Protein turnover in the whole animal.

Methods of measuring whole body protein turnover (WBPT) are discussed briefly. In a range of species from mouse to cow WBPT in the adult bears a fairly constant relationship to metabolic mass and to basal oxygen consumption. In young animals WBPT per unit metabolic mass is higher than in adults. During growth there appears to be an increase in the rate of protein breakdown as well as of synthesis. Of all tissues which have been studied, muscle shows the greatest developmental changes in the rate of protein synthesis. In the absence of food, for example during the night, the rate of protein turnover falls. Studies on the effects of different levels of protein and energy supply have not so far given a constant picture. Very few measurements of WBPT in disease states have been made. Preliminary results suggest that the loss of nitrogen after injury may be due to a block in synthesis rather than to an increase in body protein breakdown.