Nitrogen in dietary glutamate is utilized exclusively for the synthesis of amino acids in the rat intestine

Characteristic changes in plasma glutamate levels and free amino acid profiles in Japanese patients with type 1 diabetes mellitus

AIMS/INTRODUCTION

In addition to absolute insulin deficiency, dysregulated glucagon in type 1 diabetes is considered pathophysiologically important. Previously, we confirmed the presence of dysregulated glucagon in Japanese patients with type 1 diabetes, and found a significant correlation between plasma glucagon and blood urea nitrogen levels, suggesting an association between glucagon and amino acid metabolism. In this study, we evaluated plasma amino acid levels in Japanese patients with type 1 diabetes in the context of their functional relationship with glucagon.

MATERIALS AND METHODS

We assessed plasma free amino acid levels using liquid chromatography-mass spectrometry in 77 Japanese patients with type 1 diabetes, and statistically analyzed their characteristics and relationships with clinical parameters, including glucagon.

RESULTS

Participants with type 1 diabetes showed a large decrease in glutamate levels together with a characteristic change in plasma free amino acid profiles. The network structural prediction analyses showed correlations between each amino acid and glucagon in type 1 diabetes.

CONCLUSIONS

Participants with type 1 diabetes showed characteristic changes in plasma glutamate levels and free amino acid profiles compared with controls and type 2 diabetes patients. Glucagon showed a closer correlation with amino acids than with parameters of glucose metabolism, suggesting that type 1 diabetes includes dysregulation in amino acids through dysregulated glucagon from remaining pancreatic α-cells, together with that in glucose by insulin deficiency.

Low-protein diets supplemented with glutamic acid or aspartic acid ameliorate intestinal damage in weaned piglets challenged with hydrogen peroxide

Glutamic acid (Glu) and aspartic acid (Asp) are acidic amino acids with regulatory roles in nutrition, energy metabolism, and oxidative stress. This study aimed to evaluate the effects of low-protein diets supplemented with Glu and Asp on the intestinal barrier function and energy metabolism in weaned piglets challenged with hydrogen peroxide (H₂O₂). Forty piglets were randomly divided into 5 groups: NC, PC, PGA, PG, and PA (n = 8 for each group). Pigs in the NC and PC groups were fed a low-protein diet, while pigs in the PGA, PG, or PA groups were fed the low-protein diet supplemented with 2.0% Glu +1.0% Asp, 2.0% Glu, or 1.0% Asp, respectively. On day 8 and 11, pigs in the NC group were intraperitoneally injected with saline (1 mL/kg BW), while pigs in the other groups were intraperitoneally administered 10% H₂O₂ (1 mL/kg BW). On day 14, all pigs were sacrificed to collect jejunum and ileum following the blood sample collection in the morning. Notably, low-protein diets supplemented with Glu or Asp ameliorated the intestinal oxidative stress response in H₂O₂-challenged piglets by decreasing intestinal expression of genes (P < 0.05) (e.g., manganese superoxide dismutase [MnSOD], glutathione peroxidase [Gpx]-1, and Gpx-4) encoding oxidative stress-associated proteins, reducing the serum concentration of diamine oxidase (P < 0.05), and inhibiting apoptosis of the intestinal epithelium. Glu and Asp supplementation attenuated the upregulated expression of energy metabolism-associated genes (such as hexokinase and carnitine palmitoyltransferase-1) and the H₂O₂-induced activation of acetyl-coenzyme A carboxylase (ACC) in the jejunum and adenosine monophosphate-activated protein kinase–acetyl-ACC signaling in the ileum. Dietary Glu and Asp also ameliorated intestinal barrier damage as indicated by restored intestinal histology and morphology. In conclusion, low-protein diets supplemented with Glu and Asp protected against oxidative stress-induced intestinal dysfunction in piglets, suggesting that this approach could be used as a nutritional regulatory protectant against oxidative stress.

Glutamate metabolism in a human intestinal epithelial cell layer model

Plasma glutamate concentrations are constant despite dynamic changes in diets. Most likely, virtually all the dietary glutamate is metabolized in the gut. The present study investigated permeability and metabolism of dietary glutamate in a Caco-2 intestinal epithelial cell layer model by tracing the fate of [U-¹³C] or [¹⁵N]glutamate added to the apical medium. For comparison, several other labelled essential and non-essential amino acids were tested as well. Almost all the labelled glutamate in the apical medium (98% and 96% at 24 h of the culture, respectively) was incorporated in the cell layer, while it barely appeared at the basolateral side, indicating an almost complete utilization of glutamate. Indeed, the ¹³C was incorporated into alanine, proline, ornithine, and glutamine, and the ¹⁵N was incorporated into alanine, glutamine, ornithine, proline, branched chain amino acids and also found as ammonia indicative of oxidation. In contrast, substantial apical-to-basolateral transport of amino acids (8-85% of uptake) other than glutamate and aspartate was evident in studies using amino acid tracers labelled with ¹³C, ¹⁵N or D. These results suggest that the intestinal epithelial cell monolayer utilizes dietary glutamate which adds to maintaining glutamate homeostasis in the body.

Dispensable Amino Acids, except Glutamine and Proline, Are Ideal Nitrogen Sources for Protein Synthesis in the Presence of Adequate Indispensable Amino Acids in Adult Men

BACKGROUND

Nutritionally, there is a dietary requirement for indispensable amino acids (IAAs) but also a requirement for nitrogen (N) intake for the de novo synthesis of the dispensable amino acids (DAAs). It has been suggested that there might be a dietary requirement for specific DAAs.

OBJECTIVES

Experiment 1 tested whether 9 of the DAAs (Ala, Arg, Asn, Asp, Gln, Glu, Gly, Pro, Ser) are ideal N sources using the indicator amino acid oxidation (IAAO) technique. Experiment 2 examined whether there is a dietary requirement for Glu in adult men.

METHODS

Seven healthy men (aged 20-24 y) participated in 11 or 2 test diet intakes, in experiment 1 and 2, respectively, in a repeated measures design. In experiment 1, a base diet consisting of the IAA provided at the RDA was compared with test intakes with the base diet plus addition of individual DAAs to meet a 50:50 ratio of IAA:DAA on an N basis. In experiment 2, the diets corresponded to the amino acid pattern present in egg protein, in which all Glu and Gln was present as Glu, or removed, with Ser used to make the diets isonitrogenous. On each study day the IAAO protocol with l-[1-13C]phenylalanine was used to measure whole-body protein synthesis.

RESULTS

In experiment 1, repeated measures ANOVA with post hoc multiple comparisons showed that 7 of the 9 DAAs (Ala, Arg, Asn, Asp, Glu, Gly, Ser) decreased IAAO significantly (P < 0.05) compared with base IAA diet, the exceptions being Gln and Pro. In experiment 2, a paired t test did not find significant (P > 0.05) differences in the IAAO in response to removal and replacement of Glu intake.

CONCLUSIONS

The results suggest that in healthy men most DAAs are ideal N sources for protein synthesis, in the presence of adequate IAAs, and that endogenous synthesis of Glu is sufficient.Registered clinicaltrials.gov identifier: NCT02009917.

Dietary glutamate and the brain: In the footprints of a Jekyll and Hyde molecule

Glutamate is a crucial neurotransmitter of the mammalian central nervous system, a molecular component of our diet, and a popular food-additive. However, for decades, concerns have been raised about the issue of glutamate's safety as a food additive; especially, with regards to its ability (or otherwise) to cross the blood-brain barrier, cause excitotoxicity, or lead to neuron death. Results of animal studies following glutamate administration via different routes suggest that an array of effects can be observed. While some of the changes appear deleterious, some are not fully-understood, and the impact of others might even be beneficial. These observations suggest that with regards to the mammalian brain, exogenous glutamate might exert a double-sided effect, and in essence be a two-faced molecule whose effects may be dependent on several factors. This review draws from the research experiences of the authors and other researchers regarding the effects of exogenous glutamate on the brain of rodents. We also highlight the possible implications of such effects on the brain, in health and disease. Finally, we deduce that beyond the culinary effects of exogenous glutamate, there is the possibility of a beneficial role in the understanding and management of brain disorders.

Natural products as safeguards against monosodium glutamate-induced toxicity

Monosodium glutamate is a sodium salt of a nonessential amino acid, L-glutamic acid, which is widely used in food industry. Glutamate plays an important role in principal brain functions including formation and stabilization of synapses, memory, cognition, learning, as well as cellular metabolism. However, ingestion of foodstuffs rich in monosodium glutamate can result in the outbreak of several health disorders such as neurotoxicity, hepatotoxicity, obesity and diabetes. The usage of medicinal plants and their natural products as a therapy against MSG used in food industry has been suggested to be protective. Calendula officinalis, Curcuma longa, Green Tea, Ginkgo biloba and vitamins are some of the main natural products with protective effect against mentioned monosodium glutamate toxicity through different mechanisms. This review provides a summary on the toxicity of monosodium glutamate and the protective effects of natural products against monosodium glutamate -induced toxicity.

Glutamate effects on sucking piglet intestinal morphology and luminal metabolites

This study was conducted to measure the effects of orally administered glutamate (Glu) on suckling piglet (SP) intestinal morphology and volatile fatty acids (VFA). Forty-eight newborn piglets with similar initial weights (1.55 ± 0.20 kg) were selected from six sows (eight piglets/sow) and randomly assigned into four groups. There was daily administration of the following: 0.18 g/kg body weight (BW) of sodium chloride (CN group); 0.06 g/kg BW monosodium glutamate (LMG group); 0.50 g/kg BW monosodium glutamate (MMG group); and 1.00 g/kg BW monosodium glutamate (HMG group). Four piglets (one/group) were randomly selected from each sow for tissue sampling at days 7 and 21. MMG group jejunal villus height and crypt depth significantly increased (p < 0.05) compared to the CN group as of days 7 and 21. HMG group jejunal villus height and crypt depth reduced (p < 0.05) compared to the MMG group. LMG group jejunum goblet cell count was greater (p < 0.05) than that of the MMG or HMG groups. MMG and HMG group ileal villus height was greater (p < 0.05) than either CN or LMG groups as of day 7. HMG ileal crypt depth decreased (p < 0.05) compared to LMG and MMG groups. The MMG group had greater (p < 0.05) acetic acid, isobutyric acid, butyric acid and pentanoic acid contents in their caecum than the other groups as of day 21. It also had greater acetic acid, propanoic acid, isobutyric acid, butyric acid and isopentanoic acid contents in the colon than the other groups on day 21. No significant VFA content differences in the caecum or the colon were observed among groups on day 7. These results indicated that oral administration with monosodium glutamate (MSG) at 0.50 g/kg BW/day improved SP intestinal morphology and increased caecal and colonal VFA contents.

Exocrine pancreas glutamate secretion help to sustain enterocyte nutritional needs under protein restriction

Glutamine (Gln) is the most concentrated amino acid in blood and considered conditionally essential. Its requirement is increased during physiological stress, such as malnutrition or illness, despite its production by muscle and other organs. In the malnourished state, Gln has been suggested to have a trophic effect on the exocrine pancreas and small intestine. However, the Gln transport capacity, the functional relationship of these two organs, and the potential role of the Gln-glutamate (Glu) cycle are unknown. We observed that pancreatic acinar cells express lower levels of Glu than Gln transporters. Consistent with this expression pattern, the rate of Glu influx into acinar cells was approximately sixfold lower than that of Gln. During protein restriction, acinar cell glutaminase expression was increased and Gln accumulation was maintained. Moreover, Glu secretion by acinar cells into pancreatic juice and thus into the lumen of the small intestine was maintained. In the intestinal lumen, Glu absorption was preserved and Glu dehydrogenase expression was augmented, potentially providing the substrates for increasing energy production via the TCA cycle. Our findings suggest that one mechanism by which Gln exerts a positive effect on exocrine pancreas and small intestine involves the Gln metabolism in acinar cells and the secretion of Glu into the small intestine lumen. The exocrine pancreas acinar cells not only avidly accumulate Gln but metabolize Gln to generate energy and to synthesize Glu for secretion in the pancreatic juice. Secreted Glu is suggested to play an important role during malnourishment in sustaining small intestinal homeostasis. NEW & NOTEWORTHY Glutamine (Gln) has been suggested to have a trophic effect on exocrine pancreas and small intestine in malnourished states, but the mechanism is unknown. In this study, we suggest that this trophic effect derives from an interorgan relationship between exocrine pancreas and small intestine for Gln-glutamate (Glu) utilization involving the uptake and metabolism of Gln in acinar cells and secretion of Glu into the lumen of the small intestine.

Intestinal Fatty Acid Binding Protein and Citrulline as Markers of Gut Injury and Prognosis in Patients With Acute Pancreatitis

OBJECTIVES

Severe acute pancreatitis (AP) is associated with high mortality due to systemic inflammatory response syndrome in the early phase and secondary infection in the later phase. Concomitant intestinal ischemia often results in gut injury. We studied intestinal fatty acid binding protein (IFABP) and citrulline levels as markers of gut injury to predict prognosis in AP.

METHODS

Acute pancreatitis patients at admission and controls were studied. Serum IFABP was measured by enzyme-linked immunosorbent assay and plasma citrulline by high-performance liquid chromatography technique. Ultrastructural changes in duodenal biopsy were also compared between the 2 groups.

RESULTS

The IFABP concentration was significantly higher in AP cases (n = 94) compared with controls (n = 100) (mean [standard deviation], 592.5 [753.6] vs 87.8 [67.6] pg/mL; P < 0.001) and in patients with severe AP versus mild AP (738.3 [955.3] vs 404.0 [263.3] pg/ mL, P = 0.03). Citrulline concentration was lower in AP versus controls (29.9 [33.8] vs 83.9 [60.1] μg/L, P < 0.001). We propose a model by which these biomarkers (IFABP >350 pg/mL and citrulline <18 μg/L) are able to predict poor prognosis in 33.9% of patients with AP. The gut injury was also validated via ultrastructural changes.

CONCLUSIONS

Intestinal fatty acid binding protein is a promising prognostic marker in acute pancreatitis.

The Nitrogen Moieties of Dietary Nonessential Amino Acids Are Distinctively Metabolized in the Gut and Distributed to the Circulation in Rats

Background: Although previous growth studies in rodents have indicated the importance of dietary nonessential amino acids (NEAAs) as nitrogen sources, individual NEAAs have different growth-promoting activities. This phenomenon might be attributable to differences in the nitrogen metabolism of individual NEAAs. Objective: The aim of this study was to compare nitrogen metabolism across dietary NEAAs with the use of their ¹⁵N isotopologues.Methods: Male Fischer rats (8 wk old) were given 1.0 g amino acid-defined diets containing either ¹⁵N-labeled glutamate, glutamine (amino or amide), aspartate, alanine, proline, glycine, or serine hourly for 5-6 h. Then, steady-state amino acid concentrations and their ¹⁵N enrichments in the gut and in portal and arterial plasma were measured by an amino acid analyzer and LC tandem mass spectrometry, respectively.Results: The intestinal ¹⁵N distribution and portal-arterial balance of ¹⁵N metabolites indicated that most dietary glutamate nitrogen (>90% of dietary input) was incorporated into various amino acids, including alanine, proline, and citrulline, in the gut. Dietary aspartate nitrogen, alanine nitrogen, and amino nitrogen of glutamine were distributed similarly to other amino acids both in the gut and in the circulation. In contrast, incorporation of the nitrogen moieties of dietary proline, serine, and glycine into other amino acids was less than that of other NEAAs, although interconversion between serine and glycine was very active. Cluster analysis of ¹⁵N enrichment data also indicated that dietary glutamate nitrogen, aspartate nitrogen, alanine nitrogen, and the amino nitrogen of glutamine were distributed similarly to intestinal and circulating amino acids. Further, the analysis revealed close relations between intestinal and arterial ¹⁵N enrichment for each amino acid. The steady-state ¹⁵N enrichment of arterial amino acids indicated that substantial amounts of circulating amino acid nitrogen are derived from dietary NEAAs.Conclusions: The present results revealed similarities and differences among NEAAs in terms of their intestinal nitrogen metabolism in rats and indicated substantial entry of dietary NEAA nitrogen into circulating amino acid nitrogen, presumably primarily through metabolism in the gut.

Monosodium glutamate inhibits the lymphatic transport of lipids in the rat

It is not well understood how monosodium glutamate (MSG) affects gastrointestinal physiology, especially regarding the absorption and the subsequent transport of dietary lipids into lymph. Thus far, there is little information about how the ingestion of MSG affects the lipid lipolysis, uptake, intracellular esterification, and formation and secretion of chylomicrons. Using lymph fistula rats treated with the infusion of a 2% MSG solution before a continuous infusion of triglyceride, we show that MSG causes a significant decrease in both triglyceride and cholesterol secretion into lymph. Intriguingly, the diminished lymphatic transport of triglyceride and cholesterol was not caused by an accumulation of these labeled lipids in the intestinal lumen or in the intestinal mucosa. Rather, it is a result of increased portal transport in the animals fed acutely the lipid plus 2% MSG in the lipid emulsion. This is a first demonstration of MSG on intestinal lymphatic transport of lipids.

Glutamate promotes nucleotide synthesis in the gut and improves availability of soybean meal feed in rainbow trout

Glutamate (Glu) plays various roles directly or through conversions to other amino acids in intracellular metabolisms such as energy source for enterocytes and precursor for nucleic acids. In this study, we examined the effect of single and chronic oral administration of Glu on cell proliferation in intestine and growth in rainbow trout fed soybean meal (SBM) based diet. In the single dose study, 30, 120 and 360 min after oral administration of 50 and 500 mg/kg Glu, the blood and intestine tissues were collected for amino acid concentration and gene expression analysis. Cell-proliferation was detected 24 h after administration using bromo-deoxy uridine (BrdU) in intestine. In the chronic experiment, fish were fed SBM-based diet added 1 and 2 % of Glu for 8 weeks. Final body weight, plasma amino acid concentrations, gene expression and cell-proliferation in the intestine were analyzed. The expressions of some nucleic acid-synthesis related genes were significantly increased 30 min after administration of 50 mg/kg of Glu. After 8 weeks of feeding, the fish fed SBM-based diet showed significantly lower body weight and microvillus thickness in proximal intestine. Supplementation of 2 % of Glu in the SBM-based feed improved both of them. Though it was not significant difference, Glu tended to increase cell-proliferation in the proximal intestine dose-dependently in both single and chronic administration. Our experiment indicates that Glu has positive effect on rainbow trout fed SBM-based feed by reforming proximal intestine through altering cell-proliferation.

Evidence of alterations in brain structure and antioxidant status following 'low-dose' monosodium glutamate ingestion

OBJECTIVE

The study investigated the effects of low dose monosodium glutamate (MSG) on the brain, with a view to providing information on its effects on neuronal morphology and antioxidant status in mice.

METHODOLOGY

Sixty male mice (20-22 g) were divided into six groups of ten animals each. Vehicle (distilled water), a standard (l-glutamate at 10mg/kg body weight) or MSG (10, 20, 40 and 80mg/kg body weight) were administered orally for 28days. Sections of the cerebrum, hippocampus and cerebellum were processed and stained using hematoxylin and eosin, examined under a microscope and captured images analysed. Plasma and brain levels of glutamate, glutamine, and antioxidants were assayed. Data obtained were analysed using descriptive and inferential statistics.

RESULTS

MSG ingestion did not significantly alter body weight. Relative brain weight increased at 40 and 80mg/kg compared to vehicle. Histological and histomorphometric changes consistent with neuronal damage were seen in the cerebrum, hippocampus and cerebellum at 40 and 80mg/kg. Plasma glutamate and glutamine assay showed significant increase at 40 and 80mg/kg while no significant difference in total brain glutamate or glutamine levels were seen. Levels of brain superoxide dismutase and catalase decreased with increasing doses of MSG, while nitric oxide (NO) increased at these doses.

CONCLUSION

The study showed morphological alterations consistent with neuronal injury, biochemical changes of oxidative stress and a rise in plasma glutamate and glutamine. These data therefore still support the need for cautious consideration in the indiscriminate use of MSG as a dietary flavor enhancer.

Measurement of (15)N enrichment of glutamine and urea cycle amino acids derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate using liquid chromatography-tandem quadrupole mass spectrometry

6-Aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) is an amino acid-specific derivatizing reagent that has been used for sensitive amino acid quantification by liquid chromatography-tandem quadrupole mass spectrometry (LC-MS/MS). In this study, we aimed to evaluate the ability of this method to measure the isotopic enrichment of amino acids and to determine the positional (15)N enrichment of urea cycle amino acids (i.e., arginine, ornithine, and citrulline) and glutamine. The distribution of the M and M+1 isotopomers of each natural AQC-amino acid was nearly identical to the theoretical distribution. The standard deviation of the (M+1)/M ratio for each amino acid in repeated measurements was approximately 0.1%, and the ratios were stable regardless of the injected amounts. Linearity in the measurements of (15)N enrichment was confirmed by measuring a series of (15)N-labeled arginine standards. The positional (15)N enrichment of urea cycle amino acids and glutamine was estimated from the isotopic distribution of unique fragment ions generated at different collision energies. This method was able to identify their positional (15)N enrichment in the plasma of rats fed (15)N-labeled glutamine. These results suggest the utility of LC-MS/MS detection of AQC-amino acids for the measurement of isotopic enrichment in (15)N-labeled amino acids and indicate that this method is useful for the study of nitrogen metabolism in living organisms.

Monosodium L-Glutamate and Dietary Fat Differently Modify the Composition of the Intestinal Microbiota in Growing Pigs

BACKGROUND

The Chinese have been undergone rapid transition to a high-fat diet-consuming lifestyle, while monosodium L-glutamate (MSG) is widely used as a daily food additive. It has been reported that fat alters the composition of intestinal microbiota. However, little information is available on the effects of oral MSG on intestinal microbiota, and no study was done focusing on the interaction effect of fat and MSG with respect to intestinal microbiota. The present study thus aimed to determine the effects of MSG and/or fat on intestinal microbiota, and also to identify possible interactions between these two nutrients.

METHODS

Four iso-nitrogenous and iso-caloric diets were provided to growing pigs. The microbiota from jejunum, ileum, cecum, and colon were analyzed.

RESULTS

Our results show that both MSG and fat clearly increased the intestinal microbiota diversity. MSG and fat modified the composition of intestinal microbiota, particularly in the colon. Both MSG and fat promoted the colonization of microbes related to energy extraction in gastrointestinal tract via different ways. MSG promoted the colonization of Faecalibacterium prausnitzii and Roseburia, while fat increased the percentage of Prevotella in colon and other intestinal segments.

CONCLUSION

Our results will help to understand how individual or combined dietary changes modify the microbiota composition to prevent obesity.

Dietary glutamate supplementation ameliorates mycotoxin-induced abnormalities in the intestinal structure and expression of amino acid transporters in young pigs

The purpose of this study was to investigate the hypothesis that dietary supplementation with glutamic acid has beneficial effects on growth performance, antioxidant system, intestinal morphology, serum amino acid profile and the gene expression of intestinal amino acid transporters in growing swine fed mold-contaminated feed. Fifteen pigs (Landrace×Large White) with a mean body weight (BW) of 55 kg were randomly divided into control group (basal feed), mycotoxin group (contaminated feed) and glutamate group (2% glutamate+contaminated feed). Compared with control group, mold-contaminated feed decreased average daily gain (ADG) and increased feed conversion rate (FCR). Meanwhile, fed mold-contaminated feed impaired anti-oxidative system and intestinal morphology, as well as modified the serum amino acid profile in growing pigs. However, supplementation with glutamate exhibited potential positive effects on growth performance of pigs fed mold-contaminated feed, ameliorated the imbalance antioxidant system and abnormalities of intestinal structure caused by mycotoxins. In addition, dietary glutamate supplementation to some extent restored changed serum amino acid profile caused by mold-contaminated feed. In conclusion, glutamic acid may be act as a nutritional regulating factor to ameliorate the adverse effects induced by mycotoxins.

Brain amino acid sensing

The 20 different amino acids, in blood as well as in the brain, are strictly maintained at the same levels throughout the day, regardless of food intake. Gastric vagal afferents only respond to free glutamate and sugars, providing recognition of food intake and initiating digestion. Metabolic control of amino acid homeostasis and diet-induced thermogenesis is triggered by this glutamate signalling in the stomach through the gut-brain axis. Rats chronically fed high-sugar and high-fat diets do not develop obesity when a 1% (w/v) monosodium glutamate (MSG) solution is available in a choice paradigm. Deficiency of the essential amino acid lysine (Lys) induced a plasticity in rats in response to Lys. This result shows how the body is able to identify deficient nutrients to maintain homeostasis. This plastic effect is induced by activin A activity in the brain, particularly in certain neurons in the lateral hypothalamic area (LHA) which is the centre for amino acid homeostasis and appetite. These neurons respond to glutamate signalling in the oral cavity by which umami taste is perceived. They play a quantitative role in regulating ingestion of deficient nutrients, thereby leading to a healthier life. After recovery from malnutrition, rats prefer MSG solutions, which serve as biomarkers for protein nutrition.

The immune modifying effects of amino acids on gut-associated lymphoid tissue

The intestine and the gut-associated lymphoid tissue (GALT) are essential components of whole body immune defense, protecting the body from foreign antigens and pathogens, while allowing tolerance to commensal bacteria and dietary antigens. The requirement for protein to support the immune system is well established. Less is known regarding the immune modifying properties of individual amino acids, particularly on the GALT. Both oral and parenteral feeding studies have established convincing evidence that not only the total protein intake, but the availability of specific dietary amino acids (in particular glutamine, glutamate, and arginine, and perhaps methionine, cysteine and threonine) are essential to optimizing the immune functions of the intestine and the proximal resident immune cells. These amino acids each have unique properties that include, maintaining the integrity, growth and function of the intestine, as well as normalizing inflammatory cytokine secretion and improving T-lymphocyte numbers, specific T cell functions, and the secretion of IgA by lamina propria cells. Our understanding of this area has come from studies that have supplemented single amino acids to a mixed protein diet and measuring the effect on specific immune parameters. Future studies should be designed using amino acid mixtures that target a number of specific functions of GALT in order to optimize immune function in domestic animals and humans during critical periods of development and various disease states.