Intragastric bolus feeding of meals containing elementary, partially hydrolyzed or intact protein causes comparable changes in interorgan substrate flux in the pig

SNAPIG: a model to study nutrient digestion and absorption kinetics in growing pigs based on diet and ingredient properties

Current feed formulation and evaluation practices rely on static values for the nutritional value of feed ingredients and assume additivity. Hereby, the complex interplay among nutrients in the diet and the highly dynamic digestive processes are ignored. Nutrient digestion kinetics and diet × animal interactions should be acknowledged to improve future predictions of the nutritional value of complex diets. Therefore, an in silico nutrient-based mechanistic digestion model for growing pigs was developed: "SNAPIG" (Simulating Nutrient digestion and Absorption kinetics in PIGs). Aiming to predict the rate and extent of nutrient absorption from diets varying in ingredient composition and physicochemical properties, the model represents digestion kinetics of ingested protein, starch, fat, and non-starch polysaccharides, through passage, hydrolysis, absorption, and endogenous secretions of nutrients along the stomach, proximal small intestine, distal small intestine, and caecum + colon. Input variables are nutrient intake and the physicochemical properties (i.e. solubility, and rate and extent of degradability). Data on the rate and extent of starch and protein hydrolysis of different ingredients per digestive segment were derived from in vitro assays. Passage of digesta from the stomach was modelled as a function of feed intake level, dietary nutrient solubility and diet viscosity. Model evaluation included testing against independent data from in vivo studies on nutrient appearance in (portal) blood of growing pigs. When simulating diets varying in physicochemical properties and nutrient source, SNAPIG can explain variation in glucose absorption kinetics (postprandial time of peak, TOP: 20-100 min observed vs 25-98 min predicted), and predict variation in the extent of ileal protein and fat digestion (root mean square prediction errors (RMSPE) = 12 and 16%, disturbance error = 12 and 86%, and concordance correlation coefficient = 0.34 and 0.27). For amino acid absorption, the observed variation in postprandial TOP (61 ± 11 min) was poorly predicted despite accurate mean predictions (58 ± 34 min). Recalibrating protein digestion and amino acid absorption kinetics require data on net-portal nutrient appearance, combined with observations on digestion kinetics, in pigs fed diets varying in ingredient composition. Currently, SNAPIG can be used to forecast the time and extent of nutrient digestion and absorption when simulating diets varying in ingredient and nutrient composition. It enhances our quantitative understanding of nutrient digestion kinetics and identifies knowledge gaps in this field of research. Already useful as research tool, SNAPIG can be coupled with a postabsorptive metabolism model to predict the effects of dietary and feeding-strategies on the pig's growth response.

Transorgan fluxes in a porcine model reveal a central role for liver in acylcarnitine metabolism

Acylcarnitines are derived from mitochondrial acyl-CoA metabolism and have been associated with diet-induced insulin resistance. However, plasma acylcarnitine profiles have been shown to poorly reflect whole body acylcarnitine metabolism. We aimed to clarify the individual role of different organ compartments in whole body acylcarnitine metabolism in a fasted and postprandial state in a porcine transorgan arteriovenous model. Twelve cross-bred pigs underwent surgery where intravascular catheters were positioned before and after the liver, gut, hindquarter muscle compartment, and kidney. Before and after a mixed meal, we measured acylcarnitine profiles at several time points and calculated net transorgan acylcarnitine fluxes. Fasting plasma acylcarnitine concentrations correlated with net hepatic transorgan fluxes of free and C2- and C16-carnitine. Transorgan acylcarnitine fluxes were small, except for a pronounced net hepatic C2-carnitine production. The peak of the postprandial acylcarnitine fluxes was between 60 and 90 min. Acylcarnitine production or release was seen in the gut and liver and consisted mostly of C2-carnitine. Acylcarnitines were extracted by the kidney. No significant net muscle acylcarnitine flux was observed. We conclude that liver has a key role in acylcarnitine metabolism, with high net fluxes of C2-carnitine both in the fasted and fed state, whereas the contribution of skeletal muscle is minor. These results further clarify the role of different organ compartments in the metabolism of different acylcarnitine species.

Enhanced Lacto-Tri-Peptide Bio-Availability by Co-Ingestion of Macronutrients

Some food-derived peptides possess bioactive properties, and may affect health positively. For example, the C-terminal lacto-tri-peptides Ile-Pro-Pro (IPP), Leu-Pro-Pro (LPP) and Val-Pro-Pro (VPP) (together named here XPP) are described to lower blood pressure. The bioactivity depends on their availability at the site of action. Quantitative trans-organ availability/kinetic measurements will provide more insight in C-terminal tri-peptides behavior in the body. We hypothesize that the composition of the meal will modify their systemic availability. We studied trans-organ XPP fluxes in catheterized pigs (25 kg; n=10) to determine systemic and portal availability, as well as renal and hepatic uptake of a water-based single dose of synthetic XPP and a XPP containing protein matrix (casein hydrolyte, CasH). In a second experiment (n=10), we compared the CasH-containing protein matrix with a CasH-containing meal matrix and the modifying effects of macronutrients in a meal on the availability (high carbohydrates, low quality protein, high fat, and fiber). Portal availability of synthetic XPP was 0.08 ± 0.01% of intake and increased when a protein matrix was present (respectively 3.1, 1.8 and 83 times for IPP, LPP and VPP). Difference between individual XPP was probably due to release from longer peptides. CasH prolonged portal bioavailability with 18 min (absorption half-life, synthetic XPP: 15 ± 2 min, CasH: 33 ± 3 min, p<0.0001) and increased systemic elimination with 20 min (synthetic XPP: 12 ± 2 min; CasH: 32 ± 3 min, p<0.0001). Subsequent renal and hepatic uptake is about 75% of the portal release. A meal containing CasH, increased portal 1.8 and systemic bioavailability 1.2 times. Low protein quality and fiber increased XPP systemic bioavailability further (respectively 1.5 and 1.4 times). We conclude that the amount and quality of the protein, and the presence of fiber in a meal, are the main factors that increase the systemic bioavailability of food-derived XPP.

Catabolism through the threonine dehydrogenase pathway does not account for the high first-pass extraction rate of dietary threonine by the portal drained viscera in pigs

In pigs the extensive threonine utilization by the splanchnic tissues explains the relative inefficiency of dietary threonine conversion for body protein accretion. Two experiments were conducted to estimate the contribution of the portal drained viscera (PDV) and the liver to threonine metabolism and especially catabolism in growing pigs. In the first experiment, four pigs were surgically prepared for chronic catheter insertion in the portal, hepatic and jugular veins and in the carotid artery. They were continuously infused with l-[1-13C]threonine through the jugular catheter. The PDV and total splanchnic viscera (PDV and the liver) extracted 14·3 and 18·8 % of arterial threonine input, respectively. In a second experiment, we studied the metabolism of dietary threonine in the PDV and the liver in six female growing pigs. Animals were surgically prepared as in the first experiment except that l-[1-13C]threonine and [15N]glycine were continuously infused in the duodenum for 10 h. Unlabelled and labelled threonine and glycine PDV, liver and splanchnic tissues balance were calculated from plasma samples taken during the last 2 h of this infusion. Splanchnic tissues extracted 60 % of infused labelled threonine, 88 % of which was extracted by PDV so that threonine extraction by the liver was low. Both the liver and the pancreas can degrade threonine through the l-threonine 3-dehydrogenase pathway but not the intestine. Our data suggest that threonine catabolism through the l-threonine 3-dehydrogenase pathway was only a minor component of total threonine utilization in the splanchnic tissues.

Type and timing of protein feeding to optimize anabolism

PURPOSE OF REVIEW

The delivery rate of amino acids to an organism significantly affects protein anabolism. The rate can be controlled by the type and the timing of feeding. Our aim was to bring new insights to the way they may act.

RECENT FINDINGS

During young and adult ages, when food supply is liberal, subjects can adapt to various modes of protein feeding. However, during food restriction, protein anabolism is favored when the delivery of amino acids is evenly distributed over the day, either with frequent meals, or through the use of slowly absorbed proteins like casein. In contrast, during aging, quickly absorbed protein sources become more efficient. During recovery after exercise, the timing of protein feeding after the end of exercise may or may not influence its anabolic effect, depending on the subject's age and the type of exercise.

SUMMARY

The synchronization of variations in anabolic capability with amino acid supply partly explains the effects of the type and timing of protein feeding. This effect is modulated by the amount of amino acids required to increase whole-body proteins and by the signaling properties of some amino acids to stimulate protein synthesis. Indeed, the anabolic effect of amino acids is determined by their interaction with other anabolic factors (other nutrients or physiological factors, whose efficiency is mainly related to their effect on protein degradation). It is clear that benefits can be obtained from adapted protein feeding patterns.

Enhanced renal vein ammonia efflux after a protein meal in the pig

BACKGROUND/AIMS

The intake of dietary protein has been associated with increased arterial ammonia levels. However, the origin of this rise in ammonia levels is unknown. This study was designed to examine whether this increase is caused by ammonia formed by the gut escaping hepatic clearance, or ammonia formed by the kidney and subsequently released into the circulation.

METHODS

Splanchnic and renal fluxes of ammonia and amino acids were studied in 10 pigs that were fed in a randomized cross-over design with a protein meal (n = 8), a meal with an equimolar amount of free amino acids (n = 8) or an iso-osmolar NaCl solution (n = 6).

RESULTS

After the protein meal, and less pronounced after the amino acid meal, arterial ammonia levels increased from approximately 25 to 75 micromol/l. Arterial pH changes and splanchnic ammonia release were negligible. The renal vein ammonia efflux increased after the protein meal (0.67+/-0.10 to 1.94+/-0.35 micromol/kg bw/min) and to a lesser degree after the amino acid meal (to 1.20+/-0.39 micromol/kg bw/ min). Renal uptake of alanine, and not glutamine, increased stoichiometrically, paralleling the enhanced renal vein ammonia efflux.

CONCLUSIONS

Arterial ammonia increases after a meal in pigs, coinciding with a negligible splanchnic ammonia release, but increased renal vein ammonia efflux. Thus, post-prandial plasma ammonia levels appear to be mainly related to renal ammoniagenesis. Alanine appears to be the main precursor for this renal ammoniagenesis in the pig.

Effects of alimentary intact proteins and their oligopeptide hydrolysate on growth, nitrogen retention, and small bowel adaptation in inflammatory turpentine rat

The effects of dietary proteins given as whole proteins (WP) or as a peptide hydrolysate (PH) on growth, nitrogen retention, and small bowel adaptation were assessed using two groups of male Wistar rats. Measurements were made 18, 42, and 66 h after acute inflammation induced by subcutaneous injections of 0.125 mL turpentine and in two control groups (n = 12). The two diets had the same caloric, nitrogen, vitamin, and mineral content. The WP diet resulted in better weight gain, nitrogen retention, and small intestinal adaptation by control rats than did the PH diet. Loss of body weight after 18 h of acute inflammation was significantly lower and nitrogen retention significantly higher in animals on the WP diet than in those on the PH diet. Small intestine morphology was maintained with the WP diet, whereas villus height was significantly lower after 66 h, and there were fewer mitoses per crypt in the rats on the PH diet. Glucoamylase activity at all times, and N-aminopeptidase activity at 18 h, were significantly higher in rats on the WP diet. The putrescine (at 42 h) and spermidine (at 18 h) concentrations in the mucosa were higher in the rats on the WP diet. These data suggest that synthetic diets should be tested for their nutritional value during acute inflammation before they are used in human nutrition.

Infusion of soy and casein protein meals affects interorgan amino acid metabolism and urea kinetics differently in pigs

For routine evaluation of the quality of dietary protein, amino acid scoring patterns were used. Evaluation of this pattern for soy and casein revealed that these proteins are of almost equal quality. However, in vivo studies showed a large difference. To study the biological effects of meals with casein and soy protein, the contributions of individual amino acids to net protein retention and amino acid kinetics in gut, liver and muscle in healthy pigs were investigated. Isonitrogenous enteral nutrition, infused at a rate of 10 mL. kg body wt-1. h-1 and consisting of maltodextrin (137 g/L) with added casein (53 g/L) or soy protein (68 g/L), was given to conscious, healthy female multicathetized pigs (20-22 kg, n = 12). A primed-constant infusion protocol with L-[ring-2,6-3H]phenylalanine, L-[3,4-3H]valine and [15N-15N]urea was used to measure amino acid and urea kinetics in gut, liver and muscle. Measurements were done postabsorptively and 2-6 h after initiation of the enteral nutrition. During the meal, appearance of amino acids into the portal vein and the uptake by the liver was lower with casein infusion. Muscle uptake did not differ. Gut protein synthesis tended to be lower with soy infusion (P = 0.1). Liver protein synthesis and degradation were higher with casein infusion (P < 0.05), while in muscle, soy infusion stimulated protein turnover (P < 0.05). In comparison to the postabsorptive condition, liver urea production was unchanged after casein infusion, while it was significantly increased after soy infusion. These results suggest that the quality of soy protein is inferior to that of casein protein.

In vivo inter-organ protein metabolism of the splanchnic region and muscle during trauma, cancer and enteral nutrition

The study of protein kinetics has entered a new era by the recognition that whole body protein turnover only poorly reflects the true events occurring in several organs and with regard to the multitude of proteins present in the body. It is also increasingly recognized that the simultaneous synthesis and degradation of proteins is important in regulation and adaptation during several metabolic conditions like starvation, feeding, after trauma, and during exercise. Especially important is the recognition that the kinetics of individual proteins may change in opposite directions, thereby leading to fluxes of alpha-amino-nitrogen that serve to adapt to and survive a changing environment. At present, much emphasis is put upon molecular biological regulation. However, it is important that the metabolic processes that occur in the intact organism are still poorly defined. New technology allows the exploration of these processes, which should therefore prompt the initiation of further research in this area.