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Journal of Chemistry
Volume 2018, Article ID 7986591, 7 pages
https://doi.org/10.1155/2018/7986591
Research Article

Long-Term Supplementation of Laying Hen Diets with Various Selenium Sources as a Method for the Fortification of Eggs with Selenium

Department of Animal Nutrition and Feed Science, Wrocław University of Environmental and Life Sciences, 51-630 Wrocław, Poland

Correspondence should be addressed to Maja Słupczyńska; lp.ude.rwpu@aksnyzcpuls.ajam

Received 8 June 2018; Revised 14 August 2018; Accepted 10 September 2018; Published 13 November 2018

Guest Editor: Magdalena Jastrzębska

Copyright © 2018 Maja Słupczyńska et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The following study focuses on a comparison of the effectiveness of egg content enrichment with selenium (Se) via application of sodium selenite (Na-selenite), selenium-enriched yeast (Se-Yeast), or selenomethionine (Se-Met) in laying hen diets. Two hundred sixteen laying hens were divided into four treatments, each comprising eighteen replications, and each with three hens per cage. Animals were fed a basal diet without Se supplementation (control: selenium content 0.058 mg/kg), with the addition of Na-selenite, Se-Yeast, or Se-Met in amounts equivalent to 0.3 mg/kg of added selenium. The egg quality, the selenium content in eggs after the third and the fifth months of using Se supplementation, and the selenium level in the liver were determined. Enrichment of egg content with selenium was the most effective (382 μg/kg) via application of dietary Se-Met. Application of Na-selenite and Se-Yeast led to a similar effect on Se-accretion in egg content (255.9 and 258.9 μg/kg, respectively). Additionally, the calculated average Se concentration in one fresh egg was also higher in eggs from hens that received selenium additives in their diet and was far higher, almost three times higher for Se-Met addition, than the concentrations in controls. Se-accretion in the liver wet tissue was greater following application of Se-Yeast in the diet than following other treatments. These results indicate that the use of selenomethionine in the laying hen diet is the best method of enriching eggs with this micronutrient. In turn, the eggs obtained in this way can be an excellent source of highly bioavailable selenium in the human diet.

1. Introduction

The vital role of selenium (Se) originates from the fact that it is incorporated in several key enzymes and selenoproteins that are crucial for numerous biological activities, i.e., antioxidation, anti-inflammation, thyroids, fertility, DNA synthesis, and reproduction [14]. The best known selenium-dependent enzymes are glutathione peroxidases (GSH-Px); however, there is also a known selenium-independent glutathione peroxidase [5]. Some forms of gastrointestinal GSH-Px, plasma GSH-Px kinase, and iodothyronine deiodinases are synthesized and activated when the Se level in a diet is optimal [2, 3, 68]. Selenium protects the liver against necrosis and prevents muscular degeneration, neurodegenerative disorders, pancreatic fibrosis, microcytic anemia, degeneration of the testis, and, in birds, against the poor hatchability of eggs [9]. The best known disorder in livestock associated with Se deficiency is white muscle disease, with clinical signs that include lesions in skeletal and/or heart muscles. A low intake of Se in the human diet may lead to a number of diseases such as severe heart diseases, hypothyroidism, reduced male fertility, a weakened immune system, and enhanced susceptibility to infections, and it may even lead to cancer [1013]. Dietary, daily reference values for selenium in human nutrition found in available reports vary greatly. The proposed adequate intake of selenium for children ranges from 15 to 70 μg/day, for adults from 60 to 70 μg/day, and for pregnant and lactating women, that value ranges from 70 to 85 μg/day. The tolerable upper intake level for adults, including pregnant and lactating women, ranges from 200 to 500 μg/day [14]. Selenium is also known to be chronically toxic, and selenosis has been reported both in humans and in animals. Human chronic selenosis may occur when dietary selenium intake is above 1,000 μg/day. However, toxic levels of selenium are difficult to establish since selenium toxicity is affected by the kind of selenium compounds present in the food as well as mutual interactions with other components of the diet [1416]. Unfortunately, as has been stated in many countries across the world, human food ingredients can contain low levels of selenium and selenium deficiency in human nutrition has become a global phenomenon [1719].

There are several potential options to improve human selenium intake, including dietary modification and diversification or direct Se supplementation. In the last few years, a dynamic increase has occurred in terms of the attention paid to the creation of functional or designer foods containing more biologically and physiologically active components than standard food. These products have prosalubrious properties and prevent diseases connected with nutritional factors [2, 20]. Therefore, the production of selenium-enriched (biofortificated) animal-origin products such as eggs, meat, or milk, where selenium is present in higher amounts than in standard products, as well as Se occurrence in varied available forms, is the best way to increase both daily selenium uptake and the prevention of disorders related to its deficiency in the diet. The enrichment of products of animal origin with selenium may take place by increasing its concentration in the diet of animals (not exceeding the maximum levels permitted in their feed) or by selenium forms in animal nutrition which are characterized by high bioavailability. Selenium-enriched eggs are perfect candidates for the category of functional foods because eggs are affordable and are considered a traditional form of food in most countries, as well as being consumed by people of all ages more or less regularly [19, 21]. Selenium-enriched eggs contain superstandard levels of Se in the form of the essential amino acid, selenomethionine (Se-Met). The level of Se in a standard egg is about 11 μg, while the selenium-enriched egg contains around 32.6 μg of Se; therefore, consumption of two selenium-enriched eggs a day may satisfy over 70% of the Se daily dosage recommended for humans [22].

An inorganic selenium compound—sodium selenite (Na-selenite) or selenate, the major commercial Se supplements—has been used in animal feed as a source of selenium for the last few decades [3, 23]. Currently, selenium may be added to feed mixtures or to premixes both in an inorganic form and in organic forms: selenium-enriched yeasts (Se-Yeast), selenocysteine (Se-Cys), selenomethionine (Se-Met), or selenocystine [6, 7, 24]. Selenium in selenium-enriched yeast is typically in the form of the selenoamino acids: Se-Met (approx. 60–85% of total Se species) or Se-Cys (approx. to 2–4% of total Se) and as an inorganic selenium(IV) ion (less than 1% of total Se) [16]. Selenoamino acids that are used as selenium additives are the natural organic bonds present in animal feedstuffs [1, 3, 25, 26]. Major dietary ingredients contain selenium exclusively in organic forms and Se-Met that may comprise even more than 50% of total selenium, as well as smaller amounts of Se-Cys and selenate [27]. The bioavailability of Se from various forms applied in poultry feed is relatively high and mainly depends on the absorption pathways. Selenium from Na-selenite is passively absorbed by diffusion from intestinal content [28] and that from Se-Met is actively absorbed via methionine transport mechanisms. Noy et al. [29] evaluated the selenium absorption coefficient from poultry diets at a level from 60 to 86%. However, the effectiveness of Se-Yeast as a source of selenium closely depends on the kind of yeast [30], mainly Saccharomyces cerevisiae strain, that is used in the production of selenium-enriched yeast.

The level of selenium requirement recommended for laying hens varies and ranges from 0.05 to 0.08 mg/kg according to NRC standards [31], and the maximum allowable level given by the Association of American Feed Control Officials is 0.3 mg of Se/kg of complete feed mixture [32]. Usually, to meet Se requirements in poultry, supplemental Se in commercial feed is delivered with premixes at a level of 0.2-0.3 mg/kg complete feed, while the Se amount in the feed ingredients is not considered [23, 33]. However, complex poultry fodder mixes are recommended to include an Se supplement of 0.5 mg/kg of the fodder mix, which is in accordance with the maximum tolerated dietary concentrations of selenium given by EFSA [22, 34].

Given the above information about issues concerning the production of functional foods enriched in selenium, especially eggs, the objective of this study was to investigate the level of selenium transfer into the egg content of hens fed with diets supplemented with Se at the recommended level of 0.3 mg/kg. The sources of selenium in the diets prepared for experimental purposes were sodium selenite, selenium-enriched yeast, or selenomethionine. Considering the age of hens and the period of Se-inclusion into the diet may impact the transfer of selenium into eggs, a second experimental factor in the present studies was the length of the period selenium application. Additionally, some characteristic egg parameters and selenium liver concentrations were determined, as these parameters are also known to be modified by selenium supplementation.

2. Materials and Methods

2.1. Animals and Management

Two hundred sixteen Lohmann Brown pullets at the age of eighteen weeks were housed in battery cages and, before laying, were fed with the standard mixture for that stage of maturity that contains about 145 g/kg of crude protein and 11.5 MJ/kg of metabolizable energy. Directly after the laying of first eggs at the age of twenty weeks, hens with an average 1.27 kg body weight were allocated to cages according to body weight and laying at the start time. Afterwards, the cages were randomly divided into four treatments, each comprising eighteen replications and each with three hens per cage (fifty-four hens per treatment). Free access to water (nipples) was ensured. In that period, hens were fed with complete mixture containing about 180.3 g/kg of crude protein and 11.6 MJ ME/kg (according to Lohmann Brown recommendations). According to experimental assumptions, the amounts of selenium added to the feed mixture with premixes were the same (0.3 mg/kg) and the only factor that differentiated particular feeding groups was the selenium source. Selenium was added in the form of sodium selenite, selenium-enriched yeast, or selenomethionine. The environmental conditions in the autumn-winter period were controlled. The room temperature varied between 16 and 18°C, and the lighting program was set up for fourteen hours of light daily. The management was in accordance with the European Union guidelines, and the local Ethics Commission for Experiments with Animals accepted all experimental procedures.

2.2. Diets and Feeding Program

The dietary composition (Table 1) was based on our own chemical analyses of feed components and was calculated using linear optimization. The mixture of feed components prepared according to our recipe was determined for selenium content—this amounted to 0.058 mg/kg.

Table 1: Ingredients and nutrient levels of the basal/experimental diets.

The basal diet was divided into four parts, three of which were enriched with selenium using either Na-selenite, Se-Yeast, or Se-Met in amounts appropriate to the anticipated selenium supplement at the level of 0.3 mg of Se/kg of full mixture (Table 2). All the feed mixtures were prepared prior to the experiment. A homogenous mixture of selenium within the feed was obtained by preparation of premixtures with a selenium-free mineral-vitamin premix that was made for the experiment. According to the Lohmann Brown recommendations, the feed full mixture diet contained 180.3 g/kg of crude protein and 11.6 MJ ME/kg and was given at the dose of 110 g per hen per day.

Table 2: Scheme of selenium supplementation.
2.3. Egg Characteristics: Data Collection

The number of eggs was registered per cage daily, and all eggs from each cage were weighed. After the third and the fifth months of feeding with experimental diets, thirty eggs from each treatment were randomly sampled for determination of egg quality characteristics using a PM 600 RX processor apparatus (Technical Service and Supplies Ltd., York, England). Each measurement was repeated twice. Egg yolk color was determined using the La Roche scale (max. 15 points). For determination of selenium content, thirty-six eggs from each treatment were randomly collected (two eggs from replicate/cage), and the selenium content was determined in eighteen pooled samples for each treatment (two eggs per pooled sample). At the end of the experiment, after twelve hours starvation from each treatment, eight hens were randomly selected, weighed, and killed by cervical dislocation. In the sampled livers, selenium content was determined.

2.4. Analytical Methods

The chemical compositions of the feed compounds and complete mixtures were determined according to standard methods [35]. Nitrogen content was assayed using a Kjeltec 2300 Foss Tecator apparatus (Häganäs, Sweden, AOAC, 976.06); crude protein, by multiplying the nitrogen content by 6.25 (AOAC, 2003.06); crude fiber, by the Henneberg–Stohmann method using a Fibertec Tecator apparatus (Häganäs, Sweden, PN-EN ISO 6865). For mineral determination, feed samples and experimental mixtures were mineralized with nitric acid (HNO3) using a MarsX apparatus (CEM Corporation Matthews, USA). Phosphorus was analyzed after prior wet mineralization with nitric acid (HNO3) and perchloric acid (HClO4) according to the ammonium vanadomolybdate method using a Specol 11 spectrophotometer (Carl Zeiss, Jena) at a wave length of 470 nm. Calcium and sodium were determined by atomic absorption spectrophotometry using an AA 240 FS apparatus with SIPS 20 (Varian, Mulgrave, Australia). For the determination of sulfur-containing amino acids, the feed samples were oxidized (0°C, 16 h) with formic acid and hydrogen peroxide (H2O2) (9 : 1 (v : v)) prior to HCl hydrolysis. After oxidation, samples were hydrolyzed with 6 M hydrochloric acid (HCl) for 24 h at 110°C, and then, sulfuric amino acids were separated according to the Moore and Stein method [36] using an AAA 400 Ingos analyzer (Prague, Czech Republic). Selenium in feed components, complete mixtures, eggs, and samples of liver was determined by hydride-generation atomic absorption spectrometry (HG-AAS). The egg’s whole content and liver samples from experimental animals were freeze-dried and homogenized. Se concentrations in eggs and livers were determined after prior mineralization with HNO3. Afterwards, the solvent was evaporated and the residue dissolved with 25% H2SO4. Selenium content was assessed using a Vapor Generation Accessory 77 (Agilent Technologies, Inc., USA) for atomic absorption. A selenium hollow cathode lamp was used as a source of linear radiation (NARVA, Berlin, Germany). Data obtained for the freeze-dried material were converted, and selenium content was expressed on a fresh-weight basis.

2.5. Statistical Analysis

All collected data were evaluated using one- or two-factorial ANOVA within StatSoft Statistica® computer software [37]. The differences between parameters were tested according to the following statistical model: yij = µ + αi + eij (for differences between treatments) or yijk = µ + βj + (αβ)ij + eijk (for Se supplement and period of analyzed parameters in eggs), where yij or yijk is the variance associated with parameter α; µ is the overall mean; αi is treatment effect; βj is the time of analyzed parameters; (αβ)ij is the interaction effect; and eij or eijk is an error term.

3. Results and Discussion

A significant () effect was found for all selenium additives used in the experiment on the concentration of this element in laying hen eggs (Table 3).

Table 3: Se content in fresh egg content (μg/kg).
Table 4: Effects of dietary selenium supplements on egg characteristics.

In comparison with eggs received from hens from the control group (156.2 μg·Se/kg) and in eggs from hens fed a diet containing Na-selenite, the selenium concentration increased by 64% (255.9 μg·Se/kg), for the Se-Yeast-treated group by 65% (258.9 μg·Se/kg) and for Se-Met by about 144% (382.2 μg·Se/kg). The length of the period for which hens were fed supplemented diets had no significant influence on the accretion of selenium in the eggs. Additionally, the calculated average Se concentration in one fresh egg was also higher in eggs from hens that received selenium additives in their diets, and this was the highest—almost three times higher in the case of Se-Met addition—when compared with the control. The data are in agreement with results previously published by Delezie et al. [38], who also stated that dietary Se-Met is more effectively transferred to the egg than Se-Yeast or Na-selenite. Our results also agree with those obtained by Gajčević et al. [39]. The distinctive influence on Se transfer to egg embryo was obtained by Cantor et al. [40]. Payne et al. [41] and Reis et al. [42] suggested that Se-Yeast inclusion into hen diets is a useful strategy to improve the nutritional status of egg and embryo. As reported by Invernizzi et al. [43], when selenium was added as Se-Yeast or in an inorganic form, the Se egg content either doubled or increased by 74%, respectively. Se-Yeast increased Se egg content by 47% more than the increase observed in Na-selenite. The great variety of data characterizing Se concentrations in egg, egg white, or yolk content presented in the literature may have resulted from the various kinds of Se supplements used, the levels of added selenium, the genotypes of laying hens, the laying phases, the periods of Se addition, the kind of diets, and many other factors. According to Pappas et al. [26], the maintenance systems modulating the selenium content in eggs also have a meaningful input. A similar effect to that stated in our own experiments was confirmed by Zia et al. [44] who reported that egg selenium concentration was improved in the eggs of the animals receiving both the organic (Se-Yeast) and the inorganic (Na-selenite) selenium sources, whereas the effect of Se-Yeast treatment was superior to that of Na-selenite. Bargellini et al. [20] found the Se content of egg yolk increased by 39–61% when using 0.1 mg/kg of Na-selenite or Se-iodine, with the suggestion that this addition may enhance the Se absorption. According to literature data, it may be stated that the main reason for the increased Se deposition in eggs via Se-Yeast addition is that the majority of Se in Se-Yeast is Se-Met, a Se analog of methionine [45]. Se-Met is deposited in the egg to a greater extent than selenium from Na-selenite and is actively absorbed and incorporated into eggs as effectively as methionine [46]. Given this, it is clear that the highest level of selenium is to be found in the egg content of hens fed with diets enriched in Se-Met.

The estimated parameters of egg characteristics (Table 4) revealed that use of selenium feed additives does not cause significant changes, except for yolk color which was clearly higher according to the La Roche scale () in eggs derived from hens fed with diets supplemented with selenium.

In the presented experimental data, selenium source did not influence egg characteristics. The data are comparable with the results obtained by Invernizzi et al. [43]— these authors stated that addition of selenium into hen diets had no effect on shell thickness, which ranged from 0.359 mm for eggs received from hens from the control group (without selenium addition) to 0.369 mm for eggs received from hens fed with diets with Se-Yeast. These authors additionally stated that the kind of selenium additive applied significantly affected shell strength—this parameter, in their experiment, was the highest for egg shells in the group that received an addition of Se-Yeast, and this amounted to 39.03 N. In the presented data (Table 4), shell strength was also the highest in eggs from animals that received Se-Yeast addition, but these differences were not significant statistically. Heinz and Hoffman [47] did not find any significant differences in the mass or thickness of the eggshell in eggs from Mallard ducks, as a result of the kind and level of selenium. Only one parameter, the yolk color index, was influenced by Se addition. A similar effect of selenium addition was reported by Mohiti-Asli et al. [48]. The effect of selenium on yolk color may be explained by its antioxidative properties. The color of egg yolk is produced by xanthophyll pigments (oxycarotenoids) derived from the diet. Oxycarotenoids, when oxidized, lose their pigmenting power; therefore, it may be assumed that selenium, as well as vitamin E, has a beneficial effect on egg color stabilization when added to laying hen diets [48]. The length of the period of supplementation of Se influenced changes in the egg characteristics and caused an increase in the share of egg yolk and a decrease in the share of egg white. No available data were found on the effect of selenium on yolk and white share in eggs. Our result may be an effect of the laying period (phase) and physiological changes in egg composition including white and yolk share.

The accumulation of selenium in the liver varied between treatments from 313.5 to 462.6 μg·Se/kg (Table 5).

Table 5: Selenium content in hen livers.

The highest Se concentration in hen livers was found in birds fed with diets containing Se-Yeast (462.2 μg/kg), but the lowest were in treatments in which hens were fed with Se-Met diets (). Selenium content in wet liver tissue, as presented in numerous publications, changed significantly; however, in general, the presented data from these experiments indicated that application of organic forms of selenium significantly increased selenium concentrations in the liver. Pan et al. [49] found that, when using Na-selenite, the Se level in terms of the wet weight of the liver amounted to 645 μg/kg and for Se-Yeast, the level was 669 μg/kg () in comparison to the control (582 μg/kg). In fact, our data are in agreement with these findings concerning selenium-enriched yeast application because the content of selenium in the liver of birds receiving diets with Se-Yeast was significantly higher () when compared to the control and Na-selenite groups of animals; however, unexpectedly, no statistical differences were stated between the control and Na-selenite groups. In our data, the content of selenium in the liver of hens fed with diets enriched with selenium in the form of Se-Met was significantly () lower than in the remaining groups. Our data do not agree with those contained in most publications on this topic. A significant effect on liver selenium concentration of Se-Met addition into hen diets was reported, among others, by Wang et al. [50], who found Se concentrations in the liver of 315 μg/kg when Na-selenite was added and 596 μg/kg when Se-Met was included in the diet. Also, Windisch et al. [51] stated that, by using Se-Met, the cumulation of this trace element was significantly () greater than that by the use of Na-selenite or Se-Cys. This obtained result, according to the authors, may have various causes depending on the selenium form and on the selenium metabolism pathways used. According to Suchý et al., Se-Met, in comparison to the inorganic form, is absorbed more quickly in the small intestine and is resorbed independent of its levels in the organism and, additionally, this substance is not stored in the liver and is extensively used and reused [22]. Selenomethionine is nonspecifically incorporated into proteins (Se-Met in place of methionine), particularly in organs with high rates of protein synthesis [52]. Given this fact, in laying hens this form of selenium is incorporated to a greater extent into egg proteins. This was confirmed in the presented experiment by the higher content of selenium in eggs from Se-Met supplemented hen diets.

6. Conclusions

In conclusion, the best method for the enrichment of egg content with selenium is the application of dietary Se-Met as the source of selenium in laying hen diets. Selenium content in eggs from this group of animals was higher than that in a standard egg (11 μg): 17.20 μg/egg. Consumption of such selenium-enriched eggs may increase daily selenium intake and may be considered to be safe. Consumption of two selenium-enriched eggs a day satisfies about 60% of the daily Se dose recommended for humans.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

Research was realised within the project “BIOFOOD–innovative, functional products of animal origin” (no. POIG.01.01.02-014-090/09) co-financed by the European Union from the European Regional Development Fund within the Innovative Economy Operational Programme 2007–2013. The publication was supported by the Wrocław Centre of Biotechnology Programme, the Leading National Research Centre (KNOW), for years 2014–2018.

References

  1. F. W. Edens, “Practical applications for selenomethionine: broiler breeder reproduction, nutritional biochemistry in the feed and food industries,” in Proceedings of Alltech’s 18th Annual Symposium, pp. 29–42, Nottingham University Press, LexingtonLexington, LY, USA, May 2002.
  2. C. A. Zuberbuehler, R. E. Messikommer, M. M. Arnold et al., “Effects of selenium depletion and selenium repletion by choice feeding on selenium status of young and old laying hens,” Physiology and Behavior, vol. 87, no. 2, pp. 430–440, 2006. View at Publisher · View at Google Scholar · View at Scopus
  3. F. W. Edens and K. M. Gowdy, “Selenium sources and selenoproteins in practical poultry production,” in Nutritional Biotechnology in the Feed and Food Industries, pp. 35–55, Nottingham University Press, Nottingham, UK, 2004. View at Google Scholar
  4. A. R. Kennedy, “Selenomethionine protects against adverse biological effects induced by space radiation,” Free Radical Biology and Medicine, vol. 36, no. 2, pp. 259–266, 2004. View at Publisher · View at Google Scholar · View at Scopus
  5. F. Carmagnol, P. M. Sinet, and H. Jerome, “Selenium-dependent and non-selenium-dependent glutathione peroxidases in human tissue extracts,” Biochimica et Biophysica Acta, vol. 759, no. 1-2, pp. 49–57, 1983. View at Publisher · View at Google Scholar · View at Scopus
  6. P. F. Surai, “Selenium in poultry nutrition. 1. Antioxidant properties, deficiency and toxicity,” World’s Poultry Science Journal, vol. 58, no. 3, pp. 333–347, 2002. View at Publisher · View at Google Scholar · View at Scopus
  7. P. F. Surai, “Selenium in poultry nutrition 2. Reproduction, egg and meat quality and practical applications,” World’s Poultry Science Journal, vol. 58, no. 4, pp. 431–450, 2002. View at Publisher · View at Google Scholar · View at Scopus
  8. A. Fischer, A. Wagner, O. P. Waltz, and J. Pallauf, “Selenium status, antioxidant capacity and cell damage in growing pigs fed increasing levels of dietary selenium,” in Proceedings of the Society of Nutritional Physiology, vol. 11, p. 64, Göttingen, Germany, March 2002.
  9. P. F. Surai and N. H. C. Sparks, “Designer eggs: from improvement of egg composition to functional food,” Trends in Food Science and Technology, vol. 12, no. 1, pp. 7–16, 2001. View at Publisher · View at Google Scholar · View at Scopus
  10. D. L. Hatfield, P. A. Tsuji, B. A. Carlson, and V. N. Gladyshev, “Selenium and selenocysteine: roles in cancer, health, and development,” Trends in Biochemistry Sciences, vol. 39, no. 3, pp. 112–120, 2014. View at Publisher · View at Google Scholar · View at Scopus
  11. M. Roman, P. Jitaru, and C. Barbante, “Selenium biochemistry and its role for human health,” Metallomics, vol. 6, no. 1, pp. 25–54, 2014. View at Publisher · View at Google Scholar · View at Scopus
  12. P. F. Bellinger, A. V. Raman, M. A. Reeves, and M. J. Berry, “Regulation and function of selenoproteins in human disease,” Biochemical Journal, vol. 422, no. 1, pp. 11–22, 2009. View at Publisher · View at Google Scholar · View at Scopus
  13. M. P. Burke and K. Opeskin, “Fulminant heart failure due to selenium deficiency cardiomyopathy (Keshan disease),” Medicine, Science and the Law, vol. 42, no. 1, pp. 10–13, 2002. View at Publisher · View at Google Scholar · View at Scopus
  14. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA), “Scientific opinion on dietary reference values for selenium,” EFSA Journal, vol. 12, no. 10, p. 3846, 2014. View at Publisher · View at Google Scholar
  15. S. J. Fairweather-Tait, Y. Bao, M. R. Broadley et al., “Selenium in human health and disease,” Antioxidants and Redox Signaling, vol. 14, no. 7, pp. 1337–1383, 2011. View at Publisher · View at Google Scholar · View at Scopus
  16. EFSA Publication, “Scientific opinion of the panel on food additives, flavourings, processing aids and materials in contact with food (AFC) on a request from the commission on selenium-enriched yeast as source for selenium,” EFSA Journal, vol. 766, pp. 1–43, 2008. View at Google Scholar
  17. R. Stoffaneller and N. L. Morse, “A review of dietary selenium intake and selenium status in europe and the Middle East,” Nutrients, vol. 7, no. 3, pp. 1494–1537, 2015. View at Publisher · View at Google Scholar · View at Scopus
  18. Z. Kralik, G. Kralik, M. Grčević, I. Kralik, and V. Gartner, “Physical-Chemical characteristics of designer and conventional eggs,” Brazilian Journal of Poultry Science, vol. 20, no. 1, pp. 119–126, 2018. View at Publisher · View at Google Scholar · View at Scopus
  19. V. I. Fisinin, T. T. Papazyan, and P. F. Surai, “Producing selenium-enriched eggs and meat to improve the selenium status of the general population,” Critical Reviews in Biotechnology, vol. 29, no. 1, pp. 18–28, 2009. View at Publisher · View at Google Scholar · View at Scopus
  20. A. Bargellini, I. Marchesi, L. Rizzi et al., “Selenium interactions with essential and toxic elements in egg yolk from commercial and fortified eggs,” Journal of Trace Elements in Medicine and Biology, vol. 22, no. 3, pp. 234–241, 2008. View at Publisher · View at Google Scholar · View at Scopus
  21. V. I. Fisinin, T. T. Papazyan, and P. F. Surai, “Producing specialist poultry products to meet human nutrition requirements: selenium enriched eggs,” World’s Poultry Science Journal, vol. 64, no. 1, pp. 85–98, 2008. View at Publisher · View at Google Scholar · View at Scopus
  22. P. Suchý, E. Straková, and I. Herzig, “Selenium in poultry nutrition: a review,” Czech Journal of Animal Science, vol. 59, no. 11, pp. 495–503, 2014. View at Publisher · View at Google Scholar
  23. P. F. Surai, Selenium in Nutrition and Health, Nottingham University Press, Nottingham, UK, 2006.
  24. L. Jiakui and W. Xiaolong, “Effect of dietary organic versus inorganic selenium in laying hens on the productivity, selenium distribution in egg and selenium content in blood, liver and kidney,” Journal of Trace Elements in Medicine and Biology, vol. 18, no. 1, pp. 65–68, 2004. View at Publisher · View at Google Scholar · View at Scopus
  25. G. N. Schrauzer, “Selenium yeasts: composition, quality, analysis, and safety,” Pure and Applied Chemistry, vol. 78, no. 1, pp. 105–109, 2004. View at Publisher · View at Google Scholar · View at Scopus
  26. A. C. Pappas, F. Karadas, P. F. Surai et al., “Interspecies variation in yolk selenium concentrations among eggs of free-living birds: the effect of phylogeny,” Journal of Trace Elements in Medicine and Biology, vol. 20, no. 3, pp. 155–160, 2006. View at Publisher · View at Google Scholar · View at Scopus
  27. F. Cubadda, F. Aureli, S. Ciardullo et al., “Changes in selenium speciation associated with increasing tissue concentrations of selenium in wheat grain,” Journal of Agricultural and Food Chemistry, vol. 58, no. 4, pp. 2295–2301, 2010. View at Publisher · View at Google Scholar · View at Scopus
  28. K. P. McConnell and G. Cho, “Transmucosal movement of selenium,” American Journal of Physiology, vol. 208, no. 6, pp. 1191–1195, in F. W. – “Practical application of selenomethionine,” in Nutritional Biotechnology in the Feed and Food Industries. Proceedings Alltech 18th Annual Symposium, Nottigham University, pp. 29–42, 1965.
  29. Y. Noy, N. Frisch, N. Rand, and D. Sklan, “Trace mineral requirements in turkeys,” World’s Poultry Science Journal, vol. 50, no. 3, pp. 253–269, 1994. View at Google Scholar
  30. F. Ryszka, Z. Dobrzański, and B. Dolińska, “Optimization of the process of selenium, chromium and zinc incorporation into yeasts Saccharomyces cerevisiae,” Chemical Products in Agriculture and Environment, Czech-Pol Trade, Prague, Brussels, Sweden, 2002. View at Google Scholar
  31. NRC, Nutrient Requirements of Poultry, National Academy Press, Washington (DC), USA, 9th edition, 1994.
  32. AAFCO, AAFCO Official Publication, Association of American Feed Control Officials Incorporated, Olympia, WA, USA, 2003.
  33. P. F. Surai and V. I. Fisinin, “Selenium in poultry breeder nutrition: an update,” Animal Feed Science and Technology, vol. 191, pp. 1–15, 2014. View at Publisher · View at Google Scholar · View at Scopus
  34. EFSA FEEDAP Panel EFSA Panel on Additives and Products or Substances used in Animal Feed, “Scientific opinion on the safety and efficacy of selenium compounds (E8) as feed additives for all animal species: sodium selenite (coated granulated preparation), based on a dossier submitted by Doxal Italia S.p.A,” EFSA Journal, vol. 13, no. 11, pp. 1–27, 2015. View at Publisher · View at Google Scholar
  35. AOAC, Official Methods of Analysis, Association of Official Analytical Chemists (AOAC), Gaithersburg, ND, USA, 18th edition, 2007.
  36. S. Moore and N. H. Stein, “Chromatographic determination of amino acids by the use of automatic recording equipment,” Methods in Enzymology, vol. 6, pp. 819–831, 1963. View at Publisher · View at Google Scholar · View at Scopus
  37. StatSoft Inc., “STATISTICA (data analysis software system),” October 2011, http://www.statsoft.com. View at Google Scholar
  38. E. Delezie, M. Rovers, A. Van der Aa, A. Ruttens, S. Wittocx, and L. Segers, “Comparing responses to different selenium sources and dosages in laying hens,” Poultry Science, vol. 93, no. 12, pp. 3083–3090, 2014. View at Publisher · View at Google Scholar · View at Scopus
  39. Z. Gajčević, G. Kralik, E. Has-Schön, and V. Pavić, “Effects of organic selenium supplemented to layer diet on table egg freshness and selenium content,” Italian Journal of Animal Science, vol. 8, no. 2, pp. 189–199, 2016. View at Publisher · View at Google Scholar
  40. A. H. Cantor, N. D. Paton, A. J. Pescatore, M. J. Ford, and C. A. Smith, “Effect of selenium yeast in the hen’s diet on transfer of selenium to the egg and the developing embryo,” Krmiva, vol. 45, pp. 327–334, 2003. View at Google Scholar
  41. R. L. Payne, T. K. Lavergne, and L. L. Southern, “Effect of inorganic versus organic selenium on hen production and egg selenium concentration,” Poultry Science, vol. 84, no. 2, pp. 232–237, 2005. View at Publisher · View at Google Scholar · View at Scopus
  42. R. N. Reis, S. L. Vieira, P. C. Nascimento, J. E. Peña, R. Barros, and C. A. Torres, “Selenium contents of eggs from broiler breeders supplemented with sodium selenite or zinc-L-selenium-methionine,” Journal of Applied Poultry Research, vol. 18, no. 2, pp. 151–157, 2009. View at Publisher · View at Google Scholar · View at Scopus
  43. G. Invernizzi, A. Agazzi, M. Ferroni et al., “Effects of inclusion of selenium-enriched yeast in the diet of laying hens on performance, eggshell quality, and selenium tissue deposition,” Italian Journal of Animal Science, vol. 12, no. 1, pp. 1–8, 2013. View at Publisher · View at Google Scholar · View at Scopus
  44. M. W. Zia, A. Khalique, S. Naveed, and J. Hussain, “Impact of selenium supplementation on productive performance and egg selenium status in native Aseel chicken,” Italian Journal of Animal Science, vol. 15, no. 4, pp. 649–657, 2016. View at Publisher · View at Google Scholar · View at Scopus
  45. M. P. Kelly and R. F. Power, “Fractional and identification of the major selenium compounds yeast,” Journal of Dairy Science, vol. 78, pp. 237–242, 1995. View at Google Scholar
  46. G. F. Combs and S. B. Combs, Selenium in Foods and Feeds, the Role of Selenium in Nutrition, Academic Press, New York, NY, USA, 1986.
  47. G. H. Heinz and D. J. Hoffman, “Comparison of the effects of seleno-L-methionine, seleno-DL-methionine and selenized yeast on reproduction of mallards,” Environmental Pollution, vol. 91, no. 2, pp. 169–175, 1996. View at Publisher · View at Google Scholar · View at Scopus
  48. M. Mohiti-Asli, F. Shariatmadari, and H. Lotfollahian, “The influence of dietary vitamin E and selenium on egg production parameters, serum and yolk cholesterol and antibody response of laying hen exposed to high environmental temperature,” Archiv für Geflügelkunde, vol. 74, no. 1, pp. 43–50, 2010. View at Google Scholar
  49. C. Pan, K. Huang, Y. Zhao et al., “Effect of selenium source and level in hens diet on tissue selenium deposition and egg selenium concentration,” Journal of Agricultural and Food Chemistry, vol. 55, no. 3, pp. 1027–1032, 2007. View at Publisher · View at Google Scholar · View at Scopus
  50. Y. B. Wang, X. A. Zhan, D. Yuan, X. W. Zhang, and R. J. Wu, “Influence of dietary selenomethionine supplementation on performance and selenium status of broiler breeders and their subsequent progeny,” Biological Trace Elements Research, vol. 143, no. 3, pp. 1497–1507, 2011. View at Publisher · View at Google Scholar · View at Scopus
  51. W. Windisch, S. Gabler, and M. Kirchgessner, “Effect of selenite, seleno-cysteine and seleno-methionine on the selenium metabolism of 75Se labeled rats,” Journal of Animal Physiology and Nutrition, vol. 78, no. 1–5, pp. 67–74, 1998. View at Google Scholar
  52. G. N. Schrauzer, “Selenomethionine: a review of its nutritional significance, metabolism and toxicity,” Journal of Nutrition, vol. 130, no. 7, pp. 1653–1656, 2000. View at Publisher · View at Google Scholar