there is no obvious physiological benefit from any particular amplitude of diurnal cycling as long as overall balance is achieved, there is no obvious reason to be concerned if dietary protein or indispensable amino acid levels result in lower amplitudes of diurnal cycling and lower rates of postprandial protein synthesis. In fact, Pacy et al. (1994) reported that the mean daily rate of protein synthesis, S, was higher on very high protein intakes, (S=4.55 ± 0.5 g/kg/d), compared with 2 weeks on a very low protein diet of 0.36 g/kg/d (S=4.09 ± 0.63 g/kg/d), although P > 0.05, but even this was explainable by the fact that overall daily leucine balances became positive at 0.82 g protein/kg/d. Thus turnover per se (i.e., replacement) was not increased, and it was concluded that any influence of protein intake on protein turnover was below the detection limits of current methods. In general, previous results provided little support for a nutritional sensitivity of whole body protein turnover within the range of intakes likely to be consumed.
Marchini et al. (1993) reported no significant changes in protein synthesis in the postabsorptive or postprandial state on diets with the FAO, MIT, or egg amino acid patterns. The only kinetic responses commented upon were a reduced rate of postabsorptive proteolysis after 3 weeks on the FAO diet and a lack of a feeding-induced inhibition. Although this explains the lack of postprandial gain and overall negative balance, it appears that no changes in turnover occurred in these studies.
The one exceptional body of data is the large reduction in protein synthesis observed in subjects fed very low levels of individual amino acids in the MIT studies (Young and Marchini, 1990). However, Pacy and coworkers (1994) have suggested that since such responses are only observed when the tracer is the same as the limiting amino acid, and not observed in response to an identical dietary protocol when turnover is measured by a nonlimiting labeled amino acid (e.g., Zello et al., 1992), a methodological problem associated with either compartmentation or the nonsteady state may exist.
On this basis, it appears that the data suggesting that protein synthesis falls at inadequate intakes of individual amino acids are artifactual. Thus, with balanced amino acid mixtures and whole proteins, whole body protein synthesis changes only minimally at intakes below those capable of supporting overall balance. Thus, protein turnover is not a useful indicator of nutritional adequacy that can be used in the debate.
A minimal requirement for protein and amino acids may be identifiable with sufficient time allowed for adaptation. This value is likely to be much lower than the amounts provided by natural diets that also provide sufficient energy and other nutrients. Thus, the magnitude of this minimal requirement becomes to some extent only an issue of scientific curiosity. It might be considered that the requirement for balance is less important than the functional