Effects of dietary protein quality on energy metabolism and thyroid hormone status in growing pigs

To estimate long-term effects of dietary protein quality on energy metaboli sm and thyroid hormone status in growing pigs two experiments were carried out, each using 6 growing German Landrace barrows (40 to 90 kg body weight (BW)) per treatment group, which were fed semisynthetic isoenergetic diets based on either casein or soy protein isolate at 1875 kJ ME/(kg BW0.62 x d) . Casein was tested with (CAS+) amino acid (AA) supplementation (methionine + cystine, threonine, tryptophane) and soy protein isolate was tested with out (SPI-) AA supplementation at the recommended protein supply of 100% (no rmal protein level NP)) and at a protein supply of 50% of NP. During experi ments pigs were housed individually in metabolic cages at 23 +/- 1 degreesC . At both protein supply levels, SPI- in comparison to CAS+ caused a lower protein energy retention (PER), which was compensated mainly by an increase d fat energy retention (FER). The reduction of the protein supply to 50% ca used a lower PER by 30 to 50% in both dietary qualities, which was compensa ted by a significantly higher FER. However, the heat production (HP) was ne ither affected by the protein quality nor by the quantity, and resulted in nearly similar values of 60% of ME intake. The thyroid hormone concentratio ns were dependent primarily on the amount of protein supply, and after decr ease of supply to 50% secondly on the dietary protein quality. The increase d thyroid hormone concentrations at the 50% protein level were in euthyroid range of pigs and obviously not associated with HP.