Minimal enteral nutrient requirements for intestinal growth in neonatal piglets: how much is enough?

Early aggressive nutrition in preterm infants

Increasingly, neonatologists are realizing that current feeding practices for preterm infants are insufficient to produce reasonable rates of growth, and earlier and larger quantities of both parenteral and enteral feeding should be provided to these infants. Unfortunately, there is very little outcome data to recommend any particular nutritional strategy to achieve better growth. Instead, the rationale for feeding regimens in many nurseries has been quite variably extrapolated from animal data and human studies conducted in gestationally more mature and/or stable neonates. Additionally, there are no well-controlled, prospective studies that validate any nutritional regimen for the very preterm and or sick, unstable neonate. The goal of this review is to present available data to help define the risks and benefits of early parenteral and enteral nutrition, particularly in very preterm neonates, concluding with a more aggressive approach to feeding these infants than has been customary practice.

Oral IGF-I alters the posttranslational processing but not the activity of lactase-phlorizin hydrolase in formula-fed neonatal pigs

To determine the cellular mechanism whereby oral insulin-like growth factor I (IGF-I) increases intestinal lactase-phlorizin hydrolase (LPH) activity, we studied 2-d-old pigs fed cow's milk formula (control, n = 5), formula + low IGF-I (0.5 mg/L; n = 6) or formula + high IGF-I (12.0 mg/L, n = 6) for 15 d. On d 15, intestinal protein synthesis and lactase processing were measured in vivo in fed pigs using a 6-h intravenous, overlapping infusion of multiple stable isotopes (2H(3)-Leu, 13C(1)-Leu, 13C(1)-Phe, 2H(5)-Phe, 13C(6)-Phe and 13C(9)-Phe). Morphometry and cell proliferation also were measured in the jejunum and ileum. Neither dose of IGF-I affected the masses of wet tissue, protein or DNA, or the villus height, cell proliferation or LPH-specific activity. Oral IGF-I decreased the synthesis and abundance of prolactase-phlorizin hydrolase (pro-LPH), but increased brush-border (BB)-LPH synthesis in the ileum. The BB-LPH processing efficiency was twofold to threefold greater in IGF-fed than in control pigs. In all pigs, villus height and the total mucosal and specific activity of LPH activity were greater in the ileum than in the jejunum, yet the synthesis of BB-LPH were significantly lower in the ileum than in the jejunum. We conclude that oral IGF-I increases the processing efficiency of pro-LPH to BB-LPH but does not affect LPH activity. Moreover, the posttranslational processing of BB-LPH is markedly lower in the ileum than in the jejunum.

Glutamine and the bowel

Since the pioneering work of Windmueller and Spaeth, the importance of glutamine to the support of intestinal mucosal metabolic function has become generally accepted. Nevertheless, the mechanisms underlying this role still remain obscure. This paper explores a number of questions: 1) Is glutamine essential for intestinal function? 2) To what extent does this relate to its intermediary metabolism? 3) What is the importance of glutamine as a biosynthetic precursor? 4) Is glutamine supplementation of the nutrient mixture presented to patients of any metabolic or clinical benefit? As a result of this exploratory exercise, the following general conclusions were reached: 1) Much suggestive biochemical and physiologic evidence exists that implies that glutamine, especially systemic glutamine, supports the function of the intestinal mucosal system. 2) Despite the extensive metabolism of this amino acid by the intestinal tissues, most evidence suggests that if glutamine does play a physiologic role in the bowel, it is not compellingly related to its intermediary metabolism. 3) There is, on the other hand, evidence that the mucosal cells not only utilize extracellular glutamine but synthesize the amino acid. Given that inhibition of glutamine synthesis inhibits both proliferation and differentiation of mucosal cell cultures, this suggests some more subtle regulatory role. This notion is supported by the demonstration that glutamine will activate a number of genes associated with cell cycle progression in the mucosa. 4) Despite the accumulated evidence, the mechanisms underlying glutamine's function and the question whether glutamine supplementation uniformly benefits mucosal health remain equivocal at best.

Glucagon-like peptide 2: a nutrient-responsive gut growth factor

Glucagon-like peptide 2 (GLP-2) is a 33-amino acid peptide derived from the tissue-specific, post-translational processing of the proglucagon gene expressed in the intestinal enteroendocrine L-cell. The primary stimulus for GLP-2 secretion is nutrient intake, and involves direct luminal stimulation of the L-cell as well as indirect enteroendocrine and neural mechanisms. The biological activity of GLP-2 in circulation is regulated by the proteolytic cleavage of the N-terminus by dipeptidylpeptidase IV. Several studies have shown that GLP-2 has specific trophic effects on the small and large intestine, which are mediated by stimulation of cell proliferation and inhibition of apoptosis and proteolysis. GLP-2 also has been shown to suppress gastric motility and acid secretion, increase hexose transport activity and suppress food intake, specifically when infused centrally. The actions of GLP-2 are mediated by a G-protein-linked, membrane receptor (GLP-2R) that is localized largely to the gastrointestinal tract, but also is found in the brain. The secretion of GLP-2 and expression of the GLP-2R are present in the late gestation fetus. However, the developing intestine does not become responsive to the trophic effect of GLP-2 until after birth. Based on its efficacy in preventing atrophy and stimulating growth in the neonatal gut, GLP-2 may be a promising therapeutic adjuvant for treatment of infants with compromised gut function.

Secretion of trophic gut peptides is not different in bolus- and continuously fed piglets

In neonates, bolus feeding is associated with greater rates of intestinal growth than is continuous feeding. We tested whether the concentrations and secretion rates of trophic gut peptides are higher in bolus-fed than in continuously fed piglets. Five 21-d-old piglets were surgically implanted with gastric, arterial and portal catheters and a portal blood flow probe. At postnatal d 30 and 31, pigs received an equal amount of primed continuous or bolus feeding of a cow's milk formula in a randomized, crossover design. During a 6-h period, portal blood flow and arterial and portal concentrations of glucagon-like peptide-2 (GLP-2), peptide YY (PYY) and gastric inhibitory polypeptide (GIP) were measured. All hormone levels were significantly increased within 1 h of the start of the experiment, independent of the feeding modality. There were no differences between bolus and continuous feeding in either the arterial concentrations or secretion rates of GLP-2, PYY and GIP. In both treatment groups, the increases in the plasma concentrations of GLP-2 and GIP after feeding were substantially greater than those for PYY. We conclude that the production or circulating concentrations of GLP-2, PYY and GIP are not significantly different in bolus- and primed continuously fed piglets.

GLP-2 stimulates intestinal growth in premature TPN-fed pigs by suppressing proteolysis and apoptosis

We wished to determine whether exogenous glucagon-like peptide (GLP)-2 infusion stimulates intestinal growth in parenterally fed immature pigs. Piglets (106-108 days gestation) were given parenteral nutrient infusion (TPN), TPN + human GLP-2 (25 nmol. kg(-1). day(-1)), or sow's milk enterally (ENT) for 6 days. Intestinal protein synthesis was then measured in vivo after a bolus dose of [1-(13)C]phenylalanine, and degradation was calculated from the difference between protein accretion and synthesis. Crypt cell proliferation and apoptosis were measured in situ by 5-bromodeoxyuridine (BrdU) and terminal dUTP nick-end labeling (TUNEL), respectively. Intestinal protein and DNA accretion rates and villus heights were similar in GLP-2 and ENT pigs, and both were higher (P < 0.05) than in TPN pigs. GLP-2 decreased fractional protein degradation rate, whereas ENT increased fractional protein synthesis rate compared with TPN pigs. Percentage of TUNEL-positive cells in GLP-2 and ENT groups was 48 and 64% lower, respectively, than in TPN group (P < 0.05). However, ENT, but not GLP-2, increased percentage of BrdU-positive crypt cells above that in TPN piglets. We conclude that GLP-2 increases intestinal growth in premature, TPN-fed pigs by decreasing proteolysis and apoptosis, whereas enteral nutrition acts via increased protein synthesis and cell proliferation and decreased apoptosis.

Ontogeny of the glucagon-like peptide-2 receptor axis in the developing rat intestine

Glucagon-like peptide-2 (GLP-2) is secreted by enteroendocrine cells in the small and large intestines and exerts intestinotropic effects in the gastrointestinal mucosal epithelium of the adult rodent. The actions of GLP-2 are mediated by the GLP-2 receptor, a new member of the G protein-coupled receptor superfamily. To ascertain whether the GLP-2/GLP-2 receptor axis is expressed and functional in the developing intestine, we have studied the synthesis of GLP-2 and the expression of the GLP-2 receptor (GLP-2R) in the fetal and neonatal rat gut. GLP-2 immunoreactivity (GLP-2-IR) was detected in the fetal rat intestine, and fetal rat intestinal cell cultures secreted correctly processed GLP-2(1-33) into the medium. High levels of GLP-2(1-33) were also detected in the circulation of 13-day-old neonatal rats (P < 0.001 vs. adult). Analysis of GLP-2 receptor expression by RT-PCR demonstrated GLP-2R messenger RNA transcripts in fetal intestine and in neonatal stomach, jejunum, ileum, and colon. The levels of GLP-2R messenger RNA transcripts were comparatively higher in the fetal and neonatal intestine (P < 0.05-001 vs. adult) and declined to adult levels by postnatal day 21. Subcutaneous administration of a degradation-resistant GLP-2 analog, h[Gly2]-GLP-2 once daily for 10 days increased stomach (0.009 +/- 0.0003 vs. 0.007 +/- 0.002 g/g body mass, h[Gly2]-GLP-2-treated vs. controls; P < 0.05) and small bowel weight (0.043 +/- 0.0037 vs. 0.031 +/- 0.0030 g/g body mass; P < 0.05). h[Gly2]-GLP-2 also increased both small (2.4 +/- 0.05 vs. 1.8 +/- 0.17 cm/g body mass; P < 0.05) and large bowel length (0.32 +/- 0.01 vs. 0.25 +/- 0.02 cm/g body mass, h[Gly2]-GLP-2-treated vs. saline-treated controls, respectively; P < 0.05) in neonatal rats. These findings demonstrate that both components of the GLP-2/GLP-2 receptor axis are expressed in the fetal and neonatal intestine. The ontogenic regulation and functional integrity of this axis raises the possibility that GLP-2 may play a role in the development and/or maturation of the developing rat intestine.

Enteral nutrient intake level determines intestinal protein synthesis and accretion rates in neonatal pigs

Our objective was to determine the minimum enteral intake level necessary to increase the protein accretion rate (PAR) in the neonatal small intestine. Seven-day-old piglets received an equal total daily intake of an elemental diet, with different proportions given enterally (0, 10%, 20%, 40%, 60%, 80%, and 100%). After 7 days, piglets were infused intravenously with [(2)H(3)]leucine for 6 h, and the fractional protein synthesis rate (FSR) was measured in the proximal (PJ) and distal jejunum (DJ) and the proximal (PI) and distal ileum (DI). The jejunal FSR increased from 45%/day to 130%/day between 0 and 60% enteral intake, whereas the FSR in the ileum was less sensitive to enteral intake level. At 0% enteral intake, PAR was significantly negative in the PJ, DJ, and PI (range -70 to -43 mg/day) and positive in the DI (49 mg/day), whereas intestinal protein balance occurred at 20% enteral intake. At 100% enteral intake, the PAR was greatest in the DI, even though the rates of protein turnover were 50% lower than in the PJ. We conclude that there is net intestinal protein loss at 0% enteral intake, protein balance at 20% enteral intake, and maximal intestinal protein accretion at 60% enteral intake.