Free and protein-bound glutamine have identical splanchnic
extraction in healthy human volunteers
Julio J.
Boza,
Martial
Dangin,
Denis
Moënnoz,
Franck
Montigon,
Jacques
Vuichoud,
Andrée
Jarret,
Etienne
Pouteau,
Gerard
Gremaud,
Sylviane
Oguey-Araymon,
Didier
Courtois,
Alfred
Woupeyi,
Paul-André
Finot, and
Olivier
Ballèvre
Nestlé Research Center, Vers-Chez-Les-Blanc, 1000 Lausanne
26, Switzerland
 |
ABSTRACT |
The objectives of the present study
were to determine the splanchnic extraction of glutamine after
ingestion of glutamine-rich protein (15N-labeled oat
proteins) and to compare it with that of free glutamine and to
determine de novo glutamine synthesis before and after glutamine
consumption. Eight healthy adults were infused intravenously in the
postabsorptive state with L-[1-13C]glutamine
(3 µmol · kg
1 · h
1) and
L-[1-13C]lysine (1.5 µmol · kg
1 · h
1) for
8 h. Four hours after the beginning of the infusion, subjects consumed (every 20 min) a liquid formula providing either 2.5 g of
protein from 15N-labeled oat proteins or a mixture of free
amino acids that mimicked the oat-amino acid profile and
contained
L-[2,5-15N2]glutamine and
L-[2-15N]lysine. Splanchnic extraction of
glutamine reached 62.5 ± 5.0% and 66.7 ± 3.9% after
administration of 15N-labeled oat proteins and the mixture
of free amino acids, respectively. Lysine splanchnic extraction was
also not different (40.9 ± 11.9% and 34.9 ± 10.6% for
15N-labeled oat proteins and free amino acids,
respectively). The main conclusion of the present study is that
glutamine is equally bioavailable when given enterally as a free amino
acid and when protein bound. Therefore, and taking into consideration
the drawbacks of free glutamine supplementation of ready-to-use
formulas for enteral nutrition, protein sources naturally rich in this
amino acid are the best option for providing stable glutamine.
lysine; oats; nitrogen-15 intrinsic labeling
 |
INTRODUCTION |
HISTORICALLY,
GLUTAMINE has not been used as a nutritional supplement because
it is considered a nonessential amino acid. In healthy humans there
appears to be no need for glutamine supplementation. However, studies
conducted in animal models of infection have shown that oral or
parenteral glutamine supplementation was correlated with survival
rates, decreased bacterial translocation, and enhanced gut morphology
(11, 17, 23). In humans, during stress associated with
injury, sepsis, etc., there is a marked increase in glutamine consumption by the gastrointestinal tract, immunologic cells, inflammatory tissue, and kidneys. Requirements for glutamine by these
tissues may outstrip the synthetic capacity of the skeletal muscle.
Thus, in these situations, the intracellular pools of glutamine in
muscle are markedly reduced (29). Next, as tissue stores
become depleted, plasma concentrations are reduced. Finally, as the
deficiency state is manifested, alterations in tissue function are
observed, and these changes are associated with alterations in the
protein economy (e.g., negative protein balance) (23). In
recent studies, it has been shown that the addition of glutamine to
parenteral formulas maintained concentrations of this amino acid in
blood, improving nitrogen balance and cell proliferation (13,
14). However, evidence of the effects of glutamine
supplementation of enteral formulas in clinical nutrition remains
scarce (6, 18).
Free glutamine has an unfavorable chemical property that hampers its
use in routine clinical ready-to-feed enteral formulas, namely,
instability, especially during heat sterilization and prolonged
storage. This limitation led to an intensive search for new alternative
substrates. The best way of providing stable glutamine is to feed
proteins or peptides rich in this particular amino acid that can be
mixed with milk proteins to obtain a balanced amino acid profile.
However, there is not enough evidence in the literature about the
metabolic fate of protein-bound versus free glutamine. The objectives
of the present study were to determine the splanchnic extraction of
glutamine after ingestion of glutamine-rich protein
(15N-labeled oat proteins) and to compare it with that of
free glutamine and to determine the de novo glutamine synthesis before
and after glutamine consumption.
 |
MATERIALS AND METHODS |
Isotopes.
KNO2 (50% 15N),
(NH4)2SO4 (50% 15N),
and Ca(NO3)2-4 H2O (50%
15N) from Euriso-Top (St. Aubin, France) were used for the
intrinsic labeling of the oats. For the metabolic experiments,
L-[2,5-15N2]glutamine (99%
15N), L- [2-15N]lysine (99%
15N), L-[1-13C]glutamine (99%
13C), and L-[1-13C]lysine (99%
13C) were purchased from MassTrace (Woburn, MA). Chemical
and isotope purities were determined by gas chromatography-mass
spectrometry (GC-MS).
Production of 15N-labeled oat protein concentrate.
Spring oats (variety MH FD 2-6; Momont) were used. Seeds
(2,500) were grown in an inert support medium without any
nitrogen content. The oats were grown in a greenhouse with artificial
light (13:11-h light-dark cycle) at 26-28°C in a hydroponic
culture containing a nutritive solution. The oats were grown for 4 mo in these conditions, and 222 g of oat grains were harvested. The protein content of the grains was 11.8%, and the total 15N
enrichment was 50.6%. The 15N-labeled oat grains were
dehulled, milled, and digested with the aid of amylase. After digestion
at 68°C for 120 min, the hydrolysate was heat treated at 90°C for
30 min, then cooled at 4°C before separation by centrifugation at
10,000 rpm for 20 min. The supernatant was discarded, and the solids
were washed, separated by centrifugation, and freeze-dried.
The 15N-labeled oat protein concentrate was mixed with
unlabeled oat protein concentrate to obtain sufficient amounts of the appropriate enrichments (~5% 15N). This mixture is
referred to as 15N-oats. The final glutamine and lysine
enrichments and contents were checked by GC-MS after enzymatic
hydrolysis. The amino acid profile is shown in Table
1. The chemical composition of the 15N-oats was as follows: 35.2% protein, 4.2% fat, 2.7%
ash, 14.7% fiber, 41% carbohydrates, and 2.2% moisture. Similarly, a
free amino acid mixture mimicking the amino acid profile of the oat protein concentrate was prepared from L-amino acids
(Ajinomoto) and is referred to as 15N-AA. Five percent of
the glutamine and lysine in this amino acid mixture was represented as
L-[2,5-15N2]glutamine (99%
15N) and L-[2-15N]lysine
(99% 15N), respectively. To resemble the oat protein
concentrate composition in terms of macronutrients, this free amino
acid mixture was added to tapioca starch, sucrose, and soy oil.
Finally, citron flavor (0.7%) and citric acid (0.6%) were also added
to improve the taste.
Subjects.
Eight healthy adults (4 men and 4 women, 26-44 yr old)
participated in the study. Subjects were defined as healthy after
medical history and physical examinations. No subject was taking any
nonroutine medication 1 wk before the study or had suffered any recent
gastrointestinal, cardiovascular, or infectious disease. No subject was
suffering from any chronic or metabolic disease (diabetes,
hypertension, celiac disease). They were instructed to maintain their
usual levels of dietary intake and physical activity 1 wk before and during the study. The age, weight, height, and body mass index of the
subjects are given in Table 2. The
protocol was approved by the Ethical Committee of the Nestlé
Research Centre, and informed written consent was obtained from the
subjects.
Experimental protocol.
Before each study, sterile and apyrogen (Limulus lysate
assay) tracer solutions were prepared by accurately weighing each compound and dissolving it in normal saline. The solutions were filtered through 0.22-µm filters.
Two separate protocols were performed in the eight volunteers
in random order, with a 2-wk gap. The protocol is shown in Fig. 1. After an overnight fast lasting
10 h, a catheter was inserted in a retrograde fashion into a
dorsal vein of the hand for arterialized blood sampling after
introduction of the hand into a 55°C heated, ventilated box
(4). A second catheter was inserted into a vein of the
contralateral arm for tracer infusion. A primed (60 × infusion rate/min) continuous infusion of
L-[1-13C]glutamine (3 µmol · kg
1 · h
1) and
L-[1-13C]lysine (1.5 µmol · kg
1 · h
1) began at
7:00 AM, and continued for 8 h. After 4 h, the protein meal
(containing 30 g of protein or protein equivalent from
15N-oats or 15N-AA dissolved in 500 ml of
mineral water) was offered, divided into 12 equal repeated meals, one
every 20 min, to simulate a constant oral infusion. Blood samples were
taken before any infusion (basal); every 15 min between hours
3 and 4, corresponding with the isotopic plateau of the
intravenous tracer before the meal (referred to as fasted state); every
30 min after meal ingestion; and every 15 min between hours
7 and 8, corresponding with the isotopic plateau of the
intravenous and oral tracer (referred to as fed state). Plasma was
immediately separated in a refrigerated centrifuge and kept at
80°C
until analysis.

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Fig. 1.
Protocol design of studies with the labeled meals. After
the first isotopic plateau, corresponding to the Fasted state, the
protein meal [15N-oats or 15N-free amino acids
(AA)] was offered in 12 equal repeated meals (arrows), 1 every 20 min
(Fed). Blood samples (arrows) were obtained before any infusion
(Basal), at the isotopic plateau of the intravenous infusion (Fasted),
and after meal consumption (Fed).
|
|
Analytical procedures.
Plasma (0.75 ml) was deproteinized with 0.6 ml of sulfosalicylic acid
(5% wt/vol) and centrifuged at 10,000 g for 2 min.
Supernatants were acidified with 0.1 ml of 1 M HCl and desalted on
cation-exchange resin (Bio-Rad 50W-X8, 100-200 mesh,
H+) 1-ml columns. Water washes (2 × 5 ml)
were discarded, and amino acids were eluted with 5 ml of 4 M
NH4OH. The eluate was then dried under vacuum conditions
(2). Plasma 13C and 15N
enrichments of glutamine and lysine were determined as their tertiary
butyldimethylsilyl derivative on a Finnigan MAT 252 isotope ratio mass
spectrometer (Bremen, Germany) hooked to a HP5890 series II gas
chromatograph (Hewlett Packard, Palo Alto, CA) via a gas chromatograph
combustion interface. The interface consisted of a NiO-CuO-Pt
combustion furnace reactor (940°C) and a copper reduction furnace
(600°C) (5). Mean 15N and 13C
enrichments of plasma glutamine and lysine at the two plateaus for the
two protocols (15N-oats and 15N-AA) are shown
in Table 3. Plasma amino acid
concentrations were determined by ion exchange liquid
chromatography (LS 6300, Beckman) (5).
Protein degradation.
Whole body protein catabolism was calculated from the endogenous rate
of appearance of lysine, Endo Ra-Lys, which was the only
source of lysine apart from that coming from the oral nutrition. Therefore, from the rate of protein breakdown (Endo Ra-Lys
in µmol · kg
1 · h
1) and
the abundance of this essential amino acid in body proteins (720 µmol
lysine/g body proteins) (22), protein degradation in grams
of protein per kilogram per hour can be estimated
|
(1)
|
Glutamine and lysine turnover.
The estimation of the de novo synthesis of glutamine was calculated as
follows. Because lysine is an essential amino acid, its rate of
appearance is entirely derived from protein breakdown in the
postabsorptive state. However, the endogenous rate of glutamine appearance (Endo Ra-Gln) has two inflow components: protein
breakdown (BGln) and de novo synthesis (DGln)
|
(2)
|
Therefore, BGln is estimated with the assumption
that the release of an essential amino acid (lysine) depends on the
rate of protein breakdown and the content of this amino acid in body protein (92.5 mmol glutamine/100 g protein and 72.0 mmol lysine/100 g
protein) (22)
|
(3)
|
Then
|
(4)
|
The total rate of appearance of glutamine (Total
Ra-Gln) was calculated as follows (8)
|
(5)
|
where
is the
glutamine tracer infusion rate (in
µmol · kg
1 · h
1),
Ei(13C-Gln) is
[13C]glutamine enrichment in the intravenous infusion
(mole percent excess; MPE) and
Ep(13C-Gln) is the average
[13C]glutamine enrichment in plasma (MPE) during isotopic
equilibrium. The endogenous rate of appearance of glutamine (Endo
Ra-Gln) expressed in micromoles per kilogram per hour is
|
(6)
|
where Exo Ra-Gln is the amount of glutamine arising
from the protein meal (in
µmol · kg
1 · h
1). During
the postabsorptive state, Exo Ra-Gln is 0;
is the glutamine tracer
infusion rate (in
µmol · kg
1 · h
1).
Using the same rationale, the total and endogenous rates of appearance
in plasma of lysine (Ra-Lys), in micromoles per
kilogram per hour, were calculated from their respective tracer
infusion rates (I13C-Lys), the enrichment
(MPE) of the intravenously enriched tracer
[Ei(13C-Lys)], and
Ep(13C-Lys) is the average
[13C]lysine enrichment in plasma (MPE) during isotopic
equilibrium. The endogenous rate of appearance of lysine (Endo
Ra-Lys), expressed in micromoles per kilogram per hour, was
calculated from the total rates of appearance in plasma of lysine, as
well as from the amounts of lysine during the protein meal
administration (also in
µmol · kg
1 · h
1; Exo
Ra-Lys)
|
(7)
|
|
(8)
|
Splanchnic extraction.
The appearance rates of glutamine and lysine were calculated as
explained in Glutamine and lysine turnover, either
when the intravenous route was used to infuse the tracers or when the
enteral route was used. However, when the latter route is used, some
fraction of the tracers is taken up by the gut or liver during its
first pass. Splanchnic extraction of glutamine and lysine
(SEGln and SELys) therefore represents the
fraction sequestered by the splanchnic bed on the first pass, which
never mixes with the systemic blood. It was calculated as
follows (20)
|
(9)
|
where Ep-o(15N-Gln) is the average
[15N]glutamine enrichment in plasma (MPE) of the oral
tracer, Ei-o(15N-Gln) is the
[15N]glutamine enrichment in the oral tracer (MPE),
io(15N-Gln) is the oral tracer infusion rate of
[15N]glutamine
(µmol · kg
1 · h
1),
Ep-iv(13C-Gln) is the average
[13C]glutamine enrichment in plasma (MPE) of the
intravenous tracer, Ei-iv(13C-Gln) is the
[13C]glutamine enrichment in the intravenous tracer
(MPE), and iiv(13C-Gln) is the intravenous
infusion rate of [13C]glutamine
(µmol · kg
1 · h
1)
|
(10)
|
where Ep-o(15N-Lys) is the average
[15N]lysine enrichment in plasma (MPE) of the oral
tracer, Ei-o(15N-Lys) is the
[15N]lysine enrichment in the oral tracer (MPE),
io(15N-Lys) is the oral tracer infusion rate of
[13C]lysine
(µmol · kg
1 · h
1),
Ep-iv(13C-Lys) is the average
[13C]lysine enrichment in plasma (MPE) of the intravenous
tracer, Ei-iv(13C-Lys) is the
[13C]lysine enrichment in the intravenous tracer (MPE),
and iiv(13C-Lys) is the intravenous infusion
rate of [13C]lysine
(µmol · kg
1 · h
1).
Statistics.
13C and 15N enrichments and concentrations of
glutamine and lysine in plasma for each subject represent the average
of five blood samples taken during isotopic equilibrium. Values are
means ± SD. Comparison between groups was assessed by a paired
Student's t-test. Differences were considered statistically
significant if P < 0.05.
 |
RESULTS |
The mean values of the plasma amino acid concentrations are shown
in Table 4. Compared with the values in
the fasted state (before meal administration), nutrient delivery caused
a significant elevation (P < 0.05) of most plasma
amino acids, with no differences related to the type of meal (oats vs.
free amino acids).
The rate of glutamine appearance in plasma increased significantly
(P < 0.05) during enteral feeding compared with the
fasting state. The fraction of the rate of appearance of glutamine in plasma as a result of protein breakdown was estimated from the glutamine and lysine content of body protein as stated in the MATERIALS AND METHODS section. As expected from
measurements performed in lysine, the release of glutamine resulting
from protein breakdown decreased during enteral feeding (17% decrease,
but not significant for 15N-oats; 27% decrease for
15N-AA) compared with fasting values. The fraction of the
rate of appearance of glutamine that was not accounted for by protein breakdown can be attributed to de novo glutamine synthesis under conditions in which there is no exogenous supply of glutamine. During
enteral nutrition, de novo glutamine synthesis decreased significantly
(32% 15N-AA and 35% 15N-oats) compared with
values obtained during the fasting state (P < 0.05;
Table 5). Figures
2 and 3
show, respectively, the rate of appearance of glutamine resulting from
protein breakdown (BGln) and the de novo synthesis of
glutamine before (fasted state) and after (fed state) the continuous
15N-labeled meal ingestion (15N-Oats or
15N-AA) for all volunteers.

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Fig. 2.
Rate of appearance of glutamine resulting from protein
breakdown before (Fasted) and after (Fed) continuous
15N-labeled meal ingestion in human volunteers.
A: 15N-oats meal. B:
15N-AA meal. Each line represents data from an individual
volunteer.
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Fig. 3.
De novo synthesis of glutamine before and after
continuous meal ingestion in human volunteers. A:
15N-oats meal. B: 15N-AA meal. Each
line represents data from an individual volunteer.
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|
As plasma lysine concentrations showed, total lysine rates of
appearance in plasma showed a trend to increase when meal consumption began compared with the rates in the fasting state and were similar with both types of nutrition (Table 6).
On the whole, nutrient meals caused a significant change in the rate of
protein breakdown (P < 0.05), and this decrease did
not differ between the two regimens (Table
7).
Table 8 shows the splanchnic extraction
of glutamine and lysine in human volunteers after oral consumption of
either 15N-oats or 15N-AA. The splanchnic
extraction of glutamine was 66.7 ± 3.9% (15N-AA) and
62.5 ± 5.0% (15N-oats).The splanchnic extraction of
lysine was 34.9 ± 10.6% (15N-AA) and 40.9 ± 11.9% (15N-oats).
 |
DISCUSSION |
The present study used isotope-labeled probes to assess the
metabolism of glutamine and its response to exogenous glutamine delivered through the enteral route as free amino acid or as a glutamine-rich protein source. The main conclusion of the present study
is that despite a 65% local utilization by the splanchnic bed,
glutamine is bioavailable when given enterally, irrespective of the
molecular form in which it is consumed (free amino acid or protein
bound). Moreover, exogenous protein is able to modulate whole body
glutamine metabolism by decreasing both de novo glutamine synthesis
(32% or 35% depending on meal type) and glutamine resulting from
protein breakdown (0% or 27% depending on meal type). To our
knowledge, this study is the first to quantify the splanchnic extraction of glutamine provided as protein-bound glutamine as well as
its influence on whole body glutamine metabolism. The results clearly
show that oral administration of free glutamine and protein-bound
glutamine (15N-oats) are equivalent in terms of splanchnic extraction.
Despite the high extraction of glutamine in the splanchnic bed, oral
administration of 15N-oats or 15N-AA in
repeated meals caused an 11-13% significant increase in the total
rate of glutamine appearance in plasma compared with fasting values. As
suggested from earlier studies (10, 17, 20), this
demonstrates that the enteral route may be used effectively to supply
the glutamine required for extrasplanchnic tissues. Although we failed
to measure muscle glutamine concentrations, a rise in circulating
glutamine is likely to stimulate rates of glutamine uptake by muscle
(1) and could potentially correct depletion of the
glutamine pool, a situation that occurs during stress and severe
disease (15). On the contrary, protein administration led
to a significant reduction of the de novo synthesis of glutamine, the
main glutamine supplier during the postabsorptive state. This hypothesis has been proven in cultured muscle cells in which increasing glutamine concentrations inhibited glutamine synthetase activity (26). Likewise, the glutamine kinetic results are in
agreement with those of Hankard et al. (15), who observed
in human volunteers that enteral administration of glutamine produced a
sharp reduction in de novo synthesis of glutamine. In very low birth
weight infants, Darmaun et al. (9) observed similar
results. It seems that there is a feedback mechanism between glutamine
arriving in plasma from exogenous sources and that arriving by de novo
glutamine synthesis. In the study by Hankard et al. (15),
the oral administration of glutamine did not affect the appearance in
plasma of glutamine resulting from protein breakdown.
In the present study, compared with fasting values, the appearance in
plasma of glutamine as a result of protein breakdown decreased 27%
after 15N-AA consumption, and the difference was not
significant after 15N-oats consumption, likely because of
the low number of volunteers in each group, which led to a lack of
statistical significance. Nevertheless, in the present study, subjects
received test meals containing not only glutamine but whole protein or
a mixture of free amino acids plus carbohydrates and fat. This could
have inhibited protein breakdown and also affected the de novo
synthesis of glutamine compared with fasting values as Table 7 shows
for both test meals. On the contrary, in the study by Hankard et al.
(15), volunteers received a nasogastric administration of
glutamine only.
The most important goal of the present study was to compare the
metabolic fate of enteral glutamine provided within a protein to that
of free glutamine. With regard to the first-pass extraction of
glutamine in human volunteers, our results are in accordance with other
data in the literature (9, 15, 20). Unfortunately, all
these studies were done with free glutamine, and the plasma glutamine
response to enteral administration of protein-bound glutamine and its
splanchnic extraction remain undetermined. The molecular form of the
protein source can affect, per se, protein kinetics and the metabolic
fate of amino acids as a result of several factors, such as gastric
emptying and sites and rates of absorption. However, this was not the case.
According to our results, glutamine resulting from oats behaved the
same way as free glutamine in terms of splanchnic extraction and plasma
glutamine response in human volunteers. It should be emphasized,
however, that all amino acids were labeled in the 15N-oats.
It has been shown elsewhere (8) that branched-chain amino
acids (BCAA) are the major donors of
-amino N for glutamine synthesis. Therefore, part of the 15N-labeled BCAA that
result from 15N-oats could have contributed to de novo
synthesized [15N]glutamine. Our isotope detection method
(isotope ratio mass spectrometry) did not allow us to differentiate
whether the enriched glutamine was labeled in one (from de novo
synthesis by transamination mainly from 15N-labeled BCAA)
or two nitrogens (glutamine present in 15N-oats).
Estimating the 15N isotopic enrichment of plasma glutamine
would lead to an overestimation of the labeled glutamine that appeared
in plasma as a result of the test meal. This was not the case when
volunteers received 15N-AA, a preparation that only
contained two 15N-labeled amino acids, glutamine and
lysine, the latter of which cannot be transaminated. Darmaun and
Déchelotte (8) estimated that the contribution of
leucine accounted for 9% of de novo glutamine synthesis in the
postabsorptive state in human volunteers. However, the data of Darmaun
and Déchelotte (8) were derived from determinations performed solely in plasma. Glutamine synthesis obviously occurs in the
intracellular space. Therefore, estimating the isotopic enrichment in
the intracellular space from measurements made in plasma clearly leads
to an overestimation of the true precursor enrichment. We failed to
measure 15N enrichments of plasma BCAA, which would have
allowed us to estimate the contribution of transamination to
15N enrichments of plasma glutamine, but we know the
initial 15N enrichment of the meal containing the
15N-labeled oat proteins, the relative content of each one
of the BCAA in the oat protein, as well as the oat protein
administration rate for each volunteer. From these numbers, and
assuming similar amino acid fluxes, we can quantify theoretically the
contribution of transamination from BCAA resulting from the labeled
meal to the 15N enrichment of the amine group of glutamine.
We have estimated this contribution to be 7.9 ± 1.9%. Therefore,
taking into account that ~8% of 15N enrichment in plasma
glutamine comes from transamination from BCAA and not directly from
glutamine present in labeled oats, splanchnic extraction of glutamine
after administration of oat protein can account for 62.5 ± 5.0%,
which is even closer to that seen in volunteers receiving the meal
based on free amino acids (66.7 ± 3.9%).
Despite the limitations of the study, approximately one-third of the
enterally administered glutamine eventually reached the systemic
circulation, demonstrating that in the clinical setting the enteral
route may supply glutamine efficiently for the extrasplanchnic tissues.
Apart from that, however, the other two-thirds remained in the
splanchnic tissues (especially the gut) for protein synthesis and
oxidation. Because gut proteins are not especially rich in glutamine
and because other amino acids have lower splanchnic extraction rates
compared with that of glutamine, it is logical to think that the
majority of the glutamine captured in the small intestine is used for
purposes other than mucosal protein synthesis. Enterocytes, like other
rapidly dividing cells, have been shown to utilize glutamine to a
greater extent than any other fuel source (27). Glutamine
requirements for the gut and the immune system are increased in
critical illnesses such as sepsis or trauma. Therefore, an adequate
supply of this amino acid at the right level is crucial, first, to
maintain the functionality and the integrity of the small intestine and
second, to limit the muscle protein catabolism to yield glutamine via
transamination of BCAA (19). The splanchnic extraction of
lysine was 34.9 ± 10.6% (15N-AA) and 40.9 ± 11.9% (15N-oats). These results are in agreement with
those obtained previously by Hoerr et al. (16) who
estimated the first-pass splanchnic uptake of lysine in young
volunteers in the fed state. They quantified it to 35%. First-pass
splanchnic uptake of lysine in young pigs (in the fed state) has been
found to be 32% of the total lysine intake (P. Reeds, personal communication).
Repeated meal feeding, either with native proteins or with free amino
acids, led to a decrease in protein breakdown as assessed by Endo
Ra-Lys. These results are consistent with data reported in
numerous continuous feeding studies (3, 12, 24). This inhibition of protein breakdown is unlikely to be due to insulin because repeated meals are not able to produce detectable insulin increases (B. Beaufrère, personal communication). Increases in plasma amino acid concentrations per se are known to inhibit protein breakdown (7, 28), even in the presence of basal insulin levels (7). Although the plasma hyperaminoacidemia was not very high (Table 3), it was prolonged for 4 h. This would suggest that a significant duration of hyperaminoacidemia is necessary to
inhibit protein breakdown. Boirie et al. (4) have
suggested that there is a relationship between the regulation of
postprandial protein kinetics and the rate of digestion and absorption
of dietary proteins and thus amino acid appearance in plasma. They
found that whey proteins are more rapidly digested than native micellar casein. In addition, postprandial leucine oxidation was significantly higher after the ingestion of whey proteins than of casein despite identical leucine intakes. The authors concluded that different digestion rates and amino acid profiles result in different protein kinetics.
In the present study, the only major difference between meals was the
molecular form of the protein source (native protein or free amino
acids), although the amino acid profile was identical. This fact, along
with the experimental design that included 12 repeated meals, meant
that plasma amino acid appearance was the same for both meals for
almost the entire feeding period and, subsequently, had the same effect
on whole body protein kinetics. Metges et al.
(21) recently studied the kinetics of
L-[1-13C]leucine when ingested with free
amino acids and unlabeled or intrinsically labeled casein in human
volunteers in the fed state. They did not find that the form of dietary
nitrogen or of the leucine tracer administered had an effect on
estimates of splanchnic extraction. However, they found higher
leucine oxidation and lower nonoxidative leucine disposal when the
L-amino acid diet was given compared with the intrinsically
labeled casein. Moreover, they found higher mean plasma leucine,
isoleucine, and valine concentration during the ingestion of the free
amino acid diet that may be causally related to the higher
rate of leucine oxidation. In the present study, we did not find
differences in lysine kinetics between 15N-oats and
15N-AA.
Also, the question arises as to whether the nature of the specific
labeled protein could have an effect on amino acid utilization. That
is, would a different intrinsically labeled protein, as in our study,
oats, have demonstrated a lower rate of amino acid oxidation or a
higher nonoxidative amino acid disposal (21)? Thus a
recent study (25) in elderly women showed that when
dietary protein intake was increased through the addition of vegetable protein, postabsorptive protein breakdown was not inhibited to the same
extent as that occurring when animal protein was given. Therefore, all
these findings reveal that the immediate metabolic fate of absorbed
amino acids is determined by a complex interaction of factors including
the molecular form of the amino acid ingested, the amino acid profile,
the composition of the meal, the level of intake, and the pattern of
meal ingestion (21).
The main conclusion of the present study is that despite a two-thirds
local utilization by the splanchnic bed, glutamine is bioavailable when
given enterally, irrespective of the molecular form in which it is
consumed (free amino acid or protein bound). Therefore, and taking into
consideration the drawbacks of free glutamine supplementation of
ready-to-use formulas for enteral nutrition, protein sources naturally
rich in this amino acid are the best option for providing stable glutamine.
 |
ACKNOWLEDGEMENTS |
We thank Dr. C. García-Ródenas for helpful discussion
and help in the experimental design and Dr. C. Schindler (Service de
Pharmacie-CHUV, Lausanne, Switzerland) for preparing the tracer solutions for intravenous perfusion as well as for performing the
sterility and apyrogenicity tests of the labeled tracers.
 |
FOOTNOTES |
Address for reprint requests and other correspondence: P.-A.
Finot, Nestlé Research Cntr., Vers-chez-les-Blanc, 1000 Lausanne 26, Switzerland.
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.
Received 12 October 2000; accepted in final form 20 February 2001.
 |
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