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

The impact of dietary Laminaria digitata and alginate lyase supplementation on the weaned piglet liver: A comprehensive proteomics and metabolomics approach

The brown seaweed Laminaria digitata, a novel feedstuff for weaned piglets, has potentially beneficial prebiotic properties. However, its recalcitrant cell wall challenges digestion in monogastrics. Alginate lyase is a promising supplement to mitigate this issue. This study's aim was to investigate the impact of incorporating 10% dietary Laminaria digitata, supplemented with alginate lyase, on the hepatic proteome and metabolome of weaned piglets. These diets introduced minor variations to the metabolome and caused significant shifts in the proteome. Dietary seaweed provided a rich source of n-3 PUFAs that could signal hepatic fatty acid oxidation (FABP, ACADSB and ALDH1B1). This may have affected the oxidative stability of the tissue, requiring an elevated abundance of GST for regulation. The presence of reactive oxygen species likely inflicted protein damage, triggering increased proteolytic activity (LAPTM4B and PSMD4). Alginate lyase supplementation augmented the number of differentially abundant proteins, which included GBE1 and LDHC, contributing to maintain circulating glucose levels by mobilizing glycogen stores and branched-chain amino acids. The enzymatic supplementation with alginate lyase amplified the effects of the seaweed-only diet. An additional filter was employed to test the effect of missing values on the proteomics analysis, which is discussed from a technical perspective. SIGNIFICANCE: Brown seaweeds such as Laminaria digitata have prebiotic and immune-modulatory components, such as laminarin, that can improve weaned piglet health. However, they have recalcitrant cell wall polysaccharides, such as alginate, that can elicit antinutritional effects on the monogastric digestive system. The aim of this study was to evaluate the effect of a high level of dietary L. digitata and alginate lyase supplementation on the hepatic metabolism of weaned piglets, using high throughput Omics approaches.

Ingestion of Free Amino Acids Compared with an Equivalent Amount of Intact Protein Results in More Rapid Amino Acid Absorption and Greater Postprandial Plasma Amino Acid Availability Without Affecting Muscle Protein Synthesis Rates in Young Adults in a Double-Blind Randomized Trial

BACKGROUND

The rate of protein digestion and amino acid absorption determines the postprandial rise in circulating amino acids and modulates postprandial muscle protein synthesis rates.

OBJECTIVE

We sought to compare protein digestion, amino acid absorption kinetics, and the postprandial muscle protein synthetic response following ingestion of intact milk protein or an equivalent amount of free amino acids.

METHODS

Twenty-four healthy, young participants (mean ± SD age: 22 ± 3 y and BMI 23 ± 2 kg/m2; sex: 12 male and 12 female participants) received a primed continuous infusion of l-[ring-2H5]-phenylalanine and l-[ring-3,5-2H2]-tyrosine, after which they ingested either 30 g intrinsically l-[1-13C]-phenylalanine-labeled milk protein or an equivalent amount of free amino acids labeled with l-[1-13C]-phenylalanine. Blood samples and muscle biopsies were obtained to assess protein digestion and amino acid absorption kinetics (secondary outcome), whole-body protein net balance (secondary outcome), and mixed muscle protein synthesis rates (primary outcome) throughout the 6-h postprandial period.

RESULTS

Postprandial plasma amino acid concentrations increased after ingestion of intact milk protein and free amino acids (both P < 0.001), with a greater increase following ingestion of the free amino acids than following ingestion of intact milk protein (P-time × treatment < 0.001). Exogenous phenylalanine release into plasma, assessed over the 6-h postprandial period, was greater with free amino acid ingestion (76 ± 9%) than with milk protein treatment (59 ± 10%; P < 0.001). Ingestion of free amino acids and intact milk protein increased mixed muscle protein synthesis rates (P-time < 0.001), with no differences between treatments (from 0.037 ± 0.015%/h to 0.053 ± 0.014%/h and 0.039 ± 0.016%/h to 0.051 ± 0.010%/h, respectively; P-time × treatment = 0.629).

CONCLUSIONS

Ingestion of a bolus of free amino acids leads to more rapid amino acid absorption and greater postprandial plasma amino acid availability than ingestion of an equivalent amount of intact milk protein. Ingestion of free amino acids may be preferred over ingestion of intact protein in conditions where protein digestion and amino acid absorption are compromised.

Amino Acid Absorption Profiles in Growing Pigs Fed Different Protein Sources

Simple Summary

The speed by which amino acids are absorbed into the blood after intake of different protein sources may affect their metabolism and utilization. A better understanding of the absorption pattern can be used to optimize the formulation of diets for pigs and to reduce the nitrogen excretion to the environment. We studied the amino acid appearance in blood of growing pigs after a meal, as influenced by protein source (wheat, soybean meal, enzyme-treated soybean meal, hydrothermally-treated rapeseed meal, casein, or hydrolyzed casein). The amino acid concentration in plasma was influenced by both time after feeding and the protein source. Overall, the highest concentrations were found at 60 min after feeding for all diets, and soybean meal had a prolonged AA absorption compared to especially casein and hydrolyzed casein. We conclude that the AA absorption profiles did not indicate clear differences among protein sources, allowing categorizing in fast and slow proteins sources, but the results show differences in the duration of AA absorption.

Abstract

The aim of the present study was to determine postprandial amino acid (AA) appearance in the blood of growing pigs as influenced by protein source. Seven growing pigs (average body weight 18 kg), in a 7 × 5 Youden square design, were fitted with a jugular vein catheter and fed seven diets containing wheat, soybean meal, enzyme-treated soybean meal, hydrothermally-treated rapeseed meal, casein, hydrolyzed casein, and a crystalline AA blend with the same AA profile as casein. The latter was not eaten by the pigs, therefore being excluded. Blood samples were collected at −30, 30, 60, 90, 120, 180, and 360 min after a meal and analyzed for free AA. Overall, plasma AA concentrations were highest 60 min after feeding. There were no differences in plasma AA concentration between casein and hydrolyzed casein, but soybean meal resulted in lower AA plasma concentrations compared with enzyme-treated soybean meal at 60 and 120 min after feeding. There were no differences between hydrothermally-treated rapeseed meal and soybean meal. In conclusion, the ingredients could not clearly be categorized as being slow or fast protein with regard to protein digestion and absorption of AA, but soybean meal resulted in a prolonged appearance of plasma AA compared to casein and hydrolyzed casein.

Anabolic Properties of Mixed Wheat-Legume Pasta Products in Old Rats: Impact on Whole-Body Protein Retention and Skeletal Muscle Protein Synthesis

The mechanisms that are responsible for sarcopenia are numerous, but the altered muscle protein anabolic response to food intake that appears with advancing age plays an important role. Dietary protein quality needs to be optimized to counter this phenomenon. Blending different plant proteins is expected to compensate for the lower anabolic capacity of plant-based when compared to animal-based protein sources. The objective of this work was to evaluate the nutritional value of pasta products that were made from a mix of wheat semolina and faba bean, lentil, or split pea flour, and to assess their effect on protein metabolism as compared to dietary milk proteins in old rats. Forty-three old rats have consumed for six weeks isoproteic and isocaloric diets containing wheat pasta enriched with 62% to 79% legume protein (depending on the type) or milk proteins, i.e., casein or soluble milk proteins (SMP). The protein digestibility of casein and SMP was 5% to 14% higher than legume-enriched pasta. The net protein utilization and skeletal muscle protein synthesis rate were equivalent either in rats fed legume-enriched pasta diets or those fed casein diet, but lower than in rats fed SMP diet. After legume-enriched pasta intake, muscle mass, and protein accretion were in the same range as in the casein and SMP groups. Mixed wheat-legume pasta could be a nutritional strategy for enhancing the protein content and improving the protein quality, i.e., amino acid profile, of this staple food that is more adequate for maintaining muscle mass, especially for older individuals.

Effect of Polymeric Diets in Patients on Continuous Ambulatory Peritoneal Dialysis

Objective

To determine if a diet complemented with calcium caseinate is better than a natural high protein diet for increasing serum albumin levels in patients on continuous ambulatory peritoneal dialysis (CAPD).

Patients and Methods

A 4-month clinical trial involving 100 patients older than 18 years was performed. Patients were randomized into two groups: group A, high protein diet (1.4 g natural protein/kg target weight/day and 35 kcal/kg target weight/day); and group B, calcium caseinate (0.7 g calcium caseinate plus 0.7 g natural protein diet/kg target weight/day and 35 kcal/kg target weight/day). Serum levels of albumin, total proteins (TP), BUN, creatinine, glucose, urea, sodium, and potassium, and hematocrit, leukocytes, erythrocytes, and hemoglobin were analyzed at baseline and every 30 days.

Results

The final mean albumin value was, for group A, 3.04 ± 0.39 g/dL, and for group B, 3.1±2 0.41 g/dL ( p < 0.05); TP for group A, 6.29 ± 0.47 g/dL, and for group B, 6.49 ± 0.51 g/dL ( p < 0.05); leukocytes for group A, 688 ± 8 1282/mm3, for group B, 7288 ± 1878/mm3 ( p = 0.05); BUN for group A, 47 ± 11 mg/dL, for group B, 50 ± 16 mg/dL ( p = 0.05). Regression analysis showed a treatment effect in serum albumin and TP levels from the third month in both groups. In group B, a constant elevation of serum albumin of 0.19 mg/dL and TP of 0.27 mg/dL was observed in every month of treatment with calcium caseinate. In the regression analysis of group A we observed a smaller increase in serum albumin, 0.06 mg/dL, and in TP, 0.11 mg/dL, in each month of treatment with the high protein diet. Both differences are significant ( p < 0.05).

Conclusion

Calcium caseinate used in CAPD patients suffering from malnutrition increases serum albumin levels.

Amine Metabolism Is Influenced by Dietary Protein Source

Growth in world population will inevitably leads to increased demand for protein for humans and animals. Protein from insects and blood plasma are being considered as possible alternatives, but more research on their nutritional quality and health effects is needed. Here, we studied the effect of dietary protein source on metabolism and metabolic amine profiles in serum and urine of mice. Groups of mice were fed semi-purified diets containing 300 g/kg of soybean meal, casein, partially delactosed whey powder, spray-dried plasma protein, wheat gluten meal, and yellow mealworm. Feed and water intake as well as body weight gain were measured for 28 days. After 14 and 28 days, serum and urine samples were collected for measurement of a large panel of amine metabolites. MetaboAnalyst 3.0 was used for analysis of the raw metabolic data. Out of 68 targeted amine metabolites, we could detect 54 in urine and 41 in blood serum. Dietary protein sources were found to have profound effects on host metabolism, particularly in systemic amine profiles, considered here as an endophenotype. We recommend serum over urine to screen for the amine metabolic endophenotype based on partial least squares discriminant analysis. We concluded that metabolites like alpha-aminobutyric acid and 1-methylhistidine are sensitive indicators of too much or too little availability of specific amino acids in the different protein diets. Furthermore, we concluded that amine metabolic profiles can be useful for assessing the nutritional quality of different protein sources.

Dietary supplementation of branched-chain amino acids increases muscle net amino acid fluxes through elevating their substrate availability and intramuscular catabolism in young pigs

Branched-chain amino acids (BCAA) have been clearly demonstrated to have anabolic effects on muscle protein synthesis. However, little is known about their roles in the regulation of net AA fluxes across skeletal muscle in vivo. This study was aimed to investigate the effect and related mechanisms of dietary supplementation of BCAA on muscle net amino acid (AA) fluxes using the hindlimb flux model. In all fourteen 4-week-old barrows were fed reduced-protein diets with or without supplemental BCAA for 28 d. Pigs were implanted with carotid arterial, femoral arterial and venous catheters, and fed once hourly with intraarterial infusion of p-amino hippurate. Arterial and venous plasma and muscle samples were obtained for the measurement of AA, branched-chain α-keto acids (BCKA) and 3-methylhistidine (3-MH). Metabolomes of venous plasma were determined by HPLC-quadrupole time-of-flight-MS. BCAA-supplemented group showed elevated muscle net fluxes of total essential AA, non-essential AA and AA. As for individual AA, muscle net fluxes of each BCAA and their metabolites (alanine, glutamate and glutamine), along with those of histidine, methionine and several functional non-essential AA (glycine, proline and serine), were increased by BCAA supplementation. The elevated muscle net AA fluxes were associated with the increase in arterial and intramuscular concentrations of BCAA and venous metabolites including BCKA and free fatty acids, and were also related to the decrease in the intramuscular concentration of 3-MH. Correlation analysis indicated that muscle net AA fluxes are highly and positively correlated with arterial BCAA concentrations and muscle net BCKA production. In conclusion, supplementing BCAA to reduced-protein diet increases the arterial concentrations and intramuscular catabolism of BCAA, both of which would contribute to an increase of muscle net AA fluxes in young pigs.

Effect of Amino Acid Infusion on Human Postoperative Colon Protein Synthesisin Situ

BACKGROUND

Amino acids are an integral part of parenteral nutrition because of their anabolic action helping to conserve body protein after surgical stress. At the gastrointestinal tract, an adequate supply of amino acids may be particularly important because of the gut's high rate of protein turnover, cell division, and proliferation. However, no information is available about the effects of amino acids on human intestinal protein metabolism after surgery.

METHODS

Studies were performed in postabsorptive patients 8-10 days after major abdominal surgery. Mass spectrometry techniques (capillary gas chromatography/combustion isotope ratio mass spectrometry) were used to directly determine the incorporation rate of 1-[13C]-leucine into colon mucosal protein. All subjects had a colostomy, which allowed easy access to the colon mucosa, and consecutive sampling from the same tissue was performed during continuous isotope infusion (0.16 micromol/kg min). Isotopic enrichments were determined at baseline and after a 4-hour infusion of amino acids or after infusion of saline (control group).

RESULTS

Compared with baseline, infusion of amino acids reduced fractional colon protein synthesis significantly by -29.2 +/- 8.3%. This decrease was also significantly different from the corresponding (insignificant) change during saline infusion (+19.4 +/- 26.9%, p < .05 vs amino acid group).

CONCLUSIONS

After surgery, an amino acid infusion acutely reduces postoperative colon protein synthesis. This effect possibly may be attributed to interactions of specific amino acids (glutamine) with an altered intestinal immune system and enterocyte activity.

Nutritional regulation of the anabolic fate of amino acids within the liver in mammals: concepts arising from in vivo studies

At the crossroad between nutrient supply and requirements, the liver plays a central role in partitioning nitrogenous nutrients among tissues. The present review examines the utilisation of amino acids (AA) within the liver in various physiopathological states in mammals and how the fates of AA are regulated. AA uptake by the liver is generally driven by the net portal appearance of AA. This coordination is lost when demands by peripheral tissues is important (rapid growth or lactation), or when certain metabolic pathways within the liver become a priority (synthesis of acute-phase proteins). Data obtained in various species have shown that oxidation of AA and export protein synthesis usually responds to nutrient supply. Gluconeogenesis from AA is less dependent on hepatic delivery and the nature of nutrients supplied, and hormones like insulin are involved in the regulatory processes. Gluconeogenesis is regulated by nutritional factors very differently between mammals (glucose absorbed from the diet is important in single-stomached animals, while in carnivores, glucose from endogenous origin is key). The underlying mechanisms explaining how the liver adapts its AA utilisation to the body requirements are complex. The highly adaptable hepatic metabolism must be capable to deal with the various nutritional/physiological challenges that mammals have to face to maintain homeostasis. Whereas the liver responds generally to nutritional parameters in various physiological states occurring throughout life, other complex signalling pathways at systemic and tissue level (hormones, cytokines, nutrients, etc.) are involved additionally in specific physiological/nutritional states to prioritise certain metabolic pathways (pathological states or when nutritional requirements are uncovered).

Post-Prandial Protein Handling: You Are What You Just Ate

Background

Protein turnover in skeletal muscle tissue is highly responsive to nutrient intake in healthy adults.

Objective

To provide a comprehensive overview of post-prandial protein handling, ranging from dietary protein digestion and amino acid absorption, the uptake of dietary protein derived amino acids over the leg, the post-prandial stimulation of muscle protein synthesis rates, to the incorporation of dietary protein derived amino acids in de novo muscle protein.

Design

12 healthy young males ingested 20 g intrinsically [1-¹³C]-phenylalanine labeled protein. In addition, primed continuous L-[ring-²H₅]-phenylalanine, L-[ring-²H₂]-tyrosine, and L-[1-¹³C]-leucine infusions were applied, with frequent collection of arterial and venous blood samples, and muscle biopsies throughout a 5 h post-prandial period. Dietary protein digestion, amino acid absorption, splanchnic amino acid extraction, amino acid uptake over the leg, and subsequent muscle protein synthesis were measured within a single in vivo human experiment.

Results

55.3±2.7% of the protein-derived phenylalanine was released in the circulation during the 5 h post-prandial period. The post-prandial rise in plasma essential amino acid availability improved leg muscle protein balance (from -291±72 to 103±66 μM·min^-1·100 mL leg volume^-1; P<0.001). Muscle protein synthesis rates increased significantly following protein ingestion (0.029±0.002 vs 0.044±0.004%·h^-1 based upon the muscle protein bound L-[ring-²H₅]-phenylalanine enrichments (P<0.01)), with substantial incorporation of dietary protein derived L-[1-¹³C]-phenylalanine into de novo muscle protein (from 0 to 0.0201±0.0025 MPE).

Conclusion

Ingestion of a single meal-like amount of protein allows ~55% of the protein derived amino acids to become available in the circulation, thereby improving whole-body and leg protein balance. About 20% of the dietary protein derived amino acids released in the circulation are taken up in skeletal muscle tissue following protein ingestion, thereby stimulating muscle protein synthesis rates and providing precursors for de novo muscle protein synthesis.

Trial Registration

trialregister.nl 3638

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.

Natural isotopic signatures of variations in body nitrogen fluxes: a compartmental model analysis

Body tissues are generally ¹⁵N-enriched over the diet, with a discrimination factor (Δ¹⁵N) that varies among tissues and individuals as a function of their nutritional and physiopathological condition. However, both ¹⁵N bioaccumulation and intra- and inter-individual Δ¹⁵N variations are still poorly understood, so that theoretical models are required to understand their underlying mechanisms. Using experimental Δ¹⁵N measurements in rats, we developed a multi-compartmental model that provides the first detailed representation of the complex functioning of the body's Δ¹⁵N system, by explicitly linking the sizes and Δ¹⁵N values of 21 nitrogen pools to the rates and isotope effects of 49 nitrogen metabolic fluxes. We have shown that (i) besides urea production, several metabolic pathways (e.g., protein synthesis, amino acid intracellular metabolism, urea recycling and intestinal absorption or secretion) are most probably associated with isotope fractionation and together contribute to ¹⁵N accumulation in tissues, (ii) the Δ¹⁵N of a tissue at steady-state is not affected by variations of its P turnover rate, but can vary according to the relative orientation of tissue free amino acids towards oxidation vs. protein synthesis, (iii) at the whole-body level, Δ¹⁵N variations result from variations in the body partitioning of nitrogen fluxes (e.g., urea production, urea recycling and amino acid exchanges), with or without changes in nitrogen balance, (iv) any deviation from the optimal amino acid intake, in terms of both quality and quantity, causes a global rise in tissue Δ¹⁵N, and (v) Δ¹⁵N variations differ between tissues depending on the metabolic changes involved, which can therefore be identified using simultaneous multi-tissue Δ¹⁵N measurements. This work provides proof of concept that Δ¹⁵N measurements constitute a new promising tool to investigate how metabolic fluxes are nutritionally or physiopathologically reorganized or altered. The existence of such natural and interpretable isotopic biomarkers promises interesting applications in nutrition and health.

Body proteins ensure vital functions, and their constancy is maintained through the tight coordination of many nitrogen metabolic fluxes, but our understanding of how this flux system is regulated, and sometimes dysregulated, remains fragmentary and incomplete. Besides, body tissues are generally naturally enriched in the heavier stable nitrogen isotope (¹⁵N) over the diet: this ¹⁵N bioaccumulation (Δ¹⁵N) varies depending on tissues and metabolic orientations, likely as the result of isotope effects associated to some metabolic pathways. We used a novel approach, combining multi-tissue Δ¹⁵N measurements and their analysis using modeling, to understand how body Δ¹⁵N values relate to nitrogen fluxes. The multi-tissue model we have developed provides a clearer understanding of the metabolic processes that generate isotopic fractionation, and of how tissue Δ¹⁵N values are modulated in response to changes in the body distribution of specific nitrogen fluxes. We show that Δ¹⁵N values tend to rise when the amino acids intake does not optimally fit the metabolic demand, and that Δ¹⁵N values constitute natural and interpretable signatures of nutritionally-induced variations in nitrogen fluxes. This approach constitutes a new promising tool to investigate how nitrogen metabolism is nutritionally or physiopathologically reorganized or altered, and promises interesting applications in many areas (nutrition, pathology, ecology, paleontology, etc).

Nutritional recovery with okara diet prevented hypercholesterolemia, hepatic steatosis and glucose intolerance

We assessed the biological value of an okara diet and its effects on the hormonal and metabolic profile of rats submitted to protein restriction during intra-uterine life and lactation and recovered after weaning. Male rats from mothers fed either 17% or 6% protein during pregnancy and lactation were maintained on 17% casein (CC, LC), 17% okara (CO, LO) or 6% casein (LL) diets over 60 d. The nutritional quality of the okara protein was similar to that of casein. The okara diet was effective in the nutritional recovery of rats in growing that were malnourished in early life. Furthermore, the okara diet reversed the hypercholesterolemia and the hepatic steatosis observed in the malnutrition and prevented glucose intolerance in an animal model prone to diabetes mellitus.

Hepatic adaptation compensates inactivation of intestinal arginine biosynthesis in suckling mice

Suckling mammals, including mice, differ from adults in the abundant expression of enzymes that synthesize arginine from citrulline in their enterocytes. To investigate the importance of the small-intestinal arginine synthesis for whole-body arginine production in suckling mice, we floxed exon 13 of the argininosuccinate synthetase (Ass) gene, which codes for a key enzyme in arginine biosynthesis, and specifically and completely ablated Ass in enterocytes by crossing Ass (fl) and Villin-Cre mice. Unexpectedly, Ass (fl/fl) /VilCre (tg/-) mice showed no developmental impairments. Amino-acid fluxes across the intestine, liver, and kidneys were calculated after determining the blood flow in the portal vein, and hepatic and renal arteries (86%, 14%, and 33%, respectively, of the transhepatic blood flow in 14-day-old mice). Relative to control mice, citrulline production in the splanchnic region of Ass (fl/fl) /VilCre (tg/-) mice doubled, while arginine production was abolished. Furthermore, the net production of arginine and most other amino acids in the liver of suckling control mice declined to naught or even changed to consumption in Ass (fl/fl) /VilCre (tg/-) mice, and had, thus, become remarkably similar to that of post-weaning wild-type mice, which no longer express arginine-biosynthesizing enzymes in their small intestine. The adaptive changes in liver function were accompanied by an increased expression of genes involved in arginine metabolism (Asl, Got1, Gpt2, Glud1, Arg1, and Arg2) and transport (Slc25a13, Slc25a15, and Slc3a2), whereas no such changes were found in the intestine. Our findings suggest that the genetic premature deletion of arginine synthesis in enterocytes causes a premature induction of the post-weaning pattern of amino-acid metabolism in the liver.

The nature of the dietary protein impacts the tissue-to-diet 15N discrimination factors in laboratory rats

Due to the existence of isotope effects on some metabolic pathways of amino acid and protein metabolism, animal tissues are ¹⁵N-enriched relative to their dietary nitrogen sources and this ¹⁵N enrichment varies among different tissues and metabolic pools. The magnitude of the tissue-to-diet discrimination (Δ¹⁵N) has also been shown to depend on dietary factors. Since dietary protein sources affect amino acid and protein metabolism, we hypothesized that they would impact this discrimination factor, with selective effects at the tissue level. To test this hypothesis, we investigated in rats the influence of a milk or soy protein-based diet on Δ¹⁵N in various nitrogen fractions (urea, protein and non-protein fractions) of blood and tissues, focusing on visceral tissues. Regardless of the diet, the different protein fractions of blood and tissues were generally ¹⁵N-enriched relative to their non-protein fraction and to the diet (Δ¹⁵N>0), with large variations in the Δ¹⁵N between tissue proteins. Δ¹⁵N values were markedly lower in tissue proteins of rats fed milk proteins compared to those fed soy proteins, in all sampled tissues except in the intestine, and the amplitude of Δ¹⁵N differences between diets differed between tissues. Both between-tissue and between-diet Δ¹⁵N differences are probably related to modulations of the relative orientation of dietary and endogenous amino acids in the different metabolic pathways. More specifically, the smaller Δ¹⁵N values observed in tissue proteins with milk than soy dietary protein may be due to a slightly more direct channeling of dietary amino acids for tissue protein renewal and to a lower recycling of amino acids through fractionating pathways. In conclusion, the present data indicate that natural Δ¹⁵N of tissue are sensitive markers of the specific subtle regional modifications of the protein and amino acid metabolism induced by the protein dietary source.

Exercising before protein intake allows for greater use of dietary protein-derived amino acids for de novo muscle protein synthesis in both young and elderly men

BACKGROUND

Sarcopenia seems to be attributed to a blunted muscle protein synthetic response to food intake and exercise. This blunted response could be the result of impaired protein digestion and absorption kinetics and lead to lower postprandial plasma amino acid availability.

OBJECTIVE

The objective was to compare in vivo dietary protein digestion and absorption kinetics and subsequent postprandial muscle protein synthesis rates at rest and after exercise between young and elderly men.

DESIGN

Young and elderly men consumed a 20-g bolus of intrinsically L-[1-(13)C]phenylalanine-labeled protein at rest or after exercise. Continuous infusions with L-[ring-(2)H(5)]phenylalanine were applied, and blood and muscle samples were collected to assess in vivo protein digestion and absorption kinetics and subsequent postprandial muscle protein synthesis rates.

RESULTS

Exogenous phenylalanine appearance rates expressed over time did not differ between groups. No differences were observed in plasma phenylalanine availability between the young (51 ± 2%) and elderly (51 ± 1%) men or between the rest (52 ± 1%) and exercise (49 ± 1%) conditions. Muscle protein synthesis rates calculated from the oral tracer were 0.0620 ± 0.0065%/h and 0.0560 ± 0.0039%/h for the rest condition and 0.0719 ± 0.0057%/h and 0.0727 ± 0.0040%/h for the exercise condition in young and elderly men, respectively (age effect: P = 0.62; exercise effect: P < 0.05; interaction of age and exercise: P = 0.52).

CONCLUSIONS

Dietary protein digestion and absorption kinetics are not impaired after exercise or at an older age. Exercising before protein intake allows for a greater use of dietary protein-derived amino acids for de novo muscle protein synthesis in both young and elderly men. This trial was registered at clinicaltrials.gov as NCT00557388.

Purified c-phycoerythrin: safety studies in rats and protective role against permanganate-mediated fibroblast-DNA damage

We have evaluated in vitro cytotoxicity of cyanobacterial phycoerythrin (C-PE) on three human cell lines by cell proliferation and neutral red uptake assays. No toxic effects of C-PE were observed to any of the cell lines tested. The protective role of purified C-PE to potassium permanganate-mediated human fibroblast-DNA damage was assessed by comet assay at 0 (control), 10 and 20 microg C-PE ml(-1) doses in pre-, simultaneous and post-mutagen exposure conditions. Significant DNA damage was detected only in post-mutagen exposure conditions. Our findings confirmed that the C-PE is non-toxic and provides protection against permanganate-mediated DNA damage. The preliminary acute (2000 mg C-PE kg(-1) body weight, b.w.) and 90 day sub-chronic (0, 5, 15 and 25 mg C-PE kg(-1) b.w./day) oral toxicity studies of purified C-PE in male albino rats showed no mortality or treatment-related major clinical signs, and all the doses of C-PE were well tolerated. The no observed adverse effect level and no observed effect level were found to be 15 and 5 mg C-PE kg(-1) b.w./day respectively.

Effects of different dietary protein sources on expression of genes related to protein metabolism in growing rats

Protein metabolism is known to be affected by dietary proteins, but the fundamental mechanisms that underlie the changes in protein metabolism are unclear. The aim of the present study was to test the effects of feeding growing rats with balanced diets containing soya protein isolate, zein and casein as the sole protein source on the expression of genes related to protein metabolism responses in skeletal muscle. The results showed that feeding a zein protein diet to the growing rats induced changes in protein anabolic and catabolic metabolism in their gastrocnemius muscles when compared with those fed either the reference protein casein diet or the soya protein isolate diet. The zein protein diet increased not only the mRNA levels and phosphorylation of mammalian target of rapamycin (mTOR), but also the mRNA expression of muscle atrophy F-box (MAFbx)/atrogin-1 and muscle ring finger 1 (MuRF1), as well as the forkhead box-O (FoxO) transcription factors involved in the induction of the E3 ligases. The amino acid profile of proteins seems to control signalling pathways leading to changes in protein synthesis and proteolysis.

Absorption kinetics are a key factor regulating postprandial protein metabolism in response to qualitative and quantitative variations in protein intake

We have previously demonstrated that increasing the habitual protein intake widened the gap in nutritional quality between proteins through mechanisms that are not yet fully understood. We hypothesized that the differences in gastrointestinal kinetics between dietary proteins were an important factor affecting their differential response to an increased protein intake. To test this hypothesis, we built a 13-compartment model providing integrative insight into the sequential dynamics of meal nitrogen (Nm) absorption, splanchnic uptake, and metabolism, and subsequent peripheral transfer and deposition. The model was developed from data on postprandial Nm kinetics in certain accessible pools, obtained from subjects having ingested a (15)N-labeled milk or soy protein meal, after adaptation to normal (NP) or high (HP) protein diets. The faster absorption of Nm after soy vs. milk caused its earlier and stronger splanchnic delivery, which favored its local catabolic utilization (up to +30%) and limited its peripheral accretion (down to -20%). Nm absorption was also accelerated after HP vs. NP adaptation, and this kinetic effect accounted for most of the HP-induced increase (up to +20%) in splanchnic Nm catabolic use, and the decrease (down to -25%) in peripheral Nm anabolic utilization. The HP-induced acceleration in Nm absorption was more pronounced with soy than with milk, as were the HP effects on Nm regional metabolism. Our integrative approach identified Nm absorption kinetics, which exert a direct and lasting impact on Nm splanchnic catabolic use and peripheral delivery, as being critical in adaptation to both qualitative and quantitative changes in protein intake.

Dietary protein digestion and absorption rates and the subsequent postprandial muscle protein synthetic response do not differ between young and elderly men

Impaired digestion and/or absorption of dietary protein lowers postprandial plasma amino acid availability and, as such, could reduce the postprandial muscle protein synthetic response in the elderly. We aimed to compare in vivo dietary protein digestion and absorption and the subsequent postprandial muscle protein synthetic response between young and elderly men. Ten elderly (64 +/- 1 y) and 10 young (23 +/- 1 y) healthy males consumed a single bolus of 35 g specifically produced, intrinsically l-[1-(13)C]phenylalanine-labeled micellar casein (CAS) protein. Furthermore, primed continuous infusions with l-[ring-(2)H(5)]phenylalanine, l-[1-(13)C]leucine, and l-[ring-(2)H(2)]tyrosine were applied and blood and muscle tissue samples were collected to assess the appearance rate of dietary protein-derived phenylalanine in the circulation and the subsequent muscle protein fractional synthetic rate over a 6-h postprandial period. Protein ingestion resulted in a rapid increase in exogenous phenylalanine appearance in both the young and elderly men. Total exogenous phenylalanine appearance rates (expressed as area under the curve) were 39 +/- 3 mumol.6 h.kg(-1) in the young men and 38 +/- 2 mumol.6 h.kg(-1) in the elderly men (P = 0.73). In accordance, splanchnic amino acid extraction did not differ between young (72 +/- 2%) and elderly (73 +/- 1%) volunteers (P = 0.74). Muscle protein synthesis rates, calculated from the oral tracer, were 0.063 +/- 0.006 and 0.054 +/- 0.004%/h in the young and elderly men, respectively, and did not differ between groups (P = 0.27). We conclude that protein digestion and absorption kinetics and the subsequent muscle protein synthetic response following the ingestion of a large bolus of intact CAS are not substantially impaired in healthy, elderly men.

Ingestion of a protein hydrolysate is accompanied by an accelerated in vivo digestion and absorption rate when compared with its intact protein

BACKGROUND

It has been suggested that a protein hydrolysate, as opposed to its intact protein, is more easily digested and absorbed from the gut, which results in greater plasma amino acid availability and a greater muscle protein synthetic response.

OBJECTIVE

We aimed to compare dietary protein digestion and absorption kinetics and the subsequent muscle protein synthetic response to the ingestion of a single bolus of protein hydrolysate compared with its intact protein in vivo in humans.

DESIGN

Ten elderly men (mean +/- SEM age: 64 +/- 1 y) were randomly assigned to a crossover experiment that involved 2 treatments in which the subjects consumed a 35-g bolus of specifically produced L-[1-(13)C]phenylalanine-labeled intact casein (CAS) or hydrolyzed casein (CASH). Blood and muscle-tissue samples were collected to assess the appearance rate of dietary protein-derived phenylalanine in the circulation and subsequent muscle protein fractional synthetic rate over a 6-h postprandial period.

RESULTS

The mean (+/-SEM) exogenous phenylalanine appearance rate was 27 +/- 6% higher after ingestion of CASH than after ingestion of CAS (P < 0.001). Splanchnic extraction was significantly lower in CASH compared with CAS treatment (P < 0.01). Plasma amino acid concentrations increased to a greater extent (25-50%) after the ingestion of CASH than after the ingestion of CAS (P < 0.01). Muscle protein synthesis rates averaged 0.054 +/- 0.004% and 0.068 +/- 0.006%/h in the CAS and CASH treatments, respectively (P = 0.10).

CONCLUSIONS

Ingestion of a protein hydrolysate, as opposed to its intact protein, accelerates protein digestion and absorption from the gut, augments postprandial amino acid availability, and tends to increase the incorporation rate of dietary amino acids into skeletal muscle protein.

Parameter estimation for linear compartmental models--a sensitivity analysis approach

Linear compartmental models are useful, explanatory tools, that have been widely used to represent the dynamic behavior of complex biological systems. This paper addresses the problem of the numerical identification of such models, i.e., the estimation of the parameter values that will generate predictions closest to experimental observations. Traditional local optimization techniques find it difficult to arrive at satisfactory solutions to such a parameter estimation problem, especially when the number of parameters is large and/or few data are available from experiments. We present herewith a method based on a prior sensitivity analysis, which enables division of a large optimization problem into several smaller and simpler subproblems, on which only sensitive parameters are estimated, before the whole optimization problem is tackled from starting points that are already close to the optimum values. This method has been applied successfully to a linear 13-compartment, 21-parameter model describing the postprandial metabolism of dietary nitrogen in humans. The effectiveness of the method has been demonstrated using simulated and real data obtained in the intestine, blood and urine of healthy humans after the ingestion of a [(15)N]-labeled protein meal.

Pharmacokinetics of proline-rich tripeptides in the pig

Tripeptides may possess bioactive properties. For instance, blood pressure lowering is attributed to the proline-rich tripeptides Ile-Pro-Pro (IPP), Leu-Pro-Pro (LPP), and Val-Pro-Pro (VPP). However, little is known about their absorption, distribution, and elimination characteristics. The aim of this study was to characterize the pharmacokinetic behavior of IPP, LPP, and VPP in a conscious pig model. Synthetic IPP, LPP, and VPP were administered intravenously or intragastrically (4.0 mg kg(-1) BW in saline) to 10 piglets (approximately 25 kg body weight) in the postabsorptive state. After intravenous dosing, the elimination half-life for IPP was significantly higher (P<0.001) than for LPP and VPP (2.5+/-0.1, 1.9+/-0.1, and 2.0+/-0.1 min, respectively). After intragastric dosing, however, the elimination half-lives were not significantly different between the peptides (9+/-1, 15+/-4, and 12+/-6 min, respectively). Maximum plasma concentrations were about 10 nmol l(-1) for the three tripeptides. The fraction dose absorbed was 0.077+/-0.010, 0.059+/-0.009, and 0.073+/-0.015%, for IPP, LPP, and VPP, respectively. Proline-rich tripeptides reach the blood circulation intact, with an absolute bioavailability of about 0.1% when administered via a saline solution. Because half-lives of absorption and elimination were maximally about 5 and 15 min, respectively, this suggests that under these conditions a bioactive effect of these tripeptides would be rather acute.

Urea-nitrogen production and salvage are modulated by protein intake in fed humans: results of an oral stable-isotope-tracer protocol and compartmental modeling

BACKGROUND

The influence of protein source on postprandial urea kinetics is poorly understood, despite its nutritional significance with respect to nitrogen homeostasis. Furthermore, traditional tracer infusion studies underestimate acute postprandial change in urea kinetics.

OBJECTIVE

We investigated postprandial, non-steady state urea kinetics and their modulation by qualitative and quantitative factors of protein intake by the combined use of robust clinical data on nitrogen postprandial distribution and mathematical modeling.

DESIGN

In healthy subjects standardized to a normal protein intake for 7 d, dietary and total nitrogen kinetics were measured for 8 h in plasma proteins, body, and urinary urea after the ingestion of a (15)N-labeled milk (n = 8), soy (n = 8), or wheat (n = 8) protein meal. In subjects who received the soy protein meal, these postprandial measurements were repeated after a further 7-d adaptation to a high protein intake. A 4-compartment model was developed to calculate from these data the postprandial kinetics of production, urinary excretion, and intestinal hydrolysis of urea nitrogen from both dietary and endogenous sources.

RESULTS

Urinary urea excretion was not influenced by the protein source in the meal but was influenced by the protein level in the diet. By contrast, urea production and hydrolysis were higher when ingesting plant versus animal protein, together with a higher efficiency of urea hydrolysis (50-60% versus 25% of the urea produced being hydrolyzed, respectively).

CONCLUSIONS

We conclude that urea hydrolysis is an acute nitrogen-sparing mechanism that can counterbalance a postprandial higher urea production, and the efficiency of this recycling is higher when the usual protein intake is lower.

Increasing habitual protein intake results in reduced postprandial efficiency of peripheral, anabolic wheat protein nitrogen use in humans

BACKGROUND

The postprandial retention of dietary protein decreases when the prevailing protein intake increases.

OBJECTIVE

We investigated the influence of the prevailing protein intake on the regional utilization and anabolic use of wheat protein during the postprandial non-steady state in humans.

DESIGN

Healthy adults (n = 8) were adapted for 7 d, first to a normal-protein diet (NP: 1 g x kg(-1) x d(-1)) and then to a high-protein diet (HP: 2 g x kg(-1) x d(-1)). After each adaptation period, the subjects received the same single, solid mixed meal containing [15N]-labeled wheat protein. The postprandial kinetics of dietary nitrogen were then measured for 8 h in blood and urine. These data were further analyzed by using a multicompartmental model to predict the postprandial kinetics of dietary nitrogen in unsampled pools.

RESULTS

The postprandial whole-body retention of wheat protein nitrogen, measured 8 h after meal ingestion, decreased by 10% when the subjects switched from the NP diet to the HP diet. According to modeling results, this resulted from an increased splanchnic utilization of dietary nitrogen for urea production, whereas its incorporation into splanchnic proteins was unchanged, leading to a 20-30% decrease in peripheral availability and anabolic use in HP-adapted compared with NP-adapted subjects having ingested the same protein load.

CONCLUSIONS

By combining clinical experimentation with compartmental modeling, we provide a global overview of postprandial dietary protein metabolism. Increasing prior protein intake was shown to reduce the postprandial retention of wheat protein nitrogen, mainly by diminishing the efficiency of its peripheral availability and anabolic use.

Pros and cons of L-arginine supplementation in disease

The amino acid arginine and one of its metabolites NO have gathered broad attention in the last decade. Although arginine is regarded as a conditionally essential amino acid in disease, L-arginine supplementation in severe illness has not found its way into clinical practice. This might be due to the invalid interpretation of results from studies with immune-enhancing diets containing L-arginine amongst other pharmaconutrients. However, not much attention is given to research using L-arginine as a monotherapy and the possibility of the alternative hypothesis: that L-arginine supplementation is beneficial in disease. The present review will discuss data from studies in healthy and diseased animals and patients with monotherapy of L-arginine to come to an objective overview of positive and negative aspects of L-arginine supplementation in disease with special emphasis on sepsis, cancer, liver failure and wound healing.

Gaining insight into microbial physiology in the large intestine: a special role for stable isotopes

The importance of the human large intestine for nutrition, health, and disease, is becoming increasingly realized. There are numerous indications of a distinct role for the gut in such important issues as immune disorders and obesity-linked diseases. Research on this long-neglected organ, which is colonized by a myriad of bacteria, is a rapidly growing field that is currently providing fascinating new insights into the processes going on in the colon, and their relevance for the human host. This review aims to give an overview of studies dealing with the physiology of the intestinal microbiota as it functions within and in interaction with the host, with a special focus on approaches involving stable isotopes. We have included general aspects of gut microbial life as well as aspects specifically relating to genomic, proteomic, and metabolomic studies. A special emphasis is further laid on reviewing relevant methods and applications of stable isotope-aided metabolic flux analysis (MFA). We argue that linking MFA with the '-omics' technologies using innovative modeling approaches is the way to go to establish a truly integrative and interdisciplinary approach. Systems biology thus actualized will provide key insights into the metabolic regulations involved in microbe-host mutualism and their relevance for health and disease.

Body fat deposition does not originate from carbohydrates in milk-fed calves

Milk-fed heavy calves utilize dietary protein with a low efficiency and often develop hyperglycemia and insulin resistance. Distributing the daily nutrient intake over an increasing number of meals increases protein deposition and improves glucose homeostasis. Therefore, we examined effects of feeding frequency (FF) and feeding level (FL) on the diurnal pattern of substrate oxidation and on the fate of dietary carbohydrates in milk-fed heavy calves. Eighteen milk-fed calves weighing 136 +/- 3 kg were assigned to FF (1, 2, or 4 meals daily) at each of 2 FL (1.5 or 2.5 times maintenance), except for calves at FF1 (only at a low FL). Urea, leucine, and glucose kinetics were assessed for each treatment by use of [(13)C]urea, [1-(13)C]leucine, [U-(13)C], and [2-(13)C]glucose, respectively. FF altered the diurnal pattern, but not the total, of urea production production. Although urea production correlated well with nitrogen retention, oxidation of oral l-[1-(13)C]leucine did not. Dietary glucose was almost completely oxidized (80% based on [(13)C]glucose and 94% from indirect calorimetry measurements) regardless of FL. Fatty acid synthesis from glucose appeared to be negligible based on similar recoveries of (13)CO(2) from orally supplied [U-(13)C]glucose and [2-(13)C]glucose. The increased fat deposition at the higher FL originated almost exclusively from greater transfer of fatty acids to body lipid stores. These findings contrast with both glucose and lipid metabolism in growing pigs and indicate that alternative adaptive mechanisms operate in heavy milk-fed calves.

A Milk Diet Partly Containing Soy Protein Does Not Change Growth but Regulates Jejunal Proteins in Young Goats

Soy protein is known to alter intestinal function and structure. We determined in young goats whether a diet partly containing soy protein differently affects intestinal morphology and the jejunal and hepatic proteome as compared with a milk diet. Fourteen male 2-wk-old White German dairy goat kids were fed comparable diets based on whole cow's milk in which 35% of the crude protein was casein (milk protein group; MP) or soy protein supplemented by indispensable AA (SPAA) for 34 d (n = 7/group). Body weight gain and food efficiency were not different. Jejunal and hepatic tissue was collected to determine intestinal morphology by microscopy and protein repertoire by 2-dimensional gel electrophoresis and mass spectrometry. Jejunal crypt depth was reduced and villus height to crypt depth ratio was higher in SPAA than in milk protein. Out of 131 proteins identified, 32 proteins were found to be differently expressed in both groups. In SPAA, down-regulated jejunal proteins were involved in processes related to cytoskeleton generation, protein, lipid, and energy metabolism. Downregulated hepatic proteins were related to glycolysis and Krebs cycle. Thirteen proteins were upregulated in SPAA. Among these, 2 hepatic proteins were related to carbohydrate breakdown. The other 11 jejunal proteins were involved in cytoskeleton assembly, proteolysis, and carbohydrate breakdown. In addition, glutathione-S-transferase was found to be upregulated in the medial jejunum. In conclusion, a SPAA diet as compared with a milk diet was related to changes in jejunal morphology and jejunal proteins relevant for protein turnover, energy metabolism, and cytoskeleton assembly with no apparent impact on animal BW gain.

Activation of skeletal muscle protein breakdown following consumption of soyabean protein in pigs

Diets with protein of inferior quality may increase protein breakdown in skeletal muscle but the experimental results are inconsistent. To elucidate the relationship, pigs were fed isoenergetic and isonitrogenous diets based on soyabean-protein isolate or casein for 15 weeks, with four to six animals per group. A higher plasma level of urea (2.5-fold the casein group value, P=0.01), higher urinary N excretion (2.1-fold the casein group value, P=0.01), a postabsorptive rise in the plasma levels of urea, 3-methylhistidine and isoleucine in soyabean protein-fed pigs suggested recruitment of circulatory amino acids by protein breakdown in peripheral tissues. Significant differences between dietary groups were detected in lysosomal and ATP-dependent proteolytic activities in the semimembranosus muscle of food-deprived pigs. A higher concentration of cathepsin B protein was found, corresponding to a rise in the cathepsin B activity, in response to dietary soyabean protein. Muscle ATP-stimulated proteolytical activity was 1.6-fold the casein group value (P=0.03). A transient rise in the level of cortisol (2.9-times the casein group value, P=0.02) occurred in the postprandial phase only in the soyabean group. These data suggest that the inferior quality of dietary soyabean protein induces hormonally-mediated upregulation of muscle protein breakdown for recruitment of circulatory amino acids in a postabsorptive state.

Studies on persisting effects of soy-based compared with amino acid-supplemented casein-based diet on protein metabolism and oxidative stress in juvenile pigs

Juvenile growing pigs were studied to explore whether a soy-based diet can induce persistent physiological alterations, especially in protein and energy metabolism, nutrient oxidation and redox homeostasis. In former studies we have shown that in juvenile pigs chronically fed protein diets based on either casein (CAS) or soy protein isolate (SPI), the SPI diet significantly decreases growth rate and increases oxidative stress responsiveness as compared to CAS. In addition, here we show that chronic feeding of SPI vs. CAS diet decreases whole body protein synthesis (WBPS) (p = 0.007) and hepatic gene expression associated with protein synthesis. To study persistent SPI effects, a three-period feeding experiment was designed: In the test group 18 pigs received the CAS diet for 24 days (period 1), followed by 31 days on the SPI diet (period 2) and further 31 days on the CAS diet (period 3). In the control group 18 pigs were fed the CAS diet throughout the three periods (86 days). Temporary consumption of SPI diet results in persistent changes of protein metabolism and oxidative stress responsiveness. After switching back from SPI to CAS diet the decrease of WBPS of the test group vs. control group was of borderline significance (p = 0.061), transcript levels of hepatic gene expressions of leucine aminopeptidase, endopeptidase 24.16, glutathione-S-transferase and peptide methionine sulfoxide reductase were increased. In liver tissue, total glutathione was increased and thiobarbituric acid reactive substances were decreased in the test vs. control group. In conclusion, results suggest that SPI-induced changes in protein and amino acid metabolism as well as in redox homeostasis and antioxidative potential in growing pigs persist 4 weeks after the cessation of SPI feeding.

Feeding meals containing soy or whey protein after exercise stimulates protein synthesis and translation initiation in the skeletal muscle of male rats

The purpose of this investigation was to compare the early response of skeletal muscle protein synthesis and translation initiation following the ingestion of different protein sources after endurance exercise. Treadmill-acclimated rats were designated as either nonexercised controls (NEX) or treadmill exercised for 2 h at 26 m/min (approximately 75% VO2max) and then fed either carbohydrate only (EC), carbohydrate plus soy protein (ES), or carbohydrate plus whey protein (EW). One hour after exercise, serum insulin concentrations in EC, ES, and EW were greater than in NEX (P<0.05); the concentration in EW was greater than in EC, with that in ES intermediate. Serum concentrations of branched-chain amino acids in ES and EW were higher than in EC, but serum leucine and isoleucine in EW were higher than in ES (P<0.05). Nevertheless, both ES and EW promoted the fractional rate of skeletal muscle protein synthesis significantly more than EC. Likewise, compared with EC, both ES and EW increased formation of the mRNA cap binding complex eIF4F and stimulated phosphorylation of the translational repressor, 4E-BP1, the 70kD ribosomal protein S6 kinase (S6K1), and the mammalian target of rapamycin (mTOR) kinase at serine 2448. On the other hand, phosphorylation of S6K1 and mTOR was greater in EW than in ES (P<0.05). In conclusion, general protein synthesis and the mRNA cap binding step are promoted comparably by soy protein and whey protein in the skeletal muscle of exercised rats. Furthermore, the data suggest that mTOR signaling in skeletal muscle is acutely responsive to physiological variations in dietary amino acids.

Supplementation of soy protein with branched-chain amino acids alters protein metabolism in healthy elderly and even more in patients with chronic obstructive pulmonary disease

BACKGROUND

It is often suggested that chronic wasting diseases [eg, chronic obstructive pulmonary disease (COPD)] may benefit from branched-chain amino acid (BCAA) administration via improved protein metabolism.

OBJECTIVE

The aim was to examine whether adding BCAAs to a soy protein meal would enhance protein anabolism in COPD patients and in healthy elderly persons.

DESIGN

Eight normal-weight COPD patients and 8 healthy control subjects were examined on 2 test days. Simultaneous continuous intravenous infusion of l-[ring-(2)H(5)]phenylalanine (Phe) and l-[ring-(2)H(2)]tyrosine tracers was done postabsorptively and at 2 h of ingestion of a maltodextrin soy or maltodextrin soy + BCAA protein meal (rate of ingestion: 0.02 g protein.kg body weight(-1).20 min(-1)) in a crossover design. Together with the meal, oral ingestion of 1-[(13)C]Phe was performed to measure first-pass Phe splanchnic extraction (SPE(Phe)). The endogenous rate of Phe appearance [reflecting whole-body protein breakdown (WbPB)], whole-body protein synthesis (WbPS), and net WbPS (WbPS - WbPB) were calculated. Arterialized venous blood was sampled for amino acid enrichment and concentration analyses.

RESULTS

Soy feeding induced a reduction in WbPB and an increase in WbPS. BCAA supplementation of soy protein resulted in a significantly higher (P < 0.05) increase in WbPS than did soy protein alone in COPD patients but not in the healthy elderly. BCAA supplementation did not significantly alter the change in WbPB or net WbPS. Furthermore, BCAA supplementation decreased (absolute) SPE(Phe) (P < 0.05) but did not change the percentage Phe hydroxylation in the splanchnic area, which indicates a BCAA-related reduction in splanchnic protein synthesis.

CONCLUSION

BCAA supplementation to soy protein enhances WbPS in patients with COPD and alters interorgan protein metabolism in favor of the peripheral (muscle) compartment in healthy elderly and even more in COPD patients.

Conceptual, methodological and computational issues concerning the compartmental modeling of a complex biological system: Postprandial inter-organ metabolism of dietary nitrogen in humans

A multi-compartmental model has been developed to describe dietary nitrogen (N) postprandial distribution and metabolism in humans. This paper details the entire process of model development, including the successive steps of its construction, parameter estimation and validation. The model was built using experimental data on dietary N kinetics in certain accessible pools of the intestine, blood and urine in healthy adults fed a [15N]-labeled protein meal. A 13-compartment, 21-parameter model was selected from candidate models of increasing order as being the minimum structure able to properly fit experimental data for all sampled compartments. Problems of theoretical identifiability and numerical identification of the model both constituted mathematical challenges that were difficult to solve because of the large number of unknown parameters and the few experimental data available. For this reason, new robust and reliable methods were applied, which enabled (i) a check that all model parameters could theoretically uniquely be determined and (ii) an estimation of their numerical values with satisfactory precision from the experimental data. Finally, model validation was completed by first verifying its a posteriori identifiability and then carrying out external validation.

Adequate range for sulfur-containing amino acids and biomarkers for their excess: lessons from enteral and parenteral nutrition

The adequacy range of dietary requirements of specific amino acids in disease states is difficult to determine. In health, several techniques are available allowing rather precise quantification of requirements based on growth of the organism, rises in plasma concentration, or increases in the oxidation of marker amino acids during incremental administration of the amino acid under study. Requirements may not be similar in disease with regard to protein synthesis or with regard to specific functions such as scavenging of reactive oxygen species by compounds including glutathione. Requirements for this purpose can be assessed only when such a function can be measured and related to clinical outcome. There is apparent consensus concerning normal sulfur amino acid (SAA) requirements. WHO recommendations amount to 13 mg/kg per 24 h in healthy adults. This amount is roughly doubled in artificial nutrition regimens. In disease or after trauma, requirements may be altered for methionine, cysteine, and taurine. Although in specific cases of congenital enzyme deficiency, prematurity, or diminished liver function, hypermethionemia or hyperhomocysteinemia may occur, SAA supplementation can be considered safe in amounts exceeding 2-3 times the minimal recommended daily intake. Apart from some very specific indications (e.g., acetaminophen poisoning), the usefulness of SAA supplementation is not yet established. There is a growing body of data pointing out the potential importance of oxidative stress and resulting changes in redox state in numerous diseases including sepsis, chronic inflammation, cancer, AIDS/HIV, and aging. These observations warrant continued attention for the potential role of SAA supplementation. In particular, N-acetylcysteine remains promising for these conditions.

Final report of the safety assessment of Urea

Although Urea is officially described as a buffering agent, humectant, and skin-conditioning agent-humectant for use in cosmetic products, there is a report stating that Urea also is used in cosmetics for its desquamating and antimicrobial action. In 2001, the Food and Drug Administration (FDA) reported that Urea was used in 239 formulations. Concentrations of use for Urea ranged from 0.01% to 10%. Urea is generally recognized as safe by FDA for the following uses: side-seam cements for food contact; an inhibitor or stabilizer in pesticide formulations and formulations applied to animals; internal sizing for paper and paperboard and surface sizing and coating of paper and paper board that contact water-in-oil dairy emulsions, low-moisture fats and oils, moist bakery products, dry solids with surface containing no free fats or oil, and dry solids with the surface of fat or oil; and to facilitate fermentation of wine. Urea is the end product of mammalian protein metabolism and the chief nitrogenous compound of urine. Urea concentrations in muscle, liver, and fetuses of rats increased after a subcutaneous injection of Urea. Urea diffused readily through the placenta and into other maternal and fetal organs. The half-life of Urea injected into rabbits was on the order of several hours, and the reutilization rate was 32.2% to 88.8%. Urea given to rats by a bolus injection or continuous infusion resulted in distribution to the following brain regions: frontal lobe, caudate nucleus, hippocampus, thalamus plus hypothalamus, pons and white matter (corpus callosum). The permeability constant after treatment with Urea of whole skin and the dermis of rabbits was 2.37 +/- 0.13 (x 10(6)) and 1.20 +/- 0.09 (x10(3)) cm/min, respectively. The absorption of Urea across normal and abraded human skin was 9.5% +/- 2.3% and 67.9% +/- 5.6%, respectively. Urea increased the skin penetration of other compounds, including hydrocortisone. No toxicity was observed for Urea at levels as high as 2000 mg/kg in acute oral studies using female rats or mice. No signs of toxicity were observed in male piglets dosed orally with up to 4 g/kg Urea for 5 days. Dogs dosed orally with 5 to 30 g/L Urea for 4 to 10 days had signs of toxicity, including weakness, anorexia, vomiting and retching, diarrhea and a decreased body temperature, which led to a deep torpor or coma. No significant microscopic changes were observed in the skin of male nude mice dermally exposed to 100% Urea for 24 h. No observable effect on fetal development was seen in rats and mice dosed orally with an aqueous solution of Urea (2000 mg/kg) on days 10 and 12 of gestation. The mean number of implants, live fetuses, percent fetal resorptions, mean fetal weight, and percent fetuses malformed were comparable to control group. A detergent containing 15% Urea was injected into pregnant ICR-JCl mice and dams and fetuses had no significant differences when compared to control animals. Urea given orally did not enhance the developmental toxicity of N-nitrosomethylurea. Female Sprague-Dawley rats injected in the uterine horn with 0.05 ml Urea on day 3 (preimplantation) or on day 7 (post implantation) exhibited no maternal mortality or morbidity; a dose-dependent reduction in embryo survival was seen with preimplantation treatment. Urea injected intra-amniotically induces mid-trimester abortions in humans. Urea was not genotoxic in several bacterial and mammalian assays; although in assays where Urea was used at a high concentration, genotoxicity was found, many in in vitro assays. Urea is commonly used in studies of DNA because it causes uncoiling of DNA molecules. Urea was not carcinogenic in Fisher 344 rats or C57B1/6 mice fed diets containing up to 4.5% Urea. Exposure of normal human skin to 60% Urea produced no significant irritation in one study, but 5% Urea was slightly irritating and 20% Urea was irritating in other reports. Burning sensations are the most frequently reported effect of Urea used alone or with other agents in treatment of diseased skin. Overall, there are few reports of sensitization among the many clinical studies that report use of Urea in treatment of diseased skin. The Cosmetic Ingredient Review (CIR) Expert Panel determined the data provided in this report to be sufficient to assess the safety of Urea. The Panel did note that Urea can cause uncoiling of DNA, a property used in many DNA studies, but concluded that this in vitro activity is not linked to any in vivo genotoxic activity. Although noting that formulators should be aware that Urea can increase the percutaneous absorption of other chemicals, the CIR Expert Panel concluded that Urea is safe as used in cosmetic products.

Beyond diversity: functional microbiomics of the human colon

Molecular tools have revealed wide microbial diversity in the human alimentary tract. Most intestinal microorganisms have not been cultured and the in situ functions of distinct groups of the intestinal microbiota are largely unknown but pivotal to understanding the role of these microorganisms in health and disease. Promising strategies to gain more insight into the functionality of the complex microbial communities in the human alimentary tract, including fermentation processes in the colon, are discussed. These research approaches could provide a basis for the definition of a healthy gut based on key properties of microbial functionality. This will also enable the development of direct nutritional strategies for intestinal disease prevention and health promotion.

Casein and Soy Protein Meals Differentially Affect Whole-Body and Splanchnic Protein Metabolism in Healthy Humans

Dietary protein quality is considered to be dependent on the degree and velocity with which protein is digested, absorbed as amino acids, and retained in the gut as newly synthesized protein. Metabolic animal studies suggest that the quality of soy protein is inferior to that of casein protein, but confirmatory studies in humans are lacking. The study objective was to assess the quality of casein and soy protein by comparing their metabolic effects in healthy human subjects. Whole-body protein kinetics, splanchnic leucine extraction, and urea production rates were measured in the postabsorptive state and during 8-h enteral intakes of isonitrogenous [0.42 g protein/(kg body weight . 8 h)] protein-based test meals, which contained either casein (CAPM; n = 12) or soy protein (SOPM; n = 10) in 2 separate groups. Stable isotope techniques were used to study metabolic effects. With enteral food intake, protein metabolism changed from net protein breakdown to net protein synthesis. Net protein synthesis was greater in the CAPM group than in the SOPM group [52 +/- 14 and 17 +/- 14 nmol/(kg fat-free mass (FFM) . min), respectively; P < 0.02]. Urea synthesis rates decreased during consumption of both enteral meals, but the decrease tended to be greater in the subjects that consumed CAPM (P = 0.07). Absolute splanchnic extraction of leucine was higher in the subjects that consumed CAPM [306 +/- 31 nmol/(kg FFM . min)] vs. those that consumed SOPM [235 +/- 29 nmol/(kg FFM . min); P < 0.01]. In conclusion, a significantly larger portion of soy protein is degraded to urea, whereas casein protein likely contributes to splanchnic utilization (probably protein synthesis) to a greater extent. The biological value of soy protein must be considered inferior to that of casein protein in humans.

Postprandial intestinal and whole body nitrogen kinetics and distribution in piglets fed a single meal

Our aim was to characterize the postprandial total and dietary N fluxes in the portal drained viscera (PDV) and whole body after administration of a single meal in young pigs. Seven 4-wk-old piglets, implanted with a portal flow probe and portal, arterial and venous catheters, received a primed constant [(18)O]urea intravenous infusion and were studied for 8 h after a bolus mixed meal ingestion (46 mmol N/kg body wt) intrinsically labeled with (15)N to trace dietary N fluxes. The real cecal digestibility of the formula was 94.3% (SD 1.8). PDV output of dietary N was found principally in the pool of circulating protein (51% of the measured dietary N PDV output), in the free alpha-amino N pool (44%), and to a lesser extent in ammonia (5%). Dietary N release in alpha-amino N and ammonia mainly occurred during the first 3 h. Total and exogenous postprandial urea productions were 5.8 and 2.0 mmol N/kg body wt, respectively. At the end of the postprandial period, losses of dietary N amounted to 10.3% of the dose: 5.7% through ileal losses and 4.6% by deamination and transfer to urea. Net postprandial retention of dietary N was 90.4% (SD 1.3), of which 20% was found in splanchnic zone (small intestine 10%, liver 5%, and plasma protein 3%) and 42% in peripheral zone (muscle 31%, skin 6%). In conclusion, our results show a high efficiency of dietary N utilization for muscular uptake and anabolic utilization. However, the results obtained point out the necessity to further explore the form of dietary N released into the portal blood.

Amino Acid Adequacy in Pathophysiological States

Amino acid utilization and, therefore, demand differ between the healthy state and various disease states. In the healthy state most circulating amino acids are derived from dietary proteins that are stored and broken down in the gut and released gradually into the portal circulation, and from continuous turnover of body protein. In disease states, the amino acid composition of amino acids derived from periferal protein breakdown and released in the circulation, is different, for example because a substantial part of the branched-chain amino acids is broken down to yield glutamine and alanine, which are released in the circulation. It appears to be advantageous to mimic this continuous autoinfusion in patients, dependent of parenteral of enteral tube feeding. In disease, different endpoints should be used to assess the adequacy of the administered amino acid mix. Maintenance of a positive nitrogen balance and growth is less important than support of wound healing and immune function. Several amino acids such as glutamine, cysteine, and taurine are shown or suggested to be conditionally essential in disease, and to form substrate in the stressed patient for anabolic processes in liver, immune system, and injured sites. Amino acid toxicity is rare, and protein restriction for patients with renal or liver failure is obsolete because this only aggravated malnutrition. A true example of protein toxicity consists of gastrointestinal hemorrhage that precipitates hepatic encephalopathy in liver insufficiency, most likely because hemoglobin is an unbalanced protein because it lacks the essential amino acid isoleucine.

Animal models of amino acid metabolism: a focus on the intestine

One important advantage of animal models is that they permit invasive approaches and can be especially valuable when evaluating tissue and specific features of metabolism in situ. The focus of this presentation is current models, which are providing insights into the pivotal importance of the gastrointestinal tract in amino acid metabolism. Intestinal amino acid metabolism is conceptually and technically difficult to approach and multiple processes must be accounted for: protein synthesis and degradation; transit of amino acids in both directions across the basolateral surface of enterocytes, in addition to uptake on the apical side; arterio-portal differences as well as net portal appearance during uptake of defined amino acid mixtures appearing on the luminal side; first pass amino acid metabolism. These key features are largely impossible to study without access to invasive approaches in vivo and cannot be reproduced in vitro. Douglas Burrin, Ron Ball, and Vickie Baracos and their co-workers have used the domestic piglet to study intestinal protein metabolism in situ in three distinctly different and complementary approaches. Collectively, their approaches allow a means to describe the key elements of intestinal amino acid capture (and release) and the means to probe their physiological and pathological variation. It seems evident that the portal-drained viscera represent sites of quantitatively important amino acid catabolism, and that this capacity combined with hepatic metabolism would largely limit the possibility of toxic sequelae of amino acids taken orally.

Determination of 13C and 15N enrichments of urea in plasma by gas chromatography-combustion isotope ratio mass spectrometry and gas chromatography-mass spectrometry using the 2-methoxypyrimidine derivative

We describe a GC-MS and GC-c-IRMS method for the determination of labeled urea tracer enrichments in plasma as a result of combined 13C- and 15N(2)-urea infusion experiments in piglets. Urea was converted into 2-methoxypyrimidine, a stable derivative, suited for analyses by both GC-MS and GC-c-IRMS. Using calibration curves for the respective working ranges (13C-urea: 0-1% APE; 15N(2)-urea: 0-7% MPE) enrichments were established in single point measurements; for 15N(2)-urea as values+/-0.15% MPE (95% confidence interval); for 13C-urea as values+/-0.02% APE (95% confidence interval). 15N(1)-urea enrichments were determined by measurement of the same sample with GC-c-IRMS and GC-MS. Subtraction of the 13C specific GC-c-IRMS data from the nondiscriminating GC-MS data for the sum of 13C- and 15N(1)-urea resulted in 15N(1)-urea enrichments+/-0.15% MPE (95% confidence interval). Application of the method in a combined 13C-urea bolus and 15N(2)-urea primed constant infusion experiment in piglet was demonstrated.

Contribution of plasma proteins to splanchnic and total anabolic utilization of dietary nitrogen in humans

Splanchnic tissues are largely involved in the postprandial utilization of dietary amino acids, but little is yet known, particularly in humans, about the relative contributions of different splanchnic protein pools to splanchnic and total postprandial anabolism. Our aim was to develop a compartmental model that could distinguish dietary nitrogen (N) incorporation among splanchnic constitutive, plasma (splanchnic exported), and peripheral proteins after a mixed-protein meal in humans. Eight healthy subjects were fed a single mixed meal containing 15N-labeled soy protein, and dietary N postprandial kinetics were measured in plasma free amino acids, proteins, and urea and urinary urea and ammonia. These experimental data and others previously obtained for dietary N kinetics in ileal effluents under similar experimental conditions were used to develop the compartmental model. Six hours after the mixed-meal ingestion, 31.5, 7.5, and 21% of ingested N were predicted to be incorporated into splanchnic constitutive, splanchnic exported, and peripheral proteins, respectively. The contribution of splanchnic exported proteins to total splanchnic anabolism from dietary N was predicted to be approximately 19% and to remain steady throughout the simulation period. Model behavior and its predictions were strongly in line with current knowledge of the system and the scarce, specific data available in the literature. This model provides the first data concerning the anabolism of splanchnic constitutive proteins in the nonsteady postprandial state in humans. By use of only slightly invasive techniques, this model could help to assess how the splanchnic anabolism is modulated under different nutritional or pathophysiological conditions in humans.