Mesenteric blood flow, glucose absorption and blood pressure responses to small intestinal glucose in critically ill patients older than 65 years

No difference in postprandial mesenteric blood flow between healthy younger and elderly individuals

We recently used phase-contrast magnetic resonance imaging (PC-MRI) to demonstrate an attenuated postprandial blood flow response in the superior mesenteric artery (SMA) in patients with Parkinson's disease compared to age- and sex-matched healthy controls. Since both groups showed substantial inter-individual variations, we extended the cohort of controls with a group of young individuals to investigate possible age-related effects. Seventeen healthy young subjects aged < 30 years and 17 elderly subjects aged > 50 years underwent serial PC-MRI to measure the postprandial blood flow response in the SMA after ingestion of a standardized liquid test meal (∼400 kcal). Postprandial blood flow dynamics in SMA did not differ between young and elderly subjects. A noticeable inter-individual variation in postprandial intestinal blood flow increase was found, and approximately 30% of the variation could be explained by the preprandial blood flow. Regardless of age, some subjects showed a remarkable transient SMA blood flow increase immediately after meal intake. This study provides tentative evidence that postprandial blood flow dynamics in SMA in healthy young and elderly subjects do not substantially differ, indicating that age is without impact on vascular response in SMA as an indicator for regulation of mesenteric perfusion in response to food intake.

The Influence of Timing in Critical Care Nutrition

Proper timing of critical care nutrition has long been a matter of controversy. Critical illness waxes and wanes in stages, creating a dynamic flux in energy needs that we have only begun to examine. Furthermore, response to nutrition support likely differs greatly at the level of the individual patient in regard to genetic status, disease stage, comorbidities, and more. We review the observational and randomized literature concerning timing in nutrition support, discuss mechanisms of harm in feeding critically ill patients, and highlight the role of precision nutrition for moving the literature beyond the realm of blunt population averages into one that accounts for the patient-specific complexities of critical illness and host genetics. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Postprandial Increase in Mesenteric Blood Flow is Attenuated in Parkinson's Disease: A Dynamic PC-MRI Study

Background:

Gastrointestinal dysfunction and related clinical symptoms are common in Parkinson’s disease (PD), but the underlying mechanisms are still poorly understood.

Objective:

In this study, we investigated how PD affects the postprandial vascular response in the splanchnic circulation.

Methods:

23 patients with PD in the “ON-medication” state and 23 age- and sex-matched healthy control participants underwent serial phase-contrast magnetic resonance imaging (PC-MRI) to measure the postprandial blood flow response in the superior mesenteric artery (SMA). Participants ingested a standardized liquid test meal (∼400 kcal) and underwent four PC-MRI runs within the following hour. Each PC-MRI run consisted of six consecutive measurements of SMA blood flow.

Results:

In both groups, standardized food intake triggered an increase of blood flow in the SMA, but absolute and relative increases in blood flow were attenuated in patients compared to the control group (p < 0.001). While baseline blood flow in the SMA was comparable in both groups, the postprandial maximum blood flow was attenuated in patients (p = 0.03). The temporal dynamics of the postprandial blood flow did not differ between groups. Postprandial SMA blood flow increase in patients correlated neither with subjective reports of non-motor symptoms or upper gastrointestinal complaints, nor with levodopa equivalent daily dose or disease duration. Blood glucose measurements in between the PC-MRI runs showed a smaller postprandial increase in blood glucose in the patient group (p = 0.006).

Conclusion:

This study provides first-time evidence that patients with PD have an attenuated postprandial blood flow response in the SMA, indicating an impaired functional regulation of gastrointestinal perfusion in response to food intake in PD.

Blinded, Double-Dummy, Parallel-Group, Phase 2a Randomized Clinical Trial to Evaluate the Efficacy and Safety of a Highly Selective 5-Hydroxytryptamine Type 4 Receptor Agonist in Critically Ill Patients With Enteral Feeding Intolerance

Background

Delayed gastric emptying is the leading cause of enteral feeding intolerance (EFI) in critical illness. This phase 2a study compared TAK‐954, a selective agonist of 5‐hydroxytryptamine type 4 receptors, with metoclopramide in critically ill patients with EFI (NCT01953081).

Methods

A blinded, double‐dummy trial was conducted in mechanically ventilated patients with EFI (>200 mL gastric residual volume within 24 hours before randomization). Patients were randomized to receive either 0.5 mg intravenous TAK‐954 over 1 hour then 0.9% saline injection 4 times/d (sham metoclopramide) or the active comparator 10 mg intravenous metoclopramide 4 times/d and a 1‐hour 0.9% saline infusion. After initial dosing, participants received a radiolabeled meal of liquid nutrient (Ensure; 106 kcal), and gastric emptying was measured (scintigraphy). Adverse events (AEs) were recorded from the time of consent through to day 5; serious AEs were collected to day 30.

Results

Thirteen patients (TAK‐954, n = 7; metoclopramide, n = 6) participated. Five patients in the TAK‐954 group and 4 in the metoclopramide group experienced AEs (2 and 3, respectively, were serious). All AEs except 1 (diarrhea in the metoclopramide group) were considered unrelated to study drug. Following treatment, a greater proportion of patients receiving TAK‐954 had normal gastric retention (<13% retention at 180 minutes) than those receiving metoclopramide (6/7 vs 3/6 patients, respectively).

Conclusion

A single dose of 0.5 mg intravenous TAK‐954 appears to have at least similar efficacy in accelerating gastric emptying to multiple doses of 10 mg metoclopramide and was not associated with increased AEs.

Postprandial hypotension in older survivors of critical illness

PURPOSE

In older people postprandial hypotension occurs frequently; and is an independent risk factor for falls, cardiovascular events, stroke and death. The primary aim of this pilot study was to estimate the frequency of postprandial hypotension and evaluate the mechanisms underlying this condition in older survivors of an Intensive Care Unit (ICU).

MATERIALS AND METHODS

Thirty-five older (>65 years) survivors were studied 3 months after discharge. After an overnight fast, participants consumed a 300 mL drink containing 75 g glucose, labelled with 20 MBq ^99mTc-calcium phytate. Patients had concurrent measurements of blood pressure, heart rate, blood glucose and gastric emptying following drink ingestion. Proportion of participants is presented as percent (95% CI) and continuous variables as mean (SD).

RESULTS

Postprandial hypotension was evident in 10 (29%; 95% CI 14-44), orthostatic hypotension in 2 (6%; 95% CI 0-13) and cardiovascular autonomic dysfunction in 2 (6%; 95% CI 0-13) participants. The maximal postprandial nadir for systolic blood pressure and diastolic blood pressures were -29 (14) mmHg and -18 (7) mmHg.

CONCLUSIONS

In this cohort of older survivors of ICU postprandial hypotension occurred frequently . This suggests that postprandial hypotension is an unrecognised issue in older ICU survivors.

Pantoprazole or Placebo for Stress Ulcer Prophylaxis (POP-UP): Randomized Double-Blind Exploratory Study

OBJECTIVES

Pantoprazole is frequently administered to critically ill patients for prophylaxis against gastrointestinal bleeding. However, comparison to placebo has been inadequately evaluated, and pantoprazole has the potential to cause harm. Our objective was to evaluate benefit or harm associated with pantoprazole administration.

DESIGN

Prospective randomized double-blind parallel-group study.

SETTING

University-affiliated mixed medical-surgical ICU.

PATIENTS

Mechanically ventilated critically ill patients suitable for enteral nutrition.

INTERVENTIONS

We randomly assigned patients to receive either daily IV placebo or pantoprazole.

MEASUREMENTS AND MAIN RESULTS

Major outcomes were clinically significant gastrointestinal bleeding, infective ventilator-associated complication or pneumonia, and Clostridium difficile infection; minor outcomes included overt bleeding, hemoglobin concentration profiles, and mortality. None of the 214 patients randomized had an episode of clinically significant gastrointestinal bleeding, three patients met the criteria for either an infective ventilator-associated complication or pneumonia (placebo: 1 vs pantoprazole: 2), and one patient was diagnosed with Clostridium difficile infection (0 vs 1). Administration of pantoprazole was not associated with any difference in rates of overt bleeding (6 vs 3; p = 0.50) or daily hemoglobin concentrations when adjusted for transfusion rates of packed red cells (p = 0.66). Mortality was similar between groups (log-rank p = 0.33: adjusted hazard ratio for pantoprazole: 1.68 [95% CI, 0.97-2.90]; p = 0.06).

CONCLUSIONS

We found no evidence of benefit or harm with the prophylactic administration of pantoprazole to mechanically ventilated critically ill patients anticipated to receive enteral nutrition. The practice of routine administration of acid-suppressive drugs to critically ill patients for stress ulcer prophylaxis warrants further evaluation.

Nutrient stimulation of mesenteric blood flow - implications for older critically ill patients

Nutrient ingestion induces a substantial increase in mesenteric blood flow. In older persons (aged ≥ 65 years), particularly those with chronic medical conditions, the cardiovascular compensatory response may be inadequate to maintain systemic blood pressure during mesenteric blood pooling, leading to postprandial hypotension. In older ambulatory persons, postprandial hypotension is an important pathophysiological condition associated with an increased propensity for syncope, falls, coronary vascular events, stroke and death. In older critically ill patients, the administration of enteral nutrition acutely increases mesenteric blood flow, but whether this pathophysiological response is protective, or precipitates mesenteric ischaemia, is unknown. There are an increasing number of older patients surviving admission to intensive care units, who are likely to be at increased risk of postprandial hypotension, both during, and after, their stay in hospital. In this review, we describe the prevalence, impact and mechanisms of postprandial hypotension in older people and provide an overview of the impact of postprandial hypotension on feeding prescriptions in older critically ill patients. Finally, we provide evidence that postprandial hypotension is likely to be an unrecognised problem in older survivors of critical illness and discuss potential options for management.

Occult upper gastrointestinal mucosal abnormalities in critically ill patients

BACKGROUND

The objectives of this study were to estimate the frequency of occult upper gastrointestinal abnormalities, presence of gastric acid as a contributing factor, and associations with clinical outcomes.

METHODS

Data were extracted for study participants at a single centre who had an endoscopy performed purely for research purposes and in whom treating physicians were not suspecting gastrointestinal bleeding. Endoscopic data were independently adjudicated by two gastroenterologists who rated the likelihood that observed pathological abnormalities were related to gastric acid secretion using a 3-point ordinal scale (unlikely, possible or probable).

RESULTS

Endoscopy reports were extracted for 74 patients [age 52 (37, 65) years] undergoing endoscopy on day 5 [3, 9] of ICU admission. Abnormalities were found in 25 (34%) subjects: gastritis/erosions in 10 (14%), nasogastric tube trauma in 8 (11%), oesophagitis in 4 (5%) and non-bleeding duodenal ulceration in 3 (4%). The contribution of acid secretion to observed pathology was rated 'probable' in six subjects (rater #1) and five subjects (rater #2). Prior to endoscopy, 39 (53%) patients were receiving acid-suppressive therapy. The use of acid-suppressive therapy was not associated with the presence of an endoscopic abnormality (present 15/25 (60%) vs. absent 24/49 (49%); P = 0.46). Haemoglobin concentrations, packed red cells transfused and mortality were not associated with mucosal abnormalities (P = 0.83, P > 0.9 and P > 0.9 respectively).

CONCLUSIONS

Occult mucosal abnormalities were observed in one-third of subjects. The presence of mucosal abnormalities appeared to be independent of prior acid-suppressive therapy and was not associated with reduced haemoglobin concentrations, increased transfusion requirements, or mortality.

Critical Illness Is Associated With Impaired Gallbladder Emptying as Assessed by 3D Ultrasound

OBJECTIVE

To quantify gallbladder dysfunction during critical illness.

DESIGN

Prospective observational comparison study of nutrient-stimulated gallbladder emptying in health and critical illness.

SETTING

Single-centre mixed medical/surgical ICU.

PATIENTS

Twenty-four mechanically ventilated critically ill patients suitable to receive enteral nutrition were compared with 12 healthy subjects.

INTERVENTIONS

Participants were studied after an 8-hour fast. Between 0 and 120 minutes, high-fat nutrient (20% intralipid) was infused via a postpyloric catheter into the duodenum at 2 kcal/min.

MEASUREMENTS AND MAIN RESULTS

Three-dimensional images of the gallbladder were acquired at 30-minute intervals from -30 to 180 minutes. Ejection fraction (%) was calculated as changes between 0 and 120 minutes. Blood samples were obtained at 30-minute intervals for plasma cholecystokinin. Data are mean (SD) or median [interquartile range]. In the critically ill, fasting gallbladder volumes (critically ill, 61 mL [36-100 mL] vs healthy, 22 mL [15-25] mL; p < 0.001] and wall thickness (0.45 mm [0.15 mm] vs 0.26 mm [0.08 mm]; p < 0.001] were substantially greater, and sludge was evident in the majority of patients (71% vs 0%). Nutrient-stimulated emptying was incomplete in the critically ill after 120 minutes but was essentially complete in the healthy individuals (22 mL [9-66 mL] vs 4 mL [3-5 mL]; p < 0.01]. In five critically ill patients (21%), there was no change in gallbladder volume in response to nutrient, and overall ejection fraction was reduced in the critically ill (50% [8-83%] vs 77 [72-84%]; p = 0.01]. There were no differences in fasting or incremental cholecystokinin concentrations.

CONCLUSIONS

Fasted critically ill patients have larger, thicker-walled gallbladders than healthy subjects and nutrient-stimulated gallbladder emptying is impaired with "gallbladder paresis" occurring in approximately 20%.

A computer model simulating human glucose absorption and metabolism in health and metabolic disease states

A computer model designed to simulate integrated glucose-dependent changes in splanchnic blood flow with small intestinal glucose absorption, hormonal and incretin circulation and hepatic and systemic metabolism in health and metabolic diseases e.g. non-alcoholic fatty liver disease, (NAFLD), non-alcoholic steatohepatitis, (NASH) and type 2 diabetes mellitus, (T2DM) demonstrates how when glucagon-like peptide-1, (GLP-1) is synchronously released into the splanchnic blood during intestinal glucose absorption, it stimulates superior mesenteric arterial (SMA) blood flow and by increasing passive intestinal glucose absorption, harmonizes absorption with its distribution and metabolism. GLP-1 also synergises insulin-dependent net hepatic glucose uptake (NHGU). When GLP-1 secretion is deficient post-prandial SMA blood flow is not increased and as NHGU is also reduced, hyperglycaemia follows. Portal venous glucose concentration is also raised, thereby retarding the passive component of intestinal glucose absorption.   Increased pre-hepatic sinusoidal resistance combined with portal hypertension leading to opening of intrahepatic portosystemic collateral vessels are NASH-related mechanical defects that alter the balance between splanchnic and systemic distributions of glucose, hormones and incretins.The model reveals the latent contribution of portosystemic shunting in development of metabolic disease. This diverts splanchnic blood content away from the hepatic sinuses to the systemic circulation, particularly during the glucose absorptive phase of digestion, resulting in inappropriate increases in insulin-dependent systemic glucose metabolism.  This hastens onset of hypoglycaemia and thence hyperglucagonaemia. The model reveals that low rates of GLP-1 secretion, frequently associated with T2DM and NASH, may be also be caused by splanchnic hypoglycaemia, rather than to intrinsic loss of incretin secretory capacity. These findings may have therapeutic implications on GLP-1 agonist or glucagon antagonist usage.

Peri-operative nutrition

Patients are frequently malnourished or are at risk of malnutrition before surgery. Peri-operative nutritional support can improve their outcomes. This review focuses on new developments in peri-operative nutrition, including: patient preparation and pre-operative fasting; the role of nutritional supplementation; the optimal route and timing of nutrient delivery; and the nutritional management of specific groups including critically ill, obese and elderly patients.

Effect of Critical Illness on Triglyceride Absorption

BACKGROUND

Adequate nutrition support for critically ill patients optimizes outcome, and enteral feeding is the preferred route of nutrition. Small intestinal glucose absorption is frequently impaired in critical illness. Despite lipid being a major constituent of liquid nutrient administered, there is little information about lipid absorption during critical illness.

OBJECTIVES

To determine small intestinal lipid, as well as glucose, absorption in critical illness compared with health.

MATERIALS AND METHODS

Twenty-nine mechanically ventilated critically ill patients and 16 healthy volunteers were studied. Liquid nutrient (60 mL, 1 kcal/mL), containing 200 µL (13)C-triolein and 3 g 3-O-methyl-glucose (3-OMG), was infused directly into the duodenum at a rate of 2 kcal/min. Exhaled (13)CO2 and serum 3-OMG concentrations were measured at timed intervals over 360 minutes. Lipid absorption was measured as the cumulative percentage dose (cPDR) of (13)CO2 recovered at 360 minutes. Glucose absorption was measured as the area under the 3-OMG concentration curve. Data are median (range) and analyzed using the Mann-Whitney U and Pearson correlation tests.

RESULTS

Lipid absorption was markedly less in the critically ill (cPDR(13)CO2: patients, 22.6% [0%-100%] vs healthy participants, 40.7% [5.3%-84.7%]; P = .018). While glucose absorption was less at 60 minutes in the critically ill (3-OMG60: 13.2 [3.5-29.5] vs 21.1 [9.3-31.9] mmol/L·min; P = .003), this was not apparent at 360 minutes (3-OMG360: 92.7 [54.5-147.9] vs 107.9 [64.0-168.7] mmol/L·min; P = .126). There was no relationship between lipid and glucose absorption.

CONCLUSION

Small intestinal absorption of lipid is diminished during critical illness.

Measurement of gastric emptying in the critically ill

BACKGROUND & AIMS

Enteral nutrition is important in critically ill patients and is usually administered via a nasogastric tube. As gastric emptying is frequently delayed, and this compromises the delivery of nutrient, it is important that the emptying rate can be quantified.

METHODS

A comprehensive search of MEDLINE/PubMed, of English articles, from inception to 1 July 2014. References of included manuscripts were also examined for additional studies.

RESULTS

A number of methods are available to measure gastric emptying and these broadly can be categorised as direct- or indirect-test and surrogate assessments. Direct tests necessitate visualisation of the stomach contents during emptying and are unaffected by liver or kidney metabolism. The most frequently used direct modality is scintigraphy, which remains the 'gold standard'. Indirect tests use a marker that is absorbed in the proximal small intestine, so that measurements of the marker, or its metabolite measured in plasma or breath, correlates with gastric emptying. These tests include drug and carbohydrate absorption and isotope breath tests. Gastric residual volumes (GRVs) are used frequently to quantify gastric emptying during nasogastric feeding, but these measurements may be inaccurate and should be regarded as a surrogate measurement. While the inherent limitations of GRVs make them less suitable for research purposes they are often the only technique that is available for clinicians at the bedside.

CONCLUSIONS

Each of the available techniques has its strength and limitations. Accordingly, the choice of gastric emptying test is dictated by the particular requirement(s) and expertise of the investigator or clinician.

Gastrointestinal dysfunction relating to the provision of nutrition in the critically ill

PURPOSE OF REVIEW

During critical illness, enteral nutrition remains central to clinical care and an understanding of gut dysfunction is therefore important. Contemporary data have contributed to our knowledge in this area and this review will concentrate on recently published studies.

RECENT FINDINGS

It is difficult to precisely measure gastric emptying and nutrient absorption as part of routine clinical care. However, techniques for the measurement of these parameters for research purposes have been refined, studied and validated. These methodologies allow the evaluation of novel treatments that modulate gastric emptying. Quantification and an understanding of the mechanisms of nutrient malabsorption may facilitate the development of therapeutic agents to improve absorption and/or formulae, which are more readily absorbed, thereby improving nutritional and clinical outcomes.

SUMMARY

Improved understanding of gut pathophysiology in critical illness provides opportunities for the development and testing of novel and targeted treatment strategies, with the objective to improve clinical outcomes in this group.

Intestinal blood flow in patients with chronic heart failure: a link with bacterial growth, gastrointestinal symptoms, and cachexia

BACKGROUND

Blood flow in the intestinal arteries is reduced in patients with stable heart failure (HF) and relates to gastrointestinal (GI) symptoms and cardiac cachexia.

OBJECTIVES

The aims of this study were to measure arterial intestinal blood flow and assess its role in juxtamucosal bacterial growth, GI symptoms, and cachexia in patients with HF.

METHODS

A total of 65 patients and 25 controls were investigated. Twelve patients were cachectic. Intestinal blood flow and bowel wall thickness were measured using ultrasound. GI symptoms were documented. Bacteria in stool and juxtamucosal bacteria on biopsies taken during sigmoidoscopy were studied in a subgroup by fluorescence in situ hybridization. Serum lipopolysaccharide antibodies were measured.

RESULTS

Patients showed 30% to 43% reduced mean systolic blood flow in the superior and inferior mesenteric arteries and celiac trunk (CT) compared with controls (p < 0.007 for all). Cachectic patients had the lowest blood flow (p < 0.002). Lower blood flow in the superior mesenteric artery and CT was correlated with HF severity (p < 0.04 for all). Patients had more feelings of repletion, flatulence, intestinal murmurs, and burping (p < 0.04). Burping and nausea or vomiting were most severe in patients with cachexia (p < 0.05). Patients with lower CT blood flow had more abdominal discomfort and immunoglobulin A-antilipopolysaccharide (r = 0.76, p < 0.02). Antilipopolysaccharide response was correlated with increased growth of juxtamucosal but not stool bacteria. Patients with intestinal murmurs had greater bowel wall thickness of the sigmoid and descending colon, suggestive of edema contributing to GI symptoms (p < 0.05). In multivariate regression analysis, lower blood flow in the superior mesenteric artery, CT (p < 0.04), and inferior mesenteric artery (p = 0.056) was correlated with the presence of cardiac cachexia.

CONCLUSIONS

Intestinal blood flow is reduced in patients with HF. This may contribute to juxtamucosal bacterial growth and GI symptoms in patients with advanced HF complicated by cachexia.

Glucose absorption in small intestinal diseases

Recent developments in the field of diabetes and obesity management have established the central role of the gut in glucose homeostasis; not only is the gut the primary absorptive site, but it also triggers neurohumoral feedback responses that regulate the pre- and post-absorptive phases of glucose metabolism. Structural and/or functional disorders of the intestine have the capacity to enhance (e.g.: diabetes) or inhibit (e.g.: short-gut syndrome, critical illness) glucose absorption, with potentially detrimental outcomes. In this review, we first describe the normal physiology of glucose absorption and outline the methods by which it can be quantified. Then we focus on the structural and functional changes in the small intestine associated with obesity, critical illness, short gut syndrome and other malabsorptive states, and particularly Type 2 diabetes, which can impact upon carbohydrate absorption and overall glucose homeostasis.

The effects of critical illness on intestinal glucose sensing, transporters, and absorption

OBJECTIVES

Providing effective enteral nutrition is important during critical illness. In health, glucose is absorbed from the small intestine via sodium-dependent glucose transporter-1 and glucose transporter-2, which may both be regulated by intestinal sweet taste receptors. We evaluated the effect of critical illness on glucose absorption and expression of intestinal sodium-dependent glucose transporter-1, glucose transporter-2, and sweet taste receptors in humans and mice.

DESIGN

Prospective observational study in humans and mice.

SETTING

ICU and university-affiliated research laboratory.

SUBJECTS

Human subjects were 12 critically ill patients and 12 healthy controls. In the laboratory 16-week-old mice were studied.

INTERVENTIONS

Human subjects underwent endoscopy. Glucose (30 g) and 3-O-methylglucose (3 g), used to estimate glucose absorption, were infused intraduodenally over 30 minutes. Duodenal mucosa was biopsied before and after infusion. Mice were randomized to cecal ligation and puncture to model critical illness (n = 16) or sham laparotomy (control) (n = 8). At day 5, mice received glucose (100 mg) and 3-O-methylglucose (10 mg) infused intraduodenally prior to mucosal tissue collection.

MEASUREMENTS AND MAIN RESULTS

Quantitative polymerase chain reaction was performed to measure absolute (human) and relative levels of sodium-dependent glucose transporter-1, glucose transporter-2, and taste receptor type 1 member 2 (T1R2) transcripts. Blood samples were assayed for 3-O-methylglucose to estimate glucose absorption. Glucose absorption was three-fold lower in critically ill humans than in controls (p = 0.002) and reduced by a similar proportion in cecal ligation and puncture mice (p = 0.004). In critically ill patients, duodenal levels of sodium-dependent glucose transporter-1, glucose transporter-2, and T1R2 transcript were reduced 49% (p < 0.001), 50% (p = 0.009), and 85% (p = 0.007), whereas in the jejunum of cecal ligation and puncture mice sodium-dependent glucose transporter-1, glucose transporter-2, and T1R2 transcripts were reduced by 55% (p < 0.001), 50% (p = 0.002), and 69% (p = 0.004).

CONCLUSIONS

Critical illness is characterized by markedly diminished glucose absorption, associated with reduced intestinal expression of glucose transporters (sodium-dependent glucose transporter-1 and glucose transporter-2) and sweet taste receptor transcripts. These changes are paralleled in cecal ligation and puncture mice.