Plant-based sterols and stanols in health & disease: "Consequences of human development in a plant-based environment?"

Dietary plant sterols and stanols as present in our diet and in functional foods are well-known for their inhibitory effects on intestinal cholesterol absorption, which translates into lower low-density lipoprotein cholesterol concentrations. However, emerging evidence suggests that plant sterols and stanols have numerous additional health effects, which are largely unnoticed in the current scientific literature. Therefore, in this review we pose the intriguing question "What would have occurred if plant sterols and stanols had been discovered and embraced by disciplines such as immunology, hepatology, pulmonology or gastroenterology before being positioned as cholesterol-lowering molecules?" What would then have been the main benefits and fields of application of plant sterols and stanols today? We here discuss potential effects ranging from its presence and function intrauterine and in breast milk towards a potential role in the development of non-alcoholic steatohepatitis (NASH), cardiovascular disease (CVD), inflammatory bowel diseases (IBD) and allergic asthma. Interestingly, effects clearly depend on the route of entrance as observed in intestinal-failure associated liver disease (IFALD) during parenteral nutrition regimens. It is only until recently that effects beyond lowering of cholesterol concentrations are being explored systematically. Thus, there is a clear need to understand the full health effects of plant sterols and stanols.

Invited review: the preterm pig as a model in pediatric gastroenterology.

At birth, the newborn mammal undergoes a transition from a sterile uterine environment with a constant nutrient supply, to a microbe-rich environment with intermittent oral intake of complex milk nutrients via the gastrointestinal tract (GIT). These functional challenges partly explain the relatively high morbidity and mortality of neonates. Preterm birth interrupts prenatal organ maturation, including that of the GIT, and increases disease risk. Exemplary is necrotizing enterocolitis (NEC), which is associated closely with GIT immaturity, enteral feeding, and bacterial colonization. Infants with NEC may require resection of the necrotic parts of the intestine, leading to short bowel syndrome (SBS), characterized by reduced digestive capacity, fluid loss, and dependency on parenteral nutrition. This review presents the preterm pig as a translational model in pediatric gastroenterology that has provided new insights into important pediatric diseases such as NEC and SBS. We describe protocols for delivery, care, and handling of preterm pigs, and show how the immature GIT responds to delivery method and different nutritional and therapeutic interventions. The preterm pig may also provide a sensitive model for postnatal adaptation of weak term piglets showing high mortality. Attributes of the preterm pig model include close similarities with preterm infants in body size, organ development, and many clinical features, thereby providing a translational advantage relative to rodent models of GIT immaturity. On the other hand, the need for a sow surgical facility, a piglet intensive care unit, and clinically trained personnel may limit widespread use of preterm pigs. Studies on organ adaptation in preterm pigs help to identify the physiological basis of neonatal survival for hypersensitive newborns and aid in defining the optimal diet and rearing conditions during the critical neonatal period.

Glucagon-like peptide-2 (GLP-2) increases net amino acid utilization by the portal-drained viscera of ruminating calves.

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in ruminant calves, but its impact on nutrient metabolism across the portal-drained viscera (PDV) and liver is unknown. Eight Holstein calves with catheters in the carotid artery, mesenteric vein, portal vein and hepatic vein were paired by age and randomly assigned to control (0.5% bovine serum albumin in saline; n = 4) or GLP-2 (100 µg/kg BW per day bovine GLP-2 in bovine serum albumin; n = 4). Treatments were administered subcutaneously every 12 h for 10 days. Blood flow was measured on days 0 and 10 and included 3 periods: baseline (saline infusion), treatment (infusion of bovine serum albumin or 3.76 µg/kg BW per h GLP-2) and recovery (saline infusion). Arterial concentrations and net PDV, hepatic and total splanchnic fluxes of glucose, lactate, glutamate, glutamine, ß-hydroxybutyrate and urea-N were measured on days 0 and 10. Arterial concentrations and net fluxes of all amino acids and glucose metabolism using continuous intravenous infusion of [U13-C]glucose were measured on day 10 only. A 1-h infusion of GLP-2 increased blood flow in the portal and hepatic veins when administered to calves not previously exposed to exogenous GLP-2, but after a 10-day administration of GLP-2 the blood flow response to the 1-h GLP-2 infusion was substantially attenuated. The 1-h GLP-2 infusion also did not appreciably alter nutrient fluxes on either day 0 or 10. In contrast, long-term GLP-2 administration reduced arterial concentrations and net PDV flux of many essential and non-essential amino acids. Despite the significant alterations in amino acid metabolism, glucose irreversible loss and utilization by PDV and non-PDV tissues were not affected by GLP-2. Fluxes of amino acids across the PDV were generally reduced by GLP-2, potentially by increased small intestinal epithelial growth and thus energy and amino acid requirements of this tissue. Increased PDV extraction of glutamine and alterations in PDV metabolism of arginine, ornithine and citrulline support the concept that GLP-2 influences intestine-specific amino acid metabolism. Alterations in amino acid metabolism but unchanged glucose metabolism suggests that the growth effects induced by GLP-2 in ruminants increase reliance on amino acids preferentially over glucose. Thus, GLP-2 increases PDV utilization of amino acids, but not glucose, concurrent with stimulated growth of the small intestinal epithelium in post-absorptive ruminant calves.

First-pass splanchnic metabolism of dietary cysteine in weanling pigs.

Cysteine is a semi-indispensable AA in neonates and is synthesized from the indispensable AA, methionine, by transsulfuration. We previously showed that the gastrointestinal tract (GIT) is a metabolically important site of methionine transsulfuration to cysteine, yet the metabolic fate of dietary cysteine in the GIT has not been established. Cysteine use by gut epithelial cells may play an important role for maintenance of glutathione synthesis and cellular redox function. Our aim was to quantify the extent of gastrointestinal first-pass cysteine metabolism in young pigs. Four-week-old weanling pigs (n = 10) were fed a liquid milk-replacer diet and given an intragastric and intravenous [1-(13)C]cysteine infusion on 2 separate days in a crossover design. Arterial and portal blood samples were collected for cysteine isotopic enrichment by gas chromatography-mass spectrometry and for (13)CO(2) enrichment by isotope ratio mass spectrometry. Our results indicated that dietary cysteine is metabolized during its first-pass splanchnic metabolism, accounting for about 40% of dietary cysteine intake. We also showed that intestinal absorption was the major metabolic fate of dietary cysteine, representing about 75% of intake, indicating that the GIT utilizes 25% of the dietary cysteine intake. Thus, utilization by the GIT represents about one-half (approximately 53%) of the first-pass, splanchnic uptake of dietary cysteine. Moreover, a substantial proportion of dietary splanchnic cysteine metabolism was consumed by the GIT via nonoxidative pathways. We conclude that the gut utilizes 25% of the dietary cysteine intake and that synthesis of mucosal epithelial proteins, such as glutathione and mucin, are a major nonoxidative metabolic fate for cysteine.

Glucagon-like peptide-2 (GLP-2) increases small intestinal blood flow and mucosal growth in ruminating calves.

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in nonruminants but its effect in ruminants is unknown. Eight Holstein calves with an ultrasonic flow probe around the superior mesenteric artery and catheters in the carotid artery and mesenteric vein were paired by age and randomly assigned to treatment of a control (0.5% of BSA in saline; n=4) or GLP-2 (50 µg/kg of body weight of bovine GLP-2 in BSA; n=4) given subcutaneously every 12h for 10 d. Blood flow was measured on d 0 (acute) and d 10 (chronic) and included 3 periods: baseline (saline infusion), treatment (infusion of BSA or 1,000 pmol of GLP-2/kg of body weight per h), and recovery (saline infusion). On d 11, calves were killed 2h after injection of 5-bromo-2'-deoxyuridine (BrdU). Gastrointestinal tissues were weighed and epithelial samples were obtained to determine villus height, crypt depth, and BrdU staining. Infusion of GLP-2 increased superior mesenteric artery blood flow to 175% of baseline on d 0 but to only 137% of baseline after chronic treatment. Compared with that of the control, GLP-2 increased small intestinal mass by 24% by increasing epithelial mass in the jejunum and ileum. Additionally, GLP-2 increased villus height, crypt depth, and BrdU-labeling in small intestinal segments. These results demonstrate that GLP-2 induces similar increases in small intestinal blood flow and growth in ruminants to those observed in nonruminants. Furthermore, GLP-2 increases small intestinal blood flow in ruminants but this response is attenuated after 10 d of GLP-2 administration. In cattle, GLP-2 may be an important hormone in the regulation of intestinal blood flow and epithelial growth.

Expression of mRNA for proglucagon and glucagon-like peptide-2 (GLP-2) receptor in the ruminant gastrointestinal tract and the influence of energy intake.

Glucagon-like peptide-2 (GLP-2) is a potent trophic gut hormone, yet its function in ruminants is relatively unknown. Experiment 1 was conducted as a pilot study to establish the presence of GLP-2 in ruminants and to ascertain whether it was responsive to increased nutrition, as in non-ruminants. Concentrations of intact GLP-2 in the blood and gut epithelial mRNA expression of proglucagon (GCG) and the GLP-2 receptor (GLP2R) were measured in 4 ruminally, duodenally, and ileally cannulated steers. Steers were fed to meet 0.75 x NE(M) for 21 d, and then increased to 1.75 x NE(M) requirement for another 29 d. Blood samples and ruminal, duodenal, and ileal epithelium biopsies were collected at low intake (Days -6 and -3), acute high intake (Days 1 and 3), and chronic high intake (Days 7 and 29) periods. Experiment 2 investigated the mRNA expression pattern of GCG and GLP2R in epithelial tissue obtained from the forestomachs (rumen, omasum, and abomasum) and intestines (duodenum, jejunum, ileum, and colon) of 18 forage-fed Angus steers (260 kg BW). In Experiments 1 and 2, real-time polymerase chain reaction showed that expression of GCG and GLP2R mRNA was detectable in forestomach tissues, but expression was greater (P < 0.001) in small intestinal and colon tissue. High energy intake tended (P = 0.07) to increase plasma GLP-2 during the acute period and was paralleled by a 78% increase (P = 0.07) in ileal GCG mRNA expression. After this initial adaptation, duodenal GCG mRNA expression increased (P = 0.08) during the chronic high intake period. Duodenal GLP2R mRNA expression was not affected by energy intake, but ileal GLP2R expression was increased after 29 d of high energy intake compared to both the low and acute high intake periods (P = 0.001 and P = 0.01, respectively). These data demonstrate that cattle express GCG and GLP2R mRNA primarily in small intestinal and colon tissues. Increased nutrient intake increases ileal GCG mRNA and plasma GLP-2, suggesting that GLP-2 may play a role in the trophic response of the ruminant gastrointestinal tract to increased feed intake.

Amino acids and insulin are regulators of muscle protein synthesis in neonatal pigs.

The stage of development between birth and weaning in mammals is a period of very rapid growth that is crucial for the long-term well-being of the animal. The rate of protein deposition in neonatal animals is very high because dietary protein is efficiently utilized to increase body protein mass. Our studies in neonatal pigs have shown that this high efficiency of protein deposition is largely due to the marked increase in protein synthesis after feeding, and this response is particularly profound in the skeletal muscle. The enhanced stimulation of muscle protein synthesis in neonates after feeding is independently mediated by the rise in insulin and amino acids and this response declines with age. Intracellular signaling components that respond to the postprandial rise in amino acids and insulin have been identified and their activation has been shown to be elevated in skeletal muscle of neonatal pigs after a meal and to decrease with development. The enhanced activation of these components in the amino acid and insulin signaling pathways in neonatal muscle contributes to the high rate of muscle protein synthesis and rapid gain in skeletal muscle mass in newborn pigs, which are essential determinants of efficient growth during development.

Measuring splanchnic amino acid metabolism in vivo using stable isotopic tracers.

The splanchnic bed comprises the liver and the portal-drained viscera (PDV). The PDV, which include the stomach, intestines, pancreas, and spleen, represent 4 to 6% of BW, yet they account for 20 to 35% of whole-body protein turnover and energy expenditure. Because the PDV are the first to be exposed to the diet, their nutrient needs are met first. Consequently, the extraction of dietary nutrients, especially AA, by the intestine will have a critical influence on their availability to peripheral tissues and therefore, on whole body requirements. Moreover, the systemic availability of dietary AA is a key determinant of lean body growth rate. A complicating factor in the measurement of intestinal nutrient use is that the intestinal epithelial cells receive nutrients from 2 sources: the diet and the arterial circulation. However, combining measurements of the net portal balance with those of isotopic enrichments from enterally and intravenously administered stable isotope-labeled AA provides an in vivo model that can be used to determine the proportion of AA extracted by the intestine from either source. Using this technique in fed animals demonstrated that the PDV contribute significantly to the use of essential (>60% of threonine) and nonessential (>90% of glutamate) AA provided by the diet. The relative use by the PDV of individual AA from the diet and arterial inputs varies widely, and dietary AA are the preferred fuel over dietary glucose. Stable isotope-labeled AA also enable the determination of the metabolic fate of individual AA. Using this technique, studies have shown that an insufficient protein supply or the mode of feeding affects AA use by the PDV, and consequently, may affect whole-body growth.

Glucagon-like peptide-2 protects against TPN-induced intestinal hexose malabsorption in enterally refed piglets.

Premature infants receiving chronic total parenteral nutrition (TPN) due to feeding intolerance develop intestinal atrophy and reduced nutrient absorption. Although providing the intestinal trophic hormone glucagon-like peptide-2 (GLP-2) during chronic TPN improves intestinal growth and morphology, it is uncertain whether GLP-2 enhances absorptive function. We placed catheters in the carotid artery, jugular and portal veins, duodenum, and a portal vein flow probe in piglets before providing either enteral formula (ENT), TPN or a coinfusion of TPN plus GLP-2 for 6 days. On postoperative day 7, all piglets were fed enterally and digestive functions were evaluated in vivo using dual infusion of enteral ((13)C) and intravenous ((2)H) glucose, in vitro by measuring mucosal lactase activity and rates of apical glucose transport, and by assessing the abundances of sodium glucose transporter-1 (SGLT-1) and glucose transporter-2 (GLUT2). Both ENT and GLP-2 pigs had larger intestine weights, longer villi, and higher lactose digestive capacity and in vivo net glucose and galactose absorption compared with TPN alone. These endpoints were similar in ENT and GLP-2 pigs except for a lower intestinal weight and net glucose absorption in GLP-2 compared with ENT pigs. The enhanced hexose absorption in GLP-2 compared with TPN pigs corresponded with higher lactose digestive and apical glucose transport capacities, increased abundance of SGLT-1, but not GLUT-2, and lower intestinal metabolism of [(13)C]glucose to [(13)C]lactate. Our findings indicate that GLP-2 treatment during chronic TPN maintains intestinal structure and lactose digestive and hexose absorptive capacities, reduces intestinal hexose metabolism, and may facilitate the transition to enteral feeding in TPN-fed infants.

The nutritional phenotype in the age of metabolomics.

The concept of the nutritional phenotype is proposed as a defined and integrated set of genetic, proteomic, metabolomic, functional, and behavioral factors that, when measured, form the basis for assessment of human nutritional status. The nutritional phenotype integrates the effects of diet on disease/wellness and is the quantitative indication of the paths by which genes and environment exert their effects on health. Advances in technology and in fundamental biological knowledge make it possible to define and measure the nutritional phenotype accurately in a cross section of individuals with various states of health and disease. This growing base of data and knowledge could serve as a resource for all scientific disciplines involved in human health. Nutritional sciences should be a prime mover in making key decisions that include: what environmental inputs (in addition to diet) are needed; what genes/proteins/metabolites should be measured; what end-point phenotypes should be included; and what informatics tools are available to ask nutritionally relevant questions. Nutrition should be the major discipline establishing how the elements of the nutritional phenotype vary as a function of diet. Nutritional sciences should also be instrumental in linking the elements that are responsive to diet with the functional outcomes in organisms that derive from them. As the first step in this initiative, a prioritized list of genomic, proteomic, and metabolomic as well as functional and behavioral measures that defines a practically useful subset of the nutritional phenotype for use in clinical and epidemiological investigations must be developed. From this list, analytic platforms must then be identified that are capable of delivering highly quantitative data on these endpoints. This conceptualization of a nutritional phenotype provides a concrete form and substance to the recognized future of nutritional sciences as a field addressing diet, integrated metabolism, and health.

Somatotropin regulation of protein metabolism in pigs.

A primary goal of exogenous somatotropin treatment is to increase lean body mass. This is accomplished, in part, by increasing the efficiency with which dietary amino acids are used for protein deposition. Somatotropin administration also improves protein balance by minimizing the loss of protein during fasting and maximizing the protein gained during meal absorption. Amino acid catabolism is decreased by somatotropin treatment, as indicated by decreases in blood urea nitrogen, urea synthesis, hepatic urea cycle enzyme activity, and amino acid oxidation. Stable isotope tracer/mass transorgan balance studies have recently demonstrated that somatotropin treatment increases protein anabolism in young, growing swine by increasing protein synthesis in the hind limb and portal-drained viscera in the fed state, with little effect on protein degradation. Detailed study of the tissue-specific responses indicates that somatotropin treatment increases protein synthesis in skeletal muscle by increasing the efficiency of the translational process, but only in the fed state. The somatotropin-induced stimulation of skeletal muscle protein synthesis involves mechanisms that enhance the binding of both mRNA and initiator methionyl-tRNA to the 40S ribosomal subunit. Somatotropin increases protein synthesis in the liver in both the fasted and fed states by increasing ribosome number, with no change in translation initiation. Thus, the protein synthetic response to somatotropin treatment is tissue-specific and dependent on nutritional state.

Metabolomics in the opening decade of the 21st century: building the roads to individualized health.

It is rapidly becoming possible to measure hundreds or thousands of metabolites in small samples of biological fluids or tissues. This makes it possible to assess the metabolic component of nutritional phenotypes and will allow individualized dietary recommendations. ASNS has to take action to ensure that appropriate technologies are developed and that metabolic databases are constructed with the right inputs and organization. The relations between diet and metabolomic profiles and between those profiles and health and disease must be established. ASNS also should consider the social implications of these advances and plan for their appropriate utilization.

Glucagon-like peptide 2 function in domestic animals.

Glucagon-like peptide 2 (GLP-2) is a member of family of peptides derived from the proglucagon gene expressed in the intestines, pancreas and brain. Tissue-specific posttranslational processing of proglucagon leads to GLP-2 and GLP-1 secretion from the intestine and glucagon secretion from the pancreas. GLP-2 and GLP-1 are co-secreted from the enteroendocrine L-cells located in distal intestine in response to enteral nutrient ingestion, especially carbohydrate and fat. GLP-2 secretion is mediated by direct nutrient stimulation of the L-cells and indirect action from enteroendocrine and neural inputs, including GIP, gastrin-releasing peptide (GRP) and the vagus nerve. GLP-2 is secreted as a 33-amino acid peptide and is rapidly cleaved by dipeptidylpeptidase IV (DPP-IV) to a truncated peptide which acts as a weak agonist with competitive antagonistic properties. GLP-2 acts to enhance nutrient absorption by inhibiting gastric motility and secretion and stimulating nutrient transport. GLP-2 also suppresses food intake when infused centrally. The trophic actions of GLP-2 are specific for the intestine and occur via stimulation of crypt cell proliferation and suppression of apoptosis in mucosal epithelial cells. GLP-2 reduces gut permeability, bacterial translocation and proinflammatory cytokine expression under conditions of intestinal inflammation and injury. The effects of GLP-2 are mediated by a G-protein-linked receptor that is localized to the intestinal mucosa and hypothalamus. The intestinal localization of the GLP-2R to neural and endocrine cells, but not enterocytes, suggests that its actions are mediated indirectly via a secondary signaling mechanism. The implications of GLP-2 in domestic animal production are largely unexplored. However, GLP-2 may have therapeutic application in treatment of gastrointestinal injury and diarrheal diseases that occur in developing neonatal and weanling animals.

Development of intestinal immunoglobulin absorption and enzyme activities in neonatal pigs is diet dependent.

Uptake of colostrum just after birth is essential to stimulate intestinal growth and function, and in many species, including pigs, colostrum also provides immunological protection via the absorption of immunoglobulin G (IgG). In this study, intestinal growth, IgG absorptive capacity and enzyme activities were investigated in newborn pigs in response to different diets. Newborn piglets were bottle-fed porcine colostrum (PC), bovine colostrum (BC), porcine plasma (PP), porcine milk (PM), bovine colostrum containing porcine plasma (BCP) or a milk replacer (MR) every 3 h (15 mL/kg) for up to 2 d. Bovine serum albumin (BSA) was added to the diets as a macromolecule marker. The percentage of absorbed BSA just after birth was highest for piglets fed the PC diet (30-50%), lower for those fed the BC and BCP diets (23-30%) and lowest for the PP, PM and MR diet-fed piglets (7-20%, P < 0.05 relative to those fed colostrum). Porcine IgG was absorbed more efficiently than bovine IgG. Intestinal closure occurred earlier in MR and BCP piglets (within 12 h after birth) than in PC pigs. At 2 d of age, intestinal mucosal weight (+120% increase from birth) and villus morphology were similar in the PC, BCP and MR groups. All 3 groups also had increased aminopeptidase A activity compared with values at birth (+100% increase). Compared with PC pigs, the BCP group had higher sucrase and maltase activities (+50% and +200%, respectively) and lower aminopeptidase N activity (-50%, P < 0.05). Similarly, MR pigs showed elevated sucrase activity (+40%) and lowered maltase, lactase and aminopeptidase N activities (-20% to -50%, P < 0.05) compared with PC pigs. We conclude that porcine and bovine colostrum contain factors that stimulate the intestinal endocytotic and enzymatic capacity in newborn pigs. A milk replacer can produce normal gut growth, but may be inefficient in mediating normal macromolecule transport and disaccharidase activity. Bovine colostrum mixed with porcine plasma proteins may be a useful substitute for porcine colostrum in artificial rearing of newborn pigs.

GLP-2 has differential effects on small intestine growth and function in fetal and neonatal pigs.

Glucagon-like peptide-2 (GLP-2) is a potent intestinotropic factor in neonatal and adult animals. However, the GLP-2 responsiveness of the fetal intestine has not been established. To determine how stage of development affects the responsiveness to GLP-2, we examined GLP-2 receptor (GLP-2R) expression, gut morphology, and brush-border enzyme mRNA and activities in late-gestation fetal (n = 7) and parenterally fed neonatal (n = 7) piglets given GLP-2 (12.5 nmol/kg) twice daily for 6 days. The GLP-2R was expressed in the fetal and neonatal gastrointestinal tract. The biologically active GLP-2-(1-33) was undetectable (<5 pmol/l) in plasma of 98-day-gestation fetuses but increased significantly toward full term (115 days, 11 +/- 1 pmol/l) and in neonates fed by total parenteral nutrition (23 +/- 5 pmol/l). Exogenous GLP-2 had no effect on gut growth in fetuses but increased intestinal weight and villus height in neonates (P < 0.05). Crypt cell proliferation and the enzymes sucrase-isomaltase, lactase-phloridzin hydrolase, aminopeptidase A, and dipeptidyl peptidase IV were unchanged by GLP-2 in both groups. Aminopeptidase N mRNA and activity were increased in fetuses, while maltase mRNA and activity were increased in neonates. In conclusion, exogenous GLP-2 had different effects on small intestine growth and function in fetuses and neonates. This may be related to the normal developmental changes in intestine growth and function and to a maturation of the GLP-2R signaling pathways around the time of birth.

The pattern of intestinal substrate oxidation is altered by protein restriction in pigs.

BACKGROUND & AIMS: Previous studies indicate that amino acids and glucose are the major oxidative substrates for intestinal energy generation. We hypothesized that low protein feeding would lower the contribution of amino acids to energy metabolism, thereby increasing the contribution of glucose. METHODS: Piglets, implanted with portal, arterial, and duodenal catheters and a portal flow probe, were fed isocaloric diets of either a high protein (0.9 g/[kg/h] protein, 1.8 g/[kg/h] carbohydrate, and 0.4 g/[kg/h] lipid) or a low protein (0.4 g/[kg/h] protein, 2.2 g/[kg/h] carbohydrate, and 0.5 g/[kg/h] lipid) content. They received enteral or intravenous infusions of [1-13C]leucine (n = 17), [U-13C]glucose (n = 15), or enteral [U-13C]glutamate (n = 8). RESULTS: CO2 production by the splanchnic bed was not affected by the diet. The oxidation of leucine, glutamate, and glucose accounted for 82% of the total CO2 production in high protein-fed pigs. Visceral amino acid oxidation was substantially suppressed during a low protein intake. Although glucose oxidation increased to 50% of the total visceral CO2 production during a low protein diet, this increase did not compensate entirely for the fall in amino acid oxidation. CONCLUSIONS: Although low protein feeding increases the contribution of enteral glucose oxidation to total CO2 production, this adaptation is insufficient. To compensate for the fall in amino acid oxidation, other substrates become increasingly important to intestinal energy generation.

Intestinal protein and LPH synthesis in parenterally fed piglets receiving partial enteral nutrition and enteral insulinlike growth factor 1.

BACKGROUND: Providing partial enteral nutrition (PEN) supplemented with insulinlike growth factor-1 (IGF-1) to parenterally fed piglets increases lactase-phlorizin hydrolase (LPH) activity, but not LPH mRNA. The current aim was to investigate potential mechanisms by which IGF-1 up-regulates LPH activity. METHODS: Newborn piglets (n = 15) received 100% parenteral nutrition (TPN), 80% parenteral nutrition + 20% parenteral nutrition (PEN), or PEN + IGF-1 (1.0 mg. kg-1. d-1) for 7 days. On day 7, [2H3]-leucine was intravenously administered to measure mucosal protein and brush border LPH (BB LPH) synthesis. RESULTS: Weight gain, nutrient intake, and jejunal weight and length were similar among the treatment groups. Partial enteral nutrition alone increased mucosal weight, villus width and cross-sectional area, LPH activity, mRNA expression, and high mannose LPH precursor (proLPHh) abundance compared with TPN (P<0.05). Insulinlike growth factor-1 further increased mucosal weight, LPH activity, and LPH activity per unit BB LPH approximately twofold over PEN alone (P < 0.05) but did not affect LPH mRNA or the abundance of proLPHh (one of the LPH isoforms) or mature LPH. Isotopic enrichment of [2H3]-leucine in plasma, mucosal protein, and LPH precursors, and the fractional and absolute synthesis rates of mucosal protein and LPH were similar among the treatment groups. Insulinlike growth factor-1 treatment increased total mucosal protein synthesis (60%, P < 0.05) but not LPH synthesis compared with the other two groups. CONCLUSIONS: Because IGF-1 did not affect the fractional synthesis rate of either mucosal protein or LPH, the authors suggest that enteral IGF-1 increases mucosal protein mass and LPH activity by suppressing mucosal proteolytic degradation.

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.

Differential effects of insulin on peripheral and visceral tissue protein synthesis in neonatal pigs.

We recently demonstrated in neonatal pigs that, with amino acids and glucose maintained at fasting levels, the stimulation of protein synthesis in longissimus dorsi muscle with feeding can be reproduced by a physiological rise in insulin alone. In the current report, we determine whether the response of protein synthesis to insulin in the neonatal pig is 1) present in muscles of different fiber types, 2) proportional in myofibrillar and sarcoplasmic proteins, 3) associated with increased translational efficiency and ribosome number, and 4) present in other peripheral tissues and in viscera. Hyperinsulinemic-euglycemic-amino acid clamps were performed in 7- and 26-day-old pigs infused with 0, 30, 100, or 1,000 ng. kg(-0.66). min(-1) of insulin to reproduce insulin levels present in fasted, fed, refed, and supraphysiological conditions, respectively. Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. Insulin increased protein synthesis in gastrocnemius muscle and, to a lesser degree, masseter muscle. The degree of stimulation of protein synthesis by insulin was similar in myofibrillar and sarcoplasmic proteins. Insulin increased translational efficiency but had no effect on ribosome number in muscle. All of these insulin-induced changes in muscle protein synthesis decreased with age. Insulin also stimulated protein synthesis in cardiac muscle and skin but not in liver, intestine, spleen, pancreas, or kidney. The results support the hypothesis that insulin mediates the feeding-induced stimulation of myofibrillar and sarcoplasmic protein synthesis in muscles of different fiber types in the neonate by increasing the efficiency of translation. However, insulin does not appear to be involved in the feeding-induced stimulation of protein synthesis in visceral tissues. Thus different mechanisms regulate the growth of peripheral and visceral tissues in the neonate.