BRIEF COMMUNICATION / COMMUNICATION BREVE

Glutamine and carbohydrate supplements reduceammonemia increase during endurance fieldexercise

Jacqueline Carvalho-Peixoto, Robson Cardilo Alves, and Luiz-Claudio Cameron

Abstract: Blood ammonia concentration increases during endurance exercise and has been proposed as a cause for bothperipheral and central fatigue. We examined the impact of glutamine and (or) carbohydrate supplementation on ammone-mia in high-level runners. Fifteen men in pre-competitive training ran 120 min (~34 km) outdoors on 4 occasions. On thefirst day, the 15 athletes ran without the use of supplements and blood samples were taken every 30 min. After that, eachday for 4 d before the next 3 exercise trials, we supplemented the athletes’ normal diets in bolus with carbohydrate(1 g�kg–1�d–1), glutamine (70 mg�kg–1�d–1), or a combination of both in a double-blind study. Blood ammonia level was de-termined before the run and every 30 min during the run. During the control trial ammonia increased progressively to ap-proximately 70% above rest concentration. Following supplementation, independent of treatment, ammonia was notdifferent (p > 0.05) for the first 60 min, but for the second hour it was lower than in the control (p < 0.05). Supplementa-tion in high-level, endurance athletes reduced the accumulation of blood ammonia during prolonged, strenuous exercise ina field situation.

Key words: dietary supplements, fatigue, physical endurance, elite runners.

Resume : La concentration sanguine d’ammoniac augmente au cours d’un exercice d’endurance et, d’apres des chercheurs,ce serait une des causes de la fatigue tant centrale que peripherique. Nous analysons l’effet d’un supplement de glutamineou de sucre sur l’ammoniemie observee chez des coureurs de haut niveau. Quinze hommes en periode precompetitive cou-rent a l’exterieur durant 120 min (environ 34 km) en quatre occasions. Le premier jour, les 15 athletes ont couru sansprendre de supplement et on leur a preleve des echantillons de sang toutes les 30 min. Par la suite, dans un contexted’etude experimentale a double insu et tous les jours pendant les 4 jours precedant les 3 prochaines seances de course, ona ajoute au regime normal des athletes un supplement en administrant un bolus de sucre (1 g/kg/d), de glutamine (70 mg/kg/d) ou les deux reunis. On evalue le taux sanguin d’ammoniac avant la course et toutes les 30 min durant la course. Aucours de la seance de course de controle, le taux sanguin d’ammoniac augmente progressivement et atteint une valeur ap-proximative de 70 % superieure a celle du repos. Apres la supplementation, et ce, dans toutes les autres seances de course,on n’observe pas de difference de concentration sanguine d’ammoniac durant les 60 premieres minutes (p > 0,05), mais aucours de la deuxieme heure, on observe des valeurs plus basses que dans la seance de controle (p < 0,05). La supplementa-tion donnee aux coureurs d’endurance de haut niveau reduit l’augmentation de la concentration sanguine d’ammoniac ob-servee au cours d’un effort exigeant de longue duree realise sur le terrain.

Mots-cles : supplements alimentaires, fatigue, endurance physique, coureurs d’elite.

[Traduit par la Redaction]

Introduction

Ammonia (NH3, and its anion, NH4+) is produced in

greater concentrations than normal during high-intensity ac-tivities and can be toxic for muscle and brain metabolism(Nybo et al. 2003, 2005). During exercise, ammonia can be

Received 15 November 2005. Accepted 27 May 2007. Published on the NRC Research Press Web site at apnm.nrc.ca on 13 November2007.

J. Carvalho-Peixoto. Laboratorio de Bioquımica de Proteınas, Universidade Federal do Estado do Rio de Janeiro, Brazil; Programa dePos-Graduacao em Ciencia da Motricidade Humana, Universidade Castelo Branco – Rio de Janeiro, Brazil.R.C. Alves. Laboratorio de Bioquımica de Proteınas, Universidade Federal do Estado do Rio de Janeiro, Brazil.L.-C. Cameron.1 Laboratorio de Bioquımica de Proteınas, Universidade Federal do Estado do Rio de Janeiro, Brazil; Programa de Pos-Graduacao em Ciencia da Motricidade Humana, Universidade Castelo Branco – Rio de Janeiro, Brazil; Instituto de Genetica eBioquımica, Universidade Federal de Uberlandia, Brazil.

1Corresponding author (e-mail: cameron@unirio.br).

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produced by either an increased breakdown of adenine nu-cleotides to IMP or deamination of amino acids (van Hall etal. 1995; Graham et al. 1997). It has been speculated thatthe increase of ammonia could induce central and (or) pe-ripheral fatigue (Banister and Cameron 1990; Suarez et al.2002; Nybo et al. 2005).

Various investigations have linked ammonia formationand carbohydrate availability. Muscle ammonia productionhas been proposed as linked to the depletion of glycogenstores (Schulz and Hermann 2003; Roeykens et al. 1998)and the deamination of AMP (Ament et al. 1997; Ogino etal. 2000). A diet with low proportions of carbohydrate(CHO) increased the production of ammonia and reducedlactate concentrations (Langfort et al. 2004). A low-CHOdiet was found to result in increased renal production of am-monia (Bisschop et al. 2003); also, oral intake of either glu-cose polymers or glutamine increase glycogen stores(Bowtell et al. 1999, 2000). Snow et al. (2000) verified thatan 8% CHO supplementation reduced muscle ammonia pro-duction during exhaustive exercise. The combination ofCHO and protein supplementation may increase glycogenresynthesis and protein anabolism (van Hall et al. 1998;Colombani et al. 1999; van Loon et al. 2000). Recent studieshave suggested important interactions between CHO andglutamine (Gln) (Perriello et al. 1997; Stumvoll et al.1999), and its supplementation might play a role as a sub-strate for increase glycogen stores (Bowtell et al. 1999), glu-coneogenesis (Mourtzakis et al. 2006), and anapleroticreactions (Bruce et al. 2001).

Glutamine plays a role in many important biological proc-esses and its availability can be limited during exercise(Iwashita et al. 2005). Gibala et al. (1997, 1998) have dem-onstrated that amino acid metabolism during exercise is as-sociated with anaplerosis of the Krebs cycle intermediates.Bruce et al. (2001) verified that Gln supplementation in-creased the Krebs cycle intermediates pool during exercise,probably because of the entry of glutamine carbons at thelevel of 2-oxoglutarate. Since the anaplerotic utilization ofGln is via glutamate (Glu) oxidative deamination, it wasalso showed that Glu supplementation increases bloodalanine levels and decreases ammonemia (Mourtzakis andGraham 2002).

Although prolonged exercise has been examined fre-quently in laboratory settings, we are not aware of a fieldstudy involving elite distance runners. We therefore exam-ined changes in plasma ammonia concentration in elite dis-tance runners during ~34 km runs at a high intensity. Ondifferent occasions they had received supplements of CHO,Gln, or a combination of both. We hypothesized that bothsupplements can prevent ammonemia increase in athletesowing to their contribution as energetic substrates.

Materials and methodsThe investigation met the requirements for carrying out

research in human subjects (Health National Council Brazil1996) and was approved by the Human Research Committeeof the Universidade Castelo Branco. Fifteen male endurancerunners (age, 35.5 ± 9.8 y; body mass, 63.4 ± 6.4 kg; BMI,20.6 ± 1.5 kg�m–2) volunteered and gave written consentafter the nature and procedures of the study were provided.

They were highly trained runners, who regularly trained6–7 times/week, 12 h/week for at least 3 y, had a VO2max of73.1 ± 6.0 mL�kg–1�min–1, and averaged 145.7 ± 5.6 km/week of running. Their best times for running the half andfull marathon were 71.2 ± 7.7min and 142.4 ± 15.3 min.The running velocity was controlled by a timer (FS1,Polar1). Their habitual diet provided approximately13 600 kJ (~50 kcal�kg–1) (protein ~2 g�kg–1; carbohydrates~7 g�kg–1, and fat ~1.5 g�kg–1). The study was conducted inthe pre-competitive phase of training and subjects were fam-iliarized with the testing protocols. On 4 occasions, eachseparated by 4 d, the athletes performed a 120 min run(~34 km) at a VO2 of 77.1 ± 6.0 mL�kg–1�min–1, as estimatedby maximal heart rate, and the velocity and distance of run-ning before the trials. The athletes received water ad libitumduring the exercise tests. During the run the athletes main-tained a running velocity so as to keep the cardiac frequencyat the level previously established.

The subjects consumed their normal breakfast (approxi-mately 1250 kJ (20% from protein; 40% from carbohydrate(~30 g), and 40% from fat) 1 h before the test. Since highlytrained endurance athletes often suffer from overtrainingsyndrome (Lehmann et al. 1991, 1992; Rogero et al. 2005;Elliott et al. 2007), and to ensure the health status of thesubjects, a resting venous blood sample was taken beforethe run for analysis of hematological (Cell-Dyn 1700), bio-chemical (glucose, ammonia, creatinine, bilirubins, urate,urea, triacylglycerols, cholesterol, very low-density lipopro-tein (VLDL), low-density lipoprotein (LDL), high-densitylipoprotein (HDL), sodium, potassium, chloride, phosphorus,calcium, and magnesium (Merck, Brazil)), hormonal (triio-dothyronine, thyroxine, thyroid stimulating hormone, andtestosterone (Abbott, Brazil)), and serum proteins (albumin,globulin, g-glutamyltransferase (gGT), aspartate aminotrans-ferase (AST); alanine aminotransferase (ALT); alkalinephosphatase (AP), and lactate dehydrogenase (LDH)(Labtest Diagnostica, Brazil)). Blood collection was doneseparately according to the metabolite’s specific demand(blood, serum, or plasma with or without additives).

Before the supplementation tests all of the athletes’ re-sponses were analyzed using maltodextrin to evaluate andfamiliarize them with the experimental procedure.

In all trials, the athletes ran alone to assure blood collec-tion safety. Blood was taken before exercise and every30 min during the run for analysis of ammonia measured byglutamate dehydrogenase reaction (Randox LaboratoriesLtd., Antrim, UK). Briefly, blood collection with a freevein in stasis occurred through a catheter inserted in theright upper arm vein of each athlete. The athletes stoppedand immediately had their blood collected. Since they eachbegan the run at a slightly different time, there was no over-lap during the collections. On average, 10 mL of blood wascollected using a vacuum container in a time window of 20–30 s to avoid changes in metabolic responses. After collec-tion the heparinized blood was immediately spun at 4 8C ina clinical centrifuge (3000g). The plasma was quickly frozenand stored at –20 8C until analysis.

Subsequently, the subjects were divided in groups of 5and were given different daily supplements (double-blind)of glutamine (70 mg�kg–1; Glutamin1, Support, Brazil),CHO (1 g�kg–1 of sucrose and maltodextrin; Carboplex1,

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Nutrisport, Brazil), or the two supplements combined. Thesupplements were packaged individually and made indistin-guishable by the use of artificial flavor and were consumeddaily (in the morning) after dilution in 200 mL of water. Ondays 4, 8, and 12, the exercise protocol was repeated 1 hafter consumption of the supplement. The velocity was con-trolled at ~17 km/h. The 4 runs were conducted between07:00 and 09:00 and the ambient temperature ranged from~25 8C to 28 8C. In between the tests the athletes performedtraining sessions at ~60 mL�kg–1�min–1 of VO2. The areaunder the curve (AUC) for the blood ammonia data foreach individual in each treatment was determined using thefollowing equation:

½1� AUC ¼ AiðTiþ1 � TiÞ þ 0:5ðAiþ1 � AiÞðTiþ1 � TiÞ

assuming resting ammonia data as the baseline, and where Ais ammonia concentration (mmol/L) and T is time (min).

The data were analyzed by ANOVA for randomizedblock design (time by trial, n = 15), which was performedto evaluate differences (p < 0.05) between and within trials.Following a significant F test, pair-wise differences wereidentified using Tukey’s significance post hoc procedure.Data are presented as means ± SE.

ResultsHigh-intensity exercise training may induce alterations on

the hematological, biochemical, and hormonal profile of ath-letes. The data obtained (not shown) for the subjects wereall within the normal range and were not altered during thetesting period. Glucose was slightly higher on the days whenthe CHO supplement was given. The level of total serumproteins reflected an adequate intake of proteins in the diet.Surprisingly, one individual was found to have hypercholes-terolemia (7.3 mmol�L–1). During the whole protocol period(12 days) the athletes’ maintained a consistent training pro-gram.

In the control treatment (p < 0.05), blood ammonia waselevated from rest at 60 min, and continued to increasethroughout the exercise such that the values at 90 and120 min were greater (p < 0.05) than those at 30 min. Atthe end of the exercise the ammonia level was ~70% greaterthan the rest value (Fig. 1).

Across all 3 nutritional supplement treatments the restingammonia concentrations did not differ from each other orfrom those of the control treatment. During the 120 minruns there were no differences in blood ammonia concentra-tions between treatments. After 30 min of exercise the datawere not different (p > 0.05) from the placebo condition, butelevated from the comparable rest data (p < 0.05). For thelast 90 min of the runs, the ammonia levels were signifi-cantly lower (p < 0.05) than in placebo in all 3 treatments.In contrast to the placebo condition, the ammonia levelswithin each treatment did not increase after 30 min of exer-cise. The AUC data from the 4 treatments confirmed theseresults (~85% compared with the control trial-NS).

DiscussionWe measured the effect of CHO and Gln supplementation

on the ammonemia increases in endurance-trained runners

during a sub-maximal run of 120 min. The novel aspects ofthis work are the combination of supplements, the quality ofthe athletes, and the nature of the field trials. We found thatthe supplementation of either CHO or Gln resulted in a re-duced accumulation of blood ammonia, but the effect wasnot cumulative.

As expected, the increase in blood ammonia was modestand progressive during the control trial in agreement withprevious reports (van Hall et al. 1995; Alvear-Ordenes et al.2005; Degoutte et al. 2006). The accumulation is clearly dueto an imbalance in muscle release and clearance by tissuessuch as the liver and kidneys and possibly resting muscleand the lungs. Trained individuals such as those in the

Fig. 1. Plasma ammonia concentration measured at rest and duringthe exercise. (Top) Plasma ammonia concentration measured inendurance runners (n = 15) before and every 30 min during arunning test of 2 h. Values are means ± SE. T0, before the runningtest; T30, T60, T90, and T120, during the running test. The calculatedareas under the curve were as follows: non-supplemented (NS),6290.2 mmol�L–1�min–1; CHO, 5359.8 mmol�L–1�min–1; Gln,5399.1 mmol�L–1�min–1; CHO + Gln, 5277.0 mmol�L–1�min–1.(Bottom) Calculated changes in runners’ plasma ammonia concen-tration without supplementation compared with supplemented con-ditions. Values are changes (�) in means ± SE. Data were analyzedby ANOVA for randomized block design (time � trial). Followinga significant F test, pair-wise differences were identified usingTukey’s significance post hoc procedure. *, indicates non-supple-mented significantly different from CHO, Gln, and CHO+Gln (p <0.05).

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present study are characterized by a remarkable liver ammo-nia clearance induced by training (Lo and Dudley 1987).

Both CHO and Gln supplementation reduced the accumu-lation of ammonia in the circulation. Gln is regarded as animportant anaplerotic and gluconeogenic substrate (Perrielloet al. 1997; Stumvoll et al. 1999; Haisch et al. 2000;Mourtzakis et al. 2006), so we hypothesized that Gln supple-mentation could lead to a higher ATP availability, owing toits use as an energetic substrate. In fact, previous investiga-tion showed that there was an increase in blood Gln afteroral supplementation of the amino acid (Bowtell et al.1999). In addition, an increase of serum Gln concentrationhas been reported after oral intake (Ward et al. 2003; Quanet al. 2004). As reported previously, CHO supplementationreduced the normal rise in blood ammonia in response toprolonged exercise (Snow et al. 2000). It is noteworthy thatthis effect occurred later in the exercise when glycogenstores would be lowered. Low glycogen may be associatedwith increased amino acid catabolism and stimulation ofAMP deaminase, causing greater ammonia production(Spencer et al. 1991; Sahlin et al. 1999; Schulz andHermann 2003). Snow et al. (2000) reported that intake ofan 8% CHO solution during exercise reduced muscle ammo-nia production owing to reduced amino acid catabolism, andthat there was also a small contribution of AMP catabolismto ammonia production.

Some studies have shown that a low-CHO diet may de-crease glycogen stores (Matthys et al. 2000; Andrews et al.2003; Schulz and Hermann 2003) and induce greater ammo-nia production (Roeykens et al. 1998; Langfort et al.2004).The strengths of this experimental design are that wekept the macronutrient intake constant during training andbefore tests and ensured that the CHO intake was adequate(~450 g). We also monitored a wide range of hematologicalparameters to ensure that the subjects did not enter a givenexperiment in a state of over training or dehydration. Abnor-malities due to overtraining and (or) inadequate diet couldcompromise and alter the metabolism in subjects with sucha large training volume and subsequently influence the data(Lehmann et al. 1991, 1992; Rogero et al. 2005; Elliott et al.2007).

The experimental design also had a number of limitations.The study was a field trial; as such, the ability to make in-vasive measures was very limited and our results are there-fore descriptive. The major limitations are that we were notable to make measures showing that blood Gln or glucosewere indeed elevated by the supplementations and that thecontrol trial was always the first experiment.

High levels of blood ammonia have been proposed as re-lated to the development of local fatigue and during exercisecan be released for circulation and affect the brain by its dif-fusion from cerebral-spinal fluid (Banister and Cameron1990; Cooper 2001; Nybo et al. 2005). This increase mayprovoke cerebral alterations as a result of its toxicity, whichcould then influence central fatigue, since CNS has no effec-tive urea cycle and depends on the synthesis of Gln for re-moval of excess ammonia (Suarez et al. 2002; Nybo et al.2005). Given the long duration of the exercise, that the sub-jects did not run to exhaustion, and that the increase in am-monia was modest, it is quite unlikely that the changes inammonia in the present study are associated with fatigue.

This field study of very high quality runners demonstratedthat ammonia progressively increases during a 120 min runover approximately 34 km. Previous ingestion of Gln, CHO,or both resulted in a 15% decrease in the overall ammoniaresponse, with a greater effect (up to 30%) after 60 min ofexercise.

AcknowledgementsWe wish to thank all subjects and athletes who partici-

pated in this study for their cooperation; Adalberto de SouzaRabelo from the Military Police of the State of Rio deJaneiro for the recruitment of the athletes; Valeria Nasci-mento Lebeis Pires for her professional support during theperformance tests. We also thank Dr. Foued Espindola, Dr.Veronica Salerno Pinto, and Dr. John Mercer for helpful dis-cussions.

ReferencesAlvear-Ordenes, I., Garcia-Lopez, D., De Paz, J.A., and Gonzalez-

Gallego, J. 2005. Sweat lactate, ammonia, and urea in rugbyplayers. Int. J. Sports Med. 26: 632–637. doi:10.1055/s-2004-830380. PMID:16158367.

Ament, W., Huizenga, J.R., Mook, G.A., Gips, C.H., and Verkerke,G.J. 1997. Lactate and ammonia concentration in blood andsweat during incremental cycle ergometer exercise. Int. J. SportsMed. 18: 35–39. doi:10.1055/s-2007-972592. PMID:9059903.

Andrews, J.L., Sedlock, D.A., Flynn, M.G., Navalta, J.W., and Hon-gguang, J. 2003. Carbohydrate loading and supplementation inendurance-trained women runners. J. Appl. Physiol. 95: 554–590.

Banister, E.W., and Cameron, J.C. 1990. Exercise-induced hyper-ammonemia: peripheral and central effects. Int. J. Sports Med.11: 129–142.

Bisschop, P.H., De Sain-van der Velden, M.G.M., Stelaard, F.,Kuipers, F., Meijer, A.J., Sauerwein, H.P., et al. 2003. Dietarycarbohydrate deprivation increases 24–hour nitrogen excretionwithout affecting postabsortive hepatic or whole body proteinmetabolism in healthy men. J. Clin. Endocrinol. Metab. 88:3801–3805. doi:10.1210/jc.2002-021087. PMID:12915672.

Bowtell, J.L., Gelly, K., Jackman, M.L., Patel, A., Simeoni, M.,and Rennie, M.J. 1999. Effect of oral glutamine on whole bodycarbohydrate storage during recovery from exhaustive exercise.J. Appl. Physiol. 86: 1770–1777. PMID:10368336.

Bowtell, J.L., Gelly, K., Jackman, M.L., Patel, A., Simeoni, M.,and Rennie, M.J. 2000. Effect of different carbohydrate drinkson whole body carbohydrate storage after exhaustive exercise.J. Appl. Physiol. 88: 1529–1536. PMID:10797108.

Bruce, M., Constantin-Teodosiu, D., Greenhaff, P.L., Boobis, L.H.,Williams, C., and Bowtell, J.L. 2001. Glutamine supplementa-tion promotes anaplerosis but not oxidative energy delivery inhuman skeletal muscle. Am. J. Physiol. 280: E669–E675.

Colombani, P.C., Kovacs, E., Frev-Rindova, P., Frev, W.,Langhans, W., Arnold, M., et al. 1999. Metabolic effects of aprotein-supplemented carbohydrate drink in marathon runners.Int. J. Sport Nutr. 9: 181–201. PMID:10362454.

Cooper, A.J. 2001. Role of glutamine in cerebral nitrogen metabo-lism and ammonia neurotoxicity. Ment. Retard. Dev. Disabil.Res. Rev. 7: 280–286. doi:10.1002/mrdd.1039. PMID:11754523.

Degoutte, F., Jouanel, P., Begue, R.J., Colombier, M., Lac, G.,Pequignot, J.M., et al. 2006. Food restriction, performance, bio-chemical, psychological, and endocrine changes in judo athletes.Int. J. Sports Med. 27: 9–18. doi:10.1055/s-2005-837505.PMID:16388436.

Carvalho-Peixoto et al. 1189

# 2007 NRC Canada

Elliott, M.C., Wagner, P.P., and Chiu, L. 2007. Power athletes anddistance training: physiological and biomechanical rationale forchange. Sports Med. 37: 47–57. doi:10.2165/00007256-200737010-00004. PMID:17190535.

Gibala, M.J., Lozej, M., Tarnopolsky, M.A., Maclean, C., andGraham, T.E. 1997. Low glycogen and branched-chain aminoacid ingestion do not impair anaplerosis during exercise in hu-mans. Am. J. Physiol. 272: E239–E244. PMID:9124329.

Gibala, M.J., Maclean, D.A., Graham, T.E., and Saltin, B. 1998.Tricarboxylic acid cycle intermediate pool size and estimatedcycle flux in human muscle during exercise. Int. J. Sports Med.19: 82–86. PMID:9562214.

Graham, T.E., Turcotte, L.P., Kiens, B., and Richter, E.A. 1997.Effect of endurance training on ammonia and amino acid meta-bolism in humans. Med. Sci. Sports Exerc. 29: 646–653.PMID:9140902.

Haisch, M., Fukagawa, N.K., and Mattheus, D.E. 2000. Glutamineoxidation in humans. Am. J. Physiol. 278: E593–E602.

Iwashita, S., Williams, P., Jabbour, K., Ueda, T., Kobayash, I.H.,Baie, S., et al. 2005. The impact of glutamine supplementationon glucose homeostasis during and after exercise. J. Appl. Phy-siol. 5: 1858–1865.

Langfort, J., Czarnowski, D., Zendzian-Piotrowska, M., Zarzeczny,R., and Gorski, J. 2004. Short-term low carbohydrate diet dis-sociates lactate and ammonia thresholds in men. J. StrengthCond. Res. 18: 260–265. doi:10.1519/1533-4287(2004)18<260:SLDDLA>2.0.CO;2. PMID:15142017.

Lehmann, M., Dickhuth, H.H., Gendrisch, G., Lazar, W., Thum,M., Kaminsky, R., et al. 1991. Training-overtraining: a prospec-tive, experimental study with experienced middle-and long-dis-tance runners. Int. J. Sports Med. 12: 444–452. PMID:1752709.

Lehmann, M., Baumgartl, P., Wiesenack, C., Seidel, A., Baumann,H., Fischer, S., et al. 1992. Training–overtraining: influence of adefined increase in training volume vs. training intensity on per-formance, catecholamines and some metabolic parameters in ex-perienced middle- and long-distance runners. Eur. J. Appl.Physiol. Occup. Physiol. 64: 169–177. PMID:1555564.

Lo, P.Y., and Dudley, G.A. 1987. Endurance training reduces themagnitude of exercise-induced hyperammonemia in humans. J.Appl. Physiol. 62: 1227–1230. PMID:3571078.

Matthys, D., Derave, W., Calders, P., and Pannier, J.L. 2000. Car-bohydrate avaiability affects ammonemia during exercise afterb2-adrenergic blockade. Med. Sci. Sports Exerc. 32: 940–945.PMID:10795784.

Mourtzakis, M., and Graham, T. 2002. Glutamate ingestion and itseffects at rest and during exercise in humans. J. Appl. Physiol.93: 1251–1259. PMID:12235022.

Mourtzakis, M., Saltin, B., Graham, T., and Pilegaard, H. 2006.Carbohydrate metabolism during prolonged exercise and recov-ery: interactions between pyruvate dehydrogenase, fatty acids,and amino acids. J. Appl. Physiol. 100: 822–830.

Nybo, L., Moller, K., Pedersen, B., Nielsen, B., and Secher, N.H.2003. Association between fatigue and failure to preserve cere-bral energy turnover during prolonged exercise. Acta Physiol.Scand. 179: 67–74. doi:10.1046/j.1365-201X.2003.01175.x.PMID:12940940.

Nybo, L., Dalsgaard, M.K., Steenberg, A., Moller, K., and Secher,N.H. 2005. Cerebral ammonia uptake and accumulation duringprolonged exercise in humans. J. Physiol. 563: 285–290.PMID:15611036.

Ogino, K., Kinugawa, T., Osaki, S., Kato, M., Endoh, A., Furuse,Y., et al. 2000. Ammonia response to constant exercise: differ-ences to the lactate response. Clin. Exp. Pharmacol. Physiol.27: 612–617. doi:10.1046/j.1440-1681.2000.03312.x. PMID:10901391.

Perriello, G., Nurjann, N., Stumvoll, M., Bucci, A., Welle, S., Dai-ley, G., et al. 1997. Regulation of gluconeogenesis by glutaminein normal postabsortive humans. Am. J. Physiol. 272: E437–E445. PMID:9124550.

Quan, Z.F., Yang, C., Li, N., and Li, J.S. 2004. Effect of glutamineon change in early postoperative intestinal permeability and itsrelation to systemic inflammatory response. World J. Gastroen-terol. 10: 1992–1994. PMID:15222054.

Roeykens, J., Magnus, L., Rogers, R., Meeusen, R., and De Meir-leir, K. 1998. Blood ammonia-heart rate relationship duringgraded exercise is not influenced by glycogen depletion. Int. J.Sports Med. 19: 26–31. doi:10.1055/s-2007-971875. PMID:9506796.

Rogero, M.M., Mendes, R.R., and Tirapegui, J. 2005. Neuroendo-crine and nutritional aspects of overtraining. Arq. Bras. Endocri-nol. Metabol. 49: 359–368. PMID:16543989.

Sahlin, K., Tonkonogi, M., and Soderlund, K. 1999. Plasma hypox-anthine and ammonia in humans during prolonged exercise. Eur.J. Appl. Physiol. Occup. Physiol. 80: 417–422. doi:10.1007/s004210050613. PMID:10502075.

Schulz, H., and Hermann, H. 2003. Glycogen depletion as indica-tion for ammonia determination in exercise testing. Eur. J.Sports Sci. 3: 1–9.

Snow, R.J., Carey, M.F., Stathis, C.G., Febbraio, M.A., and Har-greaves, M. 2000. Effect of carbohydrate ingestion on ammoniametabolism during exercise in humans. J. Appl. Physiol. 88:1576–1580. PMID:10797115.

Spencer, M.K., Zhen, Y., Yan, Z., and Katz, A. 1991. Carbohydratesupplementation attenuates IMP accumulation in human muscleduring prolonged exercise. Am. J. Physiol. 261: 71–76.

Stumvoll, M., Perrielo, G., Meyer, C., and Gerich, J. 1999. Role ofglutamine in human carbohydrate metabolism in kidney andother tissues. Kidney Int. 55: 778–792. doi:10.1046/j.1523-1755.1999.055003778.x. PMID:10027916.

Suarez, I., Bodega, G., and Fernandez, B. 2002. Glutamine synthe-tase in brain: effect of ammonia. Neurochem. Int. 41: 123–142.doi:10.1016/S0197-0186(02)00033-5. PMID:12020613.

van Hall, G., van Der Vusse, G.J., Soderlund, K., and Wagen-makers, A.J. 1995. Deamination of amino acids as a source forammonia production in human skeletal muscle during prolongedexercise. J. Physiol. 486: 789–794. PMID:7473239.

van Hall, G., Saris, W.H., and Wagenmakers, A.J. 1998. Effect ofcarbohydrate supplementation on plasma glutamine during pro-longed exercise and recovery. Int. J. Sports Med. 19: 82–86.doi:10.1055/s-2007-971886. PMID:9562214.

van Loon, L.J.C., Saris, W.H.M., Verhagen, H., and Wagenmakers,A.J. 2000. Plasma insulin responses after ingestion of differentamino acid or protein mixtures with carbohydrate. Am. J. Clin.Nutr. 72: 96–105. PMID:10871567.

Ward, E., Picton, S., Reid, U., Thomas, D., Gardener, C., Smith,M., et al. 2003. Oral glutamine in pediatric oncology patients: adose finding study. Eur. J. Clin. Nutr. 57: 31–36. doi:10.1038/sj.ejcn.1601517. PMID:12548294.

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