Download Limits of adaptation to high dietary protein intakes

European Journal of Clinical Nutrition (1999) 53, Suppl 1, S44±S52
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Limits of adaptation to high dietary protein intakes
AA Jackson1*
1
Institute of Human Nutrition, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, UK
Ingested protein is made available to the body following digestion and absorption as amino acids and contributes
to the body's demand for amino acids for protein synthesis and other metabolic pathways. As the pattern of
amino acids required for metabolism is substantially different from that ingested, extensive metabolic
interchange serves to improve the match. As a matter of course oxidation of amino acids contributes to
satisfying the energy needs of the body. Amino acids in excess of immediate requirements follow degradative
pathways and if the capacity of these pathways is exceeded adverse consequences ensue. In pathological states,
such as inborn errors of metabolism, there is an obvious constraint on metabolic ¯ow with serious sequelae.
Pathways may be constrained to a lesser extent due to genetic polymorphisms, metabolic programming,
limitation of cofactors or lack of associated substrates. Any of these can result in metabolic derangements,
which do not manifest as overt disease, but limit normal function. There is the need to determine the dose
response to increases in dietary protein and amino acid availability, using critical metabolic intermediates as
outcome indices in order to clarify the upper limit of intake with which the body can cope under a range of
physiological and pathological states.
Introduction
By implication adaptation is a purposive response to a
changed situation and results in a functional state better
suited to the new situation, although this may be at some
cost (Waterlow, 1985). A successful adaptation implies
minimal cost for optimal function. Usually the term has
been applied to the body's response to a reduction in
protein intake to a level where normal function might be
threatened or compromised. The implicit assumption in
discussing the response to high levels of protein intake is
that higher than usual intakes may confer some bene®t. In
approaching the question Peter Garlick has provided a very
helpful overview and in my opinion developed a fairly
convincing argument that there is no evidence for bene®t
being conferred simply by increasing the protein intake to a
high level (Garlick et al, 1999). The approach adopted asks
whether it is possible to drive the body to improved
function, or to determine adverse effects or gross pathology
using whole body measures when comparing higher levels
of protein intake with habitual intakes. If anything he has
shown that high intakes of protein, by presenting an
excessive metabolic load to the system, have the potential
to produce harm or damage. For this reason I presume the
question being posed relates to the determination of the
upper limit of protein intake which is tolerable without
toxic effects or compromise in function. The analysis raises
a number of important issues about the approach to experimental design in seeking to address questions of this nature,
which border on considerations of toxicology.
In nutrition the important role played by protein in the
diet is predominantly as a source of amino acids. Care has
to be taken to ensure that in studies exploring the response
to changes in the intake, the potential confounding effect
*Correspondence: Institute of Human Nutrition, University of
Southampton, Bassett Crescent East, Southampton SO16 7PX, UK.
and metabolic importance of the nutrients other than amino
acids which are found in protein containing foods is
recognised. There is the need to consider the extent to
which changes exposed during relatively acute, experimental studies, where diets are fed for relatively short periods
of time, provide a useful understanding of the effects of
chronic ingestion over weeks, months or years. Within this
the importance of day-to day variation may be of special
relevance.
Amino acid excess and imbalances
The primary worth to the body of the proteins consumed in
the diet is for the amino acids made available following
digestion and absorption. The responses to higher intakes of
protein and amino acid imbalances in rat and other animal
studies have been fully discussed. Some forty to ®fty years
ago Harper and his colleagues drew attention to the
possible toxic effects of either excess or imbalanced intakes
of amino acids upon the metabolic behaviour of the body
(Harper et al, 1970). Although they stated that they were
unable to develop a simple rule to be applied across the
board, there are a limited number of general principles,
which can be used to guide the approach to the human
situation.
`It is important to distinguish clearly between de®ciencies and imbalances of amino acids . . . Investigations of
imbalances are concerned with the effects of surpluses of
indispensable amino acids other than the one that is limiting for growth or maintenance . . . Again, it must be
emphasised that the term amino acid imbalance is also
de®ned operationally'. There are a number of points of
importance, which derive from this work:
1. The molar requirement by the metabolism of the body
for non-essential amino acids (NEAA) is substantially
greater than that for essential amino acids (EAA).
Limits of adaptation to high dietary protein intakes
AA Jackson
2. EAA are toxic in excess, when they are presented to the
body at a rate which exceeds the rate at which they can
be effectively disposed. Any toxic excess can have
profound metabolic consequences.
3. One of the toxic effects of EAA is that they interfere
with the normal metabolism of NEAA, and increased
intakes of NEAA might ameliorate the toxic effects of
some EAA.
4. The relative toxicity of any diet which contains excess,
or an unbalanced mixture of amino acids, will depend
upon the overall metabolic state of the animal, or the
extent to which there are alternative routes for disposal
(net protein deposition during growth), or the availability of critical NEAA for effective detoxi®cation.
These observations are of direct relevance to the present
considerations. The important issues in the determination of
dietary protein requirements are the lower limit of intake,
which ensures adequate availability of each EAA and
NEAA, and the upper limit of intake before the toxic
effect of one or other amino acid becomes manifest. Here
we need to take care about how we de®ne the upper limit of
intake for an amino acid, or determine when the capacity
for metabolising an individual amino acid has been
exceeded. We have become used to accepting toxic manifestations of an excess of a dietary intake of an amino acid
as requiring rather extreme metabolic responses and have
not been suf®ciently critical in identifying the normal limits
of metabolic capacity within and between individuals in the
population, and the need to identify the limits of adaptation
within the `normal' range of intakes. Animal studies show
that the ability to tolerate a relatively small excess of one or
the other amino acid is determined by the overall pattern of
the diet in general and other amino acids in particular. The
conclusion to be drawn from these observations is that the
ability to tolerate an increase in protein, or amino acids, is
related to the capacity of the body to meet its requirements
for endogenously formed amino acids, and to effectively
excrete any excess.
The metabolic demand for amino acids and nitrogen
The dietary requirement for protein, amino acids and
nitrogen is determined by the nature of the metabolic
demand which has to be satis®ed on an ongoing basis
(Jackson, 1986). We know clearly that the metabolism of
protein and amino acids is profoundly in¯uenced by a range
of other factors, some of which are listed in Table 1. There
is the need to determine the basic principles which underlie
the de®nition of the upper limit of intake of protein or
amino acids which can be metabolised effectively without
adverse effect and how this might translate into patterns of
food consumption. What is clear from the studies in
humans is that we have only a rudimentary understanding
of the nature of the dose response to variations in dietary
protein or amino acids, from one situation to another, as
needed to model the kind of mathematical analysis developed by Mercer (Mercer et al, 1989). In order to achieve
their objective, to characterise a dose response curve, which
identi®es levels that map to threshold, de®cient, adequate,
optimal, and toxic levels of intake, there is the need for a
limited range of outcome measures, which characterise
desirable function.
Table 1 Factors which interact to determine the extent to which a high
protein or amino acid intake in the diet might produce an adverse response
or metabolic stress in individuals
Nutrient consumption
Water
Total energy intake; overall energy balance
Other macronutrients: carbohydrate, lipids
Nature of the protein, amino acid composition,
nitrogen content of the diet
Nutritional state
Habitual intake
Body composition
State of adaptation
Physiological state
Gender
Maturity
Growth
Pregnancy=lactation
Ageing
Achieved or acquired
Genetic pro®le, developmental programming
metabolic capacity
(hepatic, renal function)
Lifestyle factors
Smoking
(`low grade' disease) Alcohol consumption
Activity=inactivity (level and type)
In characterising desirable function there are likely to be
two major factors of importance: on the one hand the extent
to which that intake matches the needs of the body at that
time, and on the other the metabolic capacity of the body to
process and excrete any excess of that component with
minimal or no stress to the system. Therefore, given that
the organism is in a dynamic state and proteins turn over: at
any given time the achievement of an external balance for
the organism (classical nitrogen balance) is a necessary but
not suf®cient criterion. In addition there is the need to
determine the extent to which the demand for a critical
range of functions has been satis®ed. For the system as a
whole the demand side is determined by the pattern and
rate of protein synthesis and the utilisation of amino acids
for other metabolic pathways (Jackson, 1986; Dewey et al,
1996). The dietary intake represents one component of the
supply side. Even in an analysis which focuses on the
dietary aspects of the supply side it is important to be
explicit about the assumptions which are being made about
other aspects of the supply side in relation to the total
demand (Jackson, 1995). Foods are consumed in order to
satisfy the demand for water, energy, macronutrients and
micronutrients. Each of these is necessary, but none is
suf®cient with complex interactions between the components. It is important to identify the extent to which any
conclusions assume the availability of other components of
the diet when drawing general principles either from
experimental metabolic studies or from observational studies in populations (see below). Therefore, the quantitative
and qualitative value of the diet can only be adequately
considered in the context of an appropriate de®nition of the
demand, which creates the need for the ingestion of food in
the ®rst place.
For the current debate, at the simplest level, the demand
is for a pattern of amino acids to enable protein synthesis
and other metabolic pathways (Jackson, 1986). The
demand for protein is only indirect and secondary to the
need for amino acids (Jackson, 1995). The pattern of amino
acids in the diet does not match that which is needed to
satisfy the metabolic demand and hence there has to be
considerable interchange and de novo synthesis within the
body. Based upon our current understanding, there are three
potential routes, through which the demand for amino acids
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Limits of adaptation to high dietary protein intakes
AA Jackson
S46
might be satis®ed: intake of preformed amino acids from
the diet mainly in the form of protein, amino acids derived
from protein degradation within the body and the endogenous formation of amino acids as a consequence of the
individual's metabolic activity. It is now necessary to add a
fourth route which is likely to be of quantitative signi®cance; the absorption from the colon of amino acids formed
as a consequence of the metabolic activity of the colonic
micro¯ora (Jackson, 1995; Jackson, 1998). The availability
of amino acids from each of these sources, and how they
might interact one with another, has to be considered for a
complete analysis of the effects of a high level of protein
intake on the metabolism and function of the body. In what
follows I want to look at aspects of the methodological
approach which has been used to determine adverse effects,
consider some examples where we have evidence for
higher protein intakes exerting adverse effects, and see
whether it is possible to derive general principles which
might guide thinking and study design in the future.
Figure 1 Urea production determined from the enrichment of urea in
urine following primed and oral doses of [15N15N]urea over 24 h in seven
normal adult males (Meakins & Jackson, 1995).
Methodological and metabolic considerations
Nitrogen balance and diurnal rhythms
The major functions of the body continue in the short term
regardless of an intermittent dietary intake. Nutrients taken
in the diet interact against this background of activity.
There has been the general perception that, when the
intake of protein changes, nitrogen balance is achieved by
adjusting the rate at which urea nitrogen is produced in the
body and lost in the urine. With time balance is restored as
nitrogen loss comes to match intake. There are two points,
which I wish to pick up on. Firstly, some aspects of the
observed diurnal variation in nitrogen balance and the
implications this carries for amino acid oxidation and
urea kinetics. Secondly, the observation that at higher
protein intakes nitrogen balance always appears positive.
Urea production
We have found that over a 24 h period the rate of urea
production changes little, (Figure 1) (Meakins & Jackson,
1995; Jackson, 1998), which is in agreement with the
observations made by Young's group (El-Khouri et al,
1994). Therefore, at the present time we consider that
variations in urea excretion represent differential disposal
of urea between the kidney and the colon at different times
of the day. In discussing how diurnal variation might be
modulated at different levels of protein intake, Garlick has
made calculations, which convince him that there are
signi®cant variations in the rates of urea production over
the 24 h cycle (Garlick et al, 1998). The difference in
interpretation can only be resolved by experimentation
and I do not see any bene®t in handling it as a debating
point at this time. However, we have found that within the
habitual range of intakes of protein over which nitrogen
balance is achieved, there is no evidence for a change in the
rate of urea production despite an obvious change in the
rate of urea excretion (Child et al, 1997; Jackson, 1998).
For these reasons, we have proposed that the rate of urea
production is constant in normal adults on their habitual
diets, whether measured over the short term, 24 h, or longer
periods of time. Figure 2, shows that in 15 separate studies
in which urea kinetics have been measured in men and
women in England and the Caribbean (Child et al, 1997),
the rate of urea production is 185 mg N=kg=d (12%) over
a ®vefold range of protein intakes, from 35 ± 155 g=d (about
Figure 2 The average rate of urea production from 15 studies carried out
in normal adults in England and Jamaica. Urea production was determined
from the enrichment of urea in urine following primed and intermittent
oral doses of [15N15N]urea (Child et al, 1997).
75 ± 355 mg N=kg=d). Furthermore when individuals are
fasted for 96 h to the point where glycogen reserves have
been exhausted, urea production is 187 mg N=kg=d (Hibbert et al, 1995). We interpret this to mean that in adults
under most normal circumstances, urea production is a
constant when expressed relative to body weight, similar
to the relationship with basal metabolic rate. It may be that
this relationship does not hold at higher levels of protein
intake. An anorectic female who, in an attempt to avoid
criticism but remain thin, consumed 400 g protein=d (about
9 g protein=kg=d) and in consequence became azotaemic
with a plasma urea of about 25 mmol=L (Richards &
Brown, 1975). In this female, urea production was
80 mmol=h (1200 mg N=kg=d), six times greater than
usual. Similarly, when individuals consume a diet in
which the level of protein=nitrogen is too low to maintain
nitrogen balance (less than 0.5 g protein=kg=d), urea production falls to very low levels (70 mg N=kg=d) (Danielsen
& Jackson, 1992; Meakins & Jackson, 1996). The break
Limits of adaptation to high dietary protein intakes
AA Jackson
point at the lower level of intake coincides with the
physiological minimum intake of protein, and therefore
indicates the point at which the diet becomes inadequate. It
would be of interest to know where the break point is for
urea production at the upper end with higher levels of
protein intake and to see if this marks the upper limit of
protein intake, which the body can normally tolerate without obvious adverse effect. The relationship described is for
normal adults and does not necessarily apply for other
physiological or pathological states, such as pregnancy,
infancy, childhood or rapid catch-up growth (Jackson et al,
1990; Wheeler et al, 1991; Steinbrecher et al, 1996;
McClelland et al, 1997).
Energy derived from amino acid oxidation
If there is a constant rate of urea production over a wide
range of protein intakes, changes in nitrogen balance can
only be achieved by varying the proportion of the urea
produced which is excreted in the urine, compared with that
being salvaged through the colonic micro¯ora. Given that
urea production is 185 mg N=kg=d, on the assumption that
basal metabolic rate is 100 kJ=kg=d, then urea appearance
represents 1.9 mg N=basal kJ, or under fasting conditions
about 20% of energy is derived from the oxidation of
protein, equivalent to about 1.2 g protein=kg=d. This serves
to de®ne an upper limit through which changes in the relative
rates of disposal of urea can be effectively used as the
mechanism through which nitrogen balance is achieved.
Assuming that urinary urea comprises about 80% of the
nitrogen lost to the body on higher protein intakes, as it is
not possible to excrete more than is produced, the maximum
intake of protein that can be accommodated would be
1.5 g protein=kg=d. For balance to be achieved while consuming intakes above this would require an increase in the
excretion of nitrogen in forms other than urea.
Apparent positive nitrogen balance on higher protein
intakes
As pointed out by Garlick, there is the reproducible consistent observation that on higher protein intakes measured
nitrogen balance is consistently positive (Garlick et al,
1998). It is not possible to remain in consistently positive
nitrogen balance over long periods of time without any
change in body size or composition. Therefore, there have
to be methodological errors in the approach, and these have
been discussed by many observers. The most obvious
possibility, an over-estimate of intake and underestimate
of losses, may be an important contributing factor, but fails
to account for all of the difference in carefully conducted
studies. It is unlikely that the losses which have not been
accounted for are gaseous, as the negative results achieved
by ourselves and others when trying to explore this consideration are suf®ciently unremarkable not to be considered worthy of publication. Skin desquamation, sweat and
other losses may make a small contribution, but it is likely
that unmeasured losses suf®cient to account for the magnitude of the apparent positive N-balance are associated with
losses in the urine and=or stool.
The Kjeldhal method is the standard approach for
determining the nitrogen content of biological samples. It
is known that not all nitrogen will be identi®ed with this
method and in particular, losses of nitrite and nitrate are not
included in the measurement. The failure to take account of
any signi®cant losses of nitrogen as nitrite or nitrate will
introduce an error into the calculation, leading to an
apparent positive balance. The magnitude of the possible
errors has been explored to a limited extent. Kurzer &
Calloway (1981) have measured losses of nitrite and nitrate
and suggest that excess nitrate excretion could account for
as much as 20% of the unexplained positive nitrogen
balances previously reported in well-controlled studies. It
is likely that this is a minimum estimate, because it is now
clear that the method they used is probably not quantitative,
and gives a substantial underestimate by as much as 30 ±
80% (Tsikas et al, 1997). The reliable measurement of
nitrite and nitrate in complex biological matrices is dif®cult
and the recovery is likely to be low and variable. It appears
to be of very great importance that a suitable internal
standard is used throughout the entire analytical process.
Furthermore, because of day to day variability characterisation of losses in stool are particularly problematic and
may require several days of collection to determine with
any reliability habitual patterns of loss (Kurzer & Calloway, 1981). When taken together these two sources of error
could make a substantial contribution to accounting for the
`missing nitrogen'. Given the fundamental relevance of
these considerations for our understanding, there is an
urgent need that these points be clari®ed in a series of
well conducted studies.
Urea, water and the kidney
The urinary excretion of urea makes a very considerable
contribution to the overall renal solute load. Therefore, the
excretion of urea is intimately related to the availability of
free water and the renal processing of water. For many
years it has been known that on diets low in protein, the
secretion of arginine vasopressin enhances the retention of
urea within the body, by increasing the active retention of
urea along the collecting duct of the kidney (SchmidtNielsen, 1970). Therefore, the retention of water through
antidiuresis and the retention of urea are closely related.
The last decade has seen the closer characterisation of a
family of aquaporins, which have the general property of
moving water and urea across membranes. The identi®cation of a urea transporter in the kidney of the rat which is
responsive to dietary protein intake (Isozaki et al, 1994;
Smith et al, 1995), the demonstration of a transporter which
is speci®cally located in the renal collecting duct and the
colon (You et al, 1993), and the ®nding of similar capabilities in the human colon (Ritzhaupt et al, 1998) opens
the opportunity for a co-ordinated response. It is possible
that on a low-protein diet, enhanced retention of urea in the
kidney is matched by an enhanced capacity to move urea
into the colon where the nitrogen can be salvaged through
bacterial activity. The converse problem is faced on a high
protein diet, and under this circumstance the changes seen
during the early newborn period are instructive.
Colonic salvage of urea-nitrogen in early life
We have already identi®ed that in normal adults the
proportion of resting energy expenditure derived from
protein oxidation is around 20%. Sheep studies indicate
that during foetal life at least 40% of energy is derived from
the oxidation of amino acids, 288 ± 360 mg N=kg=d, which
may increase to 60% with maternal food deprivation under
conditions of metabolic stress, 960 mg N=kg=d (Owens et
al, 1989). The underlying basis for this remarkable dependence upon amino acid oxidation is far from clear, but it
does appear to carry into the neonatal period, at least for the
®rst few weeks of life. One important consequence is that
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Limits of adaptation to high dietary protein intakes
AA Jackson
S48
the rate of urea production is high. In breast-fed human
infants less than six weeks of age urea production was
360 mg N=kg=d, compared with 120 mg N=kg=d in infants
older than six weeks of age (Steinbrecher et al, 1996). In
the foetus this is not likely to present a major problem
provided that the urea can pass across the placenta and be
cleared effectively to the maternal circulation. However,
following birth, for the neonate the only route available
through which urea can be lost is by renal clearance,
presenting a substantial demand for free water. Under this
circumstance colonic hydrolysis of urea with the subsequent salvage of the nitrogen represents a critical route for
the effective disposal or urea and the achievement of water
homeostasis. These observations serve to highlight the
interdependence of water and urea metabolism. Any discussion which seeks to determine the upper limit of dietary
protein which the body can tolerate needs to take water
homeostasis into account.
The toxic effect of excessive amino acid intake
Inborn errors of amino acid metabolism represent natural
experiments in which the pattern of amino acids being
presented is unbalanced, and experience shows that the
effects are severe, toxic and even lethal. The detailed nature
of the toxic effects of an excess of an amino acid depends
upon the background against which the excess is taken and
the speci®c metabolic perturbations it is likely to induce. In
general a single amino acid taken in excess has more severe
effects than the same level of amino acid taken with other
amino acids as part of a mixed meal. Harper et al (1970)
suggest that this is related to the rate or ease with which
enzyme systems required for further metabolism of the
amino acid are effectively activated.
Foetus
During foetal life there is a high demand for amino acids
(Widdowson et al, 1979; Owens et al, 1989). Clearly some
severe inborn errors of metabolism are tolerated during this
period even though they may be incompatible with normal
growth and development during post-natal life. This
emphasises the importance of the metabolic buffering for
the foetus and placenta provided by the mother's metabolism.
Figure 3 The relationship between the excretion of 5-L-oxoproline and
sulphate in the urine of ®ve normal adult females, following 5 d on a low
protein diet with supplemental methionine and=or urea (Meakins et al,
1998).
Newborn and infancy
In considering the protein requirements during early life,
approaches have been dominated by a logic which suggests
that because protein is required for normal growth and
development, improved growth and enhanced development
can be best achieved on higher protein intakes. There is a
deep-rooted belief that this is so, but the evidence suggests
that this may not be correct. The consumption of human
milk has been taken to represent the intake required for
normal patterns of growth and development during the
newborn period and the ®rst six months of life. Human
milk has a composition, which is low in nitrogen and
Figure 4 The change in plasma homocysteine (n ˆ 3) and in urinary 5-L-oxoproline (n ˆ 2) in normal adults following an oral dose of methionine
(100 mg=kg).
Limits of adaptation to high dietary protein intakes
AA Jackson
extremely low in protein. The consumption of relatively
high protein formulations based upon cow's milk has been
associated with serious complications leading on occasion
to death. Over recent years there has been a progressive
reduction in the nitrogen and protein content of formulae to
achieve adequate patterns of growth together with minimal
metabolic stress (Raiha, 1988). This can best be achieved
with formulations in which the protein content is very low,
compared with what was thought to be desirable ten years
ago. When the requirements for protein during early life
have been assessed based upon a factorial approach, with
the metabolic demand for nitrogen and amino acids being
used as the objective, the recommendations for optimal
protein intakes are very much lower than they have been in
the past (Dewey et al, 1996).
Pregnancy
There have been many studies in which dietary supplements have been provided to women during pregnancy with
the objective of improving the growth and development of
the foetus. The effect of dietary supplementation with
protein appear to give confusing and con¯icting results.
Rush has provided a useful critical review which suggests
that a difference has to be drawn between studies in which
additional protein is provided as part of a balanced increase
in the intake of energy and other nutrients, compared with
studies where the supplement results in an increase in the
protein density of the diet (Rush, 1989). Therefore the
results are reasonably consistent that supplements of high
protein density result in adverse effects on the foetus.
Supplements of higher protein density were associated
with lower birthweight whereas supplements of lower
protein density in general resulted in modest bene®t.
There is an interaction amongst any bene®t from protein
supplementation and the previous nutritional status of the
mother, and it may be that mothers who enter pregnancy
with a poor nutritional status do bene®t from modest
supplementation to bring their intake into the normal
range. The analysis of more recent studies suggest that
the interactions of protein intake with maternal nutritional
status and her dietary intake during pregnancy may be
rather complex (Godfrey et al, 1996; Campbell et al, 1996).
Normal adults
In normal adults the greatest concern about the possible
adverse effects of higher protein intake has been that they
might have adverse effects on renal function, or as a
consequence of an increase in urinary calcium excretion
upon bone health. Each of these issues is complicated and
unresolved.
At all ages there is a complex interaction between the
requirement for energy and protein and the amounts
ingested in the diets (Scrimshaw & SchuÈrch, 1991). The
protein to energy content of most diets varies within a
relatively narrow range. As food intake is primarily determined by energy expenditure, the protein intake of individuals over long periods of time is more likely to be
determined by their level of activity than their need for
protein. For equivalent levels of activity, those taking a
vegetarian diet will consume less protein than those taking
a diet which contains mixed proteins from animal and
vegetable sources (Bundy et al, 1993; Jackson & Margetts,
1992). An increase in protein intake to levels, which have
usually been associated with adverse metabolic effects
either, requires very high levels of activity and total food
consumption, or the use of protein rich supplements.
Protein rich supplements often have a relatively low
water content and any effects of the protein content on
metabolism have to take into consideration the requirements for water for effective renal responses (Hadj-Aissa et
al, 1992).
Dietary protein and renal responses
Osteoporosis is a major health problem, the consequences
of which are expensive to manage. It is characterised by
changes in calcium homeostasis, but fundamentally appears
to be a disorder of bone matrix and therefore of protein
metabolism. It is known that high protein diets result in an
increased urinary excretion of calcium in the short term and
may therefore exert an adverse in¯uence on bone health.
The relationships between dietary protein and calcium
status and possible effects on bone have recently been
reviewed in a report from a symposium (Massey, 1998;
Barzel & Massey, 1998; Heaney, 1998). Two opposing
opinions are presented and a number of questions are
raised, but very few answers are provided. In trying to
reach an understanding and resolution the greatest dif®culty
is to differentiate any effect of protein containing foods,
from proteins themselves. Protein containing foods are
complex and varied and it is not possible to know the
extent to which any observed effect can be directly attributed to protein or is a consequence of other components of
the diet. Secondly, there is the need to differentiate whether
any observed effect attributed to protein is a feature of
amino acids operating as amino acids, or a consequence of
their further metabolism and the potential load placed on
the kidney to maintain either water homeostasis or acidbase balance. Therefore, for example, a recent study sought
to explore the effects upon renal haemodynamics and renal
excretion of chronic differences in protein intake, by
comparing groups with very different lifestyles who habitually consumed different amounts of protein, varying over
a three to four fold range (Brandle et al, 1996). A group of
body builders who regularly consumed a protein concentrate as a dietary supplement represented the group with the
highest protein intake. No information is provided on the
¯uid intake or balance for the study, nor on the overall acid
load represented by the different diets. Furthermore calcium metabolism is intimately linked to phosphate metabolism and it seems unlikely that we will develop a clear
view of the interaction of dietary protein with urinary
calcium unless the possible interactions with dietary phosphate or phosphate metabolism are included in the analysis.
If we wish to re®ne our understanding of the effect of
higher dietary intakes of protein on bone structure and
function there will have to be well designed studies, which
take account of all the major variables of importance.
High protein intakes in therapy
For many years the successful treatment of severely malnourished children has been based upon the use of very low
protein diets during the period when the child is critically
ill with deranged homeostasis (Waterlow, 1961; Chan &
Waterlow, 1966; Ashworth, 1980; Jackson & Golden,
1987; Ashworth et al, 1996). Under these circumstances a
low plasma albumin has been seen as a response to
metabolic stress, and an intrinsic part of the acute phase
response, rather than as an indication of a speci®c protein
de®ciency which requires direct treatment (Golden et al,
1980). Despite the success of this approach to care, it has
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Limits of adaptation to high dietary protein intakes
AA Jackson
S50
not found wide acceptance, and the important lessons have
not, in general, been extrapolated to adult care (Bredow &
Jackson, 1994; Scho®eld & Ashworth, 1996). Two recent
reports of work in adults tend to justify the approach
adopted in children, and should cause us to re¯ect very
carefully on the potential damage that an unbalanced
protein load might have in individuals who are nutritionally
compromised or metabolically stressed.
Collins et al (1998) have compared the response to two
diets which differed primarily in the protein content, during
the rehabilitation of severely malnourished adults in the
famine in Somalia in 1992. Among patients who had
oedema, there was a threefold decrease in mortality and
accelerated reduction of oedema when the lower protein
diet (8.5% of energy from protein) was provided during the
initial phase of treatment. Therefore higher protein intakes
(16.4% of energy from protein) appeared to be associated
with excess mortality. This directly re¯ects the experience
during normal growth in early life and during the management of severe malnutrition in childhood.
On behalf of the Cochrane Injuries Group Albumin
Reviewers, Roberts (1998) has reported the results of a
systematic review and meta-analysis of randomised controlled trials in which human albumin (or plasma protein
fraction) was administered during the management of
critically ill patients. There were 30 trials with a total of
1419 patients. They showed no evidence that albumin
reduced mortality, and for patients with hypovolaemia,
burns or hypoproteinaemia the risk of death was increased
by giving intravenous albumin with an increase in relative
risk of 1.68, excess mortality of 6 per 100 patients treated.
We know that in some cases individuals might receive 50 ±
100 g protein as intravenous albumin each day.
These two papers provide direct evidence that there is an
upper limit for protein which the body can tolerate without
adverse effect. It may be that this upper limit is reduced or
made more obvious in malnourished or sick people when
the capacity for metabolising protein is impaired, but we
need to know the nature and mechanism of the adverse
effects before assuming that similar constraints do not
apply across the board.
Metabolic programming and ageing
The model in which an organ achieves its mature functional
capacity as part of the normal process of growth and
development, and loses it progressively as an integral part
of the ageing progress may be a part of the general process
which underlies the observations which relate growth in
early life to the development of chronic disease during
adulthood, the `Barker hypothesis' (Barker, 1998; Jackson,
1992). When during adult life animals are presented with
higher protein diets than they were exposed to during
sensitive periods in early life, lifespan may be reduced
(Hales et al, 1996). If the capacity to handle dietary protein
were determined in part by early life exposure then any
effects of higher protein diets during adulthood would be
expected to be different in populations in relation to their
different experiences in early life. This would mean that
great care should be used in extrapolating the results
obtained from studies carried out in individuals in developed countries compared to those in other parts of the
world where different intra-uterine exposure and different
feeding practices in early life might have resulted in very
different metabolic capacities.
Renal failure and diabetes
Based upon physiological studies in dogs and other animals
it has been known for many years that following the
consumption of protein there is an increase in renal blood
¯ow and glomerular ®ltration rate (Klahr & Purkerson,
1988; Rodriguez-Iturbe et al, 1988). The immediate factors
that exert control over this response are not completely
clear, but the response is of practical importance in those
who for one reason or another have a decrease in the
number of functioning nephrons. As the number of
nephrons decreases the work required by each remaining
nephron increases, with the result that the age related
decrease in function is accelerated (Brenner et al, 1982).
This is most obvious in individuals with glomerulosclerosis
associated with diabetic changes, but might also be part of
the normal mechanism through which the ageing kidney
progressively fails (Fancourt et al, 1992). Under these
circumstances a reduction in the protein content of the
diet may reduce the stress on the kidney and maintain the
functional capacity for a longer period of time.
Single amino acids compared with total diet
Amino acids, which cannot be utilised either for protein
synthesis or other metabolic pathways, have to be degraded
to prevent toxicity, as large pools of free amino acids are
not tolerated. Therefore, the extent to which a high protein
intake is likely to result in identi®able change will be
determined by the total capacity of each of these pathways.
The functional capacity of the degradative pathways for
individual amino acids may be determined by the availability of other individual amino acids or nutrients. For
example there is a well-recognised antagonism between
lysine and arginine. Increased intakes of lysine impair the
normal metabolism of arginine, and an increased intake of
arginine serves to offset some of the deleterious in¯uences
of excessive lysine consumption (Harper et al, 1970). The
enzyme phenylalanine hydroxylase requires iron for normal
function and in the presence of iron de®ciency there is a
reduced capacity for the enzyme to convert phenylalanine
to tyrosine (Lehmann & Heinrich, 1986; Mackler et al,
1979; Gottschall et al, 1982).
Coping with excess proteins and amino acids
It is quite clear that amino acids are potentially toxic to the
body and when presented at rates exceeding the capacity
with which they can be handled safely, a metabolic stress is
imposed upon the system. There are normal physiological
and metabolic mechanisms, which buffer the extent to
which this stress expresses itself as abnormal function,
but when suf®ciently severe or maintained for suf®cient
periods of time, the capacity to cope is overwhelmed and
then the stress is expressed as identi®able pathology. For
each amino acid the critical factors might vary, depending
upon the pathways for its formation in the body, the way it
is presented for metabolism, the demands for metabolic
pathways other than protein synthesis, and the capacity of
the pathways required for its normal metabolism, its oxidation and its degradation. As the activity of most if not all of
the pathways associated with amino acid and protein
metabolism require the active involvement of cofactors,
the overall nutritional status of the individual for minerals,
vitamins and trace elements will play an important part.
Limits of adaptation to high dietary protein intakes
AA Jackson
The most toxic amino acid is methionine (Harper et al,
1970), and the metabolism of methionine is now of considerable interest in the context of hyperhomocysteinaemia
and its relationship with occlusive vascular disease
(Refsum et al, 1998). Although generally amino acids are
more likely to be toxic when given alone than as part of a
mixed diet, this is probably less true for methionine than
any other amino acid. Nevertheless, the adverse effects of
methionine are more evident on a low protein diet than on a
more adequate protein intake. Dietary supplements of
glycine, and to a lesser extent serine are known to ameliorate the toxic effects of excess methionine (Harper et al,
1970).
Methionine: perturbed metabolism and toxicity
The normal pathway through which the body disposes of
excess methionine involves the generation of a methyl
group and the formation of homocysteine, which is further
metabolised through the transsulfuration pathway to
cysteine. To determine individuals' susceptibility to hyperhomocysteinaemia they are subjected to a methionine load
test in which the increased plasma concentration of homocysteine following a large dose of methionine (100 mg=kg)
is determined. Theoretically, 3 mol of glycine are required
in the catabolism (detoxi®cation) of 1 mol of methionine.
The capacity for forming glycine in the body is substantial,
but not in®nite. When a signi®cant drain is placed upon the
free glycine pool, the ability to generate suf®cient glycine
to satisfy all the metabolic demands may be impaired, and
this can be marked by an increase in the urinary excretion
of 5-L-oxoproline (Jackson et al, 1996). On adequate, but
low protein diets, endogenous glycine production is
reduced and there is a marked increase in urinary 5-Loxoproline (Gersovitz et al, 1980; Yu et al, 1985; Jackson
et al, 1996).
In a recent study we were seeking to determine the limits
of adaptation to a low protein diet and in considering
methionine as the ®rst limiting component of the diet,
explored the effect of methionine supplementation. A
modest supplement of methionine, compared with the
amount of methionine given in the load, impaired the
normal adaptive response to a low protein intake and was
associated with a signi®cant 5-L-oxoprolinuria (Meakins et
al, 1998). Figure 3 shows the relationship between urinary
sulphate excretion and urinary 5-L-oxoproline amongst the
subjects on the different diets. We have used the urinary
excretion of 5-L-oxoproline to determine the effect of the
methionine load test on glycine status in normal adults.
Figure 4 shows that following a methionine load there is a
three- to four-fold increase in plasma homocysteine over
the following 6 h and this is associated with a substantial
increase in the urinary excretion of 5-L-oxoproline. Given
the frequency with which hyperhomocysteinaemia is seen
in ageing populations, these data raise questions about the
maximum levels of individual amino acids which can be
tolerated by the system without important metabolic perturbations. The cumulative effects on the body of high
levels of circulating homocysteine may take many years to
develop, but they do nevertheless appear real.
Conclusions
The adaptations made by the body to high dietary protein
intakes are mainly related to the body's attempts to
minimise the damage which might be produced by the
ingestion of potentially toxic amino acid loads and the
elimination of the end products of their metabolism. Any
adverse effects induced by diets of this kind may be slow
and cumulative. To date the approaches that have been
adopted in the exploration of possible adverse effects have
been short-term and have used relatively crude measures
of outcome.
There is the need to determine the dose-response to the
ingestion of mixed protein and individual amino acids,
using more sensitive measures of outcome and for longer
periods of time. It is likely that the responses will differ
with physiological state and in pathology. Importantly, the
responses are also likely to be dependent upon an interaction with other nutrients involved in critical metabolic
pathways. The interaction of factors which determine the
maximum capacity of these pathways needs to be determined and is likely to include genetic pro®le, early life
programming, habitual dietary intake and lifestyle.
When we have this information, nutritionists will be
better placed to identify the upper limit of the adaptive
response to high protein intakes and to identify effective
interventions for the promotion of health and wellbeing, as
well as the treatment and management of disease.
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