The relationship between fasting plasma citrulline concentration and small intestinal function in the critically ill

A scoping review considering potential biomarkers or functional measures of gastrointestinal dysfunction and enteral feeding intolerance in critically ill adults

BACKGROUND & AIM

Enteral feeding intolerance (EFI) as a result of gastrointestinal (GI) dysfunction in critically ill adults can lead to suboptimal nutritional delivery, increasing the risk of hospital acquired malnutrition. There are no validated measures of EFI or consensus as to which measures could be used to define EFI. The aim of this scoping review is to explore the validity of biomarkers, physiological or functional measures of GI dysfunction and EFI in critically ill adults characterising their use in routine clinical practice to identify those with GI dysfunction to better guide nutritional support.

METHODS

Database searches were completed in Ovid MEDLINE, Embase, CINAHL and Web of Science using the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) guidelines. The search was performed until June 2022. Articles were included if they reported original studies that identify potential biomarkers or functional measures of EFI in critically ill adults. A nine-stage process was completed to extract and complete data synthesis.

RESULTS

139 unique articles were identified. Following review of titles and abstracts, 114 of these articles were excluded, three further articles were excluded after full text review and 22 articles met the inclusion criteria. A thematic analysis of the articles included identified three overarching themes of GI dysfunction: (1) Serum biomarkers, (2) Physiological markers, and (3) Functional markers. Within the category of serum biomarkers, a further three sub-categories were identified: (i) enterohormones, (ii) markers of enterocyte function, and iii) cytokines and neurotransmitters. Some associations were seen between EFI and heparin binding protein, intra-abdominal pressure, cholecystokinin and acetylcholine levels but no markers are currently suitable for daily clinical use.

CONCLUSIONS

Further larger studies are required to characterise the relationships between serum biomarkers, physiological and functional makers of GI dysfunction in critically ill adults. A robust definition of GI dysfunction should be included in any future research.

Enteral nutrition and dynamics of citrulline and intestinal fatty acid-binding protein in adult ICU patients

BACKGROUND AND AIMS

Plasma citrulline and intestinal fatty acid binding protein (I-FABP) are biomarkers reflecting enterocyte function and intestinal mucosal injury. The aim was to describe daily dynamics of citrulline and I-FABP concentrations in association with enteral nutrition (EN) in adult ICU patients. We hypothesized that success or failure of EN is reflected by differences in citrulline and I-FABP levels at admission, as well as in daily dynamics over the first week.

METHODS

The present study was a planned sub-study of the iSOFA study (ClinicalTrials.gov Identifier: NCT02613000). With delayed informed consent we included adult (18 years or older) patients admitted for unlimited care to 5 ICUs in Europe. Citrulline and I-FABP were assessed and nutritional data recorded daily during the first week of the patients' ICU stay.

RESULTS

The study included 224 patients with 693 plasma samples analyzed for citrulline and 695 for I-FABP. The median ICU stay was 2 (IQR 1-4) days and 35 patients (15.6 %) stayed in the ICU for ≥ 7 days. The majority of patients (184/224; 82.1 %) received EN or oral nutrition (ON) during their ICU stay, in 164 patients (73.2 %) nutrition was started within 48 h of admission (early enteral or oral nutrition, EEN/ON). Median biomarker concentrations on admission were: citrulline 24.5 (IQR 18.1-31.7) μmol/L and I-FABP 2763 (1326-4805) pg/mL. Reference range for citrulline was 17-46 μmol/L and for I-FABP 377-2049 pg/mL. Patients with EEN/ON demonstrated an increase in citrulline concentrations over the first week in ICU unlike those not receiving EEN/ON (P = 0.049 for the mean log-citrulline values over time between groups) as well as higher average citrulline concentrations. Success of EEN/ON (80 % of caloric target achieved by day 4) was associated with citrulline values increasing from day 4, whereas a slight decrease was observed with unsuccessful EEN/ON. However, these dynamics over time were not statistically significantly different (P = 0.654). Patients with EEN/ON unexpectedly had I-FABP values higher than those without (average values for all days P = 0.004). Median I-FABP values on day 3 were higher with successful EEN/ON (646 (IQR 313-1116) vs 278 (IQR 190-701) pg/mL, P = 0.022).

CONCLUSIONS

EEN/ON was associated with higher values and different dynamics of citrulline over the first week in ICU. No clear difference of measured biomarkers was seen when patients were compared according to success of EEN/ON. Our study does not allow suggesting certain thresholds of citrulline nor I-FABP that could be used for bedside decision-making with regard to EN. This study was a planned sub-study of the iSOFA study (ClinicalTrials.gov Identifier: NCT02613000).

Weight-for-Height Z-score Gain during Inpatient Treatment and Subsequent Linear Growth during Outpatient Treatment of Young Children with Severe Acute Malnutrition: A Prospective Study from Uganda

BACKGROUND

Linear catch-up growth after treatment of severe acute malnutrition (SAM) is low, and little is known about the association between ponderal and subsequent linear growth.

OBJECTIVE

The study assessed the association of weight-for-height z-score (WHZ) gain with subsequent linear growth during SAM treatment and examined its modifiers.

METHODS

This was a prospective study, nested in a trial (ISRCTN16454889), among 6-59-mo-old children treated for SAM in Uganda. Weight, total length (TL), and knee-heel length (KHL) were measured at admission, weekly during inpatient therapeutic care (ITC), at discharge, and fortnightly during outpatient therapeutic care (OTC) for 8 wk. Linear regression was used to assess the association between WHZ gain during ITC and linear growth during OTC.

RESULTS

Of 400 children, 327 were discharged to OTC and 290 were followed up for 8 wk. Mean WHZ gains were 0.45 in ITC and 1.24 in OTC, whereas mean height-for-age z-score (HAZ) declined by 0.41 during ITC and increased by 0.14 during OTC. WHZ gain during ITC was positively associated with HAZ, TL, and KHL gains during OTC [regression coefficients (β) (95% CI): 0.12 (0.09, 0.15) z-score; 3.1 (2.4, 3.8) mm and 0.5 (0.1, 0.7) mm, respectively]. The regression coefficients were highest for the middle tertile of WHZ gain with respect to HAZ and TL. Admission diarrhea and low plasma citrulline reduced the association between WHZ gain during ITC and HAZ and TL gain during OTC (P < 0.001). In contrast, pneumonia (P = 0.051) and elevated plasma C-reactive protein (P < 0.001) increased the association with TL gain, but reduced the association with KHL gain (P < 0.001).

CONCLUSIONS

Among children admitted with SAM, considerable WHZ gain during ITC was followed by very modest linear catch-up growth during OTC, with no indication of a WHZ gain threshold, above which linear growth was higher. To optimize linear growth in these children, early treatment of infections and conditions affecting the gut may be necessary.

Gut dysfunction in the ICU: diagnosis and management

PURPOSE OF REVIEW

Progress has been made in our understanding of gut dysfunction in critical illness. This review will outline new findings and give perspectives based on previous knowledge and concurrent advances in nutrition.

RECENT FINDINGS

The relationship between gut dysfunction and poor outcomes in critical illness has received considerable interest. It remains uncertain whether gut dysfunction is merely a marker of illness severity or if it is directly responsible for prolonged critical illness and increased mortality. This relationship is difficult to ascertain given there is no agreed method for identification and quantification; biomarkers such as intestinal fatty acid binding protein and citrulline show promise but require further study. Recent studies have investigated strategies to deliver enteral nutrition targets with impacts on gut function, including high calorie or protein formulae, intermittent regimes and novel prokinetics.

SUMMARY

Gut dysfunction is associated with poor outcomes, but it remains uncertain whether strategies to improve gut function will influence survival and recovery.

Postprandial rise of essential amino acids is impaired during critical illness and unrelated to small-intestinal function

Background

Postprandial rise of plasma essential amino acids (EAAs) determines the anabolic effect of dietary protein. Disturbed gastrointestinal function could impair the anabolic response in critically ill patients. Aim was to investigate the postprandial EAA response in critically ill patients and its relation to small‐intestinal function.

Methods

Twenty‐one mechanically ventilated patients and 9 healthy controls received a bolus containing 100 ml of a formula feed (Ensure) and 2 g of 3‐O‐Methyl‐d‐glucose (3‐OMG) via postpyloric feeding tube. Fasting and postprandial plasma concentrations of EAAs, 3‐OMG, total bile salts, and the gut‐released hormone fibroblast growth factor 19 (FGF19) were measured over a 4‐hour period. Changes over time and between groups were assessed with linear mixed‐effects analysis. Early (0–60 minutes) and total postprandial responses are summarized as the incremental area under the curve (iAUC).

Results

At baseline, fasting EAA levels were similar in both groups: 1181 (1055–1276) vs 1150 (1065–1334) μmol·L−1, P = .87. The early postprandial rise in EAA was not apparent in critically ill patients compared with healthy controls (iAUC₆₀, −4858 [−6859 to 2886] vs 5406 [3099–16,853] µmol·L⁻¹·60 minutes; P = .039). Impaired EAA response did not correlate with impaired 3‐OMG response (Spearman ρ 0.32, P = .09). There was a limited increase in total bile salts but no relevant FGF19 response in either group.

Conclusion

Postprandial rise of EAA is blunted in critically ill patients and unrelated to glucose absorption measured with 3‐OMG. Future studies should aim to delineate governing mechanisms of macronutrient malabsorption.

Impact of the route of nutrition on gut mucosa in ventilated adults with shock: an ancillary of the NUTRIREA-2 trial

PURPOSE

The effects of the route of nutrition on the gut mucosa of patients with shock are unclear. Plasma citrulline concentration is a marker of enterocyte mass, and plasma intestinal fatty acid binding protein (I-FABP) concentration is a marker of enterocyte damage. We aimed to study the effect of the route of nutrition on plasma citrulline concentration measured at day 3 of nutrition.

MATERIALS AND METHODS

Ancillary study of the NUTRIREA-2 trial. Ventilated adults with shock were randomly assigned to receive enteral or parenteral nutrition. Enterocyte biomarkers were measured at baseline, day 3, and day 8 of nutrition.

RESULT

A total of 165 patients from 13 French ICUs were included in the study: 85 patients in the enteral group and 80 patients in the parenteral group. At baseline, plasma citrulline was low without difference between groups (12.2 µmol L^-1 vs 13.3 µmol L^-1). At day 3, plasma citrulline concentration was higher in the enteral group than in the parenteral group (18.7 µmol L^-1 vs 15.3 µmol L^-1, p = 0.01). Plasma I-FABP concentration was increased at baseline, without difference between groups (245 pg mL^-1 vs 244 pg mL^-1). Plasma I-FABP concentration was higher in the enteral group than in the parenteral group at day 3 and day 8 (158 pg mL^-1 vs 50 pg mL^-1, p = 0.005 and 225 pg mL^-1 vs 50 pg mL^-1, p = 0.03).

CONCLUSION

Plasma citrulline concentration was higher after 3 days of enteral nutrition than after 3 days of parenteral nutrition. This result raises the question of the possibility that enteral nutrition is associated with a more rapid restoration of enterocyte mass than parenteral nutrition.

Biomarkers of gut barrier failure in the ICU

PURPOSE OF REVIEW

Gut barrier failure is associated with bacterial translocation, systemic inflammation, and is presumed to be associated with the development of multiple organ dysfunction syndrome. As the gut barrier function is carried out by a monolayer of enterocytes, a minimum requirement is the integrity of the enterocytes, and controlled paracellular permeability between adjacent enterocytes. Many factors can cause critically ill patients to lose gut barrier function by a mechanism of enterocyte damage; for example, small bowel ischemia or hypoxia, sepsis, systemic inflammatory response syndrome, or absence of enteral feeding.

RECENT FINDINGS

Two enterocyte biomarkers may help the intensivist to identify enterocyte damage and dysfunction, namely plasma citrulline, a biomarker of functional enterocyte mass, and plasma or urinary intestinal fatty acid-binding protein, a marker of enterocyte damage. This review focuses on results obtained with these biomarkers in the context of critical care, in particular: prevalence of enterocyte biomarker abnormalities; mechanisms associated with enterocyte damage and dysfunction; link with systemic inflammation, bacterial translocation, and clinical intestinal dysfunction; prognostic value of enterocyte biomarkers. Lastly, we also review the limits of these biomarkers.

SUMMARY

Enterocyte biomarkers may help the intensivist to identify patients presenting with intestinal damage, and who are at risk of bacterial translocation and systemic inflammatory response syndrome, as well as those with decreased enterocyte function, at risk of malabsorption. Enterocyte biomarkers should be interpreted with caution in the critically ill and should be interpreted within the overall clinical context of the patient.

Plasma citrulline in the critically ill: intriguing biomarker, cautious interpretation

In a recent issue of Critical Care, Poole and colleagues found that plasma citrulline concentration and glucose absorption were reduced in 20 critically ill patients compared with 15 controls; however, the authors found no correlation between these two variables. This study highlights the question of the accuracy of plasma citrulline for assessing small bowel function in critically ill patients. Future studies should take into account the type of intestinal failure considered, the particular metabolism of citrulline, the time of plasma citrulline measurement, as well as the range of citrullinemia considered.