|Protein-Energy Interactions (International Dietary Energy Consultative Group - IDECG, 1991, 437 pages)|
|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|
As has been discussed, both in this volume and elsewhere (MUNRO, 1964; YOUNG et al., 1981; 1983; 1992a; b), there are many reasons why human nitrogen metabolism is highly sensitive to altered dietary intakes of protein and energy. These include the supply of energy yielding substrates and of amino acids that serve as substrates for the formation of polypeptides and proteins and as a source of chemical energy for the elaboration of high energy bonds (ATP and GTP). The latter are required for the formation of the initiation complex, amino acyl-tRNAs, peptide bond formation and for the release of amino acids from intact proteins via protein degradation with entry into the tissue free amino acid pools. Protein and energy intakes can also influence endocrine function and balance and this, in turn, will determine the status of both body nitrogen and energy metabolism for a given dietary situation. Although the dietary relationships between protein and energy were recognized many years ago, as evidenced for example by the well-known distinctions between starvation and non-protein feeding as they affect nitrogen excretion, the reviews of MUNRO (1951) and CALLOWAY and SPECTOR (1954) gave a renewed impetus to research on the relationships between energy and protein intakes and to both nitrogen and energy balances. Thus, it has become clear more recently that many of the earlier data on human protein and amino acid needs (see ROSE and WIXON, 1955) were conditioned by the relatively high levels of food energy fed during the nitrogen balance experiments from which the estimates of dietary needs had been obtained. It should be noted that the high levels of energy intake used in these earlier nitrogen balance experiments, examining protein and amino acid requirements, were not accidental but were consciously aimed for in attempting to determine minimum protein needs. As was remarked by MARTIN and ROBISON in 1922:
"The most certain way of determining the minimum would be to take a diet so much in excess of the energy needs that the blood sugar is maintained high, the liver and muscles are kept well stocked with glycogen and the surplus is being stored as fat".
These authors were striving for intakes of at least 55 kcal/kg in adult subjects, a figure whose degree of excess would depend on the subject.
Another relevant area is the pathophysiology of protein-energy malnutrition (PEM). Severe deficiencies of protein and/or food energy, especially in association with infections, can lead to PEM (see OLSON, 1975). However, only imbalances between protein and energy levels lead to metabolic abnormalities (PLATT, HEARD and STEWART, 1964). The latter contribute to the development of the kwashiorkor form of PEM. The role of infection in the pathogenesis of PEM is mediated, at least in part, by hormonal changes (MUNRO 1964; WHITEHEAD and ALLEYNE, 1972) that induce catabolic changes in protein and energy metabolism (see DARMAUN et al., 1988; PONTING et al., 1990 for specific recent examples).
Furthermore high-protein diets can even improve energy kinetics in patients suffering from McArdle's disease, a hereditary failure of normal glycogen storage and metabolism (JENSEN et al., 1990). Thus, considerations of protein-energy relations, as delineated in the title of this paper, could involve a wide range of considerations in nutrition, physiology and biochemistry. However, for purposes of this review the major focus will be on the effects of changes in dietary energy intake on protein metabolism, particularly as reflected by changes in nitrogen balance. We will attempt to bring together an analysis of numerous N-balance studies that have been conducted before and since the reviews of MUNRO (1951) and CALLOWAY and SPECTOR (1954). Our purpose is to develop an updated, quantitative picture of the relations between energy and nitrogen on body N balance, particularly in healthy adult subjects.
Body protein or nitrogen balance, in the healthy mature human, is maintained within relatively narrow limits indicating a regulation that is achieved by varying the rate of N excretion in relation to changes in N intake and the individual's metabolic state. MUNRO (1951; 1964; 1978) and CALLOWAY and SPECTOR (1954) demonstrated that nitrogen balance is influenced by both protein intake and energy intake, even when the intake of energy is more than adequate; in consequence, estimates of protein requirements, based on nitrogen balance, will be affected by the level of dietary energy. In addition, the type of energy source may be important, especially in short-term N-balance studies, since there is a specific insulin-dependent effect of dietary carbohydrate on protein metabolism that is not shared by fat. This particular interaction occurs when carbohydrate and amino acids are absorbed from the same meal (MUNRO, 1978).