The role of luminal nutrients in intestinal injury from mesenteric reperfusion and platelet-activating factor in the developing rat

The Practice of Enteral Nutrition: Clinical Evidence for Feeding Protocols

Establishing full enteral nutrition in critically ill preterm infants with immature gastrointestinal function is challenging. In this article, we will summarize emerging clinical evidence from randomized clinical trials suggesting the feasibility and efficacy of feeding interventions targeting the early establishment of full enteral nutrition. We will also examine trial outcomes of higher volume feedings after the establishment of full enteral nutrition. Only data from randomized clinical trials will be discussed extensively. Future opportunities for clinical research will also be presented.

Lipid Composition, Digestion, and Absorption Differences among Neonatal Feeding Strategies: Potential Implications for Intestinal Inflammation in Preterm Infants

Necrotizing enterocolitis (NEC) is a significant cause of morbidity and mortality in the neonatal population. Formula feeding is among the many risk factors for developing the condition, a practice often required in the cohort most often afflicted with NEC, preterm infants. While the virtues of many bioactive components of breast milk have been extolled, the ability to digest and assimilate the nutritional components of breast milk is often overlooked. The structure of formula differs from that of breast milk, both in lipid composition and chemical configuration. In addition, formula lacks a critical digestive enzyme produced by the mammary gland, bile salt-stimulated lipase (BSSL). The gastrointestinal system of premature infants is often incapable of secreting sufficient pancreatic enzymes for fat digestion, and pasteurization of donor milk (DM) has been shown to inactivate BSSL, among other important compounds. Incompletely digested lipids may oxidize and accumulate in the distal gut. These lipid fragments are thought to induce intestinal inflammation in the neonate, potentially hastening the development of diseases such as NEC. In this review, differences in breast milk, pasteurized DM, and formula lipids are highlighted, with a focus on the ability of those lipids to be digested and subsequently absorbed by neonates, especially those born prematurely and at risk for NEC.

The science and necessity of using animal models in the study of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) remains one of the highest causes of mortality and of acute and long-term morbidity in premature infants. Multiple factors are involved in the pathophysiology of NEC including the immaturity of the immune system and the complex changing composition of the intestinal microbiome. This is compounded by the fact that the premature infant should ideally still be a developing fetus and has an immature intestinal tract. Because these complexities are beyond the scope of studies in single-cell cultures, animal models are absolutely essential to understand the mechanisms involved in the pathophysiology of NEC and the effects of inflammation on the immature intestinal tract. To this end, investigators have utilized many different species (e.g., rats, mice, rabbits, quails, piglets, and non-human primates) and conditions to develop models of NEC. Each animal has distinct advantages and drawbacks related to its preterm viability, body size, genetic variability, and cost. The choice of animal model is strongly influenced by the scientific question being addressed. While no model perfectly mimics human NEC, each has greatly improved our understanding of disease. Examples of recent discoveries in NEC pathogenesis and prevention underscore the importance of continued animal research in NEC.

Mathematical modeling in necrotizing enterocolitis--a new look at an ongoing problem

Necrotizing enterocolitis (NEC) is the most common and lethal disease that affects the gastrointestinal (GI) tract of the premature infant. The etiology of NEC remains undefined. The only consistent epidemiological precursors for NEC are prematurity and enteral alimentation. Various inflammatory mediators, including tumor necrosis factor (TNF)-a, interleukin (IL)-1, IL-6, IL-8, IL-10, IL-18, platelet-activating factor (PAF), and nitric oxide (NO) have been implicated in the pathogenesis of NEC, but the kinetics and role of these agents are ill-defined. Currently, there are no biomarker predictors of NEC risk and severity. Sera or tissue from early time points in the development of the disease may help delineate early inflammatory events that predispose an individual to NEC, thus providing an interventional opportunity. We suggest that the lack of diagnostic and therapeutic modalities for NEC are due to the absence of a systems view of the disease, which in turn is hindered by a lack of sensitive physiological measurements that predict perturbations in the intestinal tissue compartment and an inability to reliably test serial samples for the presence of inflammatory mediators in small volumes and in a high-throughput manner. Computational modeling is a useful tool in the study of complex systems such as the inflammatory process. Computation models provide an "existence proof" for a given mechanism, uncover subtle inconsistencies between the underlying hypotheses and quantitative data, and force one to ask how much is known. We suggest that a properly validated and calibrated mathematical model of inflammation and its pathologic consequences in NEC will be useful for predicting the physiologic and biologic response in infants suffering from the disease.

Assessment of cysteine synthesis in very low-birth weight neonates using a [13C6]glucose tracer

BACKGROUND/PURPOSE

Cysteine is an amino acid necessary for the synthesis of all proteins, the antioxidant glutathione, and the neuromodulator taurine. Whether cysteine is an essential amino acid for premature neonates remains controversial. Using a [13C6]glucose precursor in very-low-birth weight (VLBW) premature neonates, we measured the 13C content of cysteine in hepatically derived apolipoprotein (apo) B-100 and in the plasma to determine whether cysteine synthesis occurs and to relate minimum synthetic capacity to neonatal maturity.

METHODS

Twelve VLBW premature neonates (birth weight, 907 +/- 274 [SD] g; gestational age, 26.8 +/- 2.4 weeks) were studied on day of life 7.8 +/- 4.2 while on total parenteral nutrition (TPN) for 5.6 +/- 4.5 days. A 4-hour intravenous infusion of [13C6]glucose was administered. Blood samples were obtained immediately before and at the end of the infusion. Isotopic enrichment of cysteine was determined by gas chromatography/mass spectrometry. Analysis of variance, Student's t test, and linear regression were used for comparisons.

RESULTS

The 13C isotope ratio of apo B-100-derived cysteine after the [13C6]glucose infusion was significantly higher than baseline (18.57 +/- 0.38 [SEM] vs 17.54 +/- 0.25 mol%, P < .05). The 13C isotope ratio of plasma cysteine was also significantly higher than baseline (17.36 +/- 0.25 vs 16.91 +/- 0.16 mol%, P < .05). When expressed as a product/precursor ratio, the mole percent above baseline of [13C]apo B-100 cysteine/[13C6]glucose correlated with birth weight (r = 0.74, P < .01).

CONCLUSIONS

Very low-birth weight neonates are capable of cysteine synthesis as evidenced by incorporation of 13C label into hepatically derived apo B-100 cysteine and plasma cysteine from a glucose precursor. The minimum capacity for intrahepatic cysteine synthesis appears to be directly proportional to the maturity of the neonate and may impact the capabilities of VLBW neonates to counteract oxidative stresses such as bronchopulmonary dysplasia and necrotizing enterocolitis.

The role of the glutathione antioxidant system in gut barrier failure in a rodent model of experimental necrotizing enterocolitis

BACKGROUND

Evidence suggests that intestinal barrier failure in necrotizing enterocolitis results in part from overproduction of nitric oxide and other toxic oxidant species that result in enterocyte death and intestinal barrier failure. We hypothesize that the glutathione detoxifying system is important in maintaining intestinal barrier integrity by protecting against nitrosative stress.

METHODS

Newborn rats were subjected to hypoxia (5% O2, tid) and fed formula by gavage (NEC), or were breast-fed without hypoxia (BF). Rats were killed and the distal ilea were harvested for RNA, protein, and morphologic studies. RNA underwent cDNA microarray analysis. To assess glutathione in protecting against nitrosative stress, IEC-6 cells were exposed to SIN-1 and/or L-buthionine-(S,R)-sulfoximine (BSO), a glutathione inhibitor. Cells were analyzed for glutathione-S-transferase activity, apoptosis and mitochondrial function.

RESULTS

BF controls developed normal intestinal architecture, whereas NEC animals sustained damage to the intestinal epithelium. Microarray analysis demonstrated that 93 genes were overexpressed in NEC compared with controls. In the array, glutathione-S-transferase pi and alpha 2, GSH-dependent detoxifying enzymes, RNA were upregulated compared with BF controls. IEC-6 cells exposed to SIN-1/BSO produced an increase in apoptosis. Poly ADP-ribosylpolymerase cleavage and apoptosis-inducing factor (AIF) nuclear localization, markers of apoptosis, were seen in IEC-6 cells exposed to SIN-1/BSO as opposed to media controls.

CONCLUSION

These data support the hypothesis that GSH antioxidant system plays a crucial role in gut barrier protection by attenuating enterocyte death.

Role of redox in fetal development and neonatal diseases

In the cell, reducing and oxidizing molecules modulate the redox state. In embryonic and fetal growth, increased oxidative stress may be detrimental, but an oxidized state can also be beneficial. This is because redox may also affect key transcription factors that can alter gene expression during development. In addition, redox may impact on placentation and amniotic membrane integrity during pregnancy. Lastly, diseases of prematurity, such as necrotizing enterocolitis, retinopathy of prematurity, and chronic lung disease, may be modulated by redox in the premature. Because antioxidant therapies have not necessarily modified the outcome of these diseases, some debate exists as to this. Nonetheless, sufficient evidence suggests a role for redox throughout embryonic, fetal, and postnatal development. This evidence will be explored here.

Necrotizing enterocolitis in infants with very low birth weight

Necrotizing enterocolitis (NEC) is a disease in which the primary risk factor is prematurity. Despite, and partially as a result of, the tremendous strides neonatal care has taken, it is a major cause of morbidity and mortality of the newborn. The infant with very low birth weight is particularly susceptible, and the management of the condition in this group differs somewhat from other neonates. The outcomes continue to improve, but there are significant sequelae. Prevention, which would be the best "cure," is elusive, in no small part because of the multifactorial nature of the etiology of NEC.

Carboxyl ester lipase activity in milk prevents fat-derived intestinal injury in neonatal mice

Carboxyl ester lipase (bile salt-stimulated lipase) is a pancreatic enzyme capable of hydrolyzing esters of cholesterol and fat-soluble vitamins. It also efficiently digests triglycerides (TG) into free fatty acids and glycerol and is abundant in the milk of humans and several other species. We used the mouse as a model to test the hypothesis that milk-derived carboxyl ester lipase (CEL) digests milk TG and that without its activity milk lipids and their digestion intermediates can disrupt the intestinal epithelium of neonates. CEL protein and enzymatic activity were shown to be abundant in mouse milk. After 24-h administration of the CEL-specific inhibitor, WAY-121,751-5, the small intestines of treated and control neonates were analyzed histologically for signs of fat malabsorption and injury to their villus epithelium. In vehicle-fed controls, TG were digested and absorbed in the duodenum and jejunum, whereas, in inhibitor-fed littermates, large intracellular neutral lipid droplets accumulated in enterocytes of the ileum, resulting in damage to the villus epithelium. Similar results were observed in neonates nursed by CEL knockout females compared with heterozygous controls. The results suggest that lack of CEL activity causes incomplete digestion of milk fat and lipid accumulation by enterocytes in the ileum of neonatal mice.

Endotoxin and transient hypoxia cause severe acidosis in the piglet

PURPOSE

Both hypoxia and gram-negative sepsis are thought to play a role in the development of necrotizing enterocolitis (NEC). Endotoxin, a lipopolysaccharide (LPS), is a potent mediator of gram-negative sepsis. The author investigated the effect of LPS and hypoxia on arterial and mesenteric venous blood gas values in a piglet model.

METHODS

16 piglets (mean age, 9 days; mean weight, 3.2 kg) were anesthetized and mechanically ventilated. Catheters were placed in the aorta and the superior mesenteric vein (SMV). After a 30-minute stabilization period, piglets were randomly assigned to four experimental groups: normoxic ventilation (FIO2, 0.21), normoxic ventilation and LPS infusion (200 microg/kg, intravenously), hypoxic ventilation (FIO2, 0.10 for 20 minutes), or hypoxic ventilation and LPS infusion. All subjects were then monitored for an additional 30 minutes (recovery period). Multiple, paired blood gas samples were obtained from the aorta and SMV during the stabilization, experimental, and recovery periods.

RESULTS

Piglets subjected to both hypoxia and LPS experienced a much more severe acidosis in both the aorta (pH, 7.10 +/- 0.08) and SMV (pH, 7.03 +/- 0.09) than piglets subjected to either hypoxia or LPS alone (P < .05). In addition, LPS lowered the arterial oxygen saturation in piglets exposed to acute, transient hypoxia (36 +/- 4% v 59 +/- 12%, P < .05).

CONCLUSION

This study suggests that the combination of transient hypoxia and gram-negative sepsis may act synergistically to produce both a severe acidosis and decreased tissue oxygenation.