Protein synthesis and retention in some tissues of the young pig as influenced by dietary protein intake after early-weaning. Possible connection to the energy metabolism.

Changes in fractional protein synthesis rates (FSR) of 4 tissues (muscle, liver, intestine and bone) were assessed in 2 groups of young pigs from weaning, 10 days postpartum, to one week later, after feeding equal amounts of dry diets at 2 levels of protein (15 and 30%). In the meantime, protein and energy balance measurements were performed on the whole body partitioned into 4 components (carcass, liver, digestive organs, other organs + blood). Whole body energy balances were strongly negative in both groups as a result of low metabolisable energy (ME) intakes and fat mobilization. Protein balance improved, with the increase in dietary protein, at the expense of additional body fat loss. Parallel to that, an increase in the efficiency of ME for protein deposition was noticed. With the lower protein intake, protein deposition remained significantly positive in digestive tissues but not in liver and carcass. Muscle and liver RNA: protein ratios decreased after weaning at rates consistent with the normal age-dependent variations regardless of diet. FSRs were directly related to protein intake and the high supply allowed these tissues to match the preweaning values. In contrast, intestine RNA: protein ratio did not change after weaning and FSR was increased in both groups, with a trend to a higher value with the lower protein supply. Bone RNA: protein ratio and FSR both decreased after weaning on the low-protein diet; the effect of increasing dietary nitrogen could not be assessed in this tissue. The most typical effect of underfeeding associated with early-weaning seems to be an exaggeration of the normal age-dependent increase in protein synthesis per unit of RNA, provided that an adequate protein diet is fed. The relevance of these findings to the variations in the ME efficiency for protein deposition needs further investigations.