Selenium absorption and retention from a selenite- or selenate-fortified milk-based formula in men measured by a stable-isotope technique

Sensitive analysis of selenium speciation in natural seawater by isotope-dilution and large volume injection using PTV-GC-ICP-MS

Although oceans play a key role in the global selenium (Se) cycle, there is currently very little quantitative information available on the distribution of Se concentrations and Se speciation in marine environments. In general, determining Se concentration and speciation in seawater is highly challenging due to very low Se levels ((sub)ng⋅L-1), whereas matrix elements interfering Se pre-concentration and detection are up to the g⋅L-1 levels. In this study, we established a sensitive method for the determination of the various Se chemical fractions present in natural seawater, i.e. selenite (SeIV), selenate (SeVI), organic Se-II + Se0 and total Se, using species-specific isotope dilution gas chromatography coupled to inductively coupled plasma mass spectrometry (ID-GC-ICP-MS). We compared different derivatization reagents and optimized specific pre-treatment protocols, including a microwave assisted oxidation protocol for the determination of total Se and organic Se-II + Se0 using H2O2. To increase sensitivity, we developed an online pre-concentration method based on large volume injection (LVI) using a programmed temperature vaporization (PTV) inlet. Eventually, the developed method achieved low absolute and methodological detection limits, i.e., respectively, 0.1-0.3 pg and 0.9-3.1 ng.L-1 for the different fractions. The accuracy of our method was of 2% for a certified reference material (CRM) diluted in artificial seawater while the precision was better than 4% for a freshwater CRM in artificial seawater matrix as well as two common seawater CRMs certified for trace elements excluding Se. As a proof-of-concept, we quantified the various Se fractions in a large number of natural water samples from the Baltic and North Seas, encompassing a wide range of salinity (7-35 psu), which shows that its detection limits are sufficient to determine total Se, SeIV, SeVI and organic Se-II + Se0 concentrations in brackish and marine systems.

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.

Understanding soil selenium accumulation and bioavailability through size resolved and elemental characterization of soil extracts

Dietary deficiency of selenium is a global health threat related to low selenium concentrations in crops. Despite the chemical similarity of selenium to the two more abundantly studied elements sulfur and arsenic, the understanding of its accumulation in soils and availability for plants is limited. The lack of understanding of soil selenium cycling is largely due to the unavailability of methods to characterize selenium species in soils, especially the organic ones. Here we develop a size-resolved multi-elemental method using liquid chromatography and elemental mass spectrometry, which enables an advanced characterization of selenium, sulfur, and arsenic species in soil extracts. We apply the analytical approach to soils sampled along the Kohala rainfall gradient on Big Island (Hawaii), which cover a large range of organic carbon and (oxy)hydroxides contents. Similarly to sulfur but contrarily to arsenic, a large fraction of selenium is found associated with organic matter in these soils. However, while sulfur and arsenic are predominantly found as oxyanions in water extracts, selenium mainly exists as small hydrophilic organic compounds. Combining Kohala soil speciation data with concentrations in parent rock and plants further suggests that selenium association with organic matter limits its mobility in soils and availability for plants.

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.

Exposures in nail salons to trace elements in nail polish from impurities or pigment ingredients - A pilot study

Nail polishes have evolved considerably. Toxic elements, such as lead, have been found in nail polish, and it is unclear if new finishes using metallic effect pigments may be contributing to metals exposure in nail technicians. We characterized concentrations of trace elements in 40 nail polishes, 9 technicians' urine, and 20 technicians' toenail clippings from 8 nail salons in the Boston area in 2017. We also collected 24 salon surface wipes from 3 of the salons. Antimony was not disclosed as a nail polish ingredient, yet concentrations (<15 μg/g) were above existing cosmetics guidelines (0.5 μg/g) in five (13%) of the samples. Aluminum (<11,450 μg/g), barium (<11,250 μg/g), iron (<3,270 μg/g), and magnesium (<2375 μg/g) were disclosed as ingredients and were also found on salon surfaces where nail polish was stored or used. Heavy metal impurities in nail polish were not detected for cadmium. Lead and nickel were found at low concentrations (<0.40 μg/g lead, <0.67 μg/g nickel). Tin (p = 0.003) concentrations were higher in nail polish with finishes compared to without. Barium and strontium (both p = 0.0001) concentrations were higher for red nail polishes compared to all other colors. Of those elements in nail polish and salon surfaces, aluminum and iron were detected in toenails, manganese was detected in urine and toenails, and barium was detected in urine at comparable levels to the general population. Besides preventable antimony levels in nail polish, individual metals in nail polish did not appear to be from impurities but mainly from colorants (i.e., pigments) and not major contributors to nail technician exposure. It is unclear if low-level chronic metals mixtures in nail salons are of health concern.

From Selenium Absorption to Selenoprotein Degradation

Selenium is an essential dietary micronutrient. Ingested selenium is absorbed by the intestines and transported to the liver where it is mostly metabolized to selenocysteine (Sec). Sec is then incorporated into selenoproteins, including selenoprotein P (SELENOP), which is secreted into plasma and serves as a source of selenium to other tissues of the body. Herein, we provide an overview of the biology of selenium from its absorption and distribution to selenoprotein uptake and degradation, with a particular focus on the latter. Molecular mechanisms of selenoprotein degradation include the lysosome-mediated pathway for SELENOP and endoplasmic reticulum-mediated degradation of selenoproteins via ubiquitin-activated proteasomal pathways. Ubiquitin-activated pathways targeting full-length selenoproteins include the peroxisome proliferator-activated receptor gamma-dependent pathway and substrate-dependent ubiquitination. An alternate mechanism is utilized for truncated selenoproteins, in which cullin-RING E3 ubiquitin ligase 2 targets the defective proteins for ubiquitin-proteasomal degradation. Selenoproteins, particularly SELENOP, may have their Sec residues reutilized for new selenoprotein synthesis via Sec decomposition. This review will explore these aspects in selenium biology, providing insights to knowledge gaps that remain to be uncovered.

Effects of different levels of dietary hydroxy-analogue of selenomethionine on growth performance, selenium deposition and antioxidant status of weaned piglets

This study was conducted to assess the effects of the hydroxy-analogue of selenomethionine (HMSeBA) on growth performance, selenium (Se) deposition and antioxidant status of piglets. In a 28-d experiment, 252 piglets were assigned into seven treatments. These treatments were a negative control (Con-, basal diet without supplement Se), a positive control (Con+, basal diet + 0.3 mg Se from sodium selenite per kg), and five HMSeBA groups (basal diet + 0.1, 0.2, 0.3, 0.4 and 0.5 mg Se/kg from HMSeBA, respectively). Results showed that dietary HMSeBA supplementation did not affect growth performance of piglets. However, HMSeBA supplementation increased the Se concentrations in serum, liver, kidney and muscle compared with groups Con- and Con+ (p < 0.05). Compared with group Con-, supplementation with 0.2 and 0.4 mg Se from HMSeBA increased serum total antioxidant capability (T-AOC) and addition of 0.4 and 0.5 mg Se from HMSeBA increased serum glutathione peroxidase (GSH-Px) activities (p < 0.05). Compared with group Con-, the addition of 0.1, 0.2, 0.4, 0.5 mg Se from HMSeBA increased GSH-Px activities and decreased malondialdehyde (MDA) contents in the liver, and 0.3 mg Se from HMSeBA increased T-AOC and GSH-Px activities in the liver (p < 0.05). Compared with group Con+, 0.3 mg Se from HMSeBA increased serum superoxide dismutase (SOD) and hepatic T-AOC activities, and decreased the serum MDA level (p < 0.05). In general, dietary HMSeBA supplementation could improve Se deposition in serum and tissue and antioxidant capacity of piglets, suggesting that HMSeBA could be an effective Se source for piglets.

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.

Delivery of selenium to selenophosphate synthetase for selenoprotein biosynthesis

BACKGROUND

Selenophosphate, the key selenium donor for the synthesis of selenoprotein and selenium-modified tRNA, is produced by selenophosphate synthetase (SPS) from ATP, selenide, and H₂O. Although free selenide can be used as the in vitro selenium substrate for selenophosphate synthesis, the precise physiological system that donates in vivo selenium substrate to SPS has not yet been characterized completely.

SCOPE OF REVIEW

In this review, we discuss selenium metabolism with respect to the delivery of selenium to SPS in selenoprotein biosynthesis.

MAJOR CONCLUSIONS

Glutathione, selenocysteine lyase, cysteine desulfurase, and selenium-binding proteins are the candidates of selenium delivery system to SPS. The thioredoxin system is also implicated in the selenium delivery to SPS in Escherichia coli.

GENERAL SIGNIFICANCE

Selenium delivered via a protein-bound selenopersulfide intermediate emerges as a central element not only in achieving specific selenoprotein biosynthesis but also in preventing the occurrence of toxic free selenide in the cell. This article is part of a Special Issue entitled "Selenium research in biochemistry and biophysics - 200 year anniversary".

Selenite exacerbates hepatic insulin resistance in mouse model of type 2 diabetes through oxidative stress-mediated JNK pathway

Recent evidence suggests a potential pro-diabetic effect of selenite treatment in type 2 diabetics; however, the underlying mechanisms remain elusive. Here we investigated the effects and the underlying mechanisms of selenite treatment in a nongenetic mouse model of type 2 diabetes. High-fat diet (HFD)/streptozotocin (STZ)-induced diabetic mice were orally gavaged with selenite at 0.5 or 2.0mg/kg body weight/day or vehicle for 4 weeks. High-dose selenite treatment significantly elevated fasting plasma insulin levels and insulin resistance index, in parallel with impaired glucose tolerance, insulin tolerance and pyruvate tolerance. High-dose selenite treatment also attenuated hepatic IRS1/Akt/FoxO1 signaling and pyruvate kinase gene expressions, but elevated the gene expressions of phosphoenolpyruvate carboxyl kinase (PEPCK), glucose 6-phosphatase (G6Pase), peroxisomal proliferator-activated receptor-γ coactivator 1α (PGC-1α) and selenoprotein P (SelP) in the liver. Furthermore, high-dose selenite treatment caused significant increases in MDA contents, protein carbonyl contents, and a decrease in GSH/GSSG ratio in the liver, concurrent with enhanced ASK1/MKK4/JNK signaling. Taken together, these findings suggest that high-dose selenite treatment exacerbates hepatic insulin resistance in mouse model of type 2 diabetes, at least in part through oxidative stress-mediated JNK pathway, providing new mechanistic insights into the pro-diabetic effect of selenite in type 2 diabetes.

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

A human in vivo metabolism study was carried out to investigate the impact of the trimethylselenium ion (TMSe) status on metabolism and toxicokinetics of sodium selenite and selenized yeast. Nine healthy human volunteers were orally exposed to 200 µg selenium as sodium selenite and seven with selenized yeast (100 µg selenium). In each intervention group, three subjects belong to TMSe eliminators. Blood samples were withdrawn before and up to 6 h after administration. Urine samples were collected before and within 24 h after administration. Total selenium (Se) was quantified in blood plasma and urine and low molecular Se species in urine. Selenium concentration in plasma increased from 84.5 ± 13.2 µg Se/L before to 97.4 ± 13.2 µg Se/L 2-3 h after selenite supplementation and 89.5 ± 12.9 µg Se/L to 92.1 ± 13.9 µg Se/L after selenized yeast intake. The oral ingestion caused an additional Se elimination via urine of 16.9 ± 10.6 µg/24 h (TMSe elim.: 10.8 ± 6.9 µg/24 h; non-TMSe elim.: 20.0 ± 11.3 µg Se/24 h) after selenite exposure and 11.8 ± 4.1 µg/24 h (TMSe elim.: 10.8 ± 4.6 µg/24 h; non-TMSe elim.: 12.6 ± 4.2 µg Se/24 h) after selenized yeast exposure. Methyl-2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside (SeSug1) was the main metabolite in all urine samples, whereas TMSe was another main metabolite in TMSe eliminators' urine. After selenite exposure, a small amount of the dose (0.5 ± 0.2 %) was oxidized to selenate and rapidly excreted via urine. With the exception of selenite exposure in TMSe eliminators, the comparison of total Se and the sum of quantified Se species revealed a high renal portion of unidentified species. The study indicated a different metabolism of inorganic and organic Se compounds in human, but also crucial differences of Se metabolism in TMSe eliminators and non-TMSe eliminators.

Regulation of Selenium Metabolism and Transport

Selenium is regulated in the body to maintain vital selenoproteins and to avoid toxicity. When selenium is limiting, cells utilize it to synthesize the selenoproteins most important to them, creating a selenoprotein hierarchy in the cell. The liver is the central organ for selenium regulation and produces excretory selenium forms to regulate whole-body selenium. It responds to selenium deficiency by curtailing excretion and secreting selenoprotein P (Sepp1) into the plasma at the expense of its intracellular selenoproteins. Plasma Sepp1 is distributed to tissues in relation to their expression of the Sepp1 receptor apolipoprotein E receptor-2, creating a tissue selenium hierarchy. N-terminal Sepp1 forms are taken up in the renal proximal tubule by another receptor, megalin. Thus, the regulated whole-body pool of selenium is shifted to needy cells and then to vital selenoproteins in them to supply selenium where it is needed, creating a whole-body selenoprotein hierarchy.

A thiol-mediated active membrane transport of selenium by erythroid anion exchanger 1 protein

In this paper, we describe a thiol-mediated and energy-dependent membrane transport of selenium by erythroid anion exchanger 1 (AE1, also known as band 3 protein). The AE1 is the most abundant integral protein of red cell membranes and plays a critical role in the carbon dioxide transport system in which carbon dioxide is carried as bicarbonate in the plasma. This protein mediates the membrane transport of selenium, an essential antioxidant micronutrient, from red cells to the plasma in a manner that is distinct from the already known anion exchange mechanism. In this pathway, selenium bound to the cysteine 93 of the hemoglobin β chain (Hb-Cysβ93) is transported by the relay mechanism to the Cys317 of the amino-terminal cytoplasmic domain of the AE1 on the basis of the intrinsic interaction between the two proteins and is subsequently exported to the plasma via the Cys843 of the membrane-spanning domain. The selenium export did not occur in plain isotonic buffer solutions and required thiols, such as albumin, in the outer medium. Such a membrane transport mechanism would also participate in the export pathways of the nitric oxide vasodilator activity and other thiol-reactive substances bound to the Hb-Cysβ93 from red cells to the plasma and/or peripherals.

Selenium status in adult cats and dogs fed high levels of dietary inorganic and organic selenium

Cats (Felis catus) maintain greater blood Se concentrations compared with dogs (Canis familiaris) and, unlike dogs, show no signs of chronic Se toxicity (selenosis) when fed dietary organic Se (selenomethionine) concentrations of 10 μg/g DM. This study investigated the response of cats and dogs to high dietary concentrations of sodium selenite and organic Se to determine differences in metabolism between both species. In 2 consecutive studies, 18 adult cats and 18 adult dogs of with equal numbers of each sex were fed a control diet (0.6 μg Se/g DM) or the control diet supplemented to 8 to 10 μg Se/g DM from Na(2)SeO(3) or organic Se for 3 wk. All animals were fed the control diet 1 mo before the start of the study and blood samples were taken on d 0 and 21. The Se balance was assessed during the final week and a liver biopsy was obtained on the final day of the study. Measurements included plasma Se concentrations, plasma glutathione peroxidise (GPx) activities, plasma Se clearance, Se intake, and urinary Se excretion. No clinical signs of selenosis were observed in the cats or dogs, and apart from Se clearance, form of Se had no effect on any of the measurements. Apparent fecal Se absorption was greater in the dogs fed both forms of Se, while greater plasma Se concentrations were observed in the cats on both the control and supplemented diet (P = 0.034). Cats fed the supplemented diets had lower hepatic Se concentrations (P < 0.001) and excreted more Se in urine (P < 0.001) compared with dogs. Furthermore, cats fed the Na(2)SeO(3) supplement had greater Se clearance rates than dogs (P < 0.001). There was no effect of species on plasma GPx activity. We conclude that cats can tolerate greater dietary Se concentrations as they are more efficient at excreting excess Se in the urine and storing less Se in the liver.

Human urinary excretion and metabolism of (82)Se-enriched selenite and selenate determined by LC-ICP-MS

Urinary excretion of selenium after ingestion of isotope labeled selenite and selenate was studied in seven healthy volunteers, 4 men and 3 women (age 28-50 years). An aqueous solution containing 330 μL (82)Se-selenate (corresponding to 74.3 μg (82)Se) was given orally and urine samples were subsequently collected during the following 24 hours. The scheme was repeated four weeks later with a 280 μL (82)Se-selenite solution (corresponding to 74.4 μg (82)Se). The amount of total Se in the urine samples was determined by inductively coupled mass spectrometry. The mean total urinary excretion of (82)Se following (82)Se-selenate administration was 33.7% (range 15.6-42.5%) while the mean total excretion of (82)Se after (82)Se-selenite administration was 3.2% (range 2.8-3.9%) of the ingested amount. LC-ICPMS analysis of the urine samples showed that the majority of the selenium excreted after selenate ingestion was unchanged selenate for 6 of the individuals while one individual had metabolized a fraction (approx. 20%) of the selenate to selenosugar. Ingestion of 10 times larger doses of selenite in two individuals resulted in 13-23% excretion primarily excreted as selenosugar. These results show that the human metabolic pathways of selenite and selenate are different and indicate that not all selenate, although well absorbed, may be available for the beneficial health effects.

Selenium balance in the adult cat in relation to intake of dietary sodium selenite and organically bound selenium

The response of cats to dietary sodium selenite (Na(2) SeO(3)) and organically bound selenium was studied in two separate studies with four cats per treatment and three levels of selenium supplementation (targets 1.0, 1.5 and 2.0 μg/g DM) for each Se source. Whole blood and plasma selenium concentrations and glutathione peroxidase (GPx) activity were determined at 7-time points across the 32-day study. Faeces were quantitatively collected during the last 8 days and urine was collected daily during both studies. The basal diet used had a low apparent faecal selenium absorption of 25.3 ± 3.0%. Daily faecal and urinary selenium excretion increased linearly with increasing selenium intake for both Se sources. Urinary selenium concentration of the cats fed the supplemented diets increased rapidly (∼2 days) and remained constant throughout the remainder of the study. Apparent faecal selenium absorption was high for both selenium sources (73.2% and 80.0%). Plasma, and to a lesser extent, whole blood selenium concentrations increased in a dose-dependent manner with supplementation. Whole blood and plasma GPx activity were highly variable and showed a variable response to dietary selenium intake. Cats closely regulate selenium homeostasis through increasing urinary excretion whilst faecal absorption remains unaffected.

The monitoring of trace elements in blood samples from patients with inborn errors of metabolism

Patients having inborn errors of intermediary metabolism (IEMs) may have element deficiencies related to dietary treatment. Our objective was to study several elements [cobalt (Co), copper (Cu), zinc (Zn), selenium (Se), manganese (Mn), molybdenum (Mo) and magnesium (Mg)] in patients with IEMs with and without dietary treatment and to compare these results with those established in a healthy paediatric population. We studied 72 patients with IEMs (age range 2 months-44 years; median 10.5 years), with and without protein-restricted dietary treatment. Control values were established in 92 subjects (age range 1 day-42 years; median 6.5 years). Dietary treatment consisted of a natural protein-restricted diet supplemented with a special formula, depending on the specific metabolic defect. Samples were analysed with an Agilent 7500ce-ICP mass spectrometer. Significant differences were observed when we compared patients under dietary treatment and control values for Se and Co (P < 0.0001). No differences were observed for the other elements when the different groups were compared, except for Co (IEM patients without dietary treatment vs control group; P = 0.003). For Se and cobalamin, the daily intake of our patients (Se 48 ± 16 µg/day; cobalamin 3.5 µg/day) was slightly higher than the recommended daily averages (RDAs) (40 µg/day and 1.8 µg/day, respectively). We concluded that IEM patients under dietary treatment showed significantly lower selenium values in spite of correct supplementation, reinforcing the idea that these patients should be regularly monitored, at least for this element. Further investigations seem advisable about Se and Co availability in special diets.

Nail antioxidant trace elements are inversely associated with inflammatory markers in healthy young adults

Antioxidant intake may be linked to a reduction of the chronic low-grade inflammatory state related to obesity and several accompanying disorders such as insulin resistance, cardiovascular diseases, and metabolic syndrome. So, the aim of this study was to evaluate the potential associations between nail trace elements and several indicators in healthy young adults, emphasizing on the putative effect of antioxidant trace element intake on inflammation-related marker concentrations. This study enrolled 149 healthy young adults, whose anthropometrical and blood pressure values as well as lifestyle features were analyzed. Fasting blood samples were collected for the biochemical and inflammation-related measurements (C-reactive protein, tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-6, IL-18, and homocysteine). Nail samples were collected for the analysis of selenium, zinc, and copper concentrations. Our results showed that nail selenium was negatively associated with IL-18; nail zinc concentrations were inversely related to circulating IL-6, IL-18, and TNF-alpha, whereas nail copper (Cu) and Cu/selenium values were negatively correlated with homocysteine levels and the Cu/zinc ratio was unaffected. In conclusion, nail content on some trace elements related to antioxidant defense mechanisms seems to be associated with several inflammation-related markers linked to chronic diseases in apparently healthy young adults, which is of interest to understand the role of antioxidant intake.

Application of enriched stable isotopes as tracers in biological systems: a critical review

The application of enriched stable isotopes of minerals and trace elements as tracers in biological systems is a rapidly growing research field that benefits from the many new developments in inorganic mass spectrometric instrumentation, primarily within inductively coupled plasma mass spectrometry (ICP-MS) instrumentation, such as reaction/collision cell ICP-MS and multicollector ICP-MS with improved isotope ratio measurement and interference removal capabilities. Adaptation and refinement of radioisotope tracer experiment methodologies for enriched stable isotope experiments, and the development of new methodologies coupled with more advanced compartmental and mathematical models for the distribution of elements in living organisms has enabled a broader use of enriched stable isotope experiments in the biological sciences. This review discusses the current and future uses of enriched stable isotope experiments in biological systems.

A short-term intervention trial with selenate, selenium-enriched yeast and selenium-enriched milk: effects on oxidative defence regulation

Increased Se intakes have been associated with decreased risk of cancer and CVD. Several mechanisms have been proposed, including antioxidant effects through selenoproteins, induction of carcinogen metabolism and effects on the blood lipid profile. In a 4 x 1 week randomised, double-blind cross-over study, healthy young men supplemented their usual diet with selenate, Se-enriched yeast, Se-enriched milk or placebo (Se dose was 300 microg/d for selenate and Se-enriched yeast, and about 480 microg/d for Se-enriched milk) followed by 8-week washout periods. All Se sources increased serum Se levels after supplementation for 1 week. The effect of the organic forms did not differ significantly and both increased serum Se more than selenate. Conversely, thrombocyte glutathione peroxidase (GPX) was increased in the periods where subjects were supplemented with selenate but not in those where they were given Se-enriched yeast or Se-enriched milk. We found no effect on plasma lipid resistance to oxidation, total cholesterol, TAG, HDL- and LDL-cholesterol, GPX, glutathione reductase (GR) and glutathione S-transferase (GST) activities measured in erythrocytes, GPX and GR activities determined in plasma, or GR and GST activities in thrombocytes. Leucocyte expression of genes encoding selenoproteins (GPX1, TrR1 and SelP), and of electrophile response element-regulated genes (GCLC, Fra1 and NQO1) were likewise unaffected at all time points following intervention. We conclude that thrombocyte GPX is specifically increased by short-term selenate supplementation, but not by short-term supplementation with organic Se. Short-term Se supplementation does not seem to affect blood lipid markers or expression and activity of selected enzymes and a transcription factor involved in glutathione-mediated detoxification and antioxidation.

Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial

Intervention trials with different forms of selenium are under way to assess the effects of selenium supplements on the incidence of cancer and other diseases. Plasma selenium biomarkers respond to selenium administration and might be useful for assessing compliance and safety in these trials. The present study characterized the effects of selenium supplementation on plasma selenium biomarkers and urinary selenium excretion in selenium-replete subjects. Moderate (approximately 200 microg/d) to large (approximately 600 microg/d) selenium supplements in the forms sodium selenite, high-selenium yeast (yeast), and l-selenomethionine (selenomethionine) were administered. Subjects were randomized into 10 groups (placebo and three dose levels of each form of selenium). Plasma biomarkers (selenium concentration, selenoprotein P concentration, and glutathione peroxidase activity) were determined before supplementation and every 4 weeks for 16 weeks. Urinary selenium excretion was determined at 16 weeks. Supplementation with selenomethionine and yeast raised the plasma selenium concentration in a dose-dependent manner. Selenite did not. The increased selenium concentration correlated with the amount of selenomethionine administered. Neither glutathione peroxidase activity nor selenoprotein P concentration responded to selenium supplementation. Urinary selenium excretion was greater after selenomethionine than after selenite, with excretion after yeast being intermediate and not significantly different from either of the other two. We conclude that plasma selenium concentration is useful in monitoring compliance and safety of selenium supplementation as selenomethionine but not as selenite. Plasma selenium seems to reflect the selenomethionine content of yeast but not the other yeast selenium forms. As judged by urinary selenium excretion, selenium in the form of selenomethionine is better absorbed than selenite.

Validity of human nails as a biomarker of arsenic and selenium exposure: A review

Human nail clippings have been used in recent epidemiological studies as a routine bioindicator of arsenic and selenium exposure. To ensure sound application of this biomarker, however, it is important to consider properties and scientific knowledge pertaining to validation of this particular tool. In this review, the use of human nails to measure exposure to arsenic and selenium is discussed in the context of the biomarker validation framework. Literature related to both analytical procedures and intrinsic characteristics of the biomarker is reviewed. Specifically, the followings are addressed: sample collection and preparation methods, establishment of the exposure-biomarker relationship, intraindividual variability and reproducibility of measurements, and biomarker-disease investigations. Drawing from a rapidly growing body of literature, current knowledge of these biomarker validation steps is assessed. Therefore, this review brings attention to the important issue of biomarker validation, laying the framework for future studies measuring elemental composition of nails.

Stable isotope-enriched selenite and selenate tracers for human metabolic studies: a fast and accurate method for their preparation from elemental selenium and their identification and quantification using hydride generation atomic absorption spectrometry

Stable isotope tracers are safe and nutritionally relevant tools for the investigation of mineral metabolism in man. Increased research into the functional role of selenium has resulted in a need for well-characterised, isotopically enriched solutions of the element in order to determine the nutritional relevance of selenium fortification of foods. A simple method for the conversion of isotopically enriched elemental selenium (2.5-10 mg) into selenite and selenate, and their accurate characterisation and quantification is described. Analysis of selenite and selenate tracers using continuous-flow hydride generation-atomic absorption spectrometry technique was based on the specificity of the selenium hydride reaction and allowed their precise (RSD<2.5%) and accurate determination in aqueous solutions. The detection and determination limits were at 0.13 and 0.36 microg Se/l, respectively. Isotopically enriched elemental selenium was converted into selenite and selenate by a nitric acid and a combined nitric acid/hydrogen peroxide oxidation, respectively. The conversion was quantitative (>95%) and specific for both inorganic selenocompounds. Selenite and selenate labels were stable in 0.1 mol/l nitric acid for at least 18 months, i.e. making them ideally suitable for use in long-term metabolic studies. An overview of data relating to the absorption and retention of selenium by humans obtained using the two, well-characterised, tracers is presented and indicates that selenite and selenate are equally well retained in adult men and infants, despite differences in their absorption and urinary excretion characteristics.

Clinical pharmacokinetics of antioxidants and their impact on systemic oxidative stress

Dietary antioxidants play a major role in maintaining the homeostasis of the oxidative balance. They are believed to protect humans from disease and aging. Vitamin C (ascorbic acid), vitamin E (tocopherol), beta-carotene and other micronutrients such as carotenoids, polyphenols and selenium have been evaluated as antioxidant constituents in the human diet. This article addresses data provided from clinical trials, highlighting the clinical pharmacokinetics of vitamin C, vitamin E, beta-carotene, lycopene, lutein, quercetin, rutin, catechins and selenium. The bioavailability of vitamin C is dose-dependent. Saturation of transport occurs with dosages of 200-400 mg/day. Vitamin C is not protein-bound and is eliminated with an elimination half-life (t((1/2))) of 10 hours. In Western populations plasma vitamin C concentrations range from 54-91 micro mol/L. Serum alpha- and gamma-tocopherol range from 21 micro mol/L (North America) to 27 micro mol/L (Europe) and from 3.1 micro mol/L to 1.5 micro mol/L, respectively. alpha-Tocopherol is the most abundant tocopherol in human tissue. The bioavailability of all-rac-alpha-tocopherol is estimated to be 50% of R,R,R-alpha-tocopherol. The hepatic alpha-tocopherol transfer protein (alpha-TTP) together with the tocopherol-associated proteins (TAP) are responsbile for the endogenous accumulation of natural alpha-tocopherol. Elimination of alpha-tocopherol takes several days with a t((1/2)) of 81 and 73 hours for R,R,R-alpha-tocopherol and all-rac-alpha-tocopherol, respectively. The t((1/2)) of tocotrienols is short, ranging from 3.8-4.4 hours for gamma- and alpha-tocotrienol, respectively. gamma-Tocopherol is degraded to 2, 7, 8-trimethyl-2-(beta-carboxyl)-6-hyrdoxychroman by the liver prior to renal elimination. Blood serum carotenoids in Western populations range from 0.28-0.52 micro mol/L for beta-carotene, from 0.2-0.28 for lutein, and from 0.29-0.60 for lycopene. All-trans-carotenoids have a better bioavailability than the 9-cis-forms. Elimination of carotenoids takes several days with a t((1/2)) of 5-7 and 2-3 days for beta-carotene and lycopene, respectively. The bioconversion of beta-carotene to retinal is dose-dependent, and ranges between 27% and 2% for a 6 and 126mg dose, respectively. Several oxidised metabolites of carotenoids are known. Flavonols such as quercetin glycosides and rutin are predominantly absorbed as aglycones, bound to plasma proteins and subsequently conjugated to glucuronide, sulfate, and methyl moieties. The t((1/2)) ranges from 12-19 hours. The bioavailabillity of catechins is low and they are eliminated with a t((1/2)) of 2-4 hours. Catechins are degraded to several gamma-valerolactone derivatives and phase II conjugates have also been identified. Only limited clinical pharmacokinetic data for other polyphenols such as resveratrol have been reported to date.

Comparison of selenite and selenate apparent absorption and retention in infants using stable isotope methodology

The inorganic selenium compounds selenite and selenate are used for selenium fortification of infant formulas. However, information on absorption and retention of selenium from these compounds is lacking. The purpose of this study was therefore to determine apparent absorption and retention of selenium from selenate and selenite added to a milk-based infant formula in healthy infants. Labeled test meals were prepared by addition of 10 microg Se as (76)Se-selenate or (74)Se-selenite to 500 mL formula. The two batches of labeled formulas were fed as alternate feeds during the first day of the balance period, followed by unlabeled formula. Selenium isotopes were determined in feces collected for 72h after intake and in 3 consecutive 24h collections of urine. Mean apparent absorption was 97.1% for (76)Se-selenate and 73.4% for (74)Se-selenite; mean difference 23.7% (range: 13.8%-35.7%; SD 6.8%, p < 0.001). Mean urinary excretion (% of ingested dose) was 36.4% ((76)Se-selenate) and 9.7% ((74)Se-selenite); mean difference 26.7% (range: 13.9%-36.5%; SD 5.9%, p < 0.001). Mean apparent retention of selenium from (76)Se-selenate and (74)Se-selenite was not significantly different, 60.7% ((76)Se-selenate) versus 63.7% (for (74)Se-selenite). The average difference was -3.01% (range: -14.0%-12.0%; SD 9.4%, p = 0.36). Although apparent selenium absorption and urinary excretion differed for selenite and selenate, selenium was equally well retained by infants from both selenium compounds. We therefore concluded that Se fortification of infant formulas with selenate or selenite can be expected to have similar impact on the selenium nutritional status of term infants.