Effect of chronic dietary nitrate and different iodine supply on porcine thyroid function, somatomedin-C-level and growth

Abstract

Due to the lately increased oral nitrate intake of humans and animals the influence of 3% KNO3 in the diet on growth and the thyroid hormone and somatomedin-C-concentration in the serum was to be tested in an experiment with growing pigs in case of different iodine supply. The investigations were undertaken in 3 groups with 9 piglets each. The animals were 6 weeks old: 1. nitrate-exposed, 2. pair-fed to group 1 (without nitrate), 3. ad libitum without nitrate. The mean daily weight gains amounted to 242, 274 and 393 g respectively, after a five-week test period. Compared to the ad libitum control group, the T4-, T3-, rT3- and Sm-C-level of nitrate-exposed animals was significantly lower after 5 weeks. There were no statistically relevant differences between nitrate-exposed and pair-fed animals with regard to the T3- and Sm-C-level. After the 5-week test period with an iodine supply covering the requirement the rations of all 3 groups were supplemented with further 0.8 mg iodine/kg. The T4-, T3- and rT3-levels of the animals of group 1 normalized within one week. The Sm-C-levels of the nitrate-exposed and pair-fed group were still decreased. The investigations show that an increased nitrate intake via food and drinking water influences the thyroid hormone metabolism. It should be taken into consideration in the etiology of endemic struma. Furthermore, excessive nitrate intakes influence the Sm-C-concentration and thus growth due to food intake depression.