Division of Nephrology, Departments of 1 Internal Medicine and 3 Pediatrics, University Hospital Groningen and 2 Groningen University Institute of Drug Exploration, 9713 GZ Groningen, The Netherlands
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ABSTRACT |
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Protein energy
malnutrition is present in 18 to 56% of hemodialysis patients. Because
hemodialysis has been regarded as a catabolic event, we studied whether
consumption of a protein- and energy-enriched meal improves the whole
body protein balance during dialysis in chronic hemodialysis (CHD)
patients. Patients were studied on a single day between dialysis (HD
protocol) in the morning while fasting and in the afternoon while
consuming six small test meals. Patients were also studied during two
separate dialysis sessions (HD+ protocol). Patients were fasted during one and consumed the meals during the other. Whole body protein metabolism was studied by primed constant infusion of
L-[1-13C]valine. During HD
, feeding changed
the negative whole body protein balance observed during fasting to a
positive protein balance. Dialysis deepened the negative balance during
fasting, whereas feeding during dialysis induced a positive balance
comparable to the HD
protocol while feeding. Plasma valine
concentrations during the studies were correlated with whole body
protein synthesis and inversely correlated with whole body protein
breakdown. We conclude that the consumption of a protein- and
energy-enriched meal by CHD patients while dialyzing can strongly
improve whole body protein balance, probably because of the increased
amino acid concentrations in blood.
protein turnover; stable isotope; valine
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INTRODUCTION |
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SIGNS OF PROTEIN ENERGY MALNUTRITION occur frequently in patients with chronic renal failure (17, 19). Protein energy malnutrition has been shown to be a major risk factor for increased morbidity and mortality in the chronic hemodialysis (CHD) patient (2, 17). Multiple factors predispose CHD patients to protein energy malnutrition, e.g., low caloric intake, low protein intake, and the hemodialysis procedure itself. Particularly, losses of amino acids or abnormal protein metabolism during hemodialysis might contribute to the observed protein energy malnutrition. Studies examining the role of CHD itself on protein metabolism are limited (22). Several lines of evidence indicate that the CHD procedure can result in a negative whole body protein balance. Nitrogen balance has been shown to be more negative on a dialysis day compared with a nondialysis day regardless of daily protein intake (5, 23). Lim et al. (22) studied whole body protein metabolism by applying the [13C]leucine isotope dilution technique in fasting CHD patients during hemodialysis. They observed a reduction in whole body protein synthesis compared with the predialysis period, and this resulted in a doubling of the negative protein balance already present in fasting CHD patients. Furthermore, hemodialysis stimulates muscle protein losses compared with the predialysis period in fasting CHD patients (18).
In apparently healthy subjects, the consumption of a meal or the administration of an amino acid mixture reverses the negative protein balance observed after an overnight fast (for reviews see Refs. 8 and 45). Particularly, amino acids in plasma are powerful modulators of protein metabolism, as a mixture or in conjunction with insulin (42). Protein breakdown is inhibited, while protein synthesis and protein oxidation are stimulated by amino acid infusion. As a result, whole body protein balance becomes positive (29, 30).
The situation in CHD patients is less well known, and the effects of a meal during dialysis have not been studied so far. It is common clinical practice, at least in Europe, that CHD patients are allowed to eat during a 4-h dialysis session. We adapted this practice for the purpose of nutritional intervention. A milk-based protein- and energy-enriched meal was given to the patients during a dialysis session and on a nondialysis day. The meal was designed with the assumption that a maximum anabolic response would be elicited in our patients by a meal enriched in both energy and protein. We studied the effect of this oral intradialytic nutrition on whole body protein metabolism during hemodialysis with a biocompatible membrane in CHD patients. We addressed the following two questions more specifically: 1) to what extent does consumption of a protein- and energy-enriched meal result in a positive whole body protein balance in CHD patients, and 2) how effective is such a meal consumed during a dialysis session in the prevention of the negative protein balance in CHD patients during dialysis? The first question was studied in CHD patients during a nondialysis day, the second question during two dialysis sessions separated by 1 wk. Whole body protein metabolism was studied by applying stable isotope infusion techniques using [1-13C]valine as a tracer (4). The use of this tracer has certain advantages over [1-13C]leucine both analytically and metabolically. [1-13C]valine has been used previously by our laboratory in the study of whole body protein metabolism and synthesis of several specific proteins (34) in nephrotic patients. Furthermore, it has been reported that, under certain conditions, high (flooding) doses of leucine can provoke an insulinomimetic effect on protein metabolism (9, 12), whereas this is not the case for valine. At doses normally applied in the study of whole body protein metabolism, valine and leucine give similar values of the fluxes of protein breakdown, synthesis, and oxidation (38).
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SUBJECTS AND METHODS |
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Study Subjects
All nondiabetic, stable hemodialysis patients aged under 65 yr in the Dialysis Center Groningen were approached to participate in the two protocols of the present study, i.e., a nondialysis and a dialysis protocol. Twelve patients gave their permission, but only three or them agreed to participate in both protocols. The other patients considered this too great a demand since they objected to fasting during the dialysis session. In summary, three patients participated in both protocols, six patients participated only in the nondialysis protocol, and three patients participated only in the dialysis protocol (Table 1). The medical ethics committee of the University of Groningen approved all studies, and written informed consent was obtained from all participants. All participants were clinically stable, without intercurrent acute illness in the 3 mo before the study protocol and had been in dialysis for 6 mo or more. The diagnoses were chronic glomerulonephritis in three patients (1 with hypertension), nephropathy resulting from hypertension in three patients, quiescent Wegeners disease in one patient, and polycystic kidney disease in three patients, and the cause of renal failure was unknown in two cases. Medications included phosphate binders, iron, multivitamins, antihypertensive drugs, calcitriol, and recombinant human erythropoietin of which the dose had not been altered for 3 mo before the study protocol to avoid altered hematopoiesis. No patients received steroid hormones or immunosuppressive agents in the 6 mo before the study protocol. The patients were dialyzed with low-flux biocompatible dialyzers for 4 h three times weekly. Blood flow ranged from 250 to 350 ml/min, and dialysate flow was 500 ml/min. Standard dialysate with 140 meq Na+, and 34 meq bicarbonate, was used for all patients. Glucose content in dialysate was 5.6 mM in two patients and 11.2 mM in four patients during their experimental dialysis sessions. Residual renal function was 3 ml/min or less, which corresponded to a dialysis adequacy (Kt/V) value of 0.45 wk
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Materials
L-[1-13C]valine and NaH13CO3, both with an enrichment of >99 atom percent excess, were purchased from Cambridge Isotope Laboratories (Andover, MA). Chemical purities were confirmed before use. Pyrogen- and bacteria-free solutions were prepared in sterile saline by the hospital dispensary the afternoon before the study day. Meal portions consisted of 150 g yogurt (5.7 g protein, 7.4 g carbohydrate, and 5.4 g fat; Domo), 20 g cream (0.5 g protein, 0.7 g carbohydrate, and 6.3 g fat; Friesche vlag, Ede, the Netherlands), and 5 g protein-enriched milk powder (1.5 g protein, 2.4 g carbohydrate, and 0.8 g fat; Fortify, Nutricia). Consumption of a meal portion every 30 min for 3 h resulted in a dietary valine intake of 132 ± 20 µmol · kgExperimental Design
Pilot experiments.
Dialysis by itself was found to gradually increase the
13CO2 enrichment in expired air because of the
entrance of bicarbonate with a high natural enrichment from the
dialysate (4.0 ± 0.3
vs. Pee Dee Belemnite
limestone). Therefore, background enrichment in expired breath
was measured independently in five patients during a dialysis session
before this study. The time course of this change as a percentage of
the initial background enrichment of expired CO2 was used
to correct the value of the 13CO2 excess
enrichment obtained during either the [13C]bicarbonate or
[13C]valine infusion for calculations of whole body
protein metabolism. In a second pilot experiment, the extent to which
the rate of [13C]valine infusion had to be increased
during dialysis was tested. This was deemed necessary since it was
observed that the turnover in the bicarbonate pool was increased during
dialysis, and, consequently, infusion of valine had to be increased to
obtain 13CO2 enrichments in expired air, which
could be measured reliably in excess of the background enrichment that
had already been changed by exchange of plasma and dialysate
bicarbonate. Doubling the [1-13C]valine infusion rate
appeared to be sufficient.
Study protocols.
The present study comprised two protocols. In the nondialysis protocol
(HD), patients were studied on a day between two dialysis days.
Fasting whole body protein metabolism was measured in the morning after
an overnight fast (HD
fas). On the same study day, in the afternoon,
this was followed by the measurement of whole body protein metabolism
while patients were consuming the meal (HD
fed). The dialysis
protocol (HD+) could not be done on a single day and therefore
consisted of two study days 1 wk apart. Patients were dialyzed normally
on these days, and measurements were made during the dialysis session.
On one occasion, patients were studied while they remained fasting (HD+
fas), and on the other occasion patients consumed a protein-enriched
meal (HD+ fed). The HD+ protocol started after the completion of the
study of whole body protein metabolism during the HD
protocol. Before the study (3 wk), all patients visited the Dialysis Center Groningen for a dietary interview and instructions on dietary recording. Patients
consumed a protein intake of 1.0 ± 0.1 g · kg
1 · day
1,
while caloric intake was not restricted.
Nondialysis protocol.
In the HD protocol, patients had fasted overnight and were studied
during a midweek day without dialysis, having dialyzed the afternoon
before. Patients were admitted to the Hospital Research Unit at
7:30
AM. A catheter was inserted in the dorsal vein of the hand of the shunt
arm to collect baseline blood samples. Subsequently, breath samples
were taken. A schematic diagram of the study day is shown in Fig.
1A. The
NaH13CO3 infusion was started at 8:00 AM.
During the 1st h, whole body bicarbonate production (details explained
in Evaluation of Primary Data) was measured using a primed
constant infusion of NaH13CO3 (5 µmol/kg
bolus followed by a continuous infusion of 5 µmol · kg
1 · h
1).
Four breath samples were taken from 30 to 60 min after the start of the
NaH13CO3 infusion at 10-min intervals. The
NaH13CO3 infusion was discontinued immediately
after the last breath sample was taken, and the
L-[1-13C]valine infusion was started with a
bolus of 15 µmol/kg followed by a continuous infusion of 7.5 µmol · kg
1 · h
1
for the next 4 h. A second catheter was then inserted in the contralateral arm to collect blood samples. Blood and breath samples were taken simultaneously every half hour for 3 h after the start of the [13C]valine infusion. During the 4th h, blood and
breath samples were taken every 15 min. At 1:00 PM, the meal period was
started by consumption of the first portion of the protein-enriched
meal and continued for 3 h by consumption of a portion every 30 min. [13C]valine infusion continued at the same rate
during this study period. Blood and breath samples were taken every 30 min for 2 h after the start of the meal, whereas during the last
hour, samples were taken every 15 min. The study day ended at 4:00 PM.
All catheters were removed, and patients were observed until stable and
then discharged.
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Dialysis protocol.
In this protocol, patients were studied on two separate dialysis
sessions, separated by 1 wk. On one occasion, patients were studied
while they remained fasting. On the second occasion, patients consumed
six small meals, the first 1 h after the start of dialysis followed by five meals spaced by 30 min. Patients had been dialyzed 44 ± 3 h before they entered the study protocol. Studies
were performed in the afternoon, and patients consumed a late-evening snack the evening before the study day to keep the fasting period comparable to that in the HD protocol. Patients were admitted to the
dialysis center at
11:30 AM. Dialysis needles were inserted in the
arterial-venous shunt to collect baseline blood samples, and breath
samples were collected simultaneously. Dialysis started at
12:00 PM.
A primed continuous infusion of NaH13CO3 was
administered for 1 h through the venous line of the dialysis machine (5 µmol/kg bolus followed by a continuous infusion of 5 µmol · kg
1 · h
1),
and four breath samples were taken from 30 to 60 min after the start of
the infusion at 10-min intervals. The NaH13CO3
infusion was discontinued after the last breath sample was taken, and a
primed continuous infusion of
L-[1-13C]valine was
started through the venous line of the dialysis machine for 3 h
(15 µmol/kg bolus followed by a continuous infusion of 15 µmol · kg
1 · h
1).
Blood samples from the arterial line of the dialysis machine and breath
samples were taken every half hour for the first 2 h after the
start of the [13C]valine infusion. During the third and
last hour, blood and breath were sampled every 15 min (Fig.
1B). When whole body protein metabolism was studied during
the meal period, the same experimental setup was used as described
above, with the exception that, at the start of the
[13C]valine infusion, the first of the six meal portions
was consumed, whereas the remaining five were consumed every 30 min
during the next 3 h (Fig. 1C). Blood pressure was
monitored during all experimental dialysis sessions. Blood flow was
estimated from the flow given on the dialysis machine while dialysate
flow was 500 ml/min plus the ultrafiltration. Approximately 70% of the
ingested fluid was removed during the experimental dialysis session,
while the other 30% was removed during the next dialysis session.
Analytical procedures.
Blood (4 ml) was drawn for each sample in liquid-heparinized vacuum
tubes and centrifuged at 3,000 rpm. Plasma was extracted and stored at
20°C until analysis. Breath samples were collected in gas
collection tubes with a straw, as described earlier (43). Subjects exhaled normally through a straw in the glass container. After
exhalation was completed, tubes were closed immediately and stored at
room temperature until analysis. Dialysate was sampled every half hour
using a syringe to extract 4 ml of dialysate and was stored at
20°C
until analysis.
Evaluation of Primary Data
Rate of appearance of intracellular valine (Ra) was calculated at isotopic steady state using the inverted pool model described by Matthews and colleagues (26-28) for leucine kinetics. When this isotopic model is applied to [1-13C]valine, enrichment of plasma KIVA is assumed to provide an estimate of intracellular enrichment of valine. The rate of appearance (µmol valine · kg
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The rate of oxidation of valine was calculated following the
approach described by Van Goudoever et al. (40). We did
not use indirect calorimetry in our study to determine CO2
production as a measure of whole body bicarbonate production.
Measurements would be perturbed when the comparison between the HD
and the HD+ protocol is made because bicarbonate from the dialysis
fluid enters the circulation and changes the bicarbonate pool of the patient. As a consequence, an unknown fraction of the whole body bicarbonate flux is derived from the dialysis fluid (22).
In the approach of Van Goudoever et al., whole body bicarbonate flux is
estimated before the [13C]valine infusion using a primed
continuous infusion of NaH13CO3 of short
duration. In this way, a two-point calibration is obtained with
background 13CO2 enrichment at no infusion of
NaH13CO3 and the measured value of enriched
CO2 at the applied continuous infusion rate of
NaH13CO3. The [13C]bicarbonate
flux originating from the oxidation of [13C]valine was
then calculated by linear interpolation of the measured 13CO2 enrichment in expired air at steady state
during [13C]valine infusion between the two points of the
calibration. In other words, the ratio of enrichments of
13CO2 in expired air during
[13C]valine infusion over that during
NaH13CO3 infusion is a reflection of the ratio
between the rate of [13C]bicarbonate production
originating from the oxidation of [13C]valine over the
rate of continuous infusion of NaH13CO3. From
the KIVA enrichment, which represents the intracellular dilution of
valine, we calculated the amount of valine being oxidized to sustain
this calculated production of [13C]bicarbonate. This
results in the following calculations
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During the meal period, recovery of labeled CO2 will
be increased in comparison with fasting. Estimates from the literature have been used, i.e., 0.74 ± 7 to 0.84 ± 8 during fasting
and meal intake, respectively (11, 14, 21). This
represents an increase of ~13%. Correction of the rate of oxidation
of valine during the meal period is necessary because the two-point
calibration was done while the patient was fasting. O(fed) was thus
calculated according to
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Calculation of Whole Body Protein Metabolism
In Fig. 2, the steady-state isotopic model for whole body valine metabolism is depicted in a schematic diagram. In this model, influx of valine comes from whole body protein breakdown (B) and, when appropriate, from meal intake (I). Valine leaves the plasma amino acid pool by whole body protein synthesis (S), oxidation (O), and, when applicable, dialysis (D). The input fluxes in this model result in label dilution of infused [1-13C]valine in plasma. These fluxes have to be differentiated from those that result in changes in size of the plasma amino acid pool. This is of particular importance for the calculation of the rate of appearance of valine in plasma in the experiments in which the influence of protein intake has been studied. During protein intake, plasma amino acid concentrations increased gradually. Therefore, the appearance of dietary valine in the circulation comprised a flux resulting in enlargement of the plasma valine pool and a flux of dietary valine, adding to whole body protein metabolism. The appearance of dietary valine was multiplied by 0.8 to correct for first-pass metabolism (10, 13). The amount of dietary valine entrapped in the enlarged pool size of valine (
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Without dialysis during fasting (HD fas), I = 0 and
D = 0; so
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Statistics
All values are given as means ± SD. Statistical analysis was done using SPSS 10.0 (SPSS, Chicago, IL). To compare the changes in protein metabolism resulting from the meal, the fasting and fed states were compared using a paired Student's t-test. Differences between the protein metabolism parameters on a nondialysis day and during dialysis were tested using the unpaired Student's t-test. Correlations between valine concentrations and protein metabolism parameters were tested using linear regression analysis and expressed using the Pearson correlation coefficient. Statistical significance was assumed at P < 0.05. ![]() |
RESULTS |
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Demographic Data
Table 1 shows the demographic and clinical data of the patients studied. All patients were well nourished, as can be concluded from the protein intake and serum albumin concentrations. Dialysis was adequate, as shown by the equilibrated Kt/V values. There were no statistical differences between the two study groups with respect to body mass index, age, or albumin concentration. There were episodes of hypotension in two out of six patients only during dialysis with feeding, which could be reversed by discontinuing the ultrafiltration. The difference between the two dialysate glucose concentrations did not influence plasma glucose and insulin concentrations. Predialysis glucose concentrations in plasma were 5.6 mM (range 3.9-7.5 mM) during fasting and did not change during the dialysis session. During the meal period, glucose concentrations in plasma were 6.8 mM (range 5.2-8.7 mM). Predialysis insulin values in plasma were 11.6 mIU/l (range 6.1-15.3 mIU/l) during fasting and did not change during the dialysis session. During the meal period, insulin concentrations in plasma were 44.7 mIU/l (range 19.8-84.6 mIU/l). Glucose or insulin values did not correlate with the other studied variables in our subjects.Amino Acid Concentrations
Losses of amino acids in the dialysate during the fasting period were 74 ± 21 mmol · patient
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Protein Metabolism
In Fig. 4, plateau enrichments for breath CO2 enrichment and plasma KIVA enrichment are shown to illustrate their steady state in time. Figure 4A shows, during the HD
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Correlations
Protein synthesis was positively correlated (Pearson r = 0.50, P < 0.01) and protein breakdown negatively correlated (Pearson r =
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DISCUSSION |
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The aim of the study was to test the hypothesis that consumption of a protein- and energy-enriched meal restores whole body protein balance during dialysis. Therefore, we examined the effects of such a meal on whole body protein metabolism in CHD patients on a day between two dialysis days and during dialysis. We used a primed continuous infusion of [13C]valine and measurement of isotope dilution of [13C]KIVA in plasma and 13CO2 in expired air. Our study shows that, on a nondialysis day, protein balance was negative after an overnight fast. Consumption of a protein-enriched meal resulted in a positive whole body protein balance. During dialysis, fasting patients were in an even more negative protein balance than on a nondialysis day. Consumption of a protein- and energy-enriched meal during dialysis resulted in a positive protein balance to the same extent as on a nondialysis day. Dialysis led to considerable losses of plasma amino acids in dialysate, which could be supplemented by dietary amino acids but with a shift in composition between essential and nonessential amino acids.
Before interpreting our results, we would like to discuss some
methodological issues. In our study, we used high infusion rates of
[13C]valine during dialysis to measure enrichment of
13CO2 in expired air with sufficient precision
above background. Particularly, we anticipated low values of
13CO2 enrichment in expired air in patients
consuming a protein-enriched meal during a dialysis session.
Accordingly, in the HD+ protocol the rate of infusion of
[13C]valine was doubled compared with the HD protocol.
Under conditions of dialysis and dietary protein intake,
13CO2 enrichment in expired air was low because
1) isotope dilution of [13C]valine was
considerable because of the appearance of dietary valine and
2) total bicarbonate production increased because of exchange of plasma bicarbonate with extracorporeal bicarbonate in
dialysate. Enrichment of plasma [13C]KIVA during dialysis
in the absence of protein intake was ~20%. Infusion of
[13C]valine at such high rates is considered a
"flooding" dose of tracer, which could perturb the processes to be
studied (25, 36, 44). However, these perturbations will
most likely be limited in the case of valine. Several studies have
shown that a flooding dose of valine does not elicit an anabolic
response of whole body protein metabolism (7, 35).
Furthermore, infusion of leucine affects plasma valine concentration,
whereas infusion of valine does not affect plasma leucine concentration
(1, 9). During the meal period, plasma
[13C]KIVA enrichment decreased to values of ~15%.
We used an independent infusion of NaH13CO3 of
short duration to estimate whole body bicarbonate production, instead
of indirect calorimetry. During dialysis, plasma bicarbonate exchanges
with extracorporeal bicarbonate in dialysate. With indirect
calorimetry, this effect of dialysis on the whole body bicarbonate
content cannot be estimated, since this method measures the net effect of this exchange. Accordingly, the dilution of
13CO2 derived from [13C]valine
oxidation in the body bicarbonate pool cannot be determined accurately
by applying CO2 measured by indirect
calorimetry while the patient is dialyzing. In one study using indirect
calorimetry to measure whole body bicarbonate production, the
bicarbonate influx from the dialysis machine was estimated by taking
the arterial-venous difference in bicarbonate concentration across the
dialysis machine (22). Influx of bicarbonate from the
dialysis machine was calculated to be negligible compared with whole
body bicarbonate flux. This is true for net bicarbonate gain during the
dialysis procedure. However, arterial-venous differences do not measure
the unidirectional fluxes of bicarbonate exchange across the dialyzing
membrane, and this is what matters in isotope dilution studies. These
unidirectional fluxes contribute to the apparent increase in whole body
bicarbonate production, observed as the increase of isotope dilution of
CO2 resulting from dialysis (
approximately equal to
+10
) compared with nondialysis (
approximately equal to +20
).
Additionally, bicarbonate dissolved in dialysate was found to be
naturally enriched (
approximately equal to
4
) compared with
background enrichment of plasma bicarbonate in our patients (
approximately equal to
25
). Exchange of bicarbonate between plasma
and dialysate resulted in a gradual increase in
13CO2 background enrichment that reached steady
state in the last 3 h of dialysis (
approximately equal to
20
). We corrected for these changes in
13CO2 background enrichment, otherwise
oxidation rates would have been overestimated by ~20%. This
overestimation of the oxidation rate would have resulted in an
underestimation of protein synthesis. The significant changes in
13CO2 background enrichment observed in our
studies precluded comparison of whole body protein metabolism
immediately preceding the dialysis session with that during dialysis
and immediately after dialysis in a single measurement.
Turning to our results, we found that dialysis mainly decreased whole body protein synthesis and to a lesser extent whole body protein oxidation. Whole body protein breakdown was not significantly affected, or in other words, the rate of appearance of valine in plasma, corrected for infusion of labeled valine, was not affected by dialysis. In several studies a similar observation was made (3, 22). However, Ikizler et al. (18) showed an increase in protein breakdown upon dialysis. Although the reason for this discrepancy is not clear, there are differences in the execution of those studies compared with our study. Leucine oxidation rates were estimated from the appearance of 13CO2 in expired air and the total CO2 production measured by indirect calorimetry. Only Lim et al. (22) corrected for the loss of 13CO2 in dialysate, albeit with a value based on theoretical considerations. We measured the bicarbonate flux in each patient studied while dialyzing. We extended the isotopic model of whole body protein metabolism to accommodate dietary valine influx and losses of valine in dialysate. When this model is applied to the results of our measurements in fasted, nondialyzing CHD patients, interpretation is straightforward. In this case, appearance of valine in plasma, corrected for infusion of isotope, arises from endogenous sources, i.e., whole body protein breakdown, and whole body protein synthesis equals nonoxidative disposal of valine. In cases of protein intake and/or dialysis, the model becomes more complicated. We reasoned that, during dialysis, loss of valine in dialysate contributed to the nonoxidative disposal of valine. Accordingly, the associated flux was subtracted from the rate of nonoxidative disposal of valine. Ikizler et al. (18) used another modeling approach for the amino acid losses, which might have influenced their conclusion.
Consumption of a protein-enriched meal by CHD patients on a nondialysis
day resulted in a positive whole body protein balance. Whole body
protein breakdown was reduced to about two-thirds the rate observed
during fasting in these patients. Synthesis was slightly increased to
125%, and oxidation was strongly increased to 205%. In view of the
absolute values of the rates of whole body protein breakdown, the
positive whole body protein balance at the end of a meal was mainly the
consequence of the strong inhibition of whole body protein breakdown.
Similar observations have been made in apparently healthy individuals
(29, 30). A difference with earlier studies is that we
corrected for the enlarged valine pool. During the consumption of the
protein-enriched meal, valine concentration in plasma of CHD patients
increased continuously. Accordingly, dietary valine influx was
calculated as the difference between the enteral release of valine
appearing in the circulation and the flux of valine associated with the enlargement of the plasma valine pool (see Fig. 2). This correction of
the enteral release of valine for pool enlargement makes the calculation of whole body protein breakdown sensitive to changes in the
size of the plasma valine pool. It does not influence the calculation
of whole body protein synthesis. Furthermore, we assumed that enteral
release of valine was the same as the amount of ingested valine
hydrolyzed in 0.5 h and that first-pass absorption was 20%
(10, 13). This represents, most likely, an
oversimplification, but it will not change the conclusions drawn in
this study. When different values for the first-pass effects are
brought into the calculations, whole body protein breakdown will
increase proportionally in both HD and HD+ protocols.
Protein intake by CHD patients during dialysis restored the whole
protein balance completely compared with a nondialysis day. The effects
of dietary protein intake on whole body protein synthesis and oxidation
measured in the HD+ protocol were comparable to those in the HD
protocol, i.e., an increase to 128 and 200% of fasting values,
respectively, as shown in Table 3. Furthermore, the effect of protein
intake was comparable between the HD+ and HD
protocol with respect to
the rate of appearance of valine, corrected for the infusion of labeled
valine. Protein breakdown was reduced to about one-half the rate
observed during fasting in these patients. It might well be that the
high effectiveness of dietary protein in inhibiting whole body protein
breakdown during dialysis might be overestimated because of the
corrections used to account for the increase of the valine pool. The
valine concentration in plasma during dialysis increased less than on a
nondialysis day. The associated flux of dietary valine to enlarge the
plasma valine pool is thus smaller, and the calculated value of whole
body protein breakdown becomes larger. The values of whole body protein
balance thus represent a minimal estimate under the condition of a
patient during dialysis while consuming a protein-enriched meal.
Recently, Pupim et al. (31) published their study on the effect of parenteral nutrition during dialysis on whole body and forearm protein metabolism in CHD patients. Infusion of an amino acid solution, containing dextrose and lipids as well, during dialysis resulted in an inhibition of whole body protein breakdown and stimulation of protein synthesis by ~50% each. Although qualitatively the same, quantitatively there are discrepancies with our study. As yet, we do not have an explanation. It might be the consequence of differences in experimental setup or in the model used in the calculations. Pupim et al. applied an intravenous infusion of amino acids together with dextrose and lipids, whereas we used a protein- and fat-enriched meal. Similar to our observations during consumption of a protein-enriched meal, Pupim et al. observed an increase in the plasma amino acid concentrations in CHD patients during dialysis as a consequence of parenteral nutrition. It is not clear from their description of the isotopic model how they corrected for this increase in pool size.
Substantial amounts of plasma amino acids were lost during dialysis. Losses of amino acids in dialysate amounted to 7.7 ± 2.1 g of amino acids during dialysis of fasting patients, similar to published figures (16, 39, 46). Losses of amino acids were 11.7 ± 1.9 g in patients while consuming a protein-enriched meal. Similar losses were observed by Wolfson et al. (46) during their infusion of 39.5 g of amino acids with 200 g of glucose. Essential amino acid concentrations responded differently during dialysis than nonessential amino acid concentrations. When fasted patients were dialyzed, plasma essential amino acid concentration decreased 13% compared with the concentration on a nondialysis day. The decrease in concentration of plasma nonessential amino acids was more pronounced (37%). Because body protein is enriched in essential amino acids compared with plasma amino acids, breakdown of body protein will result in an increase of essential amino acids relative to nonessential amino acids in plasma, as was shown in Fig. 3. Consumption of a protein-enriched meal on a nondialysis day also changed the relative composition of plasma amino acids. The increase in concentration of plasma essential amino acids (71%) was more pronounced than the increase of plasma nonessential amino acids (31%). In view of the amino acid composition of milk proteins, enteral protein hydrolysis will release essential amino acids in relative excess to nonessential amino acids. Combining the effects of dialysis and a protein-enriched meal resulted in a 57% increase in plasma essential amino acid concentration and in a small increase of plasma nonessential amino acid (26%) concentrations. Thus, at the end of the dialysis session during which the patients consumed a protein-enriched meal, nonessential amino acids were in shortage relative to essential amino acids. This effect has not been described before. It is tempting to speculate that a misbalance in plasma free amino acid composition after dialysis prevents whole body protein metabolism to revert quickly to its normal, predialysis, condition. Hypothetically, this relatively small derangement in protein metabolism could contribute to malnutrition over longer periods of time.
Oral intradialytic nutrition by means of a protein-enriched meal appeared to be an effective treatment for dialysis-induced protein loss resulting from clearance of plasma amino acids by the dialysis machine. In the study protocol, we used a protein intake of 0.6 g protein/kg, comparable to 50% of daily protein intake in this group of patients. We think that this amount of protein might be too much for the average dialysis patient during a 4-h dialysis session. Pupim et al. (31) infused 15 g of amino acids, whereas we estimated a dietary amino acid influx in the circulation of 39 g, assuming that 80% of all protein taken was digested during the dialysis session. The effects of smaller doses of oral protein on whole body protein metabolism in CHD patients during dialysis are unknown, but our results show that an oral protein load during dialysis has a positive effect that is not less than that of the same load given without dialysis.
In conclusion, we found that consumption of a protein- and energy-enriched meal abolished the negative effect of dialysis on whole body protein balance. This offers a possibility for nutritional intervention in preventing protein energy malnutrition. It also shows that, even though a meal during dialysis may increase the occurrence of hypotension, it is metabolically useful and should therefore be standard practice.
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ACKNOWLEDGEMENTS |
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We appreciate the time from the patients and nursing staff of the Dialysis Center Groningen.
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FOOTNOTES |
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Part of this work was presented at the 34th Annual Meeting of the American Society of Nephrology, San Francisco, CA, 2001.
This work was supported by Grant no. C 97-1694 from the Dutch Kidney Foundation.
Address for reprint requests and other correspondence: R. Huisman, University Hospital Groningen, Internal Medicine, Section Nephrology, Hanzeplein 1, 9713 GZ Groningen, The Netherlands (E-mail: R.M.huisman{at}int.azg.nl).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published January 21, 2003;10.1152/ajpendo.00264.2002
Received 13 June 2002; accepted in final form 15 January 2003.
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