Effects of zinc and vitamin A supplements on plasma levels of thyroid hormones, cholesterol, glucose and egg yolk cholesterol of laying hens

The effects of zinc and vitamin A supplementation to the diet on some blood metabolites were evaluated in Hisex brown laying hens from 56 weeks to 68 weeks of age. A total of 130 birds were divided into two main groups according to vitamin A treatment (0 and 3.44 mg retinyl acetate kg(-1) feed, respectively), each consisting of 65 hens. Hens in both of the main groups were then divided into five zinc treatment groups (0, 25, 50, 100 and 200 mg zinc kg diet(-1) respectively) of 13 hens each. It was observed that plasma T4, T3 and total cholesterol levels were affected by only zinc supplementation. While 100 and 200 mg Zn kg(-1) decreased plasma T4 level compared to control value, plasma T3 level was reduced by 100 mg Zn kg(-1) compared to groups fed less Zn. Adding 50 and 200 mg Zn kg(-1) to the diet increased plasma total cholesterol level in the birds compared to other groups. Vitamin A, zinc, and their interaction did not influence the concentration of plasma high-density lipoprotein (HDL)-cholesterol, glucose and egg yolk cholesterol in laying hens.