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P.L. PELLETT* and V.R. YOUNG**
* Department of Nutrition, University of Massachusetts, Amherst, MA 01003, U.S.A.
** Laboratory of Human Nutrition, School of Science, Massachusetts Institute of Technology, Cambridge, MA 02142-1308, U.S.A.
Abstract
1. Introduction
2. The effects of different levels of energy intake on protein metabolism
3. The effects of different levels of protein intake on energy metabolism
4. Protein/energy ratios
5. Summary and conclusions
References
Human nitrogen
metabolism is highly sensitive to altered dietary intakes of
protein and energy both directly, as levels of required
substrates, and indirectly through effects on endocrine function
and balance. A reexamination of the metabolic and dietary
relationships between protein and energy becomes important when
it is realized that many of the earlier published data on human
protein and amino acid needs were conditioned by the levels of
food energy that had been fed experimentally. Changes in food
energy intake, both below and above energy needs, have been shown
to affect body nitrogen balance (NB). In this paper, a number of
published studies relating protein and energy have been evaluated
and then combined into a single data set (total no. of NB=361)
for analysis. From simple linear regressions, 33 and 36% of the
variation in NB could be explained separately by nitrogen intake
(NJ) and energy intake (EI), respectively. Multiple linear
regression increased the R2 value such
that 53% of the variation in N balance could be explained by NI
and EI in combination. Further analysis, using the full data set,
grouped according to intake ranges of NI and EI, demonstrated
that both EI and NI were individually effective in improving N
balance, even for the lowest intake groups of EI (< 30
kcal/kg) and NI (< 50 mg N/kg).
The
earlier conclusions of CALLOWAY and SPECTOR (1954) and MUNRO
(1951; 1978) that the improvement in nitrogen balance caused by
an increase in energy intake could be frustrated if intake of
protein was inadequate and, conversely, that the beneficial
effects of an increase in protein intake could be inhibited by an
inadequate energy intake, appear to be true only under more
extreme conditions of protein or energy limitation than applies
to the majority of the studies analyzed herein. When the effects
of changes in protein intake on energy metabolism were examined,
it was considered that these were generally less significant in
the context of estimating energy requirements than were the
effects of energy on protein metabolism and requirements. The
relationship between NB and dietary protein/energy ratios (Pcal%)
was non significant and confirmed that Pcal% by itself was of
little value in predicting NB. In combination with NI, however,
it could be a useful predictor (R2 =
0.57). It is concluded that the use of dietary protein/energy
ratios for assessment of the protein value of a diet requires a
considerably greater degree of sophistication in its
interpretation than the simple nature of the ratio would seem to
imply.
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).