Functional egg production. II. The roles of selenium, zinc, and iodine

Different aspects of selenium, zinc, and iodine in the nutrition of laying hens are reviewed in relation to the production of functional eggs enriched with these trace elements. Selenium can be easily transferred into the eggs. Selenium is a part of certain antioxidant selenoproteins (primarily enzyme glutathione-peroxidase) improving antioxidant status and the system of antiradical defense in laying hens; these compounds can also be transferred into eggs improving the oxidative stability of yolk and albumen during egg storage (Z.G. Wang et al., 2010). Recent results of the worldwide research proved that diets for layers (and eggs as a result) should by advisably enriched simultaneously with selenium (M. Fasiangova, G. Borilova, 2017) and vitamin E since this combination of the two most active dietary antioxidants provides the best antioxidative defense in layers and the best antioxidative status of the eggs (Z. Zduńczyk et al., 2013). The organic forms of selenium are shown to be the most effective selenium sources (compared to inorganic sources) due to less toxicity for poultry, better selenium transfer to eggs and deposition into the body selenium pool, primarily in muscle tissues, which can be activated during an oxidative stress (P.F. Surai, V.I. Fisinin, 2016). The combination was also shown to be an effective protector for polyunsaturated fatty acids in yolk lipids (A.Sh. Kavtarashvili et al., 2017). Determination of optimal proportion of selenium and vitamin E in diets for layers requires further research and justification. Zinc is an integral part of antioxidative enzyme superoxide-dismutase (SOD) and lowers oxidative stresses due to the antagonism to the ions of transition metals with high redox potentials. Enrichment of eggs with zinc via high dietary zinc levels improves quality and stability of the albumen during egg storage (H. Aliarabi et al., 2007), eggshell quality, bone development, feather condition and immunity in layers (K.M. Martin, 2016). Supplementation of diets for layers with 50-80 ppm of inorganic or 500-100 ppm of organic zinc will generally not affect their productivity (K. Sahin et al., 2009). Simultaneous enrichment of eggs with selenium and zinc using their high dietary levels is complicated by the antagonism between the two elements which will be possibly overcome due to the development and investigation of their new dietary forms and sources. High dietary iodine levels provide the possibility for the production of iodine-enriched functional eggs; according to EU legislation, however, iodine level in diets of laying birds should not surpass 5 ppm (EU Commission, 2005). Several studies reported the absence of detrimental effects of higher dietary iodine doses (5-10 ppm) on overall productivity in layers while certain egg quality parameters (eggshell thickness and strength, relative albumen weight, Haugh units) decreased with the increase in dietary iodine content (M. Lichovnikova, L. Zeman, M. Cermakova, 2003). Simultaneous enrichment of eggs with selenium and iodine is possible (Yu.A. Ponomarenko, 2015) since these two elements are not antagonists (especially in their organic forms) but rather synergists; the efficiency of different sources and doses of selenium and iodine in combined diet supplementation and transfer to eggs is still to be elucidated.