Physiological approaches to determining gut endogenous amino acid flows in the mammal

Determination of Plasma Metabolites and Basal Ileal Endogenous Amino Acid Losses in Growing Pigs Fed a Nitrogen-Free or Casein Diet

The nitrogen-free diet (NFD) method for determining basal ileal endogenous losses (BEL) of amino acids (AA) has been associated with different metabolic abnormalities that can affect the accurate determination of BEL. Consequently, the use of highly digestible proteins has been suggested. This study aimed to determine the metabolic status and BEL of AA in pigs fed either an NFD or a casein (CAS) diet. Eight cannulated, castrated male pigs (39.8 kg) were randomly assigned to a 2 × 2 crossover design. An NFD diet based on corn starch, dextrose, cellulose, oil, vitamins and minerals was used. The CAS diet was equivalent, but 18% of the corn starch was replaced with casein. Pigs were fed one of the diets for a 7-day period, and blood samples were collected at the beginning and end of each period to determine plasma metabolites. Ileal digesta samples were collected on Days 6 and 7 to estimate the BEL of the AA. Results indicated that plasma albumin was significantly higher (p < 0.05) in pigs fed the CAS diet, whereas creatinine and LDL levels were higher (p < 0.03) in pigs fed the NFD. No significant differences were observed in the levels of other plasma metabolites. The BEL of protein did not differ between diets. However, in pigs fed the CAS diet, the BEL of glutamic acid, aspartic acid, serine, glycine, histidine, threonine, alanine, tyrosine and valine significantly increased (p < 0.05), while isoleucine showed a tendency to increase (p = 0.06). In conclusion, NFD did not significantly affect energy and lipid metabolism in pigs. However, the decrease in albumin synthesis and increase in plasma creatinine levels indicate that pigs fed NFD have a negative protein balance, affecting the estimation of the BEL of AA. Therefore, it is essential to consider the metabolic state of animals when estimating the BEL of AA.

Evaluation of Protein Quality in Humans and Insights on Stable Isotope Approaches to Measure Digestibility - A Review

The recent Food and Agricultural Organization/World Health Organization/United Nations University expert consultations on protein requirements and quality have emphasized the need for the new Digestible Indispensable Amino Acid Score (DIAAS), as a measure of protein quality. This requires human measurements of the true ileal digestibility of individual indispensable amino acids (IAAs) until the end of the small intestine. Digestibility is measured using standard oro-ileal balance methods, which can only be achieved by an invasive naso-ileal intubation in healthy participants or fistulation at the terminal ileum. Significant efforts have been made over the last 2 decades to develop noninvasive or minimally invasive methods to measure IAA digestibility in humans. The application of intrinsically labeled (with stable isotopes like 13C, 15N, and 2H) dietary proteins has helped in circumventing the invasive oro-ileal balance techniques and allowed the differentiation between endogenous and exogenous protein. The noninvasive indicator amino acid oxidation (IAAO) technique, which is routinely employed to measure IAA requirements, has been modified to estimate metabolic availability (a sum of digestibility and utilization) of IAA in foods, but provides an estimate for a single IAA at a time and is burdensome for participants. The recently developed minimally invasive dual isotope tracer method measures small intestinal digestibility of multiple amino acids at once and is suitable for use in vulnerable groups and disease conditions. However, it remains to be validated against standard oro-ileal balance techniques. This review critically evaluates and compares the currently available stable isotope-based protein quality evaluation methods with a focus on the digestibility and metabolic availability measurements in humans. In view of building a reliable DIAAS database of various protein sources and subsequently supporting protein content claims in food labeling, a re-evaluation and harmonization of the available methods are necessary.

Labelling fish diets with 15 N -Leucine for monitoring feed consumption and bio-distribution in Atlantic salmon

Feeding represents 50-70% of the cost of production in salmon farming, higher than any other animal farm. The improvement of this percentage is challenging as the food is thrown into the fish tank, there is no quantification of the amount of food that is consumed by the fish. In consequence, it is difficult to adjust the food composition making it more nutritive or promoting food consumption by fish. In this study, to investigate food consumption, bio-distribution and food residues, leucine containing 15 N (a stable isotope of nitrogen) was used to label the fish food. Atlantic salmon (Salmo salar) weighing 100-120 g were maintained in 30 L tanks at a density of 14 kg/m3 . Fishes were fed daily at 1% of the fish weight with pellet labelled with 15 N-leucine. The 15 N incorporation was determined 14 hours after the feeding in all the fish organs. Results showed that 14 hours after the administration of a single dose of labelled food to Atlantic salmon enables the detection of the tracer in the whole organism allowing determining the food consumption. Through the analysis of nitrogen use efficiency (NUE), we showed that the trunk, pyloric caeca and head incorporate the highest level of the marker (72.7, 8.7 and 5.7%, respectively). This methodology would permit monitoring feeding to minimize food loss, improve administration methodologies or select the preferred foods for the fish, among others to reduce production costs.

Bioactive Peptides Originating from Gastrointestinal Endogenous Proteins in the Growing Pig: In Vivo Identification

BACKGROUND

Recent in silico and in vitro studies have shown that gastrointestinal endogenous proteins (GEP) are a source of bioactive peptides. To date, however, the presence of such peptides in the lumen of the digestive tract has not been demonstrated.

OBJECTIVE

We investigated the generation of GEP-derived bioactive peptides in the growing pig fed a proteinfree diet.

METHODS

Stomach chyme (SC) and jejunal digesta (JD) fractions from 6 growing pigs (two sampling times) were assessed for their angiotensin-I-converting enzyme (ACE-I; EC 3.4.15.1) inhibition, and antioxidant activity using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) inhibition, ferric reducing antioxidant power (FRAP) and microsomal lipid peroxidation (MLP) inhibition assays.

RESULTS

Two of the fractions prepared from JD samples inhibited ACE-I and DPPH by 81 (± 2.80)% and 94 (±0.66)%. SC fractions were found to inhibit MLP between 15-39 (±3.52-1.40)%. The study identified over 180 novel peptide sequences that were related to the determined bioactivities, including a porcine serum albuminderived peptide (FAKTCVADESAENCDKS), corresponding to f(7-23) of the human serum albumin peptide LVNEVTEFAKTCVADESAENCDKSLHTLF that was previously identified from the digests of the latter GEP.

CONCLUSION

This study provides the first in vivo evidence for GEP as a source of bioactive peptides. These new findings help advance our knowledge of the latent bioactive role of GEP-derived peptides in mammalian nutrition and health and their potential pharmaceutical applications.

Bioactive Peptides in Preventative Healthcare: An Overview of Bioactivities and Suggested Methods to Assess Potential Applications

Food derived bioactive peptides can be generated from various protein sources and usually consist of between 2-30 amino acids with bulky, side-chain aromatic amino acids preferred in the ultimate and penultimate positions at the C-terminal end of the amino acid chain. They are reported to impart a myriad of preventative health beneficial effects to the consumer once ingested and these include heart health benefits through inhibition of enzymes including renin (EC 3.4.23.15) and angiotensin- I-converting enzyme (ACE-1; EC 3.4.15.1) within the renin angiotensin aldosterone system (RAAS) anti-inflammatory (due to inhibition of ACE-I and other enzymes) and anti-cancer benefits, prevention of type-2 diabetes through inhibition of dipeptidyl peptidase IV (DPP-IV), bone and dental strength, antimicrobial and immunomodulatory effects and several others. Peptides have also reported health benefits in the treatment of asthma, neuropathic pain, HIV and wound healing. However, the structure, amino acid composition and length of these peptides, along with the quantity of peptide that can pass through the gastrointestinal tract and often the blood-brain barrier (BBB), intact and reach the target organ, are important for the realisation of these health effects in an in vivo setting. This paper aims to collate recent important research concerning the generation and detection of peptides in the laboratory. It discusses products currently available as preventative healthcare peptide options and relevant legislation barriers to place a food peptide product on the market. The review also highlights useful in silico computer- based methods and analysis that may be used to generate specific peptide sequences from proteins whose amino acid sequences are known and also to determine if the peptides generated are unique and bioactive. The topic of food-derived bioactive peptides for health is of great interest to scientific research and industry due to evolving drivers in food product innovation, including health and wellness for the elderly, infant nutrition and optimum nutrition for sports athletes and the humanisation of pets. This paper provides an overview of what is required to generate bioactive peptide containing hydrolysates, what methods should be used in order to characterise the beneficial health effects of these hydrolysates and the active peptide sequences, potential applications of bioactive peptides and legislative requirements in Europe and the United States. It also highlights success stories and barriers to the development of peptide-containing food products that currently exist.

Progress in ileal endogenous amino acid flow research in poultry

The progress in our understanding of the endogenous protein concept over the past century is reviewed. Non-dietary proteins found in the digesta at the terminal ileum of poultry, known as endogenous protein loss, are comprised of digestive secretions, mucus and sloughed gut epithelial cells. The measurement of this loss is of fundamental importance because it is an indicator of gut metabolism and is essential to adjust apparent estimates of ileal amino acid digestibility. The ileal endogenous amino acid losses comprise of two components, namely basal and specific losses. The basal losses are fixed and associated with feed dry matter intake, whereas the specific losses are variable and induced by the presence of dietary components such as fibre and anti-nutrients. Currently there is no methodology available to directly measure the specific endogenous losses and these losses are calculated by determining the basal and total (basal plus specific) losses and, then subtracting the basal losses from total losses. The seminal features, specific applications and shortcomings of available methodologies are briefly outlined as well as the practical challenges faced in using the published endogenous amino acid loss values for true digestibility corrections. The relevance of taurine as a component of endogenous protein flow in poultry is identified for the first time.

Determination of Dietary Amino Acid Digestibility in Humans

Dietary amino acid digestibility is a fundamental measure of importance in protein quality evaluation. Determining amino acid digestibility in humans, as the disappearance of an amino acid across the total digestive tract, has been discredited. Extensive cecal and colonic microbial metabolism renders fecal estimates of amino acids misleading. True ileal amino acid digestibility determined at the end of the small intestine predicts amino acid uptake more accurately. Given that ileal digestibility determination cannot be undertaken routinely in humans, a pig-based assay has been developed and validated. The growing pig values for digestibility, however, relate to healthy adult humans and there is a need to be able to determine amino acid digestibility for humans with specific physiological states. To this end, isotope-based methods for determining dietary amino acid digestibility indirectly show promise but remain to be fully validated.

Protein quality as determined by the Digestible Indispensable Amino Acid Score: evaluation of factors underlying the calculation

The Food and Agriculture Organization of the United Nations recently recommended the adoption of a new and improved scoring system (Digestible Indispensable Amino Acid Score [DIAAS]) to quantify dietary protein quality. The DIAAS is based on the relative digestible content of the indispensable amino acids (IAAs) and the amino acid requirement pattern. Factors involved in calculation of the DIAAS include: use of the content and profile of IAAs as the basis for quality; methods for determination of the protein and amino acid content of the protein source; accuracy of individual requirement values for IAAs; normalization of IAA requirements by the estimated average requirement for protein; and basing the DIAAS on the true ileal digestibility of each IAA in the test protein. This review outlines the rationale for including each of these factors in the calculation of the DIAAS and describes associated potential errors.

Dietary protein structure affects endogenous ileal amino acids but not true ileal amino acid digestibility in growing male rats

BACKGROUND

The amount of endogenous, as opposed to undigested dietary, protein in digesta is a measure of fundamental interest related to gut physiology and function.

OBJECTIVE

The objective of this study was to determine whether alimentation with proteins having differing amino acid compositions influenced endogenous ileal amino acids (EIAAs) and true ileal amino acid digestibility (TIAAD) values.

METHODS

Male rats (n = 8) were fed a purified diet containing 100 g/kg of 1 of 5 protein hydrolysates, each derived from a different semipurified intact protein source [gelatin, beef muscle (BM), casein, soy protein isolate (SPI), and lactalbumin] devoid of antinutritional factors or fiber. The rats were fed their respective hydrolysate-based diet for 1 d after receiving the same diet but containing the corresponding intact protein source for 7 d. Titanium dioxide was used as an indigestible marker. Ileal digesta were collected after the rats were killed, and EIAAs were determined (precipitate + retentate) after centrifugation and ultrafiltration of the digesta. The TIAAD values of the intact protein sources were determined using EIAA flows based on each protein hydrolysate.

RESULTS

Mean EIAA flows differed (P < 0.05) across protein hydrolysates for most amino acids, with the mean ± SEM EIAA flows across amino acids being 262 ± 17, 253 ± 12, 248 ± 18, 226 ± 14, and 191 ± 20 mg/kg dry matter intake for the gelatin, BM, casein, SPI, and lactalbumin hydrolysates, respectively. The only difference (P < 0.05) for the mean EIAA flows across amino acids within each protein hydrolysate was observed between gelatin (262 ± 17 mg/kg) and lactalbumin (191 ± 20 mg/kg) hydrolysates. Except for Trp (P < 0.001) in gelatin and lactalbumin hydrolysates, EIAA flows determined using the casein hydrolysate were not different (P ≥ 0.05) from EIAA flows determined using the other protein hydrolysates. TIAAD values were not generally different (P ≥ 0.05) regardless of the hydrolysate used to determine the EIAA flows.

CONCLUSIONS

Protein source affected EIAA flows, although the differences had little effect on TIAAD. Enzyme hydrolyzed casein is a suitable model hydrolysate for determining TIAAD with the enzyme-hydrolyzed protein-ultrafiltration technique.

Endogenous proteins in the ileal digesta of adult humans given casein-, enzyme-hydrolyzed casein- or crystalline amino-acid-based diets in an acute feeding study

BACKGROUND/OBJECTIVES

To ascertain if the form of dietary nitrogen (free amino acids (AA), small peptides, or intact protein) affects the endogenous nitrogen containing substances lost from the upper digestive tract of humans.

SUBJECTS/METHODS

Digesta were collected via a naso-ileal tube from the terminal ileum of 16 adult humans in a single parallel study following an acute feeding regimen. Subjects were given an iso-nitrogenous and isocaloric test meal containing 150 g of casein (CAS) (n=6), enzyme-hydrolyzed casein (HCAS) (n=5) or crystalline AA (n=5) dissolved in 550 ml of water, as the sole sources of nitrogen.

RESULTS

The mean concentrations and flows of total nitrogen, protein nitrogen, and soluble protein nitrogen passing the terminal ileum were significantly higher (P <0.01) for the CAS and HCAS test-meal groups compared to the AA meal group. Dietary CAS and HCAS had a considerable influence on digesta mucin concentrations and flows compared to free AA (+41%). Only 3-4% of the total nitrogen remained unidentified.

CONCLUSIONS

The form of dietary nitrogen (protein, small peptides or free AA) had an acute effect upon the secretion or reabsorption of endogenous proteins in the small intestine of healthy humans, as evident from significant differences in both the quantity and composition of the proteins found in digesta at the end of the ileum.

Endogenous proteins in terminal ileal digesta of adult subjects fed a casein-based diet

BACKGROUND

Although there are several published estimates of the endogenous amino acid composition of ileal digesta in humans, to our knowledge, there are no systematic studies of ileal digesta endogenous proteins.

OBJECTIVES

We determined the nature and composition of endogenous nitrogen-containing substances lost from the upper digestive tract of humans.

DESIGN

Digesta were collected from the terminal ileum for a period of 8 h by using a nasoileal tube in 6 adult subjects fed a single meal that contained 22% of casein as the only source of nitrogen.

RESULTS

The total nitrogen that passed the terminal ileum was 39.3 mg/g native digesta dry matter. Of this amount, 86% was proteinaceous, ~60% was bacterial protein, ~7% was soluble-free protein, ~15% was mucin protein, and ~5% was protein from intact mucosal cells. For nonprotein nitrogen, ~5% of the total nitrogen was ammonia, and ~4% of the total nitrogen was urea. Bacterial and human mucosal cellular DNA nitrogen were collectively ~0.5% of the total nitrogen. Approximately 30% of the nonprotein nitrogen (4% of the total nitrogen) remained unidentified. This amount was assumed to include free amino acids, RNAs, amines, and the tetrapyrroles bilirubin and biliverdin. Bacterial nitrogen, combined with ammonia and urea nitrogen, represented >68% of total ileal nitrogenous losses.

CONCLUSIONS

Findings are presented on the endogenous nitrogen-containing compounds that left the terminal ileum. Of particular significance is the observation that mucin was the most abundant truly endogenous component within the terminal ileal digesta. Bacterial protein, which was strictly nondietary rather than endogenous, contributed the highest proportion, by far, of nondietary protein, the result of which makes a significant contribution to published estimates of ileal endogenous amino acids and protein. The high concentration of bacterial protein and the presence of ammonia and urea nitrogen indicate potentially substantial microbial activity within the human distal small intestine.

Dietary starch source influences in growing goats: the intestinal losses of endogenous nitrogen and amino acids

Four goats (20 (SD2·5) kg) fitted with ruminal, duodenal and ileal cannulae were used in a 4 × 4 Latin square design to estimate the effects of a dietary starch source on the duodenal and ileal flows of endogenous N (EN) and endogenous amino acids (EAA) in growing goats. Goats were fed total mixed rations containing four starch sources (mainly from maize (MR), wheat (WR), paddy (PR) and sorghum (SR) treatments). There were no significant (P>0·05) effects of the dietary starch source on the intestinal flows of EN and EAA. The duodenal flows of EN were 2·40, 2·39, 2·18 and 1·56 g/d for the MR, WR, PR and SR treatments, respectively, as determined by the difference method, and the duodenal flows of EAA were 10·76, 11·29, 10·95 and 10·96 g/d by estimation with the amino acid profile method. The flows of EN and EAA at the ileum were 1·17, 1·12, 1·01, 0·70 and 4·87, 4·95, 4·94, 4·99 g/d, respectively, as estimated by the water-soluble method. The average intestinal reabsorption of EN and EAA was 57·5 %, and the endogenous Leu by the MR treatment was significantly (P < 0·05) lower than that of the other three treatments. The present results indicate that losses of endogenous protein in the intestine were not affected by the dietary starch source.

Meta-analysis of phosphorus balance data from growing pigs

Many studies have highlighted concerns over current methods of determining endogenous P losses and P requirements in growing pigs. Therefore, a database containing observations on 350 pigs was assembled from various studies. Four functions for analyzing P balance data were considered: 1) a straight line, 2) a diminishing returns function (monomolecular), 3) a sigmoidal function with a fixed point of inflection (Gompertz), and 4) a sigmoidal function with a flexible point of inflection (Richards). The nonlinear functions were specifically reparameterized to assign biological meaning to the parameters. Meta-analysis of the data was conducted to estimate endogenous P excretion, maintenance requirement, and efficiency of utilization. Phosphorus retention was regressed against either available P intake or total P intake [all variables scaled by metabolic BW (BW(0.75))]. There was evidence of non-linearity in the data, and the monomolecular function provided the best fit to the data. The Richards equation did not fit the data well and appeared overparameterized. Estimates of endogenous P excretion of 14 and 17 mg/kg of BW(0.75) x d based on available and total P analysis, respectively, were predicted by the monomolecular equation, which were within the range reported in the literature. Maintenance requirement values of 15 mg of available P/kg of BW(0.75) x d and 37 mg of total P/kg of BW(0.75) x d were obtained, based on the monomolecular equation. Average efficiencies of conversion of dietary P to retained P were 65 and 36% for available and total P, respectively, with greater efficiency values calculated for low P intakes. Although the monomolecular equation fitted the data best, more observations at high P intakes/kg of BW(0.75) are required to determine conclusively whether P retention scaled by metabolic BW is linearly related to available or total P intake.

Dietary peptides increase endogenous amino acid losses from the gut in adults

BACKGROUND

Accurate estimates of endogenous ileal total nitrogen and amino acid flows are necessary to ascertain true dietary amino acid digestibility coefficients and for the factorial estimation of dietary amino acid requirements.

OBJECTIVE

The objective was to ascertain endogenous amino acid losses from the small bowel in human subjects consuming a protein-free diet or a diet with enzyme-hydrolyzed casein (EHC; MW <5000) as the sole source of nitrogen.

DESIGN

The subjects were 8 men and women with terminal ileum ileostomies after ulcerative colitis who consumed the protein-free and EHC-based diets in a crossover design. Each subject received each test diet in single meals followed by 2 consecutive 9-h total collections of digesta. Digesta samples for the EHC treatment were centrifuged and ultrafiltered (10 000 MW cutoff), with the precipitate-plus-retentate fraction (>10 000 MW) providing a measurement of endogenous ileal amino acids.

RESULTS

The mean endogenous flows for most of the amino acids and nitrogen were significantly (P < 0.05) higher when determined with the EHC-based diet than with the protein-free diet. Mean (n = 8) endogenous ileal nitrogen flows were 2061 and 4233 mug/g dry matter intake for the protein-free and EHC-based diets, respectively.

CONCLUSION

The traditional protein-free method underestimates endogenous ileal amino acid loss in adults.

Protein metabolism and the gut

This paper reviews the recent literature concerning the importance of the gut in extraintestinal protein metabolism. A growing body of evidence suggests that the gut modulates amino acid flux and inter-organ relationships in various metabolic states. This may be particularly true during the absorptive period, when the gut: (1) controls amino acid absorption; (2) may modulate catabolism and uptake for synthesis of absorbed amino acid; and (3) consequently influences the availability of liver and extrasplanchnic amino acids, as well as their pattern and kinetics through portal flow delivery.