Gerdien C. Ligthart-Melis

Intestinal and hepatic metabolism of glutamine and citrulline in humans

Glutamine plays an important role in nitrogen homeostasis and intestinal substrate supply. It has been suggested that glutamine is a precursor for arginine through an intestinal–renal pathway involving inter‐organ transport of citrulline. The importance of intestinal glutamine metabolism for endogenous arginine synthesis in humans, however, has remained unaddressed. The aim of this study was to investigate the intestinal conversion of glutamine to citrulline and the effect of the liver on splanchnic citrulline metabolism in humans. Eight patients undergoing upper gastrointestinal surgery received a primed continuous intravenous infusion of [2‐15N]glutamine and [ureido‐13C–2H2]citrulline. Arterial, portal venous and hepatic venous blood were sampled and portal and hepatic blood flows were measured. Organ specific amino acid uptake (disposal), production and net balance, as well as whole body rates of plasma appearance were calculated according to established methods. The intestines consumed glutamine at a rate that was dependent on glutamine supply. Approximately 13% of glutamine taken up by the intestines was converted to citrulline. Quantitatively glutamine was the only important precursor for intestinal citrulline release. Both glutamine and citrulline were consumed and produced by the liver, but net hepatic flux of both amino acids was not significantly different from zero. Plasma glutamine was the precursor of 80% of plasma citrulline and plasma citrulline in turn was the precursor of 10% of plasma arginine. In conclusion, glutamine is an important precursor for the synthesis of arginine after intestinal conversion to citrulline in humans.

Specific amino acids in the critically ill patient--exogenous glutamine/arginine: a common denominator?

Objective:Glutamine and arginine are both used as nutritional supplements in critically ill patients. Although glutamine has been shown to be beneficial for the metabolically stressed patient, considerations about arginine supplementation are not unanimously determined. Our aim is to review the current knowledge on the possible interplay between glutamine and arginine generation in the stressed patient and to elaborate on whether these amino acids may function as a common denominator. Because glutamine can be given by the parenteral and enteral routes, possible different actions on the metabolic fate (e.g., generation of citrulline) with both routes are analyzed. Data Source:A summary of data on the clinical effect of glutamine and arginine metabolism is given, incorporating data on glutamine and arginine supplementation. Differences between the route of administration, parenteral or enteral, and the molecular form of supplied glutamine, free or as dipeptide, on citrulline generation by the gut and production of arginine are discussed. Results:Glutamine and arginine influence similar organ systems; however, they differ in their targets. For example, glutamine serves as fuel for the immune cells, increases human leukocyte antigen-DR expression on monocytes, enhances neutrophil phagocytosis, and increases heat shock protein expression. Arginine affects the immune system by stimulating direct or indirect proliferation of immune cells. This indirect effect is possibly mediated by nitric oxide, which also enhances macrophage cytotoxicity. Furthermore, glutamine serves as a precursor for the de novo production of arginine through the citrulline-arginine pathway. Glutamine has shown to be beneficial in the surgical and critically ill patient, whereas arginine supplementation is still under debate. The route of glutamine administration (parenteral or enteral) determines the effect on citrulline and on the de novo arginine generation. There is a marked difference between the administration of free glutamine and dipeptide enterally or parenterally. Splanchnic extraction of the hydrolyzed glutamine in mice when administering the dipeptide enterally is higher compared with administering free glutamine from the enteral site. In patients, splanchnic extraction of the dipeptide given enterally is 100% when comparing supplementation of the dipeptide intravenously. Conclusions:The beneficial effects of free glutamine or dipeptide may depend on the route of administration but also on the metabolic fate of amino acids generated (e.g., citrulline, arginine). Glutamine serves as a substrate for de novo citrulline and arginine synthesis. More research needs to be done to establish the direct clinical relevance of the different metabolic pathways. Future perspectives might include combining enteral and parenteral routes of administrating free glutamine or dipeptide.

The Route of Administration (Enteral or Parenteral) Affects the Conversion of Isotopically Labeled L-[2-15N]Glutamine Into Citrulline and Arginine in Humans.

BACKGROUND Glutamine exhibits numerous beneficial effects in experimental and clinical studies. It has been suggested that these effects may be partly mediated by the conversion of glutamine into citrulline and arginine. The intestinal metabolism of glutamine appears to be crucial in this pathway. The present study was designed to establish the effect of the feeding route, enteral or parenteral, on the conversion of exogenously administered glutamine into citrulline and arginine at an organ level in humans, with a focus on gut metabolism. METHODS Sixteen patients undergoing upper gastrointestinal surgery received an IV or enteral (EN) infusion of L-[2-(15)N]glutamine. Blood was sampled from a radial artery and from the portal and right renal vein. Amino acid concentrations and enrichments were measured, and net fluxes of [(15)N]-labeled substrates across the portal drained viscera (PDV) and kidneys were calculated from arteriovenous differences and plasma flow. RESULTS Arterial [(15)N]glutamine enrichments were significantly lower during enteral tracer infusion (tracer-to-tracee ratio [labeled vs unlabeled substrate, TTR%] IV: 6.66 +/- 0.35 vs EN: 3.04 +/- 0.45; p < .01), reflecting first-pass intestinal metabolism of glutamine during absorption. Compared with IV administration, enteral administration of the glutamine tracer resulted in a significantly higher intestinal fractional extraction of [(15)N]glutamine (IV: 0.15 +/- 0.03 vs EN: 0.44 +/- 0.08 micromol/kg/h; p < .01). Furthermore, enteral administration of the glutamine tracer resulted in higher arterial enrichments of [(15)N]citrulline (TTR% IV: 5.52 +/- 0.44 vs EN: 8.81 +/- 1.1; p = .02), and both routes of administration generated a significant enrichment of [(15)N]arginine (TTR% IV: 1.43 +/- 0.12 vs EN: 1.68 +/- 0.18). This was accompanied by intestinal release of [(15)N]citrulline across the PDV, which was higher with enteral glutamine (IV: 0.38 +/- 0.07 vs EN: 0.72 +/- 0.11 micromol/kg/h; p = .02), and subsequent [(15)N]arginine release in both groups. CONCLUSIONS In humans, the gut preferably takes up enterally administered glutamine compared with intravenously provided glutamine. The route of administration, enteral or IV, affects the quantitative conversion of glutamine into citrulline and subsequent renal arginine synthesis in humans.

Dietician‐delivered intensive nutritional support is associated with a decrease in severe postoperative complications after surgery in patients with esophageal cancer

The aim of this study was to evaluate the effect of dietician-delivered intensive nutritional support (INS) on postoperative outcome in patients with esophageal cancer. Approximately 50-80% of patients with esophageal cancer are malnourished at the time of diagnosis. Malnutrition enhances the risk of postoperative complications, resulting in delay of postoperative recovery and impairment of quality of life. Sixty-five patients with esophageal cancer were included. All patients who received surgery (n = 28) in the time frame between March 2009 and April 2010, the first year after the start of INS, were included in the INS intervention group. The control group (n = 37) consisted of patients who received surgery during the 3 years before the start of INS. Logistic regression analysis was used to compare differences in severity of postoperative complications using the Dindo classification. Linear regression was applied to evaluate differences in preoperative weight change. The adjusted odds ratio for developing serious complications after surgery of INS compared with the control group was 0.23 (95% confidence interval: 0.053-0.97; P = 0.045). Benefit was mainly observed in patients who received neoadjuvant therapy before esophagectomy (n = 35). The INS program furthermore resulted in a relative preoperative weight gain in comparison with the control group of +4.8% (P = 0.009, adjusted) in these neoadjuvant-treated patients. This study shows that dietician-delivered INS preserves preoperative weight and decreases severe postoperative complications in patients with esophageal cancer.

The gut does not contribute to systemic ammonia release in humans without portosystemic shunting

The gut is classically seen as the main source of circulating ammonia. However, the contribution of the intestines to systemic ammonia production may be limited by hepatic extraction of portal-derived ammonia. Recent data suggest that the kidney may be more important than the gut for systemic ammonia production. The aim of this study was to quantify the role of the kidney, intestines, and liver in interorgan ammonia trafficking in humans with normal liver function. In addition, we studied changes in interorgan nitrogen metabolism caused by major hepatectomy. From 21 patients undergoing surgery, blood was sampled from the portal, hepatic, and renal veins to assess intestinal, hepatic, and renal ammonia metabolism. In seven cases, blood sampling was repeated after major hepatectomy. At steady state during surgery, intestinal ammonia release was equaled by hepatic ammonia uptake, precluding significant systemic release of intestinal-derived ammonia. In contrast, the kidneys released ammonia to the systemic circulation. Major hepatectomy led to increased concentrations of ammonia and amino acids in the systemic circulation. However, transsplanchnic concentration gradients after major hepatectomy were similar to baseline values, indicating the rapid institution of a new metabolic equilibrium. In conclusion, since hepatic ammonia uptake exactly equals intestinal ammonia release, the splanchnic area, and hence the gut, probably does not contribute significantly to systemic ammonia release. After major hepatectomy, hepatic ammonia clearance is well preserved, probably related to higher circulating ammonia concentrations.

Enteral administration of alanyl-[2-15N]glutamine contributes more to the de novo synthesis of arginine than does intravenous infusion of the dipeptide in humans

BACKGROUND We previously confirmed in humans the existence of a pathway of glutamine into citrulline and arginine, which is preferentially stimulated by luminally provided glutamine. However, because glutamine is unstable, we tested this pathway with a stable dipeptide of glutamine. OBJECTIVES The objectives were to explore whether alanyl-glutamine contributes to the synthesis of arginine in humans and whether this depends on the route of administration. DESIGN The study was conducted under postabsorptive conditions during surgery. Sixteen patients received alanyl-[2-(15)N]glutamine enterally or intravenously together with intravenously administered stable-isotope tracers of citrulline and arginine. Blood was collected from an artery, the portal vein, a hepatic vein, and the right renal vein. Arterial and venous enrichments and (tracer) net balances of alanyl-glutamine and glutamine, citrulline, and arginine across the portal-drained viscera, liver, and kidneys were determined. Parametric tests were used to test results (mean +/- SEM). P < 0.05 was considered significant. RESULTS Twice as much exogenous glutamine was used for the synthesis of citrulline when alanyl-glutamine was provided enterally (5.9 +/- 0.6%) than when provided intravenously (2.8 +/- 0.3%) (P < 0.01). Consequently, twice as much exogenous glutamine was used for the synthesis of arginine when alanyl-glutamine was provided enterally (5 +/- 0.7%) than when provided intravenously (2.4 +/- 0.2%) (P < 0.01). However, results at the organ level did not explain the differences due to route of administration. CONCLUSIONS Alanyl-glutamine contributes to the de novo synthesis of arginine, especially when provided enterally. A stable-isotope study using a therapeutic dose of alanyl-glutamine is needed to investigate the clinical implications of this finding.

Weight Loss of 5% or More Predicts Loss of Fat-Free Mass During Palliative Chemotherapy in Patients With Advanced Cancer: A Pilot Study

The cutoff value of critical weight loss is still subject of discussion. In this pilot study, we investigated whether ≥5% weight loss in the past year predicts changes in nutritional status in patients with advanced cancer during treatment with palliative chemotherapy. In 20 patients with advanced cancer undergoing palliative (combination) chemotherapy, body weight, fat free mass (FFM), and cachexia were measured prior to the start and at 9 wk of treatment. History of weight loss was used to test differences in development of nutritional parameters during chemotherapy with use of independent sample t-tests. At baseline, 10 of 20 patients had lost ≥5% body weight during the past year and 5 patients were cachectic. The change in FFM in the first 9 wk of chemotherapy was significantly worse in patients with ≥5% weight loss compared to patients with <5% weight loss [mean difference: 3.5 kg (P = 0.001)]. Data also suggest that ≥5% weight loss predicts shorter survival (P = 0.03). We found that patients with ≥5% weight loss prior to chemotherapy have a deterioration in nutritional status during chemotherapy and may have a shorter survival. These results have to be confirmed in a larger study including a robust survival analysis.

Is glutamine still an important precursor of citrulline

glutamine and arginine are two amino acids that have received inexhaustible attention by different research groups all over the world over the past decades. This attention is justified by their potential to enhance recovery of surgical and critically ill patients. The beneficial potential of glutamine and arginine is furthermore supported by numerous trials and an ongoing meta-analysis in surgical and critically ill patients who received a supplemental amount of glutamine or arginine (www.criticalcarenutrition.com). In this issue of this Journal, Tomlinson et al. (28) present the results of a study in healthy humans about the precursor relation between glutamine and arginine. Apart from the clinical importance of glutamine and arginine as addressed above, the relationship between the two amino acids has gained interest due to the assumed importance of glutamine as a nitrogen and carbon donor for the synthesis of arginine. The importance of glutamine as an important precursor for the synthesis of citrulline, the sole precursor for the de novo synthesis of arginine, was previously established in animal studies by Windmueller and Spaeth and others many years ago (31–34) (Fig. 1). In addition, several studies in animals and humans confirmed the important precursor relationship between glutamine, citrulline, and arginine by showing that plasma levels of citrulline and arginine increased after provision of glutamine (6, 8, 11, 12, 22, 23, 25, 26, 35) (Fig. 2). Also, high correlations were observed between intestinal arterial glutamine uptake and venous release of citrulline (8). Debate about the safety of arginine supplementation in septic critically ill patients (9, 10, 27) and the use of high dosages of glutamine in clinical studies that potentially could lead to stimulated arginine synthesis has further contributed to the relevance of investigating the importance of glutamine as a possible precursor for the synthesis of arginine, especially as no harmful side-effects from supplementation of glutamine have been established so far. Therefore, we need to understand this relationship, which will lead to more defined and scientifically based nutritional interventions. This has resulted in a series of stable-isotope studies by us in mice and surgical patients, which established glutamine to be an important precursor for the de novo synthesis of citrulline and arginine (3, 4, 13, 29). However, we believe that more studies are needed when patients are in the fed state and in critically ill patients, administered high dosages of glutamine and arginine (15, 17, 30). If glutamine is proved to be an adequate precursor of arginine, glutamine supplementation will receive further justification in medical practice and in that case could be viewed as having a dual effect. This hypothesis was recently addressed in the review of Vermeulen et al. (30). Fig. 1. Pathways by which glutamine and arginine are considered to contribute their carbon skeleton to the synthesis of citrulline and arginine. Fig. 2. Increase in plasma citrulline after ingestion of 14 g of glutamine in 2 h in healthy controls (26). However, doubt about the interpretation of the results of mentioned stable-isotope studies and the discussed importance of glutamine as an important precursor for arginine production via the production of citrulline is raised by recent publications of Marini and colleagues (19–21). These authors showed in mice using different isotopomers of glutamine (l-[2-15N], l-[5-15N], and l-[U-13C5]) that glutamine was a poor carbon precursor of citrulline and arginine. Also, glutamine nitrogen was an important source of all nitrogen atoms in citrulline and arginine, indicating a complex role of glutamine as a nitrogen precursor. However, comparison with the previous studies in postabsorptive humans (3, 4, 13, 29) is hampered because the studies by Marini and colleagues were performed in fed mice. The study in humans by Tomlinson et al. in this issue (28) is the first step of unraveling these important research questions in humans. Their results contradict the findings of Marini and colleagues with respect to the importance of glutamine as a carbon precursor for the synthesis of arginine via the intermediate citrulline, because they observed by using the l-[1-13C]glutamine tracer that glutamine contributed 56% to the de novo synthesis of arginine from citrulline. This percentage approaches the 64% contribution of glutamine to the de novo synthesis of arginine observed by Ligthart-Melis et al. (13). Also, Tomlinson et al. do not confirm significant labeling of the ureido N by l-[2-15N]glutamine, as observed by Marini et al. (20). Both observations support the notion that results in mice are not easily translated to humans (1, 8). It may be that the metabolic pathways and necessary enzymes are present in the different species but that this does not guarantee that they are quantitatively equally important. With respect to the applied tracer methodology, Tomlinson et al. (28) seem to use the same precursor-intermediate-product approach as Marini et al. (19–21). This model relies on the intragastric infusion of tracers (precursor) and their recovery as plasma amino acid products of intestinal metabolism. Although Marini and colleagues (19–21) combine this approach with the dilution equation using an additional intravenous tracer to determine whole body rate of appearance of the amino acid product of interest, Tomlinson et al. (28) use the dilution equations straight away. Furthermore, like the mice in studies by Marini and colleagues, the humans in the study by Tomlinson et al. were studied in the fed state. This approach by Tomlinson raises some questions. First, is it justified to use dilution equations when tracers are provided by the gastrointestinal route? The consequence is an overestimation of rate of appearance or turnover of the amino acid of which the concomitant tracer is provided, because the infusion rate of the tracer is overestimated due to splanchnic extraction of the tracer. Also although less likely, absorption of the tracer could have been reduced because of amino acids coming from the simultaneously ingested meals. With this in mind, one could state that Tomlinson et al. may have underestimated the relative contribution of carbon provided by glutamine for the synthesis of citrulline. Furthermore, by conducting only a protocol involving enterally provided tracers it is not clear how much is contributed by the labeled amino acids that have recycled back to the intestines and could have a different metabolic fate (24). To be able to compare the effect of route of delivery, subjects should have received an additional tracer protocol that could be done on a different day, involving intravenous administration of the same tracers. Second, measurement in the fed state will change the relative contribution of the different metabolic pathways (2), which complicates comparison with results from studies in the fasted state (13, 14, 29). Also, the provided food will further add to the dilution of the tracers. Therefore, it is more than likely that the contribution of glutamine-to-arginine biosynthesis depends on the amount and composition of amino acids provided by the diet.

Gastric emptying, glucose metabolism and gut hormones: Evaluation of a common preoperative carbohydrate beverage

OBJECTIVE To study the gastric-emptying rate and gut hormonal response of two carbohydrate-rich beverages. A specifically designed carbohydrate-rich beverage is currently used to support the surgical patient metabolically. Fruit-based beverages may also promote recovery, due to natural antioxidant and carbohydrate content. However, gastric emptying of fluids is influenced by its nutrient composition; hence, safety of preoperative carbohydrate loading should be confirmed. Because gut hormones link carbohydrate metabolism and gastric emptying, hormonal responses were studied. METHODS In eight volunteers, gastric emptying rates of both 400 mL of a ready-to-use beverage (A: Nutricia preOp; 50.4 g carbohydrates-mainly polysaccharides; 260 mOsm/kg) and 400 mL over-the-counter fruit-based lemonade (B: Roosvicee Original; 48 g carbohydrates--mainly fruit-associated saccharides; 805 mOsm/kg) were determined scintigraphically (using hepatate Tc-99(m)) according to a crossover design. Plasma glucose, insulin, C-peptide, glucagon-like peptide (GLP-1), peptide YY, total glucagon, and ghrelin were studied. RESULTS Gastric emptying showed no differences in residual volumes. Earlier onset in emptying for beverage A versus B was observed (trend), with significantly higher glucose, insulin, C-peptide, and glucagon responses at 15-90 min. GLP-1 was inversely related to residual volume. CONCLUSION Fruit-based lemonade is a safe alternative for preoperative purposes. It induces a more limited glucose, insulin, and C-peptide response. Later onset in gastric emptying (B versus A: trend), lower glucagon release, and differences in beverage content and osmolarity may have contributed to those differences. Efficient emptying was reflected by early GLP-1 levels.

Glutamine: precursor or nitrogen donor for citrulline synthesis?

to the editor: we would like to comment on the recent publication by Marini et al. ([6][1]). The authors present an impressive, meticulous analysis of the metabolic pathways by which glutamine contributes to the synthesis of citrulline. Their study in mice shows that glutamine predominantly donates