The pattern of amino acid exchange across the brain is unaffected by intravenous glutamine supplementation in head trauma patients

Amino acid homeostasis and signalling in mammalian cells and organisms

Cells have a constant turnover of proteins that recycle most amino acids over time. Net loss is mainly due to amino acid oxidation. Homeostasis is achieved through exchange of essential amino acids with non-essential amino acids and the transfer of amino groups from oxidised amino acids to amino acid biosynthesis. This homeostatic condition is maintained through an active mTORC1 complex. Under amino acid depletion, mTORC1 is inactivated. This increases the breakdown of cellular proteins through autophagy and reduces protein biosynthesis. The general control non-derepressable 2/ATF4 pathway may be activated in addition, resulting in transcription of genes involved in amino acid transport and biosynthesis of non-essential amino acids. Metabolism is autoregulated to minimise oxidation of amino acids. Systemic amino acid levels are also tightly regulated. Food intake briefly increases plasma amino acid levels, which stimulates insulin release and mTOR-dependent protein synthesis in muscle. Excess amino acids are oxidised, resulting in increased urea production. Short-term fasting does not result in depletion of plasma amino acids due to reduced protein synthesis and the onset of autophagy. Owing to the fact that half of all amino acids are essential, reduction in protein synthesis and amino acid oxidation are the only two measures to reduce amino acid demand. Long-term malnutrition causes depletion of plasma amino acids. The CNS appears to generate a protein-specific response upon amino acid depletion, resulting in avoidance of an inadequate diet. High protein levels, in contrast, contribute together with other nutrients to a reduction in food intake.

Prolonged continuous intravenous infusion of the dipeptide L-alanine- L-glutamine significantly increases plasma glutamine and alanine without elevating brain glutamate in patients with severe traumatic brain injury

INTRODUCTION

Low plasma glutamine levels are associated with worse clinical outcome. Intravenous glutamine infusion dose- dependently increases plasma glutamine levels, thereby correcting hypoglutaminemia. Glutamine may be transformed to glutamate which might limit its application at a higher dose in patients with severe traumatic brain injury (TBI). To date, the optimal glutamine dose required to normalize plasma glutamine levels without increasing plasma and cerebral glutamate has not yet been defined.

METHODS

Changes in plasma and cerebral glutamine, alanine, and glutamate as well as indirect signs of metabolic impairment reflected by increased intracranial pressure (ICP), lactate, lactate-to-pyruvate ratio, electroencephalogram (EEG) activity were determined before, during, and after continuous intravenous infusion of 0.75 g L-alanine-L-glutamine which was given either for 24 hours (group 1, n = 6) or 5 days (group 2, n = 6) in addition to regular enteral nutrition. Lab values including nitrogen balance, urea and ammonia were determined daily.

RESULTS

Continuous L-alanine-L-glutamine infusion significantly increased plasma and cerebral glutamine as well as alanine levels, being mostly sustained during the 5 day infusion phase (plasma glutamine: from 295 ± 62 to 500 ± 145 μmol/ l; brain glutamine: from 183 ± 188 to 549 ± 120 μmol/ l; plasma alanine: from 327 ± 91 to 622 ± 182 μmol/ l; brain alanine: from 48 ± 55 to 89 ± 129 μmol/ l; p < 0.05, ANOVA, post hoc Dunn's test).

CONCLUSIONS

High dose L-alanine-L-glutamine infusion (0.75 g/ kg/ d up to 5 days) increased plasma and brain glutamine and alanine levels. This was not associated with elevated glutamate or signs of potential glutamate-mediated cerebral injury. The increased nitrogen load should be considered in patients with renal and hepatic dysfunction.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02130674. Registered 5 April 2014.

A tracer bolus method for investigating glutamine kinetics in humans

Glutamine transport between tissues is important for the outcome of critically ill patients. Investigation of glutamine kinetics is, therefore, necessary to understand glutamine metabolism in these patients in order to improve future intervention studies. Endogenous glutamine production can be measured by continuous infusion of a glutamine tracer, which necessitates a minimum measurement time period. In order to reduce this problem, we used and validated a tracer bolus injection method. Furthermore, this method was used to measure the glutamine production in healthy volunteers in the post-absorptive state, with extra alanine and with glutamine supplementation and parenteral nutrition. Healthy volunteers received a bolus injection of [1-13C] glutamine, and blood was collected from the radial artery to measure tracer enrichment over 90 minutes. Endogenous rate of appearance (endoRa) of glutamine was calculated from the enrichment decay curve and corrected for the extra glutamine supplementation. The glutamine endoRa of healthy volunteers was 6.1±0.9 µmol/kg/min in the post-absorptive state, 6.9±1.0 µmol/kg/min with extra alanyl-glutamine (p = 0.29 versus control), 6.1±0.4 µmol/kg/min with extra alanine only (p = 0.32 versus control), and 7.5±0.9 µmol/kg/min with extra alanyl-glutamine and parenteral nutrition (p = 0.049 versus control). In conclusion, a tracer bolus injection method to measure glutamine endoRa showed good reproducibility and small variation at baseline as well as during parenteral nutrition. Additionally, we showed that parenteral nutrition including alanyl-glutamine increased glutamine endoRa in healthy volunteers, which was not attributable to the alanine part of the dipeptide.

Endogenous glutamine production in critically ill patients: the effect of exogenous glutamine supplementation

Introduction

Glutamine rate of appearance (R_a) may be used as an estimate of endogenous glutamine production. Recently a technique employing a bolus injection of isotopically labeled glutamine was introduced, with the potential to allow for multiple assessments of the glutamine R_a over time in critically ill patients, who may not be as metabolically stable as healthy individuals. Here the technique was used to evaluate the endogenous glutamine production in critically ill patients in the fed state with and without exogenous glutamine supplementation intravenously.

Methods

Mechanically ventilated patients (n = 11) in the intensive care unit (ICU) were studied on two consecutive days during continuous parenteral feeding. To allow the patients to be used as their own controls, they were randomized for the reference measurement during basal feeding without supplementation, before or after the supplementation period. Glutamine R_a was determined by a bolus injection of ¹³C-glutamine followed by a period of frequent sampling to establish the decay-curve for the glutamine tracer. Exogenous glutamine supplementation was given by intravenous infusion of a glutamine containing dipeptide, L-alanyl-L-glutamine, 0.28 g/kg during 20 hours.

Results

A 14% increase of endogenous glutamine R_a was seen at the end of the intravenous supplementation period as compared to the basal measurements (P = 0.009).

Conclusions

The bolus injection technique to measure glutamine R_a to estimate the endogenous production of glutamine in critically ill patients was demonstrated to be useful for repetitive measurements. The hypothesized attenuation of endogenous glutamine production during L-alanyl-L-glutamine infusion given as a part of full nutrition was not seen.

Whole body protein kinetics during hypocaloric and normocaloric feeding in critically ill patients

Introduction

Optimal feeding of critically ill patients in the ICU is controversial. Existing guidelines rest on rather weak evidence. Whole body protein kinetics may be an attractive technique for assessing optimal protein intake. In this study, critically ill patients were investigated during hypocaloric and normocaloric IV nutrition.

Methods

Neurosurgical patients on mechanical ventilation (n = 16) were studied during a 48-hour period. In random order 50% and 100% of measured energy expenditure was given as IV nutrition during 24 hours, corresponding to hypocaloric and normocaloric nutrition, respectively. At the end of each period, whole body protein turnover was measured using d5-phenylalanine and 13C-leucine tracers.

Results

The phenylalanine tracer indicated that whole-body protein synthesis was lower during hypocaloric feeding, while whole-body protein degradation and amino acid oxidation were unaltered, which resulted in a more negative protein balance, namely −1.9 ± 2.1 versus −0.7 ± 1.3 mg phenylalanine/kg/h (P = 0.014). The leucine tracer indicated that whole body protein synthesis and degradation and amino acid oxidation were unaltered, but the protein balance was negative during hypocaloric feeding, namely −0.3 ± 0.5 versus 0.6 ± 0.5 mg leucine/kg/h (P < 0.001).

Conclusion

In the patient group studied, hypocaloric feeding was associated with a more negative protein balance, but the amino acid oxidation was not different. The protein kinetics measurements and the study’s investigational protocol were useful for assessing the efficacy of nutrition support on protein metabolism in critically ill patients.

Nutritional support for patients sustaining traumatic brain injury: a systematic review and meta-analysis of prospective studies

Background

In traumatic brain injury (TBI), the appropriate timing and route of feeding, and the efficacy of immune-enhancing formulae have not been well established. We performed this meta-analysis aiming to compare the effects of different nutritional support modalities on clinical outcomes of TBI patients.

Methods

We systematically searched Pubmed, Embase, and the Cochrane Library until October, 2012. All randomized controlled trials (RCTs) and non-randomized prospective studies (NPSs) that compared the effects of different routes, timings, or formulae of feeding on outcomes in TBI patients were selected. The primary outcomes included mortality and poor outcome. The secondary outcomes included the length of hospital stay, the length of ventilation days, and the rate of infectious or feeding-related complications.

Findings

13 RCTs and 3 NPSs were included. The pooled data demonstrated that, compared with delayed feeding, early feeding was associated with a significant reduction in the rate of mortality (relative risk [RR] = 0.35; 95% CI, 0.24–0.50), poor outcome (RR = 0.70; 95% CI, 0.54–0.91), and infectious complications (RR = 0.77; 95% CI, 0.59–0.99). Compared with enteral nutrition, parenteral nutrition showed a slight trend of reduction in the rate of mortality (RR = 0.61; 95% CI, 0.34–1.09), poor outcome (RR = 0.73; 95% CI, 0.51–1.04), and infectious complications (RR = 0.89; 95% CI, 0.66–1.22), whereas without statistical significances. The immune-enhancing formula was associated with a significant reduction in infection rate compared with the standard formula (RR = 0.54; 95% CI, 0.35–0.82). Small-bowel feeding was found to be with a decreasing rate of pneumonia compared with nasogastric feeding (RR = 0.41; 95% CI, 0.22–0.76).

Conclusion

After TBI, early initiation of nutrition is recommended. It appears that parenteral nutrition is superior to enteral nutrition in improving outcomes. Our results lend support to the use of small-bowel feeding and immune-enhancing formulae in reducing infectious complications.

[Guidelines for specialized nutritional and metabolic support in the critically-ill patient. Update. Consensus of the Spanish Society of Intensive Care Medicine and Coronary Units-Spanish Society of Parenteral and Enteral Nutrition (SEMICYUC-SENPE): neurocritical patient]

Neurocritical patients require specialized nutritional support due to their intense catabolism and prolonged fasting. The preferred route of nutrient administration is the gastrointestinal route, especially the gastric route. Alternatives are the transpyloric route or mixed enteral-parenteral nutrition if an effective nutritional volume of more than 60% cannot be obtained. Total calore intake ranges from 20-30 kcal/kg/day, depending on the period of the clinical course, with protein intake higher than 20% of total calories (hyperproteic diet). Nutritional support should be initiated early. The incidence of gastrointestinal complications is generally higher to other critically-ill patients, the most frequent complication being an increase in gastric residual volume. As in other critically-ill patients, glycemia should be closely monitored and maintained below 150 mg/dL.

Encephalopathy in acute liver failure resulting from acetaminophen intoxication: new observations with potential therapy

OBJECTIVE

Hyperammonemia is a major contributing factor to the encephalopathy associated with liver disease. It is now generally accepted that hyperammonemia leads to toxic levels of glutamine in astrocytes. However, the mechanism by which excessive glutamine is toxic to astrocytes is controversial. Nevertheless, there is strong evidence that glutamine-induced osmotic swelling, especially in acute liver failure, is a contributing factor: the osmotic gliopathy theory. The object of the current communication is to present evidence for the osmotic gliopathy theory in a hyperammonemic patient who overdosed on acetaminophen.

DESIGN

Case report.

SETTING

Johns Hopkins Hospital.

PATIENT

A 22-yr-old woman who, 36 hrs before admission, ingested 15 g acetaminophen was admitted to the Johns Hopkins Hospital. She was treated with N-acetylcysteine. Physical examination was unremarkable; her mental status was within normal limits and remained so until approximately 72 hrs after ingestion when she became confused, irritable, and agitated.

INTERVENTIONS

She was intubated, ventilated, and placed on lactulose. Shortly thereafter, she was noncommunicative, unresponsive to painful stimuli, and exhibited decerebrate posturing. A clinical diagnosis of cerebral edema and increased intracranial pressure was made. She improved very slowly until 180 hrs after ingestion when she moved all extremities. She woke up shortly thereafter.

MEASUREMENTS AND MAIN RESULTS

Despite the fact that hyperammonemia is a major contributing factor to the encephalopathy observed in acute liver failure, the patient's plasma ammonia peaked when she exhibited no obvious neurologic deficit. Thereafter, her plasma ammonia decreased precipitously in parallel with a worsening neurologic status. She was deeply encephalopathic during a period when her liver function and plasma ammonia had normalized. Plasma glutamine levels in this patient were high but began to normalize several hours after plasma ammonia had returned to normal. The patient only started to recover as her plasma glutamine began to return to normal.

CONCLUSIONS

We suggest that the biochemical data are consistent with the osmotic gliopathy theory--high plasma ammonia leads to high plasma glutamine--an indicator of excess glutamine in astrocytes (the site of brain glutamine synthesis). This excess glutamine leads to osmotic stress in these cells. The lag in recovery of brain function presumably reflects time taken for the astrocyte glutamine concentration to return to normal. We hypothesize that an inhibitor of brain glutamine synthesis may be an effective treatment modality for acute liver failure.

Glutamine supplementation

Intravenous glutamine supplementation is standard care when parenteral nutrition is given for critical illness. There are data of a reduced mortality when glutamine supplementation is given. In addition, standard commercial products for parenteral nutrition do not contain any glutamine due to glutamine instability in aqueous solutions. For the majority of critical ill patients who are fed enterally, the available evidence is insufficient to recommend glutamine supplementation. Standard formulation of enteral nutrition contains some glutamine: 2-4 g/L. However, this dose is insufficient to normalize glutamine plasma concentration.

Plasma concentration of glutamine is low in many patients with critical illness and a low level is an independent risk factor for mortality. A low plasma glutamine concentration is the best indicator of glutamine depletion. Data are emerging about how the endogenous production of glutamine is regulated. We know that skeletal muscle is the major producer of glutamine and that a part of the profound depletion of skeletal muscle seen in critical illness is a reflection of the need to produce glutamine.

Glutamine is utilized in rapidly dividing cells in the splanchnic area. Quantitatively most glutamine is oxidized, but the availability of glutamine in surplus is important for the de novo synthesis of nucleotides and necessary for cell division and protein synthesis. More knowledge about the regulation of the endogenous production of glutamine is needed to outline better guidelines for glutamine supplementation in the future.

Amino acid composition in parenteral nutrition: what is the evidence?

PURPOSE OF REVIEW

Complete parenteral nutrition solutions contain mixed amino acid products providing all nine essential amino acids and a varying composition of nonessential amino acids. Relatively little rigorous comparative efficacy research on altered parenteral nutrition amino acid composition has been published in recent years.

RECENT FINDINGS

Limited data from randomized, double-blind, adequately powered clinical trials to define optimal doses of total or individual amino acids in parenteral nutrition are available. An exception is the growing number of studies on the efficacy of glutamine supplementation of parenteral nutrition or given as a single parenteral agent. Parenteral glutamine appears to confer benefit in selected patients; however, additional data to define optimal glutamine dosing and the patient subgroups who may most benefit from this amino acid are needed. Although some promising studies have been published, little data are available in the current era of nutrition support on the clinical efficacy of altered doses of arginine, branched chain amino acids, cysteine, or taurine supplementation of parenteral nutrition.

SUMMARY

Despite routine use of parenteral nutrition, surprisingly little clinical efficacy data are available to guide total or specific amino acid dosing in adult and pediatric patients requiring this therapy. This warrants increased attention by the research community and funding agencies to better define optimal amino acid administration strategies in patient subgroups requiring parenteral nutrition.