Effects of dietary protein quality on protein turnover in growing pigs

To estimate the long-term effects of dietary protein quality (AA pattern) o n whole-body protein synthesis and degradation in growing pigs two experime nts were carried out each using four barrows per treatment group, which wer e fed semisynthetic isoenergetic diets based on either casein or soy protei n 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 (S PI+) and without (SPI-) AA supplementation, respectively. The effects of di etary protein quality were studied at the recommended protein supply of 100 % (normal protein level (NP), experiment 1 and at a protein supply of 50% o f NP (low protein level (LP), experiment 2. In experiment 1 measurements we re 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 metaboli c cages at 23 +/- 1 degrees C. Protein deposition was determined by the nit rogen balance method during 8 days. The whole-body protein synthesis was de rived from the cumulative urinary N-15 excretion up to 48 h after applicati on of a single dose of [N-15]glycine. Calculations were based on a three co mpartment model. At both protein supply levels, feeding of SPI- with the lo wer biological value resulted in significantly lower protein deposition tha n 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 b etween CAS+ and SPI- decreased with increasing BW. Because the efficiency o f protein synthesis (ratio of deposition to synthesis) was not altered in r esponse to the dietary protein quality at both protein supply levels, it is concluded that with inadequate AA supply the protein turnover runs on a lo wer basal level than with adequate AA supply. Intravenous [N-15]glycine adm inistration 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. H owever, 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 b e compared within both protein levels.