based on available intake data, adult consumers are unlikely to exceed the UL, except for regular users of food supplements containing high daily doses of selenium or regular consumers of Brazil nuts. No risk has been reported with the current levels of selenium intake in European countries from food (excluding food supplements) in toddlers and children, and selenium intake arising from the natural content of foods does not raise reasons for concern. Selenium-containing supplements in toddlers and children should be used with caution, based on individual needs.

Metabolism and Anticancer Mechanisms of Selocompounds: Comprehensive Review

Selenium (Se) is an essential micronutrient with several functions in cellular and molecular anticancer processes. There is evidence that Se depending on its chemical form and the dosage use could act as a modulator in some anticancer mechanisms. However, the metabolism of organic and inorganic forms of dietary selenium converges on the main pathways. Different selenocompounds have been reported to have crucial roles as chemopreventive agents, such as antioxidant activity, activation of apoptotic pathways, selective cytotoxicity, antiangiogenic effect, and cell cycle modulation. Nowadays, great interest has arisen to find therapies that could enhance the antitumor effects of different Se sources. Herein, different studies are reported related to the effects of combinatorial therapies, where Se is used in combination with proteins, polysaccharides, chemotherapeutic agents or as nanoparticles. Another important factor is the presence of single nucleotide polymorphisms in genes related to Se metabolism or selenoprotein synthesis which could prevent cancer. These studies and mechanisms show promising results in cancer therapies. This review aims to compile studies that have demonstrated the anticancer effects of Se at molecular levels and its potential to be used as chemopreventive and in cancer treatment.

Absorption, distribution, metabolism and excretion (ADME) of oral selenium from organic and inorganic sources: A review

BACKGROUND

Selenium is a trace element traditionally ingested either in its organic form via food or in its inorganic form through nutritional supplements, while selenium formulated as nanoparticles is a putative long-acting alternative. To understand the physiology and toxicology of the different selenium formulations, it is important to determine how their selenium content is absorbed, distributed, metabolised and excreted; therefore, we reviewed their biokinetics following oral exposure.

METHODS

We retrieved and reviewed the literature on the absorption, distribution, metabolism, and excretion of oral exposure to different forms of selenium.

RESULTS

Selenium in both the organic form (containing carbon to selenium chemical bonds) and the inorganic form is absorbed into the blood in humans. The mean normal blood level of many studies was 139 μg/L. There are indications that selenium from organic sources is more bioavailable than selenium from inorganic sources. Selenium is distributed throughout the body, including in breast milk. The elimination of selenium mainly involves the faecal and urinary pathways, whereas breath, saliva and hair are minor contributors. Urinary metabolites include trimethylselenium ions, selenosugars and Se-methylselenoneine.

CONCLUSION

Selenium is absorbed to a high extent, and selenium from organic sources is more bioavailable than from inorganic sources. Selenium, as expected as an essential trace element, is distributed throughout the body. Selenium is extensively metabolised, and various excretion metabolites have been identified in both urine and breath, while some selenium is also excreted via faeces.

Selenium in Human Health and Gut Microflora: Bioavailability of Selenocompounds and Relationship With Diseases

This review covers current knowledge of selenium in the dietary intake, its bioavailability, metabolism, functions, biomarkers, supplementation and toxicity, as well as its relationship with diseases and gut microbiota specifically on the symbiotic relationship between gut microflora and selenium status. Selenium is essential for the maintenance of the immune system, conversion of thyroid hormones, protection against the harmful action of heavy metals and xenobiotics as well as for the reduction of the risk of chronic diseases. Selenium is able to balance the microbial flora avoiding health damage associated with dysbiosis. Experimental studies have shown that inorganic and organic selenocompounds are metabolized to selenomethionine and incorporated by bacteria from the gut microflora, therefore highlighting their role in improving the bioavailability of selenocompounds. Dietary selenium can affect the gut microbial colonization, which in turn influences the host's selenium status and expression of selenoproteoma. Selenium deficiency may result in a phenotype of gut microbiota that is more susceptible to cancer, thyroid dysfunctions, inflammatory bowel disease, and cardiovascular disorders. Although the host and gut microbiota benefit each other from their symbiotic relationship, they may become competitors if the supply of micronutrients is limited. Intestinal bacteria can remove selenium from the host resulting in two to three times lower levels of host's selenoproteins under selenium-limiting conditions. There are still gaps in whether these consequences are unfavorable to humans and animals or whether the daily intake of selenium is also adapted to meet the needs of the bacteria.

Instability of urinary excreted methyl-2-acetamido-2-deoxy-1-seleno-β-d-galactopyranoside (selenosugar 1), the main elimination product of human selenium metabolism, and measures for its stabilization

BACKGROUND

The urinary excreted selenium species selenosugar 1 (SeSug1) plays a key role for monitoring of supplemental selenium exposure, e.g. by occupational exposure. In order to reproduce its contents in the long term, the integrity of SeSug1 in the urine is essential. Studies on the stability of SeSug1 in urine samples stored at -20 °C have shown that degradation of SeSug 1 occurs, requiring adequate countermeasures.

METHODS

Here, we explored the long-term stability of SeSug1 under usual storage conditions at -20 °C. For this purpose, the simultaneous determination of selenosugar 1 and methylselenic acid (MeSeA) was used to explore the stabilizing of the SeSug1 content by applying sodium azide (NaN3) as a bactericide or/and 5 M ammonium acetate buffer for pH control.

RESULTS

In untreated urine, conversion of SeSug1 to MeSeA was evident within days. Differences in urine matrices clearly showed different impact, which could be attributed to different buffer strengths by the urine itself. For durability, various concentrations of sodium azide were first applied, followed by pH buffering. A combination of 0.1% NaN3 and pH of 5.5 kept the SeSug1 content stable for over 3 months.

CONCLUSION

The formation of MeSeA as degradation product of SeSug1 could be confirmed. Based on the proportions, an oxidation-based decomposition pathway was proposed. The investigations revealed that the complex interaction of pH buffering and bactericidal activity must be taken into account in order to stabilize SeSug1 in the urine. The main effect was the addition of NaN3. However, the alkaline nature of NaN3 required a sufficient buffering of the urinary matrix at a pH of 5.5.

Determination of eleven small selenium species in human urine by chromatographic-coupled ICP-MS methods

BACKGROUND

The determination of various selenium species in urine enables a specific biomonitoring of the exposure to different selenium compounds.

METHODS

For this task a coupling of three chromatographic techniques with ICP-MS was developed for the separate quantification of eleven species in urine. The first procedure was based on reverse phase chromatography and was designed for the separate determination of methyl-2-acetamido-2-deoxy-1-seleno-b-d-galactopyranoside (SeSug1), methyl-2-acetamido-2-deoxy-1-seleno-b-d-glucopyranoside (SeSug2), selenomethionine (SeMet), methylselenocysteine (MeSeC), seleno-D,L-ethionine (SeEt), methylselenic acid (MeSeA) and methylselenoglutathione (MeSeG); the second procedure was based on anion exchange chromatography and measured selenate (Se (VI)) and selenite (Se (IV)); the third procedure was based on cationic exchange chromatography and determined methyl-2-amino-2-deoxy-1-seleno-b-d-galactopyranoside (SeSug3) and the trimethylselenium ion (TMSe). A fourth method for the more sensitive determination of TMSe was upgraded by an on-line after-column reaction process.

RESULTS

The validation of the methods yielded sensitive detection limits of the species between 0.03 and 0.10 μg Se/L. For TMSe a detection limit of 0.02 μg Se/L resulted by the fourth method. An intra-day precision of 2.7-10.6% and a relative recovery between 87 % and 108 % confirm the robustness of the methods.

CONCLUSION

The developed procedures enable a separate and sensitive determination of eleven selenium species in urine and thus permit the exploring of metabolic factors in the general population and particularly exposed individuals.

Evaluation of urinary selenium as a biomarker of human occupational exposure to elemental and inorganic selenium

PURPOSE

Selenium (Se) is an essential trace element, which however, exhibits a narrow safe range of intake. Selenium also occurs at several workplaces, which results in an inhalative selenium exposure of the employees. Thus, an efficient exposure assessment strategy is demanded. The most established parameter, selenium in plasma, mostly consists of protein-bound selenium. This study aimed to investigate urinary total selenium (Se-U) as an additional biomarker of recent human occupational exposure to elemental and inorganic selenium.

METHODS

Pre- and post-shift urine samples from employees with exposure to selenium-containing dust were analyzed to total selenium and compared with Se levels in urine samples from controls without occupational exposure to selenium as well as correlated with the recent ambient Se exposure by personal air monitoring.

RESULTS

Se-U in post-shift samples was considerably increased compared to the levels in pre-shift samples as well as to the controls. However, Se-U in pre-shift urine was elevated compared to the Se-U in controls too. Se-U in post-shift urine and even better the shift increment in Se-U correlated with the air exposure of the present shift. A rough estimation by Se-U shift increment and external exposure accounted for an inhalative resorption rate of about 1%.

CONCLUSION

Our data indicate that Se-U can display the exposure. Pre-shift Se-U levels may be based on previous exposure and indicate a slow urinary elimination kinetics. The results hint for a rather low resorption rate of selenium and inorganic selenium compounds via the lung.

Supranutritional Sodium Selenate Supplementation Delivers Selenium to the Central Nervous System: Results from a Randomized Controlled Pilot Trial in Alzheimer's Disease

Insufficient supply of selenium to antioxidant enzymes in the brain may contribute to Alzheimer's disease (AD) pathophysiology; therefore, oral supplementation may potentially slow neurodegeneration. We examined selenium and selenoproteins in serum and cerebrospinal fluid (CSF) from a dual-dose 24-week randomized controlled trial of sodium selenate in AD patients, to assess tolerability, and efficacy of selenate in modulating selenium concentration in the central nervous system (CNS). A pilot study of 40 AD cases was randomized to placebo, nutritional (0.32 mg sodium selenate, 3 times daily), or supranutritional (10 mg, 3 times daily) groups. We measured total selenium, selenoproteins, and inorganic selenium levels, in serum and CSF, and compared against cognitive outcomes. Supranutritional selenium supplementation was well tolerated and yielded a significant (p < 0.001) but variable (95% CI = 13.4-24.8 μg/L) increase in CSF selenium, distributed across selenoproteins and inorganic species. Reclassifying subjects as either responsive or non-responsive based on elevation in CSF selenium concentrations revealed that responsive group did not deteriorate in Mini-Mental Status Examination (MMSE) as non-responsive group (p = 0.03). Pooled analysis of all samples revealed that CSF selenium could predict change in MMSE performance (Spearman's rho = 0.403; p = 0.023). High-dose sodium selenate supplementation is well tolerated and can modulate CNS selenium concentration, although individual variation in selenium metabolism must be considered to optimize potential benefits in AD. The Vel002 study is listed on the Australian and New Zealand Clinical Trials Registry ( http://www.anzctr.org.au /), ID: ACTRN12611001200976.

The thioredoxin system as a target for mercury compounds

BACKGROUND

Mercury interaction with selenium in vivo has been recognized for >50 years. Several researchers attempted to use selenium to mitigate the detrimental effects of mercurial compounds but the results were controversial. Selenium pools in living organisms are quite low and the high affinity of mercury to bind selenols pointed out selenoproteins as possible targets of toxicity. Such was the case of the selenoenzyme thioredoxin reductase (TrxR) which is an integrant part of the thioredoxin system. Given the important role of this redox system for cellular functioning and the high affinity of mercury for TrxR's active site, this interaction can be key to understand the mechanism by which Hg causes cell death.

SCOPE OF THE REVIEW

This review discusses the current state of knowledge concerning the interaction between mercury compounds and the thioredoxin system, its implications for the development of toxicity and the effects of selenium co-exposure.

MAJOR CONCLUSIONS

The mechanism of toxicity of mercurials is a complex chain of events starting with inhibition of the selenoenzyme, TrxR. Selenium supplementation protects TrxR from the toxicity of inorganic forms of mercury (i.e., Hg(II)) to a certain extent, but not from methylmercury. When TrxR is inhibited, thioredoxin is reduced by alternative mechanisms involving glutathione and glutaredoxin and only when this pathway is hampered does cell death occur.

GENERAL SIGNIFICANCE

Understanding the molecular mechanism of mercury toxicity and the mechanisms of enzymatic compensation allows the design of mitigation strategies and, since TxrR and Trx exist in the plasma, puts forward the possibility for future use of changes in activity/expression of these enzymes as biomarkers of mercury toxicity, thus refining the risk assessment process.

Low internal exposure and absence of adverse effects in workers exposed to high air levels of inorganic selenium

Selenium is an essential trace element for humans, but adverse health effects may occur after elevated intake. The margin between it is small. This study aimed to assess external and internal exposure in workers of a selenium-processing plant, in which elemental and inorganic selenium occurred. Selenium was analyzed in the form of the selenium concentration in plasma (Se-P), in erythrocytes (Se-RBC) and in personal air samples (Se_total-Air) of 17 exposed workers. Internal exposure was compared to 20 controls without occupational selenium exposure. For potential effects, glucose, HbA1c, proinsulin, prothrombin time and GPX were determined. Se_total-Air had a maximum of 2394 μg/m³ (median 319 μg/m³), containing a small water-soluble fraction (median 12.7 μg/m³, range 0.07-975 μg/m³). Se-P of the exposed ranged from 62 to 123 μg/L (median 105 μg/L), whereas the median of Se-RBC was 63.4 μg/L blood (range 51.9-92.7 μg/L). Both were significantly higher than the controls. No significant difference was found for the effect parameters. Biological effect monitoring of employees occupationally exposed to very high levels of selenium and inorganic selenium compounds did not show any indication of adverse health effects. The moderate increase of the internal selenium exposure compared to the high ambient exposure to selenium and its compounds suggests an efficient air protection or an extremely low resorption of elemental and inorganic species of selenium via inhalation.

Selenoprotein N Was Required for the Regulation of Selenium on the Uterine Smooth Muscle Contraction in Mice

Selenium (Se) is an essential micronutrient affecting various aspects of health. The balance of the Se concentration has an important protective and promoter effect on physiological function in inducing muscular disorders in smooth muscle. Selenoprotein N (SelN) is closely related to Ca²⁺ release. The present study aimed to determine the effects and mechanism of action of dietary Se on uterine smooth muscle contraction via SelN using a mouse model. Quantitative polymerase chain reaction (qPCR) analysis was performed to detect mRNA levels. Western blotting was performed to detect protein levels. The results of the immunohistochemical analysis showed that Se had an effect on the uterine smooth muscle. The Se-supplement increased the release of Ca²⁺, Ca²⁺-calmodulin (CaM) expression, myosin light chain kinase (MLCK) expression, and myosin light chain (MLC) phosphorylation but did not affect ROCK and RhoA in uterine smooth muscle. Furthermore, the lack of Se showed an opposite impact. The effects of Se regulation were closely related to SelN. The interference of mouse SelN was performed on the uterine smooth muscle cell. Additionally, the results displayed the regulation of Se on the release of Ca²⁺, CaM expression, MLCK expression, and MLC phosphorylation were significant inhibited, and there was no effect on ROCK and RhoA. In conclusion, Se played an important role in regulating the process of contraction in uterine smooth muscle with SelN.

Diet composition and serum levels of selenium species: A cross-sectional study

Selenium is a trace element of both nutritional and toxicological interest, depending on its dose and chemical form. Diet is the primary source of exposure for most individuals. We sought to investigate the influence of food intake on serum levels of selenium species. Among fifty subjects randomly selected from a Northern Italian population, we assessed dietary habits using a validated semi-quantitative food frequency questionnaire. We also measured circulating levels of selenium species in serum using high pressure liquid chromatography associated with inductively-coupled plasma dynamic reaction cell mass spectrometer. Circulating levels of inorganic selenium, the most toxic selenium species, were positively associated with intake of fish, legumes and dry fruits, and inversely associated with intake of dairy products and mushrooms. Concerning the organic selenium species, selenoproteinP-bound selenium was inversely associated with intake of fish, fresh fruits, vegetables, and legumes, while selenocysteine-bound selenium positively associated with intake of fresh fruit, potato, legume and mushroom. In the present study, intakes of different foods were correlated with different types of selenium species. These results have important public health implications when assessing the nutritional and toxicological potential of diet composition with reference to selenium exposure.

A selenium species in cerebrospinal fluid predicts conversion to Alzheimer's dementia in persons with mild cognitive impairment

Background

Little is known about factors influencing progression from mild cognitive impairment to Alzheimer’s dementia. A potential role of environmental chemicals and specifically of selenium, a trace element of nutritional and toxicological relevance, has been suggested. Epidemiologic studies of selenium are lacking, however, with the exception of a recent randomized trial based on an organic selenium form.

Methods

We determined concentrations of selenium species in cerebrospinal fluid sampled at diagnosis in 56 participants with mild cognitive impairment of nonvascular origin. We then investigated the relation of these concentrations to subsequent conversion from mild cognitive impairment to Alzheimer’s dementia.

Results

Twenty-one out of the 56 subjects developed Alzheimer’s dementia during a median follow-up of 42 months; four subjects developed frontotemporal dementia and two patients Lewy body dementia. In a Cox proportional hazards model adjusting for age, sex, duration of sample storage, and education, an inorganic selenium form, selenate, showed a strong association with Alzheimer’s dementia risk, with an adjusted hazard ratio of 3.1 (95% confidence interval 1.0–9.5) in subjects having a cerebrospinal fluid content above the median level, compared with those with lower concentration. The hazard ratio of Alzheimer’s dementia showed little departure from unity for all other inorganic and organic selenium species. These associations were similar in analyses that measured exposure on a continuous scale, and also after excluding individuals who converted to Alzheimer’s dementia at the beginning of the follow-up.

Conclusions

These results indicate that higher amounts of a potentially toxic inorganic selenium form in cerebrospinal fluid may predict conversion from mild cognitive impairment to Alzheimer’s dementia.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0323-1) contains supplementary material, which is available to authorized users.

Exploring the urinary selenometabolome following a multi-phase selenite administration regimen in humans

To gain more insight into the human metabolism of the essential trace element selenium, we investigate the response of the urinary selenium metabolites to changing selenium intake by applying a stepwise selenium administration regimen based on repeated dosaging. Sodium selenite was administered orally to healthy volunteers at an incrementally increasing dosage. The supplementation regimen extended over 20 days for each volunteer, and daily morning urine samples were collected prior to, during, and following the supplementation phases. A total of 160 urine samples were analyzed for total urinary selenium and a panel of selenometabolites by using ICPMS and HPLC/ICPMS. Selenosugar 1 gave the strongest response followed by TMSe and then selenosugar 3. Se-methylselenoneine excretion was not stimulated by increased selenium intake, suggesting that it is not in equilibrium with selenium body pools. Selenate was detected in all urine samples; it showed a clear and consistent response to supplementation and an abrupt return to baseline levels upon cessation of supplementation, indicating that it arose from the oxidation of the administered selenite rather than from the oxidation of endogenous hydrogen selenide. The gap between total urinary selenium and the sum of Se species markedly increased in response to selenium administration, which highlights the presence of unknown Se species that respond to selenite supplementation. The characterization of these unknown species and their possible biological activities might be essential before considering selenium supplementation in clinical trials. We discuss the implications of the responses of the selenium metabolites and their inter-relationships for selenium metabolism.

Relationship between arsenic and selenium in workers occupationally exposed to inorganic arsenic

The interaction between arsenic (As) and selenium (Se) has been one of the most extensively studied. The antagonism between As and Se suggests that low Se status plays an important role in aggravating arsenic toxicity in diseases development. The objective of this study was to assess the Se contents in biological samples of inorganic As exposed workers (n=61) and in non-exposed subjects (n=52). Median (Me) total arsenic concentration in urine of exposed workers was 21.83μg/g creat. (interquartile range (IQR) 15.49-39.77) and was significantly higher than in the control group - (Me 3.75μg/g creat. (IQR 2.52-9.26), p<0.0001). The median serum Se concentrations in the study group and the control were: 54.20μg/l (IQR 44.2-73.10μg/l) and 55.45μg/l (IQR 38.5-69.60μg/l) respectively and did not differ significantly between the groups. In the exposed group we observed significantly higher urine concentrations of selenosugar 1 (SeSug 1) and selenosugar 3 (SeSug3) than in the control group Me: 1.68μg/g creat. (IQR 1.25-2.97 vs Me: 1.07μg/g creat. (IQR 0.86-1.29μg/g), p<0.0001 for SeSug1; Me: 0.45μg/g creat. (IQR 0.26-0.69) vs Me: 0.28μg/g creat. (IQR 0.17-0.45μg/g), p=0.0021). In the multivariate model, after adjusting to cofounders (age, BMI, job seniority time, consumption of fish and seafood and smoking habits) the high rate of arsenic urine wash out (measured as a sum of iAs+MMA+DMA) was significantly associated with the high total selenium urine excretion (B=0.14 (95%CI (confidence interval) 0.05-0.23)). Combination of both arsenic and selenium status to assess the risk of arsenic-induced diseases requires more studies with regard to both the analysis of speciation, genetics and the influence of factors such as nutritional status.

Health risk assessment of environmental selenium: Emerging evidence and challenges (Review)

New data have been accumulated in the scientific literature in recent years which allow a more adequate risk assessment of selenium with reference to human health. This new evidence comes from environmental studies, carried out in populations characterized by abnormally high or low selenium intakes, and from high-quality and large randomized controlled trials with selenium recently carried out in the US and in other countries. These trials have consistently shown no beneficial effect on cancer and cardiovascular risk, and have yielded indications of unexpected toxic effects of selenium exposure. Overall, these studies indicate that the minimal amount of environmental selenium which is source of risk to human health is much lower than anticipated on the basis of older studies, since toxic effects were shown at levels of intake as low as around 260 µg/day for organic selenium and around 16 µg/day for inorganic selenium. Conversely, populations with average selenium intake of less than 13–19 µg/day appear to be at risk of a severe cardiomyopathy, Keshan disease. Overall, there is the need to reconsider the selenium standards for dietary intake, drinking water, outdoor and indoor air levels, taking into account the recently discovered adverse health effects of low-dose selenium overexposure, and carefully assessing the significance of selenium-induced proteomic changes.

Long-term mortality patterns in a residential cohort exposed to inorganic selenium in drinking water

Selenium (Se) is a metalloid of considerable nutritional and toxicological importance in humans. To date, limited epidemiologic evidence exists about the health effects of exposure to this trace element in drinking water. We investigated the relationship between Se levels in water and mortality in the municipality of Reggio Emilia, Italy, where high levels of Se were previously observed in drinking water. From 1974 to 1985, 2065 residents consumed drinking water with Se levels close to the European standard of 10μg/l, in its inorganic hexavalent form (selenate). Follow-up was conducted for the years 1986-2012 in Reggio Emilia and a lesser exposed comparison group of around 100,000 municipal residents, with comparable socio-demographic characteristics. Overall mortality from all causes, cardiovascular disease and cancer showed little evidence of differences. However, excess rate ratios were seen for some site specific cancers such as neoplasms of buccal cavity and pharynx, urinary tract, lymphohematopoietic tissue, melanoma, and two neurodegenerative diseases, Parkinson's disease and amyotrophic lateral sclerosis. Excess mortality in the exposed cohort for specific outcomes was concentrated in the first period of follow-up (1986-1997), and waned starting 10 years after the high exposure ended. We also found lower mortality from breast cancer in females during the first period of follow-up. When we extended the analysis to include residents who had been consuming the high-selenium drinking water for a shorter period, mortality rate ratios were also increased, but to a lesser extent. Overall, we found that the mortality patterns related to long-term exposure to inorganic hexavalent selenium through drinking water were elevated for several site-specific cancers and neurodegenerative disease.

Treatment strategies in Alzheimer's disease: a review with focus on selenium supplementation

Alzheimer's disease (AD) is a neurodegenerative disorder presenting one of the biggest healthcare challenges in developed countries. No effective treatment exists. In recent years the main focus of AD research has been on the amyloid hypothesis, which postulates that extracellular precipitates of beta amyloid (Aβ) derived from amyloid precursor protein (APP) are responsible for the cognitive impairment seen in AD. Treatment strategies have been to reduce Aβ production through inhibition of enzymes responsible for its formation, or to promote resolution of existing cerebral Aβ plaques. However, these approaches have failed to demonstrate significant cognitive improvements. Intracellular rather than extracellular events may be fundamental in AD pathogenesis. Selenate is a potent inhibitor of tau hyperphosphorylation, a critical step in the formation of neurofibrillary tangles. Some selenium (Se) compounds e.g. selenoprotein P also appear to protect APP against excessive copper and iron deposition. Selenoproteins show anti-inflammatory properties, and protect microtubules in the neuronal cytoskeleton. Optimal function of these selenoenzymes requires higher Se intake than what is common in Europe and also higher intake than traditionally recommended. Supplementary treatment with N-acetylcysteine increases levels of the antioxidative cofactor glutathione and can mediate adjuvant protection. The present review discusses the role of Se in AD treatment and suggests strategies for AD prevention by optimizing selenium intake, in accordance with the metal dysregulation hypothesis. This includes in particular secondary prevention by selenium supplementation to elderly with mild cognitive impairment.

Selenium speciation in human serum and its implications for epidemiologic research: a cross-sectional study

Observational studies addressing the relation between selenium and human health, particularly cancer risk, yielded inconsistent results, while most recent randomized trials showed a fairly consistent pattern suggesting null or adverse effects of the metalloid. One of the most plausible explanations for such inconsistencies is inadequate exposure assessment in observational studies, commonly carried out by measuring total Se content without taking into account the specific exposure to the individual chemical forms of the metalloid, whose toxic and nutritional properties may vary greatly. Data on the distribution of these species in human blood and their correlation with overall selenium levels are very limited. The concentrations of organic and inorganic selenium species were analyzed in serum of fifty subjects sampled from the general population of the municipality of Modena, northern Italy, aged from 35 to 70 years. Samples were collected during a 30-month period, and determinations of selenium species were carried out using high pressure liquid chromatography coupled with inductively coupled plasma dynamic reaction cell mass spectrometry. The majority of selenium was found to be present as organic species, but the inorganic forms showed higher levels than expected. These species showed limited correlations with age, sex and body mass index, while the organic forms increased in subjects consuming selenium-containing dietary supplements and decreased in smokers. The length of the sample storage period strongly influenced the distribution of selenium compounds, with a clear tendency towards higher inorganic and lower organic selenium levels over time. In multivariate analysis adjusting for potential confounders, total serum selenium correlated with human serum albumin-bound selenium and, in males, with two organic species of the metalloid (selenocysteine and glutathione peroxidase-bound selenium), while little association existed with the other organic forms and the inorganic ones. These findings highlight the potential for exposure misclassification of observational epidemiologic investigations based on overall selenium content in blood and possibly other tissues, and the critical role of the storage conditions for speciation analysis.

Biological monitoring of exposure and effects in workers employed in a selenium-processing plant

PURPOSE

The aim of the study was to assess external and internal selenium exposure and potential biological effects in employees working in a selenium-processing plant.

METHODS

Twenty male employees of a selenium-processing plant (exposed) and 20 age-matched male individuals without occupational selenium exposure (controls) participated in the study. Exposure to selenium at the workplace was determined by personal air sampling. Internal exposure was assessed by measuring total selenium concentration in plasma after a workshift and after holidays as well as by measuring the selenium concentration in urine before and after shift and after holidays. The activity of glutathione peroxidase (GPx) in blood and the prothrombin time were determined as biological effect parameters.

RESULTS

The exposure to selenium in the workplace air ranged from 8 to 950 µg/m(3) and exceeded in a large part current occupational exposure limits. The selenium levels in plasma samples of the exposed ranged from 49 to 182 µg/L during exposure and were significantly higher than the plasma levels observed after holidays as well as in the control group. The selenium concentration in pre- and post-shift urine samples did not differ significantly, and the average urinary selenium levels of the employees (18-1,104 µg/g creatinine) were significantly higher than those measured after holidays or determined in controls. Both the concentration of selenium in plasma and in urine did not correlate with the current external exposure of the employees at the workplace. However, we found a strong and significant association between the two biomonitoring parameters. In spite of the considerable high external exposure to elemental selenium, we did not find any effect on the GPx activity in blood as well as on the prothrombin time.

CONCLUSIONS

Both the selenium levels in plasma and urine may be used as biological monitoring parameters for the assessment of chronic occupational exposure to selenium. Nevertheless, the toxicokinetics of the urinary excretion of selenium remain still unclear and require further investigations. Moreover, our findings emphasise the necessity of a re-evaluation of occupational exposure limits for elemental selenium and inorganic selenium compounds.