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Unité Institut National de la Recherche Agronomique/Institut National Agronomique Paris-Grignon Physiologie de la Nutrition et du Comportement Alimentaire, 75005 Paris, France
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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KEY WORDS: • dietary protein • nutritional value • nitrogen retention • high protein diets • humans
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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, Munro 1969
, Rennie et al. 1994
, Young and Pellett 1988
). Levels of body protein remain relatively
constant throughout adult life, despite qualitative and quantitative
variations in dietary protein intake. High protein diets provide as
much as 180–200 g/d in adults, whereas the minimum intake is currently
evaluated at 50 g/d. Under these conditions, reaching a nitrogen
balance involves adaptations that may be associated with adverse
metabolic consequences (Young 1986
, Young et al. 1987
). It is well accepted that the nutritional value of
proteins may differ substantially. Variable factors include their
essential amino acid content and digestibility. Based on the different
methods used to assess dietary protein quality in humans, the protein
digestibility–corrected amino acid score
(PDCAAS)3
takes account of these two parameters
with respect to amino acid requirements (FAO/WHO, 1990
).
However, the definition of protein and amino acid requirements is
complex and makes it difficult to assess the nutritional value of
proteins by using scores as a means of predicting net protein
utilization. |
Nitrogen balance and nitrogen requirements |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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Human nitrogen requirements are usually determined from the nitrogen
balance. The usual procedure is to regress nitrogen balance on intake
and to define the requirement as the intake level that would produce a
zero balance, i.e., equality of dietary N intake and N losses
(Fig. 1
). Nitrogen losses occur in different ways. They mainly arise from
urinary losses in the form of urea, ammonia and creatinine but also in
the form of fecal and miscellaneous losses (Calloway and Margen 1971
) (Table 1
). Minimum nitrogen losses ["obligatory nitrogen losses" (ONL)]
were measured in subjects fed a protein-free diet for 1 week. Under
these conditions, nitrogen losses were estimated at 36 mg/kg/d in
urine, 12 mg/kg/d in feces and 8 mg/kg/d in miscellaneous nitrogen
losses (sweat, sebum, desquamations, nails, hairs and saliva)
(Manatt and Garcia 1992
). Obligatory oxidative losses
and indispensable ileal amino acid losses have been reported to reach
162 and 18 mg/kg/d, respectively (Fuller et al. 1994
,
Young et al. 1989
) (Table 2
). We determined total ileal nitrogen losses as reaching 9 mg/kg/d,
i.e., 16% of ONL. Indispensable ileal amino acid losses represent
~10% of obligatory oxidative losses. From these estimations, ONL
represent 54 mg/kg/d and correspond to a protein requirement level of
0.34 g/kg/d (FAO/WHO, 1985
). Because dietary protein
utilization does not achieve 100% efficiency, it has been suggested
that an intake of 0.6 g/kg/d of well-balanced protein will achieve
a zero nitrogen balance. The adequacy of this diet has been reported in
studies conducted over 2- or 3-mo periods (FAO/WHO 1985
). A safety coefficient is added to this figure so that the
final recommendation for dietary protein is 0.75 g/kg/d. In children,
the requirements for growth must be integrated in addition to
maintenance requirements.
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): "Nitrogen balance estimates are
highly dependent on the assumed amount of N miscellaneous losses...
further studies on these losses and on the factors that influence them
are essential." First, there is a slight difference between large
values for N intake and N losses. Second, it is well recognized that
the nitrogen balance technique overestimates N intake and
underestimates N losses. This is mainly due to the difficulty in
assessing N gas losses after denitrification by the colonic microflora,
N losses through the skin (urea) and expired air (ammonia) and the
nitrate content in food and urine, which is not measured using the
Kjeldahl method. |
Increases in protein intake and nitrogen balance |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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).
Increasing the protein intake induces a series of adaptive processes
(Fig. 2
). The most conspicuous adjustment is an increase in amino acid
oxidation and in subsequent nitrogen excretion, mainly as urea and
especially pronounced in the fed state. There is a trend toward an
increase in the nitrogen balance when nitrogen intakes are increased
(Price et al. 1994
). Changes in the protein balance
mainly result from the enhanced inhibition of protein breakdown by
feeding and, to a lesser extent, from an increase in protein synthesis
(Forslund et al. 1999
) (Fig. 3
). Whole body protein synthesis is probably sensitive to amino acid
availability, whereas degradation may be sensitive to an interactive
effect by both the amino acid level and insulin. The amplitude of
diurnal body protein cycling increases, with no clear change in the
mean daily protein turnover rate (Fig. 4
) (Pacy et al. 1994
). In addition, high protein intakes
have been shown to result in a continuous, positive N balance of 1–3 g
N/d, as shown in Table 3
(Cheng et al. 1978
, Fisher et al. 1967
,
Forslund et al. 1999
, Oddoye and Margen 1979
, Price et al. 1994
). However, it is not
clear whether this apparent retention is real or originates from
intrinsic errors in calculation of the N balance.
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) (Bos et al. 2000
). The possible site of deposition
under both acute and chronic feeding conditions remains to be
determined. It is also unclear whether increasing the dietary protein
level causes a generalized increase in protein turnover. Adaptability
could be enhanced if the turnover was more rapid.
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Dietary protein nitrogen distribution during postprandial phase |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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). Acute nitrogen
deposition during the postprandial phase is likely to be particularly
critical in terms of the deposition of dietary protein in the tissues.
Assessment of the postprandial utilization of dietary proteins is an
appropriate approach, because this parameter is known to influence
protein turnover (Marchini et al. 1993
).
Studies have been undertaken to assess the acute postprandial
utilization of dietary protein during the repletion phase of the
diurnal cycle. The key steps concerning the fate of dietary nitrogen
are considered to be i) the amount of nitrogen actually
absorbed; ii) the amount that has been deaminated and
recovered, mainly in the form of urea; and iii) the level of
nitrogen retained in the body. The problems of measuring the
postprandial utilization of dietary protein nitrogen in terms of ileal
nitrogen digestibility and short-term retention of dietary protein
nitrogen can be circumvented by the use of
15N-labeled proteins. This technique makes it
possible to follow the metabolic fate of dietary nitrogen after its
ingestion in humans (Bos et al. 1999
, Gaudichon et al. 1999
, Gausserès et al. 1996
,
1997
, Mahé et al. 1992
,
1994
, Mariotti et al. 1999
). Taking into
account the different results obtained regarding evaluations of total
nitrogen and protein metabolism in adult humans consuming 100–110 g/d
of a well-balanced protein diet, of the 300 g/d protein turnover,
we measured that 75–80 g was lost through the oxidative pathways and
14 g was lost at the ileal level (Fig. 5
). The contribution of dietary nitrogen to the principal pathways was
evaluated at 70–80 and 13–20 g/d, i.e., 30–40% and 17–25%, to
anabolism and oxidative loss fluxes, respectively. This strongly
suggests a preferential orientation of dietary nitrogen toward anabolic
pathways. This preferential orientation toward body protein synthesis
is due to the adequacy of the dietary protein amino acid profile
regarding the body protein and the compartmentation of protein
metabolism. Indispensable amino acids supplied by dietary proteins
equilibrate the free amino acid pool: the first-pass metabolism of
dietary amino acids is mainly related to the splanchnic metabolism,
whereas amino acids released from the peripheral metabolism are used in
the catabolic pathways (i.e., alanine, glutamine).
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). This has
been made possible by the development and validation of an
11-compartment model that makes a particular distinction between free
and protein-bound amino acids in both the splanchnic and peripheral
areas, to describe the cascade of transient metabolic processes that
control the distribution of exogenous nitrogen throughout the body. The
results obtained by modeling the pattern of dietary nitrogen
distribution into the different body compartments after the ingestion
of a protein meal is an useful tool to enable further definition of the
notion of protein quality in a period of protein gain because it
simulates the relative ability of a protein source to promote dietary
nitrogen retention in different organs. It can also be used to
discriminate between different nutritional conditions (type of the
protein ingested, energy content of the meal) and to describe the
processes involved in the differential metabolic utilization of various
protein meals. |
Nitrogen metabolism and dietary protein characteristics |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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, Munro 1964
). Our
current knowledge is somewhat limited. We know that diets containing
poor quality protein are associated with an increase in nitrogen losses
due to the inefficient utilization of indispensable amino acids. It is
necessary to clarify which proportions of dietary and intestinal
nitrogen are absorbed as amino acids or excreted in the feces, urine or
other pathways and, most important, to determine which portion is
utilized for protein synthesis and retained in body proteins.
One of the major limitations to the use of the classic nitrogen balance
method is the existence of diurnal cycling for a transition between the
fasted and fed states, which leads alternately to nitrogen postprandial
accretion and postabsorptive loss phases. Because of this, retention
calculated on a daily basis is lower than that derived from the
postprandial phase (Millward et al. 1974
), and under
these conditions, dietary protein utilization calculated as the daily
gain should be lower than the postprandial gain. Furthermore, the
relationship between protein characteristics and protein intake
requires additional study. It has been demonstrated that differences in
the gastric emptying rate of dietary proteins are associated with
highly variable rates of amino acid absorption in the small intestine
(Gaudichon et al. 1994
, Mahé et al. 1996
). These differences are also associated with significant
differences in amino acid oxidation and nitrogen postprandial accretion
(Boirie et al. 1996
).
Methods based on digestibility and short-term protein retention are
of interest when looking at the short-term utilization of dietary
proteins, but few protein retention values are available in humans. Net
postprandial protein utilization (NPPU) is calculated using true ileal
digestibility and true 15N-labeled protein
deamination parameters and adding the dietary nitrogen collected in the
urine and that retained in the body in the form of urea, as follows:
 |
Using this approach, we calculated NPPU values of 80 and 72% for
milk protein and soy protein, respectively, measured during the 8 h after the ingestion of a standard meal by healthy human subjects
(Fig. 6
). These data strongly suggest the existence of certain differences
between the nutritional value of proteins. These differences should be
taken into account when calculating amino acid scores. According to the
method used at present, PDCAAS values that are >1 are rounded off to
1, based on the argument that (digestible) essential amino acid
concentrations in a protein that exceed those in the reference amino
acid pattern do not provide any additional nutritional value. The
reality is probably more complex, and the best approach would probably
be to compare different calculations of PDCAAS values with available in
vivo results obtained in humans. Studies are in progress to determine
both the ileal digestibility and metabolic fate of the individual
15N-amino acid ingested from milk, soy and wheat
protein and to measure the NPPU of these proteins in subjects adapted
to normal (1 g/kg/d) or high (2 g/kg/d) protein levels in their diet.
As shown in Figure 7
, preliminary results indicate that both the protein source and the
habitual level of protein level intake influence the efficiency of
postprandial dietary nitrogen accretion. As expected, the principal
differences arise from modulations to the splanchnic fate of nitrogen.
A protein source–dependent difference in interorgan amino acid
metabolism has also been described in pigs after the infusion of either
soy or casein (Deutz et al. 1998
).
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). Although it is clear
that adult humans in different cultures survive on a broad range of
protein intakes (from 0.6 g/kg/d to 
5 times that amount), the effects
of high protein diets are still poorly understood. Thus, the
significance of amino acid oxidation still requires clarification in
terms of their roles as nutrients used to provide energy. The ability
of high protein diets to increase nitrogen retention and the protein
turnover rate remains unclear. As far as an assessment of protein
quality is concerned, it seems important to consider the level of
protein in the diet. From a qualitative point of view, the standard
methods used to evaluate protein mainly enable a discrimination between
poor (unbalanced) and high quality protein diets. A comparison of
relatively well-balanced protein diets remains difficult. However,
in vivo methods are available that enable the accurate measurement of
smaller differences between protein sources and the influence of other
factors (other nutrients in the meal, habitual protein intake) on
nitrogen metabolic utilization. The postprandial retention of dietary N
could therefore represent a reference method for further validation
of the PDCAAS method.
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FOOTNOTES |
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3 Abbreviations used: NPPU, net postprandial protein utilization; ONL, obligatory nitrogen loss; PDCAAS, protein digestibility–corrected amino acid score.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONNitrogen balance and nitrogen...Increases in protein intake...Dietary protein nitrogen...Nitrogen metabolism and dietary...REFERENCES |
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