Citrulline and the gut

Citrulline and the gut Emmanuel Curisa, Pascal Crennb,c and Luc Cynoberb,d Purpose of review Citrulline, a nonprotein amino acid, is an important source of endogenous arginine. The gut is the main source of citrulline in humans. Hence, citrulline is a potential biomarker of short bowel function. Conversely, citrulline uptake by the gut is important for an oral supply of this amino acid as an alternative to arginine. This review discusses these two aspects of citrulline, as well as the recent developments in the understanding of its metabolism. Recent findings Citrullinemia is such an efficient marker when the active mass of the bowel is affected that it can be used as a prognostic marker for parenteral nutrition weaning (if citrullinemia is >20 mmol/l) and as a factor for deciding between parenteral and enteral nutrition (as long as the pathology is considered). Citrullinemia should be used with care as a marker either of the intestinal absorption or following small bowel transplantation. Summary Citrulline is easily taken up by the gut, with a broad set of transporters that can remove it from the lumen in the enterocytes. This is confirmed by pharmacokinetic studies and the efficacy is so great that oral complementation with citrulline seems more efficient than complementation with arginine to provide arginine. Keywords arginine, atrophy, citrulline, mucosal tissue, short bowel syndrome Curr Opin Clin Nutr Metab Care 10:620–626. ß 2007 Lippincott Williams & Wilkins. a Laboratoire de Biomathématiques, Faculté de Pharmacie, Université Paris Descartes, Paris, bLaboratoire de Biologie de la Nutrition, Faculté de Pharmacie, Université Paris Descartes, Paris, cDépartement de Médecine Aiguë Spécialisée, Hôpital Raymond Poincaré, Assistance Publique – Hôpitaux de Paris, Université Versailles-Saint Quentin en Yvelines, Paris and dService de Biochimie, Hôtel-Dieu, Assistance Publique – Hôpitaux de Paris, Paris, France Correspondence to M. Emmanuel Curis, Laboratoire de Biomathématiques; Faculté de Pharmacie; Université Paris Descartes, 4 Avenue de l’Observatoire, F-75006 Paris, France Tel: +33 1 53 73 98 37; fax: +33 1 53 73 97 77; e-mail: emmanuel.curis@univ-paris5.fr Sponsorship: This study was supported by a grant (EA 2498) from the French Ministère de la Recherche et de la Technologie. Current Opinion in Clinical Nutrition and Metabolic Care 2007, 10:620– 626 ß 2007 Lippincott Williams & Wilkins 1363-1950 620 Introduction Citrulline has a unique metabolism that has prompted suggestions that plasma citrulline level could be a reliable marker of gut function [1]. This led to a hypothesis that citrulline may be a ‘conditionally’ essential amino acid in short bowel syndrome, even if it is not incorporated into proteins. This latter point, together with the facts that citrulline is poorly represented in food except watermelon [2] and that it was viewed only as an intermediary product of the urea cycle, explains the moderate interest invested in this amino acid in nutrition until recently. Beside the specific topic addressed in this paper, the readers are encouraged to look at two general review papers published recently [3,4

], which cover general aspects of citrulline metabolism and pharmacological properties in health and disease. Citrulline metabolism and its relationships with the gut An overview of citrulline metabolism in mammals can be found in the paper by Curis et al. [3]. Basically, it is assumed that citrulline mainly comes from conversion of glutamine (via glutamate, glutamate semi-aldehyde, and ornithine) in the enterocyte (Fig. 1). Citrulline is then released into the blood stream. The kidneys take up the circulating citrulline and convert it into arginine, which is itself released into the blood stream. As shown in Fig. 1, other amino acids can be converted into citrulline, but the amount of these contributions is poorly understood in vivo. This action provides a way to fine tune citrulline synthesis and, more generally, nitrogen homeostasis, with these pathways differentially activated or inhibited according to the protein content of the diet, for instance [3]. Despite the general scheme of this metabolism being well known, the details have not been completely elucidated. The main questions raised are the universality of this metabolic scheme, the origin of the metabolized glutamine, and the possible uptake of citrulline by the liver. Two factors must be considered when assuming that the intestine is the main endogenous source of citrulline. First, intestinal metabolism can vary according to species, hence extrapolating results from animal to humans may be difficult. For instance, the first studies of citrulline metabolism were performed in rats [5], which led to the metabolic pathway presented in Fig. 1, but studies in sheep did not show any significant conversion of Citrulline and the gut Curis et al. 621 Figure 1 Metabolism of citrulline and related amino acids in enterocytes Arg, arginine; Cit, citrulline; D1-P5C, D1-Lpyrroline-5-carboxylate; Gln, glutamine; Gln-ase, glutaminase; Glu, glutamate; Glusal, glutamate semialdehyde; OAT, ornithine aminotransferase; OCT, ornithine carbamoyltransferase; Orn, ornithine; P5CS, D1-P5C synthase; PO, proline oxidase; Pro, proline. Percentages indicate the contribution of different pathways to the metabolism of each amino acid [3]; they are reflected in the width of the arrows. Dashed arrows are for the ‘secondary’ pathways. glutamine into citrulline [6]. In addition, there is no significant production of citrulline by the gut in strictly carnivorous animals such as cats. Various researchers have tried to confirm how citrulline metabolism occurs in humans, for instance, using stable isotopes to determine fluxes. Fujita and Yanaga [7
] showed that citrulline appearance in the portal circulation is correlated with glutamine disappearance, and that the small intestine is involved, but the technique used does not prove that citrulline is directly produced from glutamine. In fact, to our knowledge, there is no direct evidence of glutamine to citrulline conversion in humans. All the available results are in agreement with this metabolic model, however, and this remains the simplest explanation for the observed relationships between glutamine, citrulline, and arginine fluxes. In addition, experiments and clinical trials based on this metabolic scheme led to results that enforced its validity [4

,8]. The second consideration is that intestinal metabolism changes during development, and especially with weaning. In particular, argininosuccinate synthase and argininosuccinate lyase activities disappear with aging, even though a recent study suggested residual activity of these enzymes in adults [7
], allowing some recycling of citrulline into arginine. Hence, what will be presented in this article relates to adults, and the reader is warned that these ideas do not necessarily apply to newborn children. These considerations allow the formulation of a hypothesis for this metabolism. In contrast to strict carnivorous or herbivorous diets, protein intake varies from one meal to another in omnivorous diets. Hence, a fine-tuning of amino acid metabolism is important, allowing a quick response according to nitrogen supply. The activation or inactivation of the citrulline pathway as an alternate route for arginine metabolism, which itself controls ureagenesis, hence the catabolism of amino acids, could be the method used by omnivorous mammals to enable this quick adaptation, with the citrulline pathway preferred for low or normal protein intake and the arginine pathway preferred for high protein intake. It is even possible that argininosuccinate synthase and argininosuccinate lyase in the intestine may act in the conversion of glutamine into arginine, via citrulline, in situations of high protein intake, thus enhancing further ureagenesis in the liver. Further research would be useful to check these hypotheses. Recent work from Boelens et al. [9,10
] showed that glutamine from either enteral or parenteral nutrition can be used to synthesize citrulline, when provided either as a free form or as a dipeptide (alanine-glutamine). Despite some apparent differences in the efficiency of these routes of feeding, it seems that exogenous dietary glutamine as well as endogenous plasma glutamine can both be sources of citrulline (with a better efficiency for enteral glutamine). These researchers also suggest evidence of fine control of these metabolic pathways that should be further investigated to better understand the source of citrulline in the intestine. Similar results obtained in a rat model [11] support the hypothesis that the rat is a suitable model for the study of citrulline metabolism in the intestine in humans; pigs are also reliable [12]. Of interest from recent publications is the question of possible uptake of citrulline by the liver. Although it has been commonly felt that citrulline bypasses the liver, two recent papers from van de Poll and colleagues [13
,14
] suggest that the reality may be more complex. Although this does not directly concern the gut, it is important because it is one of the basic assumptions for using citrulline as a way to replete arginine pools. The first paper [13
] studies catabolic fluxes in 20 patients undergoing partial liver resection because of colorectal metastases and the second [14
] studies six patients undergoing 622 Nutrition and the gastrointestinal tract liver resections, one having pancreaticoduodenectomy, and one having duodenectomy because of gastro-intestinal malignant disease. Both papers conclude that the liver can take up citrulline from blood. The message remains unclear, however; whereas in the first paper [13
], half of the citrulline appearing in the portal vein is taken up, resulting in a decreased availability of citrulline for other organs, in the second paper [14
], the citrulline uptake is counterbalanced by citrulline release, resulting in an apparent null flux of citrulline across the liver. In the second paper [14
], the evidence for citrulline uptake is thin from a statistical point of view, although this may be related to the small number of patients or the heterogeneous nature of the patient population. In the first paper [13
], where dosages were performed just after the tumoral resection, there is undoubtedly liver uptake of citrulline; the question is to what extent this result can be extrapolated to healthy people, since it is well known that the tumor can force the surrounding tissues to adapt their metabolism, expressing new amino acid transporters. This metabolic modification is all the more probable as citrulline can be a precursor of polyamines, via arginine, in colon carcinoma cells [15]. Of note, in multicatheterized dogs, Yu et al. [16] did not find any citrulline uptake by the liver, but instead found an important citrulline release. These two papers raise important issues, and more work is needed to better understand the interaction between the liver (and tumor within this organ) and citrulline metabolism. Mass spectrometry on dried blood spot Citrulline has recently been measured by hydrophilic interaction chromatography/mass spectrometry/mass spectrometry on dried blood spot specimens to monitor graft function following intestinal transplantation [19]. The advantages of this method are that it is almost noninvasive and requires minimal blood sampling (<25 ml). Also, hydrophilic interaction chromatography/mass spectrometry/mass spectrometry detection has high sensitivity and a low limit of detection (1.5 mmol/l). Using dry blood spot specimens induces large variations compared to results obtained using plasma, however. For example, patients with a citrulline concentration of 60 mmol/l using the dry blood spot method may have a plasma concentration ranging from 38–90 mmol/l. Similar uncertainty is found for low values (dry blood spot measured at 15 mmol/l equals citrullinemia between 10 and 40 mmol/l). Finally, the authors claim a cost of US$30.00 per dosage [19] compared with US$5.00 using ion exchange chromatography (N. Neveux, personal communication). Thus, the dry blood spot method cannot be recommended in clinical practice at the present time. Plasma citrulline concentration as a marker of intestinal functionality In Western countries, healthy patients with normal intestinal mucosa function and normal renal function have a citrulline level between 30 and 50 mmol/l with a median of 40 mmol/l [20]. Analytical methods to quantify citrulline Various methods can be used to quantify citrulline, either specifically or similar to other amino acids [3]. Only the most frequently used and recently proposed methods are discussed here. Ion exchange chromatography Ion exchange chromatography with postcolumn derivatization (ninhydrin) is the reference method for amino acid analysis. The dosage is fully automatized and the performance (between-run reproducibility) for citrulline measurement is good, with a coefficient of variation of 4.3% [17]. A ‘short program’ allows specific measurement of citrulline in 30 min from run-to-run (N. Neveux et al., unpublished observation). The limitation of this method is its relatively low sensitivity, making it inadequate for arteriovenous difference measurement. Reversed liquid phase chromatography Citrulline concentration can be measured by reversed liquid phase chromatography using various precolumn derivatization agents, in particular o-phtalaldehyde [18]. The between-run reproducibility is not as good as for ion exchange chromatography but the dosage is more sensitive, allowing measurements of arteriovenous differences. Short bowel syndrome Historically, short bowel syndrome was the first pathological situation to be studied due to the quasi ‘experimental’ situation created by the removal of a significant amount of the anatomical and functional mass of intestine. Since Crenn et al. published their study [1], there have been at least seven other studies performed in adults [21–26,27
], comprising nearly 200 patients, and one performed in children [28] that clearly and constantly showed that plasma citrulline concentration gives a reliable indication of the remaining small bowel length. All of these studies have found a strong and significant positive correlation, with a correlation coefficient r ranging from 0.47 to 0.90, between postabsorptive plasma or serum citrulline concentration and remnant small bowel length. It appears that citrulline concentration reflects the overall small bowel function, including small bowel excluded from the digestive circuit; hence, determination of citrulline concentration can be used preoperatively as a reliable biomarker of the probability of parenteral nutrition weaning at a 19 mmol/l threshold [29]. There are fewer results concerning the relationship between citrulline and absorptive function. The pioneering study by Crenn et al. [1] suggests a positive relationship between citrullinemia and percentage of fat (r ¼ 0.53) and nitrogen Citrulline and the gut Curis et al. 623 (r ¼ 0.47) absorbed. Another study performed in children also suggests that citrulline concentrations provide reliable information on global gut absorption. Indeed, Rhoads et al. [28] determined in children that plasma citrulline concentration enables estimation of the percentage of enteral calories that a short gut can tolerate without diarrhea. A study of adults with short bowel syndrome [24], however, did not find any significant relationship between citrulline and macronutrient (nitrogen, fat, carbohydrate, calories), fluid, and electrolyte (sodium, potassium, phosphorus, magnesium) absorption expressed in percentage of amount ingested. The major problem with this study was that the dosage of citrulline was performed only 1h after parenteral nutrition was discontinued. Therefore, the conversion of arginine (from parenteral nutrition) into citrulline can be a confounding factor in the interpretation of the data. On the other hand, it is likely that citrulline concentration cannot reflect the various aspects of gut function. Citrulline reflects the integrity of the intestinal epithelial cells with a predominant site of production at the proximal jejunum, whereas absorption is a complex integrated function related to small bowel mucosa, biliopancreatic secretions, digestive motricity, gut lumen, and colonic absorption. In addition, the absorption process is variable in capacity and location according to the nutrients considered. Thus, citrulline concentration is an indicator of the functional enterocyte metabolic mass but not of the digestive function per se. Conversely, citrulline concentration provides a practical and clinical indication of global function, and therefore of nutritional prognosis, of a compromised digestive tract. In the study by Crenn et al. [1], a cut-off of 20 mmol/l was highly predictive (92% sensitivity, 90% specificity) to distinguish transient from permanent intestinal failure in patients with short bowel syndrome after 2 years following intestinal resection. A recent study [25] indicates that this threshold can be used as a preoperative (before digestive circuit reestablishment) marker of parenteral nutrition indication in the months following surgery. A prognosis of intestinal failure can be addressed with a valid (analytically and clinically) blood citrulline dosage [29]. One of the major points to underline is the independence between citrullinemia and nutritional status and the presence of parenteral nutrition [1]. Administration of growth hormone has no significant effect on citrulline concentration [30] despite the fact that it exerts a moderate and nonspecific improvement of macronutrient absorption. Surgical procedures of intestinal lengthening improve intestinal capacity and serum citrulline in a porcine short bowel model [31]. Villous atrophy syndrome In chronic villous atrophy (e.g., celiac disease, ‘tropical’ small bowel, and various infectious enteritis), citrulline is decreased (less than 20 mmol/l) in patients with proximal destructive lesions of villous architecture and severely decreased in patients with extensive (proximal and distal) impairment of intestinal mucosa [20]. In these patients, a citrulline value below 10 mmol/l is highly predictive of the need for at least one period of parenteral nutrition. Similar results were recently reported in patients with human immunodeficiency virus enteropathy or severe intestinal infectious disease [32]. In this situation, as in other conditions of villous atrophy syndrome, citrullinemia can be considered as an indicator of evolutivity of the intestinal disease. Patients with only mild enterocyte involvement, i.e., partial proximal villous atrophy, have a normal or moderately decrease in citrulline concentration [20]. Adult patients with celiac disease with severe mucosal impairment, as in refractory sprue, have low citrulline plasma values, whereas patients with celiac disease that clinically and histologically responds to a gluten free diet after 1 year experience normalization or quasi normalization of citrullinemia [20,33]. One of the most exciting concepts is the potential use of citrullinemia to give an objective tool for the indication of nutrition support and to predict the route of the nutrition support as recently reported in human immunodeficiency virus intestinal-associated disease [32]. In summary, a citrullinemia value below 10 mmol/l predicts a high probability (>90%) of parenteral nutrition, whereas enteral nutrition is mandatory when citrullinemia is between 10 and 20 mmol/l for approximately 50% of the patients in our experience. Nevertheless, because the indications for enteral nutrition are variable and not only associated with a digestive disease, particularly in anorexia syndrome, citrullinemia between 20 and 30 mmol/l can also be an indication for enteral nutrition in some patients. A citrulline concentration of more than 20 mmol/l allows parenteral nutrition weaning, whereas the threshold is variable for enteral nutrition weaning [29]. Crohn’s disease In Crohn’s disease, plasma citrulline is normal because there is no enterocyte damage per se, with the exception of patients with extensive involvement of the small intestine or significant intestinal resection. Systemic inflammation does not significantly influence citrullinemia in Crohn’s disease [27
]. There is no apparent relationship between citrullinemia and small bowel permeability tests (urinary lactulose/Rhamnose test) [27
]. Acute mucosal enteropathy and antineoplastic treatments Acute mucosal enteropathy can cause a significant loss of enterocytes, as experienced clinically by secondary lactose intolerance. Citrullinemia is reduced in these situations, for example, in adenovirus enteritis [34
] and in all infectious intestinal diseases with high cytopathic effect. The normalization of citrullinemia is often rapid, after 1–3 weeks. Chemotherapy in hematology (bone marrow allograft) and oncology induces a decrease in citrulline 624 Nutrition and the gastrointestinal tract levels [35], which relates with the known cytokinetic renewal of intestinal mucosa with a nadir 5–8 days after treatment initiation [36]. Citrullinemia is more sensitive and more specific than the sugar-based permeability test for detecting chemotherapy-induced gut damage in patients with hematological malignancies. After bone marrow transplantation, the decrease in citrullinemia appears to be a risk factor for infections [35]. This suggests that, during antineoplastic therapies, citrullinemia could be used to monitor the intestinal mucosa toxicity. Mucositis and epitheliitis can be treated or prevented in part by keratinocyte growth factor. In a mouse model, recombinant human keratinocyte growth factor treatment allowed maintenance of the citrulline level at a normal value during chemotherapy [37]. Acute radiation enteritis induced by total body irradiation or fractioned localized irradiation can be monitored by citrullinemia that correlates to dose received and volume of bowel in the field of radiations [38]. The relationship is not so good between citrulline concentration and clinical symptoms such as diarrhea. Small bowel transplantation The most important interest in citrulline dosage has been in intestinal transplantation because there is no powerful indicator of acute dysfunction of an intestinal graft [34
]. Citrulline was suggested several years ago as an indicator of acute epithelial rejection after small bowel transplantation [29]. Nevertheless, after intestinal transplantation, the citrulline level has to be interpreted as part of multiple parameters, including time after surgery, renal function, and graft pathology [34
]. Finally, the nonspecificity of citrullinemia variations after major treatment (with multiple influencing factors and the decline only when diffuse or severe mucosal damage has occurred [39]) requires care in the interpretation of citrulline concentration in these situations. In addition, obtaining the result quickly, preferably the same day, remains a challenge. Citrulline requirements There are situations in which it may be useful to provide citrulline to a patient. The most obvious one is after massive intestinal resection, corresponding to short bowel syndrome, in which the main site of citrulline production is greatly reduced. A decrease in plasma and intracellular arginine concentration is observed, in addition to plasma citrulline lowering [40,41]. This suggests that arginine becomes an essential amino acid after significant small bowel resection. On the other hand, as established in a rat model, citrulline could be a good candidate for generating arginine and improving nutritional status after massive intestinal resection. A model of massive intestinal resection (80% of the small bowel) in the rat demonstrated that citrulline-enriched enteral nutrition (1 g/kg/day) was able to generate large amounts of arginine in various tissues, and totally restored the nitrogen balance [42]. More recent results suggest that citrulline, while not a component of proteins, may influence protein synthesis [43
]. These considerations indicate that citrulline may become an essential amino acid after intestinal resection. Absorption of citrulline by enterocytes A recent study of CaCo2 cell cultures demonstrated that citrulline is efficiently transported across the luminal membrane [44]. A broad set of transporters (belonging to systems L, b0,þ and B0,þ) seems to be able to transport citrulline from the lumen to the cell, which is unusual for amino acids. Pharmacokinetics of citrulline Only a few pharmacokinetic studies of citrulline have been performed. All of them confirm that citrulline is very efficiently absorbed when orally administered; these studies have been summarized by Cynober [45], although this review does not include the pioneering paper by Rajantie et al [46]. Only results from the study by Moinard et al. [47] are reported here, as it is the only study with dose-ranging oral administration of citrulline. After ingestion of 10 g of citrulline by eight young healthy men, citrullinemia reaches a peak of Cmax ¼ 2756  70 mmol/l after Tmax ¼ 0.72  0.08 h; only plasma levels of citrulline, arginine, and ornithine were raised. In addition to the increased level in plasma, which is promising for the use of citrulline as an arginine precursor, giving citrulline orally is more efficient than giving arginine orally. This was first observed in animals (piglets [48
] and rats [43
]), but now studies have confirmed this observation in humans [4

,49

]. Romero et al. [4

] reported that the peak plasma arginine concentration increases by 227% when citrulline is given, but only by 90% when arginine is given. Smith et al. [49

] performed a clinical trial in young children (younger than 6 years) undergoing surgical procedures for congenital heart lesions who were given either citrulline or a placebo by way of enteral nutrition. Children receiving citrulline presented significantly higher citrullinemia, but also argininemia. The arginine level was maintained in these patients, whereas it decreased for patients receiving placebo. How to provide citrulline Since citrulline conversion accounts for 80% of the arginine de novo synthesis in the organism [3], giving citrulline seems an interesting approach to providing arginine to patients who require arginine. Two ways can be used to give citrulline: either parenterally or orally. The question is how to provide this amino acid? Due to its high solubility in water, a parenteral administration of citrulline is possible. The results presented Citrulline and the gut Curis et al. 625 above provide strong evidence that the oral route is a convenient route of administration. There are two ways to provide oral citrulline: either as a food component or as a pure amino acid supplement. Since citrulline is a nonprotein amino acid [3], it is not commonly found in food; only watermelon is a rich source of citrulline [50]. To test the hypothesis that watermelon may be a suitable source of citrulline and therefore of arginine, Collins et al. [51
] enrolled 12–23 patients (according to the treatment) to receive 0 or 780 g (i.e., 1 g citrulline) or 1560 g of (i.e., 2 g citrulline) watermelon per day as a juice (representing three or six cups of juice/day) for 3 weeks in a crossover design. Plasma arginine increased by 12% and 22% with the low and high ingestion of watermelon juice, respectively, after 3 weeks. Such an increase may be of potential interest in the context of prevention of cardiovascular risk and no side effects were observed in the 3-week trial. This study was retrospective, however, and used samples that had been stored for more than 4 years. More importantly, the practical relevance is limited by the fact that long-term compliance to a high intake (six cups/day) of watermelon juice is uncertain. Finally, the concentration of citrulline in watermelon is variable according to the strain and the degree of maturity [2]. Therefore, if this method is to be used in the future, it will be important to add pure citrulline to the juice to guarantee the citrulline content. The other strategy is to provide pure citrulline as a supplement. Pure citrulline is available, marketed by Kyowa Hakko (Tokyo, Japan), and as a malate salt, marketed by Biocodex (Compiègne, France). Such a strategy has recently been used experimentally in rats after bowel resection [42] and for refeeding old malnourished rats [43
], demonstrating in the latter study that a citrulline-enriched enteral diet increases muscle protein synthesis. Conclusion Due to its unique metabolism, citrulline has recently emerged as a promising marker of enterocyte function and as a potential pharmacological agent for oral, enteral, and parenteral nutrition of gut-compromised patients. A logical development of this concept is mandatory to obtain more evidence. References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:
of special interest

of outstanding interest Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 651–652). 1 Crenn P, Coudray-Lucas C, Thuillier F, et al. Postabsorptive plasma citrulline concentration is a marker of absorptive enterocyte mass and intestinal failure in humans. Gastroenterology 2000; 119:1496–1505. 2 Rimando AM, Perkins-Veazie PM. Determination of citrulline in watermelon rind. J Chromatogr A 2005; 1078:196–200. 3 Curis E, Nicolis I, Moinard C, et al. Almost all about citrulline in mammals. Amino acids 2005; 29:177–205. 4 Romero MJ, Platt DH, Caldwell RB, Caldwell RW. Therapeutic use of citrulline

in cardiovascular disease. Cardiovasc Drug Rev 2006; 24:275–290. A timely review on metabolism and pharmacology of citrulline, especially in endothelial cells, and potential therapeutical interest in cardiovascular diseases. 5 Windmueller HG, Spaeth AE. Source and fate of circulating citrulline. Am J Physiol 1981; 241:E473–E480. 6 Gate JJ, Parker DS, Lobley GE. The metabolic fate of the amido-N group of glutamine in the tissues of the gastrointestinal tract in 24 h-fasted sheep. Br J Nutr 1999; 81:297–306. Fujita T, Yanaga K. Association between glutamine extraction and release of citrulline and glycine by the human small intestine. Life Sci 2007; 80:1846– 1850. Clear evidence that citrulline release by the small intestine is correlated to glutamine uptake by these cells; interestingly, glycine release is also correlated to glutamine uptake. 7
8 Moinard C, Cynober L. Citrulline: a new player in the control of nitrogen homeostasis. J Nutr, In press. 9 Boelens PG, van Leeuwen PA, Dejong CH, Deutz NE. Intestinal renal metabolism of L-citrulline and L-arginine following enteral or parenteral infusion of L-alanyl-L-[2,15N]glutamine or L-[2,15N]glutamine in mice. Am J Physiol Gastrointest Liver Physiol 2005; 289:G679–G685. 10 Boelens PG, Melis GC, van Leeuwen PA, et al. Route of administration
(enteral or parenteral) affects the contribution of L-glutamine to de novo L-arginine synthesis in mice: a stable-isotope study. Am J Physiol Endocrinol Metab 2006; 291:E683–E690. Demonstration that the small intestine can use either luminal or arterial glutamine as a source of citrulline in mice. 11 Plauth M, Schneider BH, Raible A, Hartmann F. Effects of vascular or luminal administration and of simultaneous glucose availability on glutamine utilization by isolated rat small intestine. Int J Colorectal Dis 1999; 14:95–100. 12 Wu G. Synthesis of citrulline and arginine from proline in enterocytes of postnatal pigs. Am J Physiol 1997; 272:G1382–G1390. 13 van de Poll MC, Siroen MP, van Leeuwen PA, et al. Interorgan amino acid
exchange in humans: consequences for arginine and citrulline metabolism. Am J Clin Nutr 2007; 85:167–172. Nice work demonstrating that citrulline interorgan exchanges can, in some circumstances, involve the liver, despite the reason for this involvement remaining unclear. This result leads to a wide range of important studies to better understand these metabolic variations. 14 van de Poll MC, Ligthart-Melis GC, Boelens PG. Intestinal and hepatic
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protein metabolism in old malnourished rats. Am J Physiol Endocrinol Metab 2006; 291:E582–E586. Citrulline supplementation dramatically improves muscle protein synthesis during refeeding of aged malnourished rats. 44 Bahri S, Curis E, Aussel C. Caractérisation in vitro du transport instestinal de la citrulline. Nut Clin Métab 2006; 20 (Suppl 2):S111. (abstract). 30 Seguy D, Vahedi K, Kapel N, et al. Low-dose growth hormone in adult home parenteral nutrition-dependent short bowel syndrome patients: a positive study. Gastroenterology 2003; 124:293–302. 45 Cynober L. Pharmacokinetics of arginine and related amino acids. J Nutr 2007; In press. 31 Chang RW, Javid PJ, Oh JT, et al. Serial transverse enteroplasty enhances intestinal function in a model of short bowel syndrome. Ann Surg 2006; 243:223–228. 46 Rajantie J, Simell O, Perheentupa J. Oral administration of urea cycle intermediates in lysinuric protein intolerance: effect on plasma and urinary arginine and ornithine. Metabolism 1983; 32:49–51. 32 Crenn P, de Truchis P, Neveux N, et al. Plasma citrulline and HIV-associated enteropathy. Clin Nutr 2006; 26:114. (abstract). 47 Moinard C, Nicolis I, Neveux N, et al. L’étude citrudose: pharmacocinétique de la citrulline après administration chez le volontaire sain. Nutr Clin Métab 2005; 19 (Suppl):S59. (abstract). 33 Hozyasz KK, Szaflarska-Poplawska A, Oltarzewski M, et al. Whole blood citrulline levels in patients with coeliac disease. Pol Merkur Lekarski 2006; 20:173–175. 34 Gondolesi G, Ghirardo S, Raymond K, et al. The value of plasma citrulline to
predict mucosal injury in intestinal allografts. Am J Transplant 2006; 6:2786– 2790. This paper indicates the nonspecificity of citrullinemia in this complex situation. In addition, normalization of citrullinemia takes weeks to months after transplantation. 35 Blijlevens NM, Lutgens LC, Schattenberg AV, Donnelly JP. Citrulline: a potentially simple quantitative marker of intestinal epithelial damage following myeloablative therapy. Bone Marrow Transplant 2004; 34: 193–196. 36 Lutgens LC, Blijlevens NM, Deutz NE, et al. Monitoring myeloablative therapyinduced small bowel toxicity by serum citrulline concentration: a comparison with sugar permeability tests. Cancer 2005; 103:191–199. 37 Vanclee A, Lutgens LC, Oving EB, et al. Keratinocyte growth factor ameliorates acute graft-versus-host disease in a novel nonmyeloablative haploidentical transplantation model. Bone Marrow Transplant 2005; 36: 907–915. 48 Urschel KL, Shoveller AK, Uwiera RR, et al. Citrulline is an effective arginine
precursor in enterally fed neonatal piglets. J Nutr 2006; 136:1806–1813. A nice demonstration of the efficiency of an oral citrulline supplementation to provide arginine in piglets. 49 Smith HA, Canter JA, Christian KG, et al. Nitric oxide precursors and

congenital heart surgery: a randomized controlled trial of oral citrulline. J Thorac Cardiovasc Surg 2006; 132:58–65. A very nice clinical study that provides evidence of the ability of oral citrulline to restore arginine levels in surgical patients (children). Also, the study shows that patients with high citrulline plasma levels have fewer complications of postoperative pulmonary hypertension. 50 Mandel H, Levy N, Izkovitch S, Korman SH. Elevated plasma citrulline and arginine due to consumption of Citrullus vulgaris (watermelon). J Inherit Metab Dis 2005; 28:467–472. 51 Collins JK, Wu G, Perkins-Veazie P, et al. Watermelon consumption increases
plasma arginine concentrations in adults. Nutrition 2007; 23:261–266. This study explores the possibility of sustaining the arginine plasma pool with different watermelon intake levels. This strategy deserves further study with clinically relevant endpoints.