Effects of dietary protein quality on protein turnover in growing pigs

To estimate the long-term effects of dietary protein quality (AA pattern) on whole-body protein synthesis and degradation in growing pigs two experiments were carried out each using four barrows per treatment group, which were fed semisynthetic isoenergetic diets based on either casein or soy protein isolate at 1875 kJ ME/(kg (BW)(0.62) X day) Casein was tested with (CAS+) amino acid (AA) supplementation and soy protein isolate was tested with (SPI+) and without (SPI-) AA supplementation, respectively. The effects of dietary protein quality were studied at the recommended protein supply of 100% (normal protein level (NP), experiment 1 and at a protein supply of 50% of NP (low protein level (LP), experiment 2. In experiment 1 measurements were carried out with pigs of 40-90 kg BW and in experiment 2 with pigs of 40-60 kg BW. During the experiments pigs were housed individually in metabolic cages at 23 +/- 1 degrees C. Protein deposition was determined by the nitrogen balance method during 8 days. The whole-body protein synthesis was derived from the cumulative urinary N-15 excretion up to 48 h after application of a single dose of [N-15]glycine. Calculations were based on a three compartment model. At both protein supply levels, feeding of SPI- with the lower biological value resulted in significantly lower protein deposition than feeding of CAS+. This was a result of a simultaneous decrease of protein synthesis and degradation. Due to the lower requirement for essential AA of the older pigs, the differences in protein deposition and turnover rates between CAS+ and SPI- decreased with increasing BW. Because the efficiency of protein synthesis (ratio of deposition to synthesis) was not altered in response to the dietary protein quality at both protein supply levels, it is concluded that with inadequate AA supply the protein turnover runs on a lower basal level than with adequate AA supply. Intravenous [N-15]glycine administration caused approximately 30% higher estimates of whole-body protein synthesis than oral [N-15]glycine administration did in identical CAS+-fed pigs of about 90 kg BW. This finding is consistent with literature data. However, because this relation is not yet verified in SPI-fed and in younger pigs, the effect of dietary protein quality on protein turnover can only be compared within both protein levels.