Influence of casein and soy protein isolate on thyroid hormone status and on energy and protein metabolism in growing pigs

Three experiments were carried out each using 12 growing, male, castrated pigs, which were fed semi-synthetic, isoenergetic diets based on either casein or soy protein isolate (3 x 650 kJ ME/kg BW0,62 x d(-1)). Three protein sources [(casein plus amino acid (AA) supplementation and soy protein isolate (SPI) without and with AA supplementation)] and two levels of protein supply (100% and 50% of the protein requirement) were tested. Both protein supply levels supported a lower protein deposition with SPI compared with casein+AA. At the 50% protein supply level, the decreased protein deposition was compensated by an increased fat deposition. At the 100% protein supply level there was no significantly higher fat deposition but a tendency of higher heat production. In our study, thyroid hormone levels were affected by the protein:energy ratio and the quality (AA pattern) of the dietary protein but higher thyroid hormone levels were not associated with an increased heat production.