Quantitative aspects of protein fractional synthesis rates in ruminants

Protein fractional synthesis rate (FSR) is a key-factor in the characterisation of ruminant metabolism. Published data from the literature were collected and statistically analysed to isolate the factors influencing FSR. FSR varied largely depending on the tissue considered, over a range from 1 to 20. FSR, with the plasma as the precursor pool for protein synthesis, was halved compared to that of the intracellular pool. The method for supplying the amino acid also significantly affects FSR since the flooding dose technique gave higher FSR estimates than the constant infusion technique. The choice of the labelled amino acid infused influenced FSR. There is a ranking order depending on the tissue or organ. The protein and energy levels of the diets significantly increased FSR, which raises the question of the body nitrogen requirements. Moreover, FSR values were dependent on the physiological status of the animals. To conclude, FSR values should be determined simultaneously with other biological parameters in order to obtain a realistic quantitative estimate of the nitrogen turnover rates during intermediary metabolism.