Exogenous glutamine: The clinical evidence

Thomas Bongers, MRCP; Richard D. Griffiths, BSc, MD, FRCP, ILTM; Anne McArdle, BSc, PhD

I n the 1960s, glutamine was con-sidered a nonessential amino acidand was therefore not part of thethen available total parenteral nu-trition regimes. This was at least in partinfluenced by the technical difficulties inpreparing and storing glutamine-con-taining solutions. L-glutamine has a verypoor solubility, is not heat stable, andtherefore has to be stored at 4C. Sinceglutamine is readily synthesized in mosttissues and therefore classified as a di-etary nonessential amino acid, it was con-sidered appropriate to omit it.

Although it is certainly true that glu-tamine is nonessential in the healthy hu-man being, it has been suggested other-wise during situations of extreme stress,particularly of prolonged duration. Themechanism for the reduction of muscleglutamine represents a demand for in-creased rates of glutamine utilization atthe whole-body level and a relative im-

pairment of de novo synthesis in skeletalmuscle leading to a failure of systemicdelivery to other organs and a conditionaldeficiency (1). The lung and the brainalso produce glutamine, but skeletalmuscle, by virtue of its size and its abilityto synthesize glutamine de novo, is themost important source of glutamine forthe bloodstream. The initial response toseptic stress is to export glutamine to thesplanchnic bed and immune system fromthe free amino acid pool in muscle. Thisleads to protein breakdown and de novosynthesis of glutamine, which may be asmuch as 85% of the turnover (2) from thereleased amino acids. Tumor necrosisfactor- (3) and endotoxin (4) have beenshown to increase glutamine synthetasemessenger RNA in both lung and muscle,but in animals with cachectic muscle theincreases in glutamine synthetase proteinlevels are seen predominantly in lung tis-sue, signifying its greater importancewhen muscle mass is diminished. Thisbenefit is lost in the presence of concom-itant lung disease.

The major fate of enteral dietary glu-tamine is to be extracted in the first passby the gut and liver (5). The intestinallumen can also transport glutamate inlarge amounts and use it in the oxidativeprocesses or synthesis of proline, citrul-line, arginine, and glutathione (6). Else-where in the body on the arterial inter-face, glutamine rather than glutamate is

the major transported substrate acrosscell membranes. This may be the key tothe importance of the systemic plasmalevels and (parenteral) delivery of glu-tamine for many tissues. Not only do wesynthesize glutamine in many tissues,but we also hold it free in solution inskeletal muscle at a gradient of 32:1 overplasma levels by active transport mecha-nisms.

This observation was first made byVinnars et al. (7), who showed that fol-lowing surgery, trauma, or sepsis, thefree glutamine pool in muscle is reduced.Despite the rapid decrease in the intra-muscular concentration of free glu-tamine, transport out of muscle is main-tained and clearance from the plasma byother tissues is increased, indicating ac-tivated transport mechanisms (8). De-tailed stable isotope studies of glutaminemetabolism in critically ill patients sup-port the extensive and robust large ani-mal studies that show net flux of glu-tamine from skeletal muscle to vitalorgans. Jackson et al. (9) demonstrated innewly admitted intensive care unit (ICU)patients a similar production rate but in-creased metabolic clearance rate fromplasma, consistent with increased utiliza-tion by other tissues and only modestcorrection of low plasma levels with 28-g/day glutamine infusions (10). Later onduring an intensive care stay, the efflux ofglutamine cannot be maintained and

From the Division of Metabolic and Cellular Med-icine, School of Clinical Science, University of Liver-pool, UK.

Dr. Bongers has received a grant from Fresenius-Kabi. Dr. Griffiths has received travel expenses andhonoraria from Fresenius-Kabi. Dr. McArdle has notdisclosed any potential conflicts of interest.

For information regarding this article, E-mail:rdg@liverpool.ac.uk

Copyright 2007 by the Society of Critical CareMedicine and Lippincott Williams & Wilkins

DOI: 10.1097/01.CCM.0000279193.23737.06

We know that critically ill patients suffering from undernu-trition with a limited nutritional reserve have a poorer out-come. Furthermore, having a low body mass index has beenshown to be an independent predictor of excess mortality inmultiple organ failure. Therefore, nutritional support hasgained increasing interest in critical illness with the hope ofpreventing or attenuating the effects of malnutrition. A nega-tive nitrogen balance is the characteristic metabolic feature incritical illness, with the major protein loss derived from skel-etal muscle. In particular, glutamine concentrations are rapidlyreduced in plasma and muscle.

Over the last 20 yrs or so, increasing evidence is emergingto support the use of glutamine supplementation in critical

illness. Clinical trials have found a mortality and morbidityadvantage with glutamine supplementation. The advantageappears to be greater the more glutamine is given and greateragain when given parenterally. Various modes of action havebeen postulated. Glutamine seems to have an effect on theimmune system, antioxidant status, glucose metabolism, andheat shock protein response. However, the benefit of exoge-nous glutamine on morbidity and mortality is not universallyaccepted. This review critically appraises the current clinicalevidence regarding glutamine supplementation in critical ill-ness. (Crit Care Med 2007; 35[Suppl.]:S545S552)

KEY WORDS: critical illness; glutamine; heat shock protein; mor-tality; skeletal muscle

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plasma delivery declines. In children withburns, the plasma levels of glutaminewere reduced, whole body flux of glu-tamine was 40% greater, but the turn-over in the skeletal tissues was reducedmaintaining the net efflux near normal(11). A decrease in muscle glutamine pro-duction was confirmed by Biolo et al. (12)in a large study of 20 severely burnedpatients 2 wks into their critical illness. Itis suggested that muscle has competingdemands to meet the increased require-ment for systemic glutamine delivery vs.the internal glutamate supply problemsfor the transamination of pyruvate to ala-nine in the nonoxidative removal of pyru-vate. This deficiency appears to be main-tained even when moderate amounts ofglutamine are supplemented parenterally(13). A further study investigated thedose response of parenteral glutaminesupplementation. Plasma glutamine con-centration responded by normalization ofplasma glutamine levels in a dose-dependent way, while free muscle glu-tamine concentration, as well as muscleprotein synthesis and muscle proteincontent, did not change significantly(14). There is therefore strong evidenceto contend that glutamine should be re-garded as a conditionally essentialamino acid in critical illness. Despite thisevidence, the view that glutamine is aconditionally essential amino acid in crit-ical illness has been challenged recently(15). Whether we class the observed lowtissue and plasma concentrations as defi-ciency or merely as a response to criticalillness is probably secondary; more im-portant is whether a low plasma and/ortissue glutamine concentration has an ef-fect on morbidity or mortality. This ques-tion was addressed by an observationalstudy from the Netherlands published in2001 (16). In this study, plasma glu-tamine was measured in 80 critically illpatients. A low glutamine concentrationwas associated with higher mortality anda trend toward higher severity scores. Itwas further noticed that the low plasmaglutamine group was considerably older,possibly consistent with a reduced mus-cle bulk. In the light of these findings, itis interesting to remember that a lowbody mass index is an independent pre-dictor of excess mortality in multiple or-gan failure, and this becomes evident atabout 25 days into the illness (17). It isworthy of note that under normal cir-cumstances, glutamine is the most abun-dant nonessential free amino acid in thebody.

Work from other researchers sug-gested that glutamine may work as apharmacologic agent at higher concen-trations in a dose-dependent way. Cer-tainly, animal works seem to suggest anassociation between glutamine dose andresponse (18), but this does not precludethese effects alongside that of correctinga deficiency.

The preceding observations raise sev-eral questions:

Can glutamine substitution normalizeplasma and/or tissue glutamine levels?

Can glutamine substitution influenceclinical outcome?

If so, is this effect seen in the elderlytoo?

Clinical Outcome

The important question is whether ex-ogenous glutamine provision will affectoutcome. However, to understand theavailable data requires an appreciation ofthe time scales and consequences thatcould reasonably be expected. It is appar-ent when caring for patients under inten-sive care that some patients die earlyfrom the failure of a single organ that wasinvolved in the primary pathology, andthis usually involves just the brain or theheart or the lung. While patients die laterfrom multiple organ failure, which is as-sociated with the development of second-ary infections and is often a combinedsystem failure (e.g., lung, liver, and kid-ney), this is more a feature of nonrecov-ery from initial or maintained insults.For instance, although an acute inflam-matory response is an early feature ofmost ICU presentations, it is now appre-ciated that an optimized immune system,still capable of mounting normal inflam-matory signaling, is a feature of survival.Perhaps the greatest challenge with anynutritional replacement therapy is to ap-preciate the likely time scales involved toshow a clinically meaningful response.The analogy of scurvy is useful to con-sider. A benefit will be most stronglyshown in those who are most deficient fora prolonged period sufficient to affectmany systems, for example, the most se-verely ill with gut failure dependentsolely on a glutamine-deficient parenteralnutrition (analogous to 18th centurymariners needing to have been at sea for6 months without fresh fruit or vegeta-bles before scurvy developed). Glutaminealso needs to be given in a sufficient dose(e.g., low-dose continuous enteral deliv-

ery may all be consumed in first passmetabolism, analogous to the use formany years of the low vitamin C-contain-ing lime juice over the more effectivelemon or orange juice) and for suffi-ciently long periods (e.g., not continuingglutamine after the initial treatment pe-riod in those not recovering within inten-sive care; glutamine needs to be given formore than a few days and maintainedduring the voyage to affect survival). Giv-ing too little for too short a treatmentperiod may well have little impact onsurvival; furthermore, choosing an earlyend point (e.g., 28 days) may not disclosea real effect. Evidence suggests that ther-apies that conserve or restore optimalcirculation (19) and metabolism (20)have shown real outcome benefit. Provid-ing the optimal nutritional environmentis central, and the glutamine story is anillustration of such a deficiency arising(21), whereby its correction improvessurvival from multiple organ failure (22).

Parenteral GlutamineSupplementation

Various trials have investigated therole of enteral and parenteral glutaminesubstitution in critical illness. One of theearliest trials on glutamine substitutionin critical illness in a randomized con-trolled fashion was published in 1997 byGriffiths et al (23). They investigated aselect group of 84 critically ill patientswho were unable to receive enteral feed-ing and for whom major sepsis was thepredominant feature. In a double-blindmanner, they were randomized to glu-tamine, substituted total parenteral nu-trition (TPN), or isocaloric and isonitrog-enous TPN. On an intention-to-treatbasis, a significantly reduced mortalitybenefit after 6 months in the glutaminegroup was observed, an outcome chosento better reflect the known time scales ofrecovery of such patients. Survival in theglutamine group was 24 of 42 comparedwith 14 of 42 in the control group (23).Figure 1 shows the survival curve fromICU admission to 6 months.

Using previously unpublished data,the same authors (24) showed that glu-tamine recipients have a significantlylower incidence of catheter-related infec-tions (p .026) but overall only a non-significant and modest reduction in ac-quired infections. Since the opportunityfor new infections is so high in thesepatients, this is not surprising, but moreimportantly there was a later reduction in

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infection. The difference in survival notedwas almost entirely explained by reducedintensive care mortality from multipleorgan failure in those patients remainingin ICU for a longer period and requiring5 days of parenteral feed (p .05). Inthese very sick patients, colonizationwith Candida was high, but fewer glu-tamine recipients acquired infections af-ter a longer time on feeding and nonedied, whereas six control patients ac-quired Candida infections and died frommultiple organ failure (p .02). This is aclinical illustration of how glutaminemay restore the impaired T-cell-mediatedacquired immunity and allow optimal re-covery. There soon followed the largestparenteral study to date, which was pub-lished in 1999 (25). In this study, 168patients who needed parenteral nutritionwere randomized to either glutamine,supplemented TPN, or control TPN ofisocaloric and isonitrogenous values.This study could not show a significantdifference in 6-month mortality or infec-tious complications. However, there wasa nonsignificant trend toward improvedin-hospital mortality in medical and he-matological patients in the glutaminegroup. Major criticisms of this study werea lack of patient homogeneity and the factthat far fewer of the most severely illpatients in intensive care (as in the pre-vious study) and potentially at higher riskof deficiency were recruited.

Two further studies used the dipeptidealanyl-glutamine, which overcomes thesolubility and stability issues. In a French

multicenter randomized control trial,114 ICU patients predominantly undergo-ing complicated surgery or traumashowed on intention-to-treat analysis asignificant reduction in complicated out-comes (41.4% vs. 60.7%; p .05) thatwas predominately related to reduced in-fectious rate and pneumonias (26). Therewas no difference in survival in this groupof patients in whom the age and predictedrisk of death were lower. The medianduration of feed was about 67 days, butthe maximum feed duration was limitedto only 10 days. This also may be criticalto any effect on outcome since it is pos-sible that a longer period on the controland treatment parenteral feeding is re-quired for an effect on survival to becomeapparent in those who were most deficient.The time scale effect was emphasized by astudy involving 144 ICU patients in Ger-many (27). In this randomized but un-blinded study, the investigators decided apriori to analyze the data of 95 patientstreated for 5 days and 68 patientstreated for 9 days. Plasma glutamineswere low, as expected, and even by 5 dayshad not returned to normal since the feed-ing only contained 0.2 g/kg body weight ofglutamine compared with 0.35 g/kg used inearlier studies. There was a small but non-significant difference in clinical outcome inthose patients fed for a shorter period, butfor those fed for9 days, the survival mea-sured at 6 months was significantly better,22 of 33 vs. 13 of 35 (p .05).

A recent systemic review tried to ad-dress important criticisms concerning

the paucity of trials and the small num-ber of patients involved in these trials(28). The authors used electronic data-bases to search for randomized controlledtrials of glutamine supplementation insurgical and critically ill patients. Theauthors identified 14 trials. When aggre-gated, glutamine supplementation wasassociated with a risk ratio (RR) of 0.78(95% confidence interval [CI], 0.581.04) for mortality. Glutamine supple-mentation was further associated with alower rate of infectious complications(RR, 0.81; 95% CI, 0.641.00) and ashorter hospital stay (2.6 days; 95% CI,4.50.7). The mortality benefit was evenmore striking in the trials that used theparenteral route (RR, 0.71; 95% CI, 0.510.99) and higher glutamine concentra-tions (RR, 0.73; 95% CI, 0.531.00). Theaddition of trials published more recentlyhas not altered the overall conclusion,and when the three level 1 and five level2 studies were aggregated, glutamine-supplemented parenteral nutrition wasassociated with a significant reduction inmortality in critically ill patients (RR,0.67; 95% CI, 0.480.92, p .01) using arange of glutamine of 0.20.57 g/kg/day(29). The results of studies currently un-derway in North America and Scandina-via are awaited.

Enteral GlutamineSupplementation

The evidence for enteral glutaminesubstitution is less convincing so far. Alarge study from western Australia ran-domized 363 relatively well-nourishedcritically ill patients to an enteral supple-ment of about 19 g of glutamine per day(30). Neither mortality (glutamine 15%[27 of 179] vs. control 16% [30 of 184])nor severe sepsis incidence (glutamine21% [38 of 179] vs. control 23% [43 of184]) was affected. As is typical in inten-sive care, about 8% of patients requiredparenteral nutrition instead and did notreceive the feed. The lack of any effect onmortality seems to be an observation con-sistent with previous lower dose enteralglutamine studies and may reflect thesystemic availability of glutamine throughthe enteral route that is too limited for itto be sufficient to significantly influencesurvival in the sicker patients.

This is probably not surprising if weremember that rapidly dividing cells (en-terocytes) use glutamine as an energysource and so readily use enterally sup-plied glutamine. This would suggest that

Figure 1. Survival curves for study patients from intensive care unit (ICU) admission to 6 months. Survivalis similar for the first 20 days but then significantly decreases in the control parenteral nutrition groupcompared with the glutamine parenteral nutrition group (14 of 42 vs. 24 of 42; p .049). The survivalcurve from similar matched patients requiring total parenteral nutrition (TPN) before the study com-menced is shown for comparison. Reproduced with permission from Griffiths et al (23).

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glutamine given enterally will not appear,or will appear in a lesser amount, in thesystemic circulation, which in turn maybe the reason for its limited effect onclinical outcome. This was convincinglyshown in a study from 2005 that com-pared enteral with parenteral glutamineadministration (31). In this study, 20 g ofalanine-glutamine dipeptide infusion wasgiven over 4 hrs. Plasma glutamine con-centration rose significantly with enteraland parenteral substitution but signifi-cantly more so when administration wasparenteral. These findings suggest theneed for relatively higher glutaminedoses, when given enterally, to achievethe same systemic concentrations. Fur-thermore, only parenteral alanine-glu-tamine influenced the measured systemicarginine concentration. The significanceof this observation remains to be seen.

Specific Circumstances

Infection. Critically ill patients are atincreased risk of sepsis, which is a majorcause of mortality in the ICU. Moreover, asignificant number of patients are admit-ted to intensive care due to sepsis as aprimary diagnosis. It is therefore not sur-prising that sepsis and its avoidance are amajor focus in current intensive care re-search. It has been suggested for sometime now that glutamine is a major fuelsupply for immune cells. This appears tobe the case for all immune-competentcells. Moreover, optimal phagocytic andsecretory activity of immune cells may bedependent on adequate glutamine supply(32). Oehler et al. (33) investigated theeffect of glutamine depletion on humanleukocytes. They found that lymphocyteswere less able to produce an adequateheat shock protein response to 42C heatwith a reduced glutamine concentrationof 0.125 mM, which is 25% of the physi-ologic glutamine level, at the time of thestress response (33). A study from 2003investigated the role of glutamine andperipheral blood polymorphonuclearcells. The investigators showed that in-creased glutamine levels 4 mM reducedthe tumor necrosis factor- release after4 and 24 hrs of lipopolysaccharide stim-ulation (34). Furthermore, glutamineconcentrations of 4 mM led to an in-crease of heat shock protein after lipo-polysaccharide stimulation. Various clin-ical studies have shown a benefit oninfectious complications in patients withglutamine supplementation, althoughothers have not. A review from 2005 by

Dhaliwal and Heyland (35) suggested glu-tamine supplementation in critical ill-ness. The authors identified numerousrandomized controlled trials with re-duced rates of infection in patients withglutamine supplementation and con-cluded that enteral and parenteral glu-tamine supplementation is associatedwith reduced infectious morbidity in crit-ically ill patients (35).

Burn Injury. For some time now, var-ious authors have concentrated on thebenefit of glutamine in patients with se-vere burn injuries. We know that patientssuffering from major burn injuries are inan extreme catabolic state. Moreover,burn patients are at increased risk of in-fectious complications due to the loss ofthe normal barrier mechanism of theskin. It is therefore not surprising thatglutamine appears to be an ideal and nec-essary compound of nutrition in severeburn injury. In 2001, Wischmeyer et al.(36) reported on 26 severely burned pa-tients who received enteral nutrition plus40 g of parenteral glutamine or an aminoacid control. The authors specificallychose the parenteral route to guaranteesystemic delivery of glutamine. Theyshowed a significant reduction in mortal-ity and Gram-negative bacteremia (36). ACanadian study from 2003 investigatedthe role of enteral glutamine substitution(37). This double-blind trial comprised 45patients with severe burn injuries (40%total body surface area). Patients wererandomized either to receive enteral glu-tamine boluses (4.3 g every 4 hrs) or toserve as isonitrogenous control. The in-vestigators found a three-fold increase inpositive blood cultures in the controlgroup and worsened mortality (12 vs. 2patients). A further randomized double-blind controlled study from the same yearlooked into continuous enteral glutaminesubstitution in severely burned patients(0.35 g/kg of body weight/day glutaminevs. isocaloric and isonitrogenous control)(38). The researchers recruited 40 se-verely burned patients with a total bodysurface burn area between 50% and 80%.Feed was started on day 1 after burninjury. Full feeding was established at day4 and continued until day 12 after burninjury. The authors showed a significantincrease in the plasma glutamine concen-tration after day 12, a reduced infectionrate, a reduced length of hospital stay,and an overall reduced hospital cost inthe glutamine group. However, no mor-tality benefit was observed. A furtherstudy from 2005 compared enteral glu-

tamine substitution with isocaloric andisonitrogenous control. The studyshowed a reduced hospital stay but nomortality benefit (39). A recent reviewaddressed the question of glutamine sub-stitution in burn patients. The authorstated, Glutamine supplementation doesappear to confer significant clinical andcost advantages in critical illness. Burnedpatients may be one particular group whobenefit (40). However, large randomizedcontrolled trials are lacking. Further at-tention needs to be focused on the routeof administration and the duration of glu-tamine administration.

Glucose Metabolism

It has been suggested in the literaturethat glutamine, as parenteral infusion,can beneficially influence insulin-medi-ated glucose utilization (41) and inhealthy adults can beneficially influencepostprandial insulin action, glucose dis-posal, and fat oxidation (42). These find-ings reinforced the hope that glutamineis beneficial during clinical situations as-sociated with insulin resistance, which isfrequently seen in critical illness. Tworandomized controlled trials from 2006investigated this hypothesis further. Inthe large French randomized control trialstudy discussed earlier (26), as well asshowing a substantially reduced infectionand pneumonia rate the investigatorsfound a significant reduction of hypergly-cemia and a significant reduction in thenumber of patients requiring insulin. Anelegant study specifically examined insu-lin resistance in trauma. The authors ran-domized 40 patients with multiple traumato receive either 0.4 g of glutamine per kgof body weight per day or isocaloric andisonitrogenous control (43). To assess in-sulin sensitivity, euglycemic clamp wasperformed on day 4 and day 8. The inves-tigators found that improved insulin sensi-tivity in multiple trauma patients was pos-itively associated with parenteral glutaminesupplementation. In the light of these stud-ies, it is probably reasonable to accept thebeneficial effect that glutamine has on in-sulin-dependent glucose metabolism. Thisis clinically relevant when we review thecurrent evidence on glycemic control insurgical andmedical intensive care patientsand its importance in view of morbidity andmortality (20, 44).

Antioxidant Effect

Reactive oxygen species are assumedto play a key role in the underlying patho-

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physiology of multisystem organ failurein critically ill patients. When oxygenavailability is limited in the tissues ofvital organs due to hypoperfusion, thecells shift from aerobic to anaerobic me-tabolism, thereby lowering the cellularenergy charge. As a result, increasedadenosine triphosphate hydrolysis, a sub-sequent increase in adenosine mono-phosphate levels, and an accumulation ofpurine metabolites are found in ischemictissues. Furthermore, during activationof the immune response, neutrophils,macrophages, and other competent im-mune cells may activate a plasma-membrane-associated nicotinamide ade-nine dinucleotide phosphate (reducedform) oxidase system, capable of oxidiz-ing nicotinamide adenine dinucleotidephosphate to nicotinamide adenine dinu-cleotide (oxidized form), leading to fur-ther generation of superoxide radicals.Spontaneous dismutation of the superox-ide radical generates hydrogen peroxideand molecular oxygen at physiologic pH.Reactive oxygen species not only lead todirect damage of cellular components butalso trigger the release of cytokines thatfurther activate the inflammatory cascade(45, 46). Glutathione is an important en-dogenous scavenger of reactive oxygenspecies, and glutamine is an importantsubstrate for glutathione. Therefore, re-search has focused on the relationshipbetween glutamine and glutathione andfurther on the effect of low glutamineconcentrations on glutathione concen-trations during critical illness. In inten-sive care patients with septic complica-tions following surgery, muscle freeglutamine is depleted to 25% of thenormal concentration (47). In parallel,during the first week in the ICU, muscleglutathione concentration is reduced tothe same level, or greater, compared withpatients undergoing elective surgery. Im-portantly, in this situation there is a sig-nificant correlation between muscle freeglutamine and muscle total glutathioneconcentration as well as the ratio of oxi-dized glutathione to total glutathione (48).Furthermore, the same group showed thatglutamine supplementation can attenuatemuscle glutathione depletion (49).

Heat Shock Protein

The expression of stress or heat shockproteins (HSPs) is one of the most highlyconserved mechanisms of cellular protec-tion and may be central to protectingagainst the assault from systemic inflam-

mation as seen during severe critical ill-ness. An adequate HSP response is there-fore believed to be critical for cell survivalin these circumstances. Skeletal musclenormally adapts following stress, suchthat it is protected against subsequentdamage (50). This adaptation occurs fol-lowing a variety of insults. Free radicalsare being generated, which in turn leadto a rapid adaptive response in the activ-ity of protective enzymes, such as super-oxide dismutase and catalase, and an in-

crease in the cellular content of HSPs(50). An increase in these protective en-zymes and HSPs protects the tissueagainst subsequent exposure to damage(51). All HSPs act to preserve cellularintegrity. Cells stressed by a sublethalinsult that induces the expression ofHSPs are rendered more resistant to sub-sequent extreme stress. One of the possi-ble mechanisms underlying stress toler-ance involves the concept that the properfolding of proteins in a cell requires an

Figure 2. Top panel, heat shock protein (HSP) 1: function of HSPs in the unstressed cell. Bottompanel, HSP2: protective effect of an increased content of HSPs in skeletal muscle. HsF1; heat shockfactor 1.

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intricate set of folding machinery knownas molecular chaperones. HSPs are be-lieved to work as molecular chaperones.Thus, when induced following cellularstress, HSPs appear to repair denatured/injured proteins or promote their degra-dation following irreparable injury (52)(Fig. 2). The protective effect of increasedHSP concentration was very elegantlyshown in a severe sepsis model of Sprague-Dawley rats. Sepsis was induced by a two-puncture cecal ligation technique. At thetime of the operation, an adenovirus ex-pressing HSP 70 (AdHSP) was adminis-tered via endotracheal tube. The sepsis-induced lung injury was markedly reduced,as was mortality in these animals (53).

A recent study in severe trauma pa-tients found a correlation between sur-vival and the ability to mount an HSPresponse, with a survival benefit in thehigh HSP concentration group (54)clearly suggesting that the HSP responseplays a significant role in critical illness.

Glutamine appears to regulate proteinturnover in cell cultures of myotubes,increasing the half-life of long-lived pro-teins. This may be related to the increasein HSP 70 (55). Furthermore, glutamineappears to be a potent enhancer of thestress response (56 58). Preliminarywork in a rodent sepsis model confirmsthat systemic sepsis induces HSP 70 andthat glutamine infusions facilitate furtherHSP expression. This is associated with astriking preservation of the ability ofmuscle to contract. Others have shownthat glutamine infusions, over a range ofdoses (0.150.75 g/kg), are able to en-hance HSPs in multiple organs of the rat(18). This induction occurs as early as 1hr postadministration and persists for upto 72 hrs postadministration. The au-thors demonstrated that glutamine infu-sion before a septic insult was associatedwith protection against endotoxin-in-duced septic shock in the rat and couldmarkedly decrease end-organ injury andoverall mortality. Further experiments il-lustrated that glutamine given after aseptic insult enhanced HSP 70 and 25expressions, protected against acute lunginjury, and reduced end-organ injury andoverall mortality (59). The survival bene-fit from glutamine was abrogated if anHSP inhibitor was administered. Indeed,it has been suggested that glutamine de-ficiency renders the cells incapable of anadequate HSP response (33). Conse-quently, it has been suggested that glu-tamine substitution influences the HSPresponse in humans, and indeed the ob-

served survival benefit of glutamineadded to parenteral nutrition to meet adeveloping deficiency in the critically illmight reflect this. Moreover, it has beendemonstrated that glutamine substitu-tion added to parenteral nutrition corre-lates with increased HSP 70 response inthe critically ill (60). It has been postu-lated that the severity of illness correlateswith the need for glutamine substitution.This could be a further reason why en-teral glutamine supplementation doesnot appear to carry a significant benefitcompared with parenteral supplementa-tion, as severity of illness appears to cor-relate with gastrointestinal failure.

Half of severely ill intensive care pa-tients are 65 yrs old, with upward of25%75 yrs of age. The ability of cells toinduce HSPs following stress is reducedin aged humans and animals. Tissuesfrom aged animals and blood cells fromelderly humans both show a reduced pro-duction of stress proteins following ther-mal stress (61). It has recently been con-firmed that this attenuated responseoccurs in skeletal muscle of aged rodentsfollowing a physiologic stress. The HSP70 content of resting skeletal muscle wasreduced in muscles from aged rodents,and the production in response to a pe-riod of contractile activity was severelyblunted in comparison with young ani-mals (62, 63). This lack of adaptation inHSP content in the aged animals may berelated to a more general failure of adap-tation to stress. Moreover, as mentionedpreviously, it has been observed that alow glutamine concentration might beassociated with older age (16). Further-more, HSP response in trauma might beblunted in older age (54). Evidence is stilllacking whether glutamine supplementa-tion in this high-risk group can improveplasma glutamine concentration, HSP re-sponse, and ultimately outcome.

CONCLUSION

Over the last 20 yrs, increasing evi-dence has emerged to support the use ofglutamine supplementation in critical ill-ness. Clinical trials have found mortalityand morbidity advantages with glutaminesupplementation. The advantage appearsto be greater the more glutamine isgiven. Furthermore, the advantage isgreater again when glutamine is givenparenterally. Various modes of actionhave been postulated. Glutamine seemsto affect the immune system, antioxidantstatus, glucose metabolism, and heat

shock protein response. However, all tri-als on the subject so far recruited anunsatisfactory number of patients. Thisfact, together with the findings of severaltrials that could not demonstrate benefi-cial effects of glutamine supplementation(30, 64, 65), warrants a large randomizedcontrolled trial (45), however difficultthat may be. It must not be forgotten,however, that glutamine is a naturallyoccurring amino acid that appears tohave a very favorable side-effect profileeven when given in large doses. Thiswould justify its use in critical illnesswith the limited evidence available. De-spite the brave undertaking of the largetrial by the Canadian Critical Care TrialsNetwork, further questions might not beanswered by this trial.

As mentioned, glutamine appears toinfluence the HSP response. However,the HSP response appears to be attenu-ated in the elderly. Can the administra-tion of glutamine overcome the bluntedHSP response in the elderly, who clearlyare a high-risk group?

Furthermore, other researchers haveproposed a pharmacologic action of glu-tamine rather than the correction of adeficiency. Certainly animal works seemto suggest a dose response. Does glu-tamine have a dose response in humans,and, if so, how do we titrate the requireddose?

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