The Dipeptide Alanyl-Glutamine Prevents Intestinal Mucosal Atrophy in Parenterally Fed Rats

Preparation of healing-promoting and fibrosis-inhibiting asymmetric poly(ethylene glycol-b-L-phenylalanine)/cRGD-modified hyaluronate sponges and their applications in hemorrhage and nasal mucosa repair

Acute or chromic bleeding, such as epistaxis, requires hemostatic materials to assist hemostasis. Even in complex cases, hemostatic materials must have other functions, including the promotion of healing and prevention of adhesion. Herein, a series of fibrosis-suppressive functional cRGD-modified crosslinking hyaluronic acid sponges were prepared. The in vitro hemostatic efficiency and mechanism were determined using blood clotting time, blood coagulation index, lactate dehydrogenase (LDH) and thromboxane B2 (TX-B2) ELISA, and proteomics. Among the prepared sponges, both poly(ethylene-b-L-Phe) (PEBP)-and cRGD contained SPN4 and exhibited the highest platelet concentration and activation efficiency as well as the most effective coagulative effect. In addition, no significant cytotoxicity was observed for the sponges in rat airway epithelial cells. The in vivo hemostatic and adhesion-preventive effects of the sponges were evaluated using rat models of liver injury and sidewall defect-cecum abrasion. PEBP-containing sponges effectively prevented postoperative adhesion and cRGD-modified sponges exhibited excellent hemostatic effects. Finally, the comprehensive repair effects of the sponges were evaluated using a rabbit maxillary sinus mucosal injury model, based on CT, MRI examination, and pathological staining. SPN4 exhibited the best comprehensive reparative effects, including the promotion of mucosal repair and infection inhibition. Thus, SPN4 is a promising multifunctional hemostatic material.

Preparation of healing promotive alanyl-glutamine-poly(p-dioxanone) electrospun membrane integrated with gentamycin and its application for intestinal anastomosis in rats

Anastomosis surgery at the intestinal site is performed on millions of individuals every year. However, several persistent complications, such as anastomotic leakage, abnormal adhesion, and anastomotic stenosis, have been observed after the surgery. For promoting anastomotic healing and to overcome the challenges mentioned above, re-epithelialization at anastomotic sites is crucial. In this study, an epithelialization-promoting macromolecular prodrug Ala-Gln-PPDO was prepared and processed into fibrous membranes by electrospinning. Ala-Gln and gentamicin were sustainably released from the electrospun membranes with degradation of these membranes to promote the proliferation of rat intestinal epithelial cells and suppress the proliferation of Staphylococcus aureus and Escherichia coli. The comprehensive repair effects of Ala-Gln-PPDO membranes have been evaluated in rat models of intestinal anastomosis in this study. Application of Ala-Gln-PPDO membranes, especially the gentamicin-incorporated Ala-Gln-PPDO ones, could prevent adhesion between the injured intestine and surrounding intestinal tissues. In addition, they did not affect the healing strength of anastomotic stoma negatively and could promote re-epithelialization at the anastomotic sites. Furthermore, the gentamicin-incorporated Ala-Gln-PPDO membranes could relieve stenosis at anastomotic sites. The gentamicin-incorporated Ala-Gln-PPDO electrospun membrane is a promising, comprehensive implantable material for promoting healing after gastrointestinal anastomosis owing to its effects involving the promotion of re-epithelialization, prevention of adhesion, and relieving of anastomotic stenosis.

The use of dipeptide supplementation as a means of mitigating the negative effects of dietary soybean meal on Zebrafish Danio rerio

Soybean meal (SBM) inclusion in aquaculture diets has been found to negatively affect growth and induce intestinal inflammation in fish. The objective of this study was to determine the effect of health-promoting dipeptide supplementation into SBM-based feeds on growth performance, intestinal health, and muscle free amino acid composition, an indicator of dietary amino acid availability, in a zebrafish model. There were five treatment groups in this study. The first group ((+) Control) received a fishmeal-based diet. The second group ((-) Control) received SBM-based diet. The last three groups (Ala-Glu, Car, and Ans) were fed SBM-based diets, supplemented with alanyl-glutamine, carnosine, and anserine respectively. The Ala-Glu and Car groups experienced a significantly higher weight gain than the (-) Control group, weighing 35.38% and 33.96% more, respectively at the conclusion of the study. There were no significant differences in gene expression among the groups, but Ala-Glu had the highest expression of both nutrient absorption genes measured, PepT1 and fabp2. Ala-Glu had significantly longer intestinal villi, and a significantly higher villus length-to-width ratio than the (-) Control group. The Car group had a significantly higher post-prandial tissue concentration of lysine, compared to the (-) Control group. The increase in villus surface area and expression of nutrient absorption genes represent an improvement in intestinal absorptive capacity in the Ala-Glu group. The results from this study provide support for the use of alanyl-glutamine and carnosine supplementation as a means of improving growth performance of zebrafish fed with a high level SBM-based diet.

Intestinal adaptation following resection

Intestinal adaptation is a natural compensatory process that occurs following extensive intestinal resection, whereby structural and functional changes in the intestine improve nutrient and fluid absorption in the remnant bowel. In animal studies, postresection structural adaptations include bowel lengthening and thickening and increases in villus height and crypt depth. Functional changes include increased nutrient transporter expression, accelerated crypt cell differentiation, and slowed transit time. In adult humans, data regarding adaptive changes are sparse, and the mechanisms underlying intestinal adaptation remain to be fully elucidated. Several factors influence the degree of intestinal adaptation that occurs post resection, including site and extent of resection, luminal stimulation with enteral nutrients, and intestinotrophic factors. Two intestinotrophic growth factors, the glucagon-like peptide 2 analog teduglutide and recombinant growth hormone (somatropin), are now approved for clinical use in patients with short bowel syndrome (SBS). Both agents enhance fluid absorption and decrease requirements for parenteral nutrition (PN) and/or intravenous fluid. Intestinal adaptation has been thought to be limited to the first 1-2 years following resection in humans. However, recent data suggest that a significant proportion of adult patients with SBS can achieve enteral autonomy, even after many years of PN dependence, particularly with trophic stimulation.

Novel therapies for the management of short bowel syndrome in children

Short bowel syndrome (SBS) is the most common cause of intestinal failure in children. It is defined as the inability to maintain adequate nutrition enterally as a result of a major loss of the small intestine. SBS is a life-threatening entity associated with potential significant morbidity and mortality. The etiology in the pediatric age group includes necrotizing enterocolitis (32%), atresia (20%), volvulus (18%), gastroschisis (17%), and aganglionosis (6%). It is characterized by substrate malabsorption, electrolyte imbalance, intestinal bacterial overgrowth, steatorrhea, and weight loss. Current medical management includes parenteral nutrition, progressive feeds as tolerated, various medications, and surgical manipulations. However, frequently this management is not successful in achieving the goal of attaining normal growth and development without parenteral nutrition. It has been known for decades that there is a normal physiologic response of the residual intestine to massive bowel resection referred to as intestinal adaptation. The mechanisms that control this process are unknown. Unfortunately, intestinal adaptation and the current management are not always successful. As a result of new knowledge regarding the pathophysiology of SBS over the past two decades, several novel strategies have been developed in experimental animal models as well as limited clinical trials in infants and children. They can be divided into several categories that potentially influence intestinal (1) absorption, (2) secretion, (3) motility, and (4) adaptation. More recently, newer modalities have been studied including small intestine transplantation, and the use of specific intestinal growth factors. Ultimately, tissue and organ engineering will become the treatment for infants and children with SBS.

Intestinal Adaptation in Short Bowel Syndrome

Short bowel syndrome occurs when there is insufficient length of the small intestine to maintain adequate nutrition and/or hydration status without supplemental support. This syndrome most frequently occurs following extensive surgical resection of the intestine, and the extent of adaptation depends on the anatomy of the resected bowel and the amount of bowel remaining. Following resection, the intestinal tissue undergoes morphologic and functional changes to compensate for the lost function of the resected bowel. These changes are mediated by multiple interactive factors, including intraluminal and parenteral nutrients, gastrointestinal secretions, hormones, cytokines, and growth factors, many of which have been well characterized in animal models. The amount of small bowel remaining is the most important predictor of adaptive potential; neither structural nor functional adaptative changes have been demonstrated in humans or animal models with more extreme resections resulting in an end-jejunostomy. The current understanding of these processes has led to the recent use of supplemental hormones, such as growth hormone and glucagon-like peptide 2, in intestinal rehabilitation programs and may lead to the development of pharmacologic agents designed to augment the innate adaptive response.

Glutamine supplementation in infants with gastrointestinal disease: A randomized, placebo-controlled pilot trial

OBJECTIVE

Glutamine (Gln) is a non-essential amino acid that plays an important role in energy metabolism for gastrointestinal epithelia and other cells with rapid turnover. We evaluated the effects of enteral supplementation with Gln in infants undergoing surgery for congenital or acquired gastrointestinal disease.

METHODS

This was a randomized, double-masked, controlled clinical trial.

RESULTS

Twenty infants were randomly assigned to receive Gln (n = 9) or placebo amino acid (n = 11), with a goal of supplemental amino acid intake of 0.4 g.kg(-1).d(-1). Infants were weaned from parenteral nutrition, and enteral feeds were started according to a standardized feeding protocol. Median (interquartile range) durations of parenteral nutrition were 39 d (12 to 99) in the Gln group and 21 d (6 to 59) in the control group (P = 0.201). Median (interquartile range) durations needed to reach 80% of the US recommended dietary allowance for energy with enteral nutrition were 24 d (8 to 55) in the Gln group and 12.5 d (5 to 32) in the control group (P = 0.313). There were no differences in the occurrence of infections between groups. Among all infants enrolled, significant correlations were found between duration of parenteral nutrition and residual small bowel length, peak concentrations of direct bilirubin, and alanine aminotransferase. Peak direct bilirubin was associated with longer duration of parenteral nutrition, shorter gestation, older age before feeds were started, shorter bowel length, and larger amounts of parenteral energy and protein intake.

CONCLUSIONS

In this pilot trial, enteral Gln supplementation was well tolerated among infants with surgical gastrointestinal disease. There was no effect observed on the duration of parenteral nutrition, tolerance of enteral feeds, or intestinal absorptive or barrier function. Larger, multicenter trials in infants with surgical gastrointestinal disease are needed due to the variability in important outcome measurements.

Effect of glutamine supplementation on diarrhea, interleukin-8 and secretory immunoglobulin A in children with acute diarrhea

OBJECTIVE

Glutamine is an important fuel for rapidly dividing cells such as enterocytes and lymphocytes. Exogenous glutamine supplementation in catabolic states preserves intestinal mucosal structure and function, decreases bacterial translocation, and supports normal immunologic responses. This study was planned to assess the effect of glutamine supplementation on duration and severity of diarrhea and to assess its immunomodulatory effect by measuring serum interleukin-8 (IL-8) and salivary immunoglobulin A (sIgA) in children with acute diarrhea.

METHODS

In this placebo-controlled, double-blind and randomized trial, 6- to 24-month-old otherwise healthy children admitted to the Diarrheal Diseases Training and Treatment Center with acute diarrhea received either 0.3 g/kg/day of glutamine (n = 63) or placebo (n = 65) for 7 days. Serum IL-8 and sIgA levels were determined on admission and 7 days later. All cases were followed until the diarrheal episode ended. Anthropometric measurements and history of subsequent infectious diseases were monitored monthly for 3 months after treatment.

RESULTS

Mean duration of diarrhea in the glutamine treated group was significantly shorter than that of the placebo group (3.40 +/- 1.96 days, 4.57 +/- 2.48 days, respectively; P = 0.004). No differences in serum IL-8 and sIgA were found between groups on admission or 1 week later. During 3 month follow-up, mean weight gain and incidence of infectious diseases were similar in both groups.

CONCLUSION

Duration of diarrhea was shorter in children supplemented with glutamine. The beneficial impact of glutamine supplementation seems to be through effects on gastrointestinal mucosa rather than the host immune response.

Protective nutrients and functional foods for the gastrointestinal tract

Epithelial and other cells of the gastrointestinal mucosa rely on both luminal and bloodstream sources for their nutrition. The term functional food is used to describe nutrients that have an effect on physiologic processes that is separate from their established nutritional function, and some of these nutrients are proposed to promote gastrointestinal mucosal integrity. We review the recent in vitro, animal, and clinical experiments that evaluated the role of several types of gastrointestinal functional foods, including the amino acids glutamine and arginine, the essential micronutrients vitamin A and zinc, and 2 classes of food additives, prebiotics and probiotics. Many of the data from preclinical studies support a strong role for enteral nutrients in gastrointestinal health; in comparison, the data from human studies are limited. In some cases, impressive data from in vitro and animal studies have not been replicated in human trials. Other clinical trials have shown positive health benefits, but some of those studies were plagued by flaws in study design or analysis. The methods available to detect important changes in human gastrointestinal function and structure are still limited, but with the development of more sensitive measures of gastrointestinal function, the effects of specific nutrients may be more easily detected. This may facilitate the development of phase 3 clinical trials designed to more rigorously evaluate the effects of a particular nutrient by focusing on valid and reliable outcome measures. Regulatory changes in the way in which health claims can be made for dietary supplements should also be encouraged.

Intestinal adaptation in short bowel syndrome

Regaining enteral autonomy after extensive small bowel resection is dependent on intestinal adaptation. This adaptational process is characterized by hyperplastic growth of the remaining gut, which is accompanied by both an increase of cell division at the level of the crypt cells and by an increased rate of programmed cell death (apoptosis). Apart from the absorptive function, the small bowel also has a barrier function and plays an important role in interorgan metabolism. Also, these functions are greatly affected by a massive intestinal resection and subsequent recovery by intestinal adaptation. This review aims to give an overview of the debilitating effects of massive intestinal resection on gut function and subsequently discusses intestinal adaptation and possible factors stimulating adaptation.

Key nutrients and growth factors for the neonatal gastrointestinal tract

The nutritional support of gastrointestinal growth and function is an important consideration in the clinical care of neonatal infants. In most health infants, the provision of either breast milk or formula seems to support normal intestinal mucosal growth, but the most significant advantages of breast milk may be for host defense or gut barrier-related functions that are involved in reducing infection. The specific effects of various milk-borne growth factors on key mucosal immune and barrier functions are likely to provide valuable new clues to the advantages of human milk. A substantial number of preterm, low-birth weight babies or those suffering from compromised intestinal function, however, often cannot tolerate oral feedings and instead receive TPN. The consequences of TPN on gastrointestinal function and how this contributes to morbidity of these infants warrants further study, with respect to both clinical and basic research questions. Although enteral nutrition seems to be a critical stimulus for intestinal function, the minimal amounts and composition of nutrients necessary to maintain specific intestinal functions remain to be established. The experimental tools exist to start defining the specific nutrient requirements for the infant gut and some of these nutrients are known (e.g., glutamate, glutamine, and threonine). Peptide growth factors and gut hormones clearly play a role in gut growth and in several ways mediate the trophic actions of enteral nutrition. Although a number of these growth factors are good candidates for therapeutic use, their clinical application in the management of gastrointestinal insufficiency and disease has been slow. The emergence of GLP-2 as a trophic peptide that seems to target the gut is a promising candidate on the horizon.

Pharmacotherapy and growth factors in the treatment of short bowel syndrome

A review of the pharmacologic substances and growth factors that have been studied experimentally and administered clinically for the management of short bowel syndrome is presented. The medical management of short bowel syndrome is multifaceted. In the acute phase, efforts focus on fluid and electrolyte management and the reduction of gastric acid output. As enteral feeding is initiated, antimotility and antisecretory agents may be effective in reducing gastrointestinal losses. Additional modalities of management, including nutrients and growth factors, may be directed at maximizing absorptive function beyond that which occurs with intestinal adaptation. Continued research aimed at further elucidating the process of intestinal adaptation may allow us to use the various peptides and hormones that act as growth factors for the bowel mucosa. Knowledge gained from these studies combined with gene therapy techniques will result in the permanent enhancement of intestinal function beyond the normal adaptation process, eliminate the dependence on total parenteral nutrition, and avoid the need for intestine transplantation.

Interactions between nutrients and peptide growth factors in intestinal growth, repair, and function

Several lines of evidence demonstrate that general nutritional status, specific nutrients (eg, zinc, glutamine), and certain trophic growth factors (eg, growth hormone, insulin-like growth factor I, keratinocyte growth factor, and glucagon-like peptide-2) have important interactions relevant for intestinal growth and function. Adequate nutritional status is critical for endogenous growth factor synthesis in the gut and other tissues and is an important mediator of organ responsiveness to exogenous growth factor administration. Both endogenously synthesized and exogenously administered growth factors upregulate nutrient uptake and utilization by gut mucosa, skeletal muscle, and other organs. Emerging data from both animal and human studies indicate that combinations of selected growth factors and specific nutrients may improve the growth, adaptation, and repair of the intestinal mucosa. Additional studies to determine basic mechanisms of nutrient-growth factor interactions and the safety and efficacy of treatment with combinations of specific nutrients and recombinant growth factors are needed. Results of these investigations should define new methods for support of the intestinal tract during short bowel syndrome (SBS), catabolic illness, and malnutrition.

Glutamine for the gut: mystical properties or an ordinary amino acid?

Glutamine is a nonessential amino acid that can be synthesized from glutamate and glutamic acid by glutamine synthetase. It is the preferred fuel for the small intestine in the rat. Results from animal studies suggest that both glutamine-supplemented parenteral nutrition and enteral diets may prevent bacterial translocation. This effect may be modulated through the preservation and augmentation of small bowel villus morphology, intestinal permeability, and intestinal immune function. The existing data from studies with humans are less compelling. What, if any, intestinal deficits actually occur during provision of exclusive parenteral nutrition remains unclear. Furthermore, the clinical significance of these changes is largely undefined. Nevertheless, glutamine and glutamine supplementation appear to be important for the normal maintenance of intestinal morphology and function, intestinal adaptation following resection, and prevention of clinical infection related to bacterial translocation. The existing data on the use of parenteral and enteral glutamine for preservation of intestinal morphology and function and prevention of bacterial translocation in humans are reviewed in this article. Pertinent animal data are also described.

Reduction of chemotherapy-induced side-effects by parenteral glutamine supplementation in patients with metastatic colorectal cancer

In animal studies, glutamine (Gln) reduces chemotherapy-associated mucositis and mucosal atrophy. Therefore, this study examined the protective effects of a parenteral Gln supplementation in patients with metastatic colorectal carcinoma receiving 5-fluorouracil (5-FU)/calcium-folinate (CF) chemotherapy. In a prospective study, a total of 24 patients underwent three courses of 5-FU/CF chemotherapy and were randomised with (n = 12) or without (n = 12) glycyl-L-glutamine. Effects on gastrointestinal mucosa were assessed by endoscopic examinations and histomorphometric measurements. Clinical side-effects were documented according to the World Health Organisation grading. In the Gln group, a significant reduction in mucositis and ulcerations of the gastric (P < 0.01) and duodenal mucosa (P < 0.05) was documented after the third course of chemotherapy. In the same group, the villus height/crypt depth ratio was significantly higher after therapy than in the unsupplemented group (1st course P < 0.01; 3rd course P < 0.05). However, there were no significant differences in the incidence and severity of clinical side-effects. The results suggest that parenteral Gln supplementation protects the gastrointestinal mucosa against 5-FU/CF chemotherapy-induced damage.

Alanyl-glutamine-supplemented parenteral nutrition increases luminal mucus gel and decreases permeability in the rat small intestine

BACKGROUND

Effect of supplemental alanyl-glutamine in standard TPN (S-TPN) on luminal mucus gel and small intestinal permeability was investigated.

METHODS

Thirty Sprague-Dawley rats were divided into group I (n = 10), receiving standard rat diet; group II (n = 10), receiving S-TPN; and group III (n = 10), receiving alanyl-glutamine-supplemented TPN for 1 week. After 1 week, fluorescein isothiocyanate (FITC)-dextran was injected into the small intestine of the rats, and they were killed. A small intestinal sample and portal blood were obtained for morphologic and functional analysis of mucus gel and intestinal permeability.

RESULTS

In group II, thickness and optical density of mucus gel per millimeter serosal length of intestine were significantly lower than group I (p<.001) and were significantly higher in group III than in group II (p<.001). The number of goblet cells in the villi and in the crypt of the small intestine was significantly lower in group II than in group I (p<.001) and was significantly higher in group III than in group II (p<.001), with the exception of the villi of jejunum. Villous and crypt surface area per millimeter serosal length of intestine was significantly lower in group II than in group I (p<.001) and was significantly higher in group III than in group II (p<.001). Small intestinal permeability to FITC-dextran was significantly higher in group II than in group I (p<.001) and was significantly lower in group III than in group II (p<.001). Glucosamine synthetase level was significantly higher in group III than in group I and ileum of group II (p<.001).

CONCLUSIONS

Alanyl-glutamine-supplemented TPN prevents a decrease in mucus gel and an increase in small intestinal permeability associated with S-TPN.

Amino acids with anabolic properties

Experimental studies have clearly demonstrated both the indispensability in stress situations of amino acids, previously considered to be non-essential, and the importance of the specific properties of these same amino acids. Glutamine, arginine and their precursors/metabolites, ornithine and alpha-ketoglutarate, exert anabolic or anticatabolic effects through their involvement in protein metabolism, in the immune response and in cell proliferation. Clinical studies suggest that the supplementation of nutritional therapy with these amino acids can be of significant benefit for injured patients.

Glutamine dipeptides in clinical nutrition

Glutamine is a conditional indispensable amino acid during stress. However, limited solubility and instability of glutamine prevent its addition to presently available nutritional preparations. To overcome these drawbacks, we propose the dipeptide concept by which stable and highly soluble synthetic glutamine containing dipeptides are used. The synthetic dipeptides fulfill all chemical/physical properties to be considered as parenteral substrates. Numerous experimental studies show rapid clearance of parenteral supplied glutamine containing dipeptides without accumulation in tissues; the loss via the urine being inconsequential. Differences related to the dipeptide structure are not observed. There is overwhelming evidence existent that a nutritional support with supplemental glutamine dipeptide positively influences nitrogen excretion, immune status, gut integrity, morbidity, rehabilitation and outcome. Consequently, omission of glutamine from conventional TPN and its subsequent administration should be considered as a replacement of a deficiency rather than a supplementation. It might thus be conceivable that the beneficial effects observed with glutamine nutrition are simply a correction of disadvantages produced by an inadequacy of conventional amino acid solutions. The availability of stable glutamine containing preparations will certainly facilitate an adequate amino acid nutrition in routine clinical setting during episodes of stress and malnutrition.

Glutamine extraction by the gut is reduced in depleted [corrected] patients with gastrointestinal cancer

OBJECTIVE AND SUMMARY BACKGROUND DATA

Glutamine is an important fuel for the intestinal mucosa. However, glutamine pools may become depleted in the cancer-bearing host as a result of tumor consumption and diminished production due to nutritional depletion. As human data are lacking, the authors investigated glutamine extraction by different sites of the human intestine, including tumor and the potential relation with the degree of nutritional depletion.

METHODS

Thirty-two patients with gastrointestinal malignancies were studied. Blood from an artery and veins draining jejunum, ileum, colon, or tumor were sampled. Depletion was estimated by the percentage ideal body weight.

RESULTS

Fractional glutamine extraction rate in the jejunum was 24%, three times higher than in ileum and colon. Percentage ideal body weight correlated with arterial glutamine levels (r = 0.5275, p = 0.003). In addition, arterial glutamine concentrations were correlated with extraction in the ileum (r = -0.8411, p < 0.001). Colon-containing tumor did not extract more glutamine than did nontumor-containing colon.

CONCLUSIONS

Glutamine is a quantitatively more important substrate for the proximal intestine than for the distal gut. Nutritional depletion results in decreased arterial glutamine concentration, which in turn results in diminished extraction. Colon cancer does not function as a glutamine trap and does not contribute to glutamine depletion.

Influence of glutamine-supplemented parenteral nutrition on intestinal amino acid metabolism in rats after small bowel resection

Glutamine (Gln)-supplemented total parenteral nutrition (TPN) has been shown to improve mucosal adaptation after massive small bowel resection (SBR); however, its influences on intestinal amino acid metabolism remain unknown. In this study, intestinal amino acid flux, circulating plasma aminogram, mucosal glutaminase activity and protein, and DNA content were measured 7 days after massive SBR in rats receiving either standard (Std) or Gln-supplemented TPN. Sham-operated rats and rats fed chow after enterectomy served as controls. The uptake of Gln and the release of citrulline (Cit) by the remaining intestine was significantly decreased, with reduced mucosal glutaminase activity after SBR in the Chow and Std-TPN groups. Glutamine supplementation resulted in significantly increased gut Gln uptake compared with Std-TPN (P < 0.01). Mucosal glutaminase activity, mucosal protein, and DNA content was also increased by Gln; however, the gut release of Cit remained unchanged (P > 0.05). The subsequent decrease in circulating arginine (Arg) in the Gln-TPN group compared with the Std-TPN group (P < 0.05) was attributed to an insufficient exogenous supply. These findings show that Gln-supplemented TPN improves mucosal growth and gut Gln uptake after SBR. However, the intestinal production of Cit, which remained low in both TPN groups, may lead to an insufficiency of endogenous Arg synthesis. Thus, both Gln and Arg may be essential amino acids after SBR.

Glutamine: is it a conditionally required nutrient for the human gastrointestinal system?

Glutamine is a nonessential amino acid which can be synthesized from glutamate and glutamic acid by glutamine synthetase. It is the preferred fuel for the rat small intestine. Animal studies have suggested both glutamine-supplemented parenteral nutrition and enteral diets may prevent bacterial translocation. This effect is thought to be modulated via the preservation and augmentation of small bowel villus morphology, intestinal permeability and intestinal immune function. The existing data are less compelling in humans. It remains unclear what, if any, intestinal deficits actually occur in humans during provision of exclusive parental nutrition. Furthermore, the clinical significance of these changes is largely undefined in humans. The existing data on the use of parenteral and enteral glutamine for the purpose of preserving intestinal morphology and function, and the prevention of bacterial translocation in humans are reviewed. Pertinent animal data are also described.

The role of glutamine, short-chain fatty acids, and nucleotides in intestinal adaptation to gastrointestinal disease

Important first steps have been taken towards establishing how some nutrients interact with genes and affect intestinal adaptation. These mechanisms may be typical of how other nutrients influence cell function and turnover and help to maintain intestinal integrity. The dietary effects of nucleotides on intestinal cell mucosa act at the gene transcription level. The dietary effects of nucleotides on immune suppression also may act through similar mechanisms. The effects of the other trophic agents may interact at this level or at other levels. Scientific interest in how the various tropic factors work to maintain and repair the gastrointestinal tract is manifested by a growing body of research that demonstrates potential mechanisms for nutrient-gene interaction and how much interactions affect intestinal development and turnover. It seems clear that intestinal gene transcription and the activity of transcription factors are at least sometimes directly related to nutrition. The techniques of molecular biology now permit the exploration and explanation of how dietary factors, such as glutamine, SCFAs, and nucleotides, affect normal and pathologic intestinal mucosal development, function, adaptation, and repair.

Alanyl-glutamine dipeptide-supplemented parenteral nutrition improves intestinal metabolism and prevents increased permeability in rats

OBJECTIVE

The authors determined the effects of alanyl-glutamine-supplemented total parenteral nutrition (TPN) on mucosal metabolism, integrity, and permeability of the small intestine in rats.

METHODS

Male Sprague-Dawley rats were randomized to receive TPN supplemented with a conventional amino acids mixture (STD group) or the same solution supplemented with alanyl-glutamine; both solutions were isocaloric and isonitrogenous. On the seventh day of TPN, D-xylose and fluorescein isothiocyanate (FITC)-dextran were administered orally. One hour later, superior mesenteric vein (SMV) D-xylose and plasma FITC-dextran concentration were measured. Intestinal blood flow and calculated intestinal substrates flux were measured with ultrasonic transit time flowmetery.

RESULTS

Plasma FITC-dextran increased significantly in the STD group. Intestinal blood flow and SMV D-xylose concentration did not differ between the groups. Mucosa weight, villus height, mucosal wall thickness, mucosal protein, and DNA and RNA content in jejunal mucosa were significantly increased in the alanyl-glutamine group. Jejunal mucosal glutaminase activity and net intestinal uptake of glutamine (glutamine flux) were significantly higher in the alanyl-glutamine group as compared with the STD group.

CONCLUSION

Addition of alanyl-glutamine dipeptide to the TPN solution improves intestinal glutamine metabolism and prevents mucosal atrophy and deterioration of permeability.

Interrelation of intracellular proteases with total parenteral nutrition-induced gut mucosal atrophy and increase of mucosal macromolecular transmission in rats

Total parenteral nutrition (TPN) is known to induce mucosal atrophy and to increase macromolecular transmission of the small intestine. The potential participation of various proteases in that process was investigated. Male Wistar rats were randomly divided into two groups: the TPN group (n = 11) received a standard TPN (250 kcal/kg per day, 1.78 g nitrogen/kg per day) and the FED group (n = 10) received a standard rat food for 1 week. This was followed by an examination of gut macromolecular transmission of fluorescein isothiocyanate dextran 70,000 (FITC-dextran) after intragastric injection and of the activities of gut mucosal cathepsins B, H, and L and of proteasome. Mucosal wet weight and protein content decreased significantly by TPN for 1 week. In both groups, the activities of all proteases in the ileum were significantly greater than in the jejunum. In the TPN group, cathepsin L and H activities in the ileum, and cathepsin B activity in both the jejunum and the ileum, were greater than those in the FED group. The portal concentration of FITC-dextran was higher than arterial and venous concentrations in the both groups. In the TPN group, the portal FITC-dextran concentration increased significantly compared with the FED group. In conclusion, active proteolysis is not associated with TPN-induced mucosal atrophy. Cathepsins activities in the ileum increase as a result of TPN. Interrelationship is implicated between increase of lysosomal protease activity and the deterioration of the intestinal barrier function, which permits macromolecular transmission.

Effect of an enterally administered glutamine-rich protein on the catabolic response to a zymosan challenge in rats

Glutamine is considered to be a conditionally essential amino acid during critical illness and has, therefore, been advocated to be included in nutritional support supplied in these situations. We investigated whether a diet containing a protein source rich in glutamine can restore depleted glutamine pools (plasma and muscle) and counteract muscle wasting, in a rat model of critical illness (intraperitoneal injection of zymosan). A glutamine-rich protein source was obtained by mixing a wheat protein hydrolysate (25% glutamine) with a whey protein isolate (to prevent essential amino acid deficiency). Feeding healthy control rats for 2 weeks with an adequate diet containing this protein source increased the two main glutamine pools (plasma and muscle). However, no effect was observed on the following zymosan-induced changes: 1) decreased glutamine and arginine concentrations (plasma and muscle), 2) wasting of muscle protein, and 3) decreased mitochondrial content in skeletal muscle. We conclude that a diet containing a glutamine-rich protein source can be used to increase plasma and muscle glutamine concentrations in healthy rats, but is of limited value to counteract wasting of skeletal muscle in zymosan-treated rats.

Glutamine dipeptide-supplemented parenteral nutrition maintains intestinal function in the critically ill

BACKGROUND/AIMS

Long-term total parenteral nutrition is accompanied with mucosal atrophy and subsequent malabsorption syndrome. Current information attests the important role of glutamine in maintaining intestinal structure and function. The aim of this study was to investigate the effect of glutamine dipeptide supplementation on small intestinal absorption capacity during critical illness.

METHODS

Twelve intensive care unit patients were uniformly randomized to receive isonitrogenous (0.26 g nitrogen.kg-1.day-1) and isoenergetic (155 kJ.kg-1.day-1) parenteral nutrition over 9 days. The control group received a conventional amino acid solution (1.5 g amino acids.kg-1.day-1), and the test group received a complete amino acid solution containing the dipeptide L-alanyl-L-glutamine (20 g/L). On days 8 and 9, a modified D-xylose test was performed.

RESULTS

Excretion of D-xylose during the 5-hour test period was 7.4 +/- 1.1 g (test) vs. 3.8 +/- 0.9 g (control) (P < 0.05). The 2-hour serum D-xylose concentration was 38.7 +/- 3.0 (test) vs. 27.8 +/- 2.9 mg/100 mL (control) (P < 0.05). Kinetic evaluation showed higher maximum D-xylose blood concentration and higher values for the area under the curve with the peptide.

CONCLUSIONS

The results strongly suggest that glutamine dipeptide-containing total parenteral nutrition prevents intestinal atrophy and increased permeability associated with glutamine-free parenteral nutrition.

Supplemental alanylglutamine, organ growth, and nitrogen metabolism in neonatal pigs fed by total parenteral nutrition

The objective of this study was to determine whether supplemental glutamine (alanylglutamine dipeptide) is effective in preventing small intestinal mucosal atrophy associated with total parenteral nutrition and whether it affects the growth of other organs in neonatal pigs. We compared organ growth, intestinal enzyme activity, and plasma nitrogen metabolites in 4-day-old pigs randomly selected to receive total parenteral nutrition supplemented with 0 g, 2.0 g, or 4.5 g of glutamine per deciliter for a total amino acid intake of either 11 or 25 g.kg-1.d-1 for 7 days. Glutamine supplementation increased (60% to 100%) plasma concentrations of glutamine, urea nitrogen, ammonia, and both jejunal villus height and surface area, but it did not significantly affect jejunal mucosal protein and DNA masses or the relative growth of liver, kidneys, and brain. No histologic evidence of tissue toxicity was found. Supplementing large amounts of glutamine (alanylglutamine dipeptide) did not completely prevent total parenteral nutrition-associated intestinal mucosal atrophy but did improve villus morphology without affecting vital organ growth or histology.

Glutamine supplementation does not prevent small bowel mucosal atrophy after total parenteral nutrition in the rat

Glutamine supplementation to non-lipid parenteral nutrition has been demonstrated to attenuate villus atrophy and increase mucosal DNA content in the rat. This study was performed in order to determine the effects of glutamine supplementation to a balanced TPN mixture (including lipids) on epithelial cell kinetics using autoradiography. Male Sprague-Dawley rats were used. Group 1 (control) received food and an intravenous saline infusion. Group 2 received an intravenous TPN mixture including lipids but without glutamine. The same TPN mixture, glutamine replacing an isonitrogenous amount of non-essential amino acids, was given to Group 3. Animals were fed for 7 days, whereafter blood and intestinal samples were taken 1 h after injection of tritiated thymidine. Microscopy of specimens from proximal jejunum revealed a significant reduction in the number of cells in crypts and villi in both TPN groups (2 and 3) compared to orally fed animals (p < 0.001). Epithelial cell numbers were not significantly different in Group 2 and 3. Similarly, the labelling index (number of labelled cells/number of crypt cells) was not affected by glutamine administration. In plasma, glucagon concentrations in Group 2 (TPN without glutamine) seemed to decrease compared to Group 1 and 3 (p = 0.06). In this study, glutamine supplementation did not affect apithelial atrophy or cell proliferation. It is concluded, that the effects of glutamine on mucosal atrophy and renewal in jejunum may depend on the composition of the TPN mixture supplied during parenteral feeding.

The potential use of parenteral dipeptides in clinical nutrition

The metabolic effects of intravenous peptides have undergone extensive investigation in recent years. Dipeptide solutions provide a mechanism for the provision of selected amino acids that may be conditionally indispensable under certain clinical conditions. In particular, amino acids such as cystine, glutamine, and tyrosine may be difficult to provide in their free form, but their availability can be increased substantially when they are supplied in the form of a dipeptide. Animal and human studies have demonstrated that parenteral dipeptides are cleared rapidly from the plasma compartment and favorably influence nitrogen equilibrium in healthy volunteers and catabolic patients. Certain dipeptides offer the potential for tailoring tissue-specific nutrition therapy. It seems likely that parenteral peptides will offer a major change in the delivery of intravenous nutrients.

Glutamine and the preservation of gut integrity

Parenteral glutamine dipeptide improves nitrogen balance in postoperative patients on total parenteral nutrition (TPM). Animal studies show that the structure and function of the gut is preserved by glutamine. It is not known if this is the case in human beings. 20 patients admitted to hospital for total parenteral nutrition were randomly allocated to receive parenteral nutrition enriched with glycyl-L-glutamine (Gln TPN), or standard parenteral nutrition (STPN). Mucosal biopsy specimens were taken from the second part of the duodenum before starting parenteral nutrition, and after two weeks. The ratio between the urine concentrations of lactulose and mannitol after enteral administration was used to measure intestinal permeability. After two weeks of parenteral nutrition in the GlnTPN group, intestinal permeability was unchanged, whereas permeability in the STPN group increased. Villus height was unaltered in the GlnTPN group but in the STPN group it decreased. The addition of glutamine to parenteral nutrition prevents deterioration of gut permeability and preserves mucosal structure.