Theory that successfully explains the magnitude and range of estimates of protein retention (PR) efficiency from the cost of
turnover of existing protein indicates that conventional curves for growth description are inappropriate for protein growth.
A solution to this problem is found in the consideration that the rate-limiting steps for protein synthesis (PS) and breakdown
are likely to be associated with the diffusion of metabolites in and between cells. The algebraic scaling of nuclear and cellular
diffusion capacity with tissue or total body protein leads to a parameterization of the primal differential equation for PR (kg/day)
based on two terms representing PS and breakdown, viz.
PR5cQ½ðP=aÞX 1Z ð4=9ÞY ðP=aÞX 1Z :
where c is an arbitrary constant, Q is the proportion of nuclei active in cell growth or division in a tissue or the whole body,
a is the limit mass for protein (P, kg) in a tissue or the whole body, the power X1Z represents the rate-limiting steps in protein
breakdown and Y is the power of the relationship between cell volume and the amount of tissue protein. For the whole body, the
contribution of the different tissues should be weighted in proportion to their PS rates with, on average, Y51/2. The constant 4/9
arises from the scaling of the specific diffusion rate of DNA activator precursors from nuclear dimensions and from the relationship
between nuclear and cell volume. Experimental evidence on protein breakdown rate as well as protein and body mass points
of inflection indicates that the range of theoretically possible numerical values of the rate-limiting powers X1Z5( i13)/9 for
i51, 2,y,12 seems adequate for the description of the range of observed whole body protein and body mass growth patterns
for mammals. Q51 represents maximal protein retention, and for 0, Q,1, experimental evidence exists in support of a
theoretical relationship between Q and food ingestion. The conclusion follows that some knowledge of the protein limit mass (a)
and of the point of inflection (related to X1 Z) is the main requirement for the application of the theory for description and
prediction in animal nutrition and breeding.