|Protein-Energy Interactions (IDECG, 1991, 437 p.)|
|The effects of different levels of energy intake on protein metabolism and of different levels of protein intake on energy metabolism: A statistical evaluation from the published literature|
Changes in dietary intakes of protein and energy will significantly influence human nitrogen metabolism. This arises both from changes in the supply of amino acids that serve as substrates for the formation of polypeptides, and from changes in the amounts and sources of chemical energy for the elaboration of high-energy intermediaries (ATP and GTP) required for the formation of the initiation complex, amino acyl-tRNAs, peptide bond formation and for the release of amino acids from dietary and endogenous proteins into the tissue free amino acid pools. In addition, the status of both body nitrogen and energy metabolism is determined by endocrine function and balance which are themselves affected by protein and energy intakes.
While the relationships between protein and energy were well known in the early era of nutritional investigation, it was not until the extensive reviews of MUNRO (1951) and CALLOWAY and SPECTOR (1954) appeared that the relationships were again considered seriously in nutrition research, and it had become clear that many older data on human protein and amino acid needs were conditioned by the levels of food energy given during the nitrogen balance experiments from which conclusions about requirements had been drawn.
Although we now appreciate that there are a vast number of factors involved in determining the relationships between protein and energy intakes and protein metabolism, the major focus of this review has been on the effects of changes in dietary energy intake on protein metabolism, as reflected by alterations in nitrogen balance in adult subjects. Brief consideration has also been given to the needs of children, including those under the special conditions of prematurity and those recovering from malnutrition.
Nitrogen balance has been demonstrated as being influenced by changes in food energy intake, below or above energy needs. Thus, nitrogen balance must be seen as a result of levels of both energy and protein intake, and both need to be considered before any reliable interpretation of nitrogen balance can be made in relation to protein requirements. By current theory, the improvement in nitrogen balance caused by an increase in energy intake can be frustrated, however, if intake of protein is inadequate; conversely, the beneficial effects of an increase in protein intake can be inhibited by an inadequate energy intake. The results of our analysis, however, question this interpretation in that it may only be true under highly restrictive conditions.
A number of studies from the literature over the last three decades have been evaluated, and, by expressing the results in the same units, a single data set was produced which allowed a more detailed examination of the possible statistical correlations between nitrogen and energy intakes and nitrogen balance. By using simple linear regression, 33 to 36% of the variation in NB could be explained separately by NI and EI, while by using multiple linear regression some 53% of the variation in N balance could be explained by NI and EI in combination. Despite the perceived importance of dietary protein/energy ratios, the relationship between NB and Pcal% was non significant and confirmed that Pcal% by itself is of little predictive value. Nevertheless, Pcal% in combination with protein intake (NJ) was a better predictor of nitrogen balance (R2=0.57). Further analysis, using the full data set grouped for EI and NI, demonstrated that over the range for EI and NI, from ca. 30 to 60 kcal/kg and 40 to 200 mg N/kg, increases in either El or NI improve NB, even at the lowest levels of El (< 30 kcal/kg) or NI (< 50 mg N/kg).
It would appear, therefore, that the statements of both CALLOWAY and SPECTOR (1954) and MUNRO (1951; 1978), which claimed that the improvement in nitrogen balance caused by an increase in energy intake could be frustrated if intake of protein were inadequate, and that, conversely, the beneficial effects of an increase in protein intake could be inhibited by an inadequate energy intake, were applicable only under much more extreme conditions of protein or energy limitations than was generally the case for the analyzed studies. Within this range, that is from about 2100 kcal and 18 g protein to about 4200 kcal and 90 g protein/d for a 70 kg male, increases in both EI and NI appeared to be separately and individually effective in improving NB, and no stepwise progression, i.e., expected improvement in NB by increases in either NI or EI being frustrated by a lack of increase of the other, could be observed.
It is also concluded that, while there are very real effects of protein on energy metabolism, they are generally less significant, in the context of requirement estimates, than are the effects of energy on protein (nitrogen) metabolism. Finally, although Pcal% has been suggested as a useful dietary indicator for protein sufficiency, there are considerable limitations in its practicality. This emerges, in part, from the fact that it is not generally directly correlated with nitrogen-balance response; the use, therefore, of protein/energy ratios for assessment of dietary protein value requires careful and detailed evaluation of the specific circumstances involved. Nevertheless, when energy needs are met, a minimum percentage of protein can be specified.