The use of 15N-labeled dietary proteins for determining true ileal amino acid digestibilities is limited by their rapid recycling in the endogenous secretions of pigs

Comparison of the Dual Isotope Tracer Approach with Oro-Ileal Balance Method for Determination of Amino Acid Digestibility in Cannulated Pigs

BACKGROUND

The dual isotope tracer approach was developed as a less invasive alternative for the measurement of ileal amino acid (AA) digestibility.

OBJECTIVES

This study aimed to validate the dual isotope tracer approach with the standardized oro-ileal balance method in growing pigs.

METHODS

Eight pigs were fitted with jugular catheters and ileal T-cannulas. On the test day, feed containing intrinsically labeled 15N-milk protein and 13C-spirulina was provided every half hour for 240 min. Ileal digesta and 10 jugular blood samples were collected between 0 min and 540 min. Digesta samples were analyzed for isotopic enrichment, AA, and titanium concentrations for balance method calculations. Serum AA isotopic enrichment was measured for dual stable isotope tracer calculations.

RESULTS

Using the oro-ileal balance method, the mean ileal AA digestibility of milk protein concentrate was 97.8 ± 0.59% and of spirulina 81.5 ± 2.44% (mean ± standard deviation). Lysine digestibility was specifically evaluated, as it does not transaminate. Lysine digestibility of milk protein concentrate calculated according to the dual isotope tracer approach was 88.9 ± 8.35%, 9% point lower than the value obtained with the oro-ileal method (98.1 ± 0.36, P = 0.04). Moreover, Bland-Altman analysis showed that the difference between methods was higher, with lower mean lysine digestibility.

CONCLUSIONS

This study observed differences between the dual isotope tracer approach and the oro-ileal balance method for estimating lysine digestibility under the current experimental conditions with 6 pigs. This result may be due to methodological issues. Considering the use of 15N protein, conclusions on other AA that do not transaminate could not be drawn.

A conceptual mechanistic model of amino acid fluxes in the small intestine, taking the example of pig

During digestion, almost 50% of absorbed essential amino acids (AAs) are metabolised by intestinal tissue, thus not appearing directly in the portal vein. This value, which is referred to as first-pass metabolism, seems high in relation to the overall efficiency of AA use considered in growth models. Experimental studies of first-pass metabolism are complicated due to the presence of numerous metabolic fluxes in the intestine and to the dynamics of digestion and absorption. The aim of this study was to integrate current knowledge of the metabolic AA fluxes in the small intestine in a conceptual model of intestinal AA metabolism. The model was built as a series of 200 intestinal segments, each having the same structure. Each segment was composed of seven pools, representing the fate of a generic AA according to their location (i.e., luminal or intestinal), origin (i.e., dietary or endogenous), and form (i.e., as protein or as a free AA). The pools were connected by fluxes, representing the main fates of AA, such as saturable transport of luminal AA or homeostasis of free or protein-bound AA in intestinal tissue. To parameterise the model, data from the literature were used, as well as values considered as reasonable. Simulations were carried out over 24 h, with five meals during the day and fasting during the night. Representing the small intestine as a series of segments allowed to account both for its tubular structure and for changing luminal environment. During the day, the model simulated the uptake of AA from the intestine and export to the blood, while during the night it simulated the uptake of AA from the blood to ensure intestinal homeostasis. Approximately, half of dietary AAs absorbed were metabolised in first-pass by intestinal tissue (i.e., used for intestinal protein synthesis). Part of this intestinal protein was secreted in the lumen as endogenous protein, which was driven by the presence of digesta, and endogenous protein can be digested and absorbed in more distal segments. In vivo, only the apparent first-pass metabolism of AA can be measured due to the dynamics of AA recycling and the tubular structure of the small intestine. This model can be a valuable tool for research and education to simulate the impact of nutrition on intestinal AA metabolism.

Potential New Methods to Analyze Basal and Total Endogenous Protein Losses of Host and Bacterial Origin in Pigs

BACKGROUND

Current systems for assessing protein quality such as the Digestible Indispensable Amino Acid Score correct apparent amino acid (AA) digestibility for basal endogenous protein losses (bEPL), ignoring the potential influence of the diet on these losses. However, the quantification of total endogenous protein losses (tEPL) poses a challenge.

OBJECTIVES

To evaluate different methods for quantifying tEPL and bEPL, and to assess their potential in discriminating between tEPL originating from bacteria and host.

METHODS

Using an incomplete Youden square design, 12 ileal cannulated pigs received 10 different protein sources, and a nitrogen-free (NF) diet. Ileal digesta were collected on days 6 and 7 of each 1-wk feeding period, to quantify endogenous protein losses (EPL) and analyze apparent ileal digestibility. Ileal EPL were estimated based on 1) 16S-+18S gene copy quantitative polymerase chain reaction, 2) diaminopimelic acid (DAPA)+18S, 3) differential AA profiles in digesta, EPL, and bacteria, equaling tEPL, and 4) an NF diet and 5) whey protein isolate (WPI), equaling bEPL.

RESULTS

Ileal bEPL based on the NF and WPI method correlated moderately to highly (r = 0.69, P < 0.05), but the NF method probably underestimated bEPL. In pigs fed the WPI diet, EPL based on the WPI and AA profile method were highly correlated (r = 0.88, P < 0.01). Overall, tEPL based on the AA profile method were moderately correlated with the 16S+18S method (r = 0.58, P < 0.001), and DAPA+18S (r = 0.57, P < 0.001). Low correlations were observed between bacterial tEPL based on the AA profile method and 16S or DAPA. Host tEPL based on the AA profile method and 18S were weakly correlated (r = 0.39, P < 0.001).

CONCLUSIONS

The AA profile method seems the most appropriate method for tEPL quantification, whereas the WPI method is preferred for bEPL quantification. Despite challenges in distinguishing between bacterial and host EPL, it is evident that bacterial proteins substantially (on average 37%-83%, depending on method) contribute to the EPL.

Determination of ileal endogenous nitrogen losses and true ileal nitrogen digestibility during non-steady-state conditions of the 15N-isotope dilution technique

The aim was to determine ileal endogenous nitrogen losses (ENL) and true ileal N-digestibility (TD-N) under non-steady-state conditions of the 15N-isotope dilution technique (15N-IDT), using diets generating low and high ENL and compare results to those obtained under steady-state conditions. Twelve growing pigs (mean LW 22.4 kg) fitted with a post-valve T-caecum cannula were fed an enzyme-hydrolysed casein (EHC)-based diet or an EHC diet + 4% quebracho tannins (QT) and were labelled via continuous 15N-leucine i.v. infusion or twice daily oral 15N-leucine administration. Digesta were collected daily over three consecutive hours with blood plasma sampled on the four consecutive days after cessation of 15N-labelling. There was a significant effect of sampling day on the dilution factor. Endogenous N losses were significantly lower for the EHC than the EHC+QT diet (2.41 vs. 8.69 g/kg DMI), while no significant effect of sampling day was observed. The TD-N of the EHC+QT diet did not differ from the TD-N of the EHC diet (95.1 vs. 92.0%). A significant effect of sampling day was observed for TD-N with day 1 and 2, being higher than day 4. Non-steady-state conditions overestimated ENL by 25-28% as compared to 3 h collections in steady-state conditions, but the relative overestimation was similar for the EHC diet as for the EHC+QT diet. TD-N did not differ significantly compared to 12 h steady-state measurements, but comparison to 3 h steady-state measurements showed that non-steady-state conditions overestimated TD-N for the EHC+QT diet by 9%. However, on day 4 this overestimation disappeared. Using the 15N-IDT during non-steady-state conditions can provide valuable additional data on endogenous N losses and TD-N.

Use of Isotope-Labeled Body or Dietary Proteins to Determine Dietary Amino Acid Digestibility

Amino acid (AA) digestibility in humans has been determined conventionally based on oro-ileal AA disappearance. With this approach, it is necessary to account for undigested AAs of body origin (endogenous AAs) found in the ileal digesta. Determination of the endogenous AAs under physiological conditions is not straightforward, and the use of isotopes (labeled foods or body tissues) has been pivotal to advancing our understanding. The application of isotopes for determining gut endogenous AAs and AA digestibility is discussed as well as the types of digestibility coefficient generated (apparent, true, real) dependent upon methodology. Recently a new dual isotope-based method for determining ileal AA digestibility in humans has been developed that obviates the collection of ileal digesta. The dual isotope method, which awaits full validation, offers considerable promise for making noninvasive measures of AA digestibility in humans of different ages and physiological states.

Real ileal amino acid digestibility of pea protein compared to casein in healthy humans: a randomized trial

BACKGROUND

It is necessary to propose plant alternatives to animal proteins that are of good nutritional quality. Pea is a good candidate owing to its high protein content and its well-balanced amino acid (AA) profile.

OBJECTIVES

This study aimed to assess the real ileal AA and nitrogen digestibility (RIDAA and RIDN) of pea protein isolate as compared to milk casein in humans. It also aimed to evaluate their nutritional quality through calculation of the digestible indispensable amino acid score (DIAAS) and to determine the net postprandial protein utilization (NPPU).

METHODS

Fifteen healthy volunteers were included in a randomized, single-blinded, 2-arm, parallel-design trial. They were equipped with a naso-ileal tube. They ingested the test meals, which consisted of 9 successive portions of mashed potatoes containing either pea protein or casein, intrinsically labeled with nitrogen 15. Ileal content, plasma, and urine samples were collected regularly over an 8-h postprandial period.

RESULTS

The mean RIDAA values were 93.6% ± 2.9% for pea protein and 96.8% ± 1.0% for casein, with no difference between the sources (P = 0.22). Leucine, valine, lysine, and phenylalanine were significantly less digestible in pea than in casein. The RIDN values were 92.0% ± 2.7% and 94.0% ± 1.7% for pea protein and casein, respectively, and were not different (P = 0.11). The DIAAS was 1.00 for pea protein and 1.45 for casein. The NPPU was 71.6% ± 6.2% and 71.2% ± 4.9% for pea protein and casein, respectively (P = 0.88).

CONCLUSIONS

Although some AAs are less digestible in pea protein than in casein, the real ileal digestibility and the NPPU were not different. The DIAAS of 1.00 obtained for pea protein demonstrated its ability to meet all AA requirements. This study shows the potential of pea isolate as a high-quality protein. This study was registered at clinicaltrials.gov as NCT04072770.

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.

Intact and hydrolyzed casein lead to similar ileal endogenous protein and amino acid flows in adult humans

BACKGROUND

Endogenous amino acids (AAs) contribute to terminal ileal digesta and must be corrected for in determining coefficients of true ileal digestibility. Such estimates are also needed for the factorial calculation of AA requirements.

OBJECTIVE

The effect of the form of delivery of dietary AAs on endogenous nitrogen and AA flows at the end of the ileum was studied.

METHODS

Isotopically labeled 15N-casein (33-34 g), in either a hydrolyzed (HC) or intact (C) form, was included as the sole source of nitrogen in a mixed meal (320 mmol N) consumed by healthy adult humans equipped with a triple-lumen sampling tube in the small intestine. Ileal endogenous AA flows were determined by isotope dilution. An additional meal (A) containing a free AA mixture (306 mmol N) simulating the AA composition of casein was included. Serine was omitted from the AA mixture to allow direct determination of its ileal endogenous flow.

RESULTS

Endogenous N and AA flows did not differ (P > 0.05) for diets C and HC, with mean respective N flows of 728 and 617 mg/8 h (± pooled SD: 144 mg/8 h). Endogenous serine flow was similar (P > 0.05) for diets C, HC, and A [181, 169, and 191 mg/8 h (± 56 mg/8 h)]. Recycling of the 15N marker was determined to be ∼11%, suggesting that the 15N endogenous loss values could underestimate endogenous protein and AA losses by ∼6% (the proportion of recycled 15N divided by the sum of endogenous N and recycled 15N).

CONCLUSIONS

The 3 different forms of dietary AA delivery (free AAs, HC, or C) elicited similar ileal endogenous AA flows in the adult human.This study was registered at clinicaltrials.gov as NCT00873951.

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.

Quantifying the contribution of dietary protein to whole body protein kinetics: examination of the intrinsically labeled proteins method

Intrinsically labeled dietary proteins have been used to trace various aspects of digestion and absorption, including quantifying the contribution of dietary protein to observed postprandial amino acid and protein kinetics in human subjects. Quantification of the rate of appearance in peripheral blood of an unlabeled (tracee) amino acid originating from an intrinsically labeled protein (exogenous R_a) requires the assumption that there is no dilution of the isotope enrichment of the protein-bound amino acid in the gastrointestinal tract or across the splanchnic bed. It must also be assumed that the effective volume of distribution into which the tracer and tracee appear can be reasonably estimated by a single value and that any recycling of the tracer is minimal and thus does not affect calculated rates. We have assessed these assumptions quantitatively using values from published studies. We conclude that the use of intrinsically labeled proteins as currently described to quantify exogenous R_a systematically underestimates the true value. When used with the tracer-determined rates of amino acid kinetics, underestimation of exogenous R_a from the intrinsically labeled protein method likely translates to incorrect conclusions regarding protein breakdown, including the effect of a protein meal and the anabolic impact of the speed of digestion and absorption of amino acids. Estimation of exogenous R_a from the bioavailability of ingested protein has some advantages as compared with the intrinsically labeled protein method. We therefore conclude that the bioavailability method for estimating exogenous R_a is preferable to the intrinsically labeled protein method.

A Casein Hydrolysate Does Not Enhance Ileal Endogenous Protein Flows Compared With the Parent Intact Casein When Fed to Growing Pigs

BACKGROUND

The form of dietary nitrogen (free peptides or intact proteins) may influence the amount of endogenous amino acids found at the terminal ileum of the pig, and it has been speculated that hydrolyzed dietary protein may lead to increased endogenous amino acids.

OBJECTIVE

To compare the effect of dietary free peptides on ileal endogenous nitrogen and amino acid flows [ileal endogenous nitrogen flow (ENFL), ileal endogenous amino acid flow (EAAFL)] with that of peptides released naturally from dietary protein during digestion, from the same intact parent protein source.

METHODS

Six pigs (mean body weight: 34 kg) were equipped with a postvalve T-caecum cannula. Semisynthetic test diets contained the same 15N-labeled intact casein (C) or hydrolyzed casein (HC). Pigs received the test diets every sixth day and the corresponding unlabeled diets in the intervening 5-d periods. Digesta were pooled from 4 to 10 h postprandially. EAAFL and ENFL, calculated with reference to the dietary marker titanium dioxide, were determined by isotope dilution for C and HC.

RESULTS

Ileal EAAFL and ENFL (mean flows n = 5 of 1828 and 1912 μg/g of dry matter intake for diets HC and C, respectively) did not differ (P > 0.05) between pigs fed HC and C. Centrifugation and ultrafiltration of the HC digesta allowed an estimation of label recycling into gut endogenous proteins. Some 20% of ileal endogenous protein (diet HC, ultrafiltered digesta) was 15N-labeled due to tracer recycling.

CONCLUSIONS

The administration of a casein hydrolysate had no effect on ileal endogenous protein flows compared with C. There was no evidence of enhanced ileal endogenous protein losses with the HC diet.

Estimation of endogenous amino acid losses in growing chickens given soya-bean meal supplemented or not with DL-methionine

An experiment was carried out in growing chickens to study the effect of supplementation of a semi-synthetic diet containing soya-bean meal as the sole protein source with DL-methionine, to improve its biological value, on the excretion of endogenous protein and amino acids measured in lower ileum and total tract using traditional methods. Thirty-two White Rock male broilers (10 days old) were randomly divided into eight groups each of four birds, of similar body weight (mean live weight: 142·8 (s.e. 0·68) g), and individually housed in metabolism cages. Following a paired-feeding design based on metabolic body weight (kgM0·75), each group of birds was given, for an experimental period of 20 days, each of four levels of protein (60, 120, 180 or 240 g/kg; 5 days each) in two groups of isoenergetic (14·5 kJ metabolizable energy per g dry matter) and semi-synthetic diets based on soya-bean meal, either not supplemented or supplemented with 2 g/kg DL-methionine (diets S and SM, respectively). After 3 days of each treatment excreta were collected for 48 h, frozen and stored at –20ºC. At the end of the fourth treatment three chickens of each group were killed and their lower ileal contents collected. The remaining chick of each treatment was fasted for 24 h and given a protein-free diet for 8 days and excreta were collected for the last 4 days. Then (day 39 of age), chickens were killed and lower ileum contents removed and stored at –20ºC. Samples of excreta and lower ileum contents were subjected to nitrogen (N) analysis by Kjeldahl procedure and amino acid (AA) analysis by high-performance liquid chromatography. Supplementation with DL-methionine of the soya-bean meal-based diets halved total tract endogenous AA losses. Regression analysis produced a higher estimation of ileal and faecal endogenous AA excretion than feeding a protein-free diet. Endogenous AA excretion determined in the lower ileum was higher than in excreta no matter which estimation procedure was utilized. In conclusion, supplementation of dietary protein with the first limiting AA to improve its protein quality, causes an important drop in endogenous AA losses, that may have an important effect on the N economy and energy requirements in poultry. The use of regression analysis on excreta data where graded amounts of protein are given to growing chickens, seems a suitable method for determining endogenous AA losses provided that good quality proteins are used.

Effects of meat cooking, and of ingested amount, on protein digestion speed and entry of residual proteins into the colon: a study in minipigs

The speed of protein digestion impacts on postprandial protein anabolism. After exercise or in the elderly, fast proteins stimulate protein synthesis more efficiently than slow proteins. It has been shown that meat might be a source of fast proteins. However, cooking temperature, acting on the macrostructure and microstructure of the meat could affect both the speed, and efficiency, of protein digestion. This study aims to evaluate, in vivo, the effect of meat cooking on digestion parameters, in the context of a complete meal. Six minipigs fitted with an ileal cannula and an arterial catheter were used. In order to measure the true ileal digestibility, tested meat was obtained from a calf, the muscle proteins of which were intrinsically labelled with ¹⁵N-amino acids. Three cooking temperatures (60, 75 and 95°C; core temperature for 30 min), and three levels of intake (1, 1.45, and 1.90 g protein/kg body weight) were tested. Following meat ingestion, ileal digesta and arterial blood were collected over a 9-h period. The speed of digestion, evaluated from the kinetics of amino acid appearance in blood within the first 3 h, was greater for the cooking temperature of 75°C, than for 60 or 95°C. The true ileal digestibility, which averaged 95%, was not affected by cooking temperature or by the level of meat intake. The amino acid composition of the digesta flowing at the ileum was not affected by cooking temperature. These results show that cooking temperature can modulate the speed of meat protein digestion, without affecting the efficiency of the small intestinal digestion, and consequently the entry of meat protein residues into the colon.

Animal models for determining amino acid digestibility in humans – a review

Animal models have been commonly used for determining amino acid digestibility in humans. This allows digestibility assays to be undertaken more efficiently than those undertaken using humans directly. The laboratory rat, usually considered as a suitable animal model, has been widely used, especially as the rat is easy to raise and relatively inexpensive to house. Although more technically demanding, the pig has also been promoted as a useful model for human nutrition studies. It may be a better model than the rat, as it is a meal eater, its upper digestive tract is anatomically and physiologically closer to that of humans and it eats most foods consumed by humans. Amino acid digestibility may be determined either at the faecal or the ileal level, the latter being considered the most accurate. This contribution evaluates the suitability of the rat and pig as animal models for assessing ileal and faecal amino acid digestibility in humans. The drawbacks and advantages of using these animal models are discussed. The review is based mainly on results from controlled studies comparing both species; however, as the number of these studies is limited, data from indirect comparisons also provide insight.

3H-leucine single-injection method for determining endogenous amino acid losses of broilers

OBJECTIVE

A 3H-leucine (3H-Leu) single-injection method was proposed for determining the endogenous amino acid losses of broilers. This method was based on the hypothesis that the ratio of the specific radioactivity (SR) of endogenous Leu in excreta (SRe) to that of free Leu in trichloroacetic acid-soluble plasma (SRp) remains constant after a single subcutaneous injection of 3H-Leu into birds fed different diets.

METHODS

Two experiments were designed to clarify this hypothesis. In experiment 1, 40 female broilers were randomly divided into four groups and were force-fed a nitrogen-free diet (NFD), NFD plus enzyme-hydrolyzed casein (EHC), 5% crude protein (CP) and SBM (soybean meal), or 20% CP-SBM. In experiment 2, 24 broilers were randomly divided into four groups and were fasted or force-fed the NFD, 20% CP-SBM, or 20% CP and cottonseed meal (CSM) diet. After the forced feeding, broilers were administered 3H-Leu by a single subcutaneous injection at a rate of 30 μCi/kg of body weight. Blood samples were taken at 5 min, 30 min, 4 h, 24 h, 36 h, and 48 h after the injection. The excreta were totally collected and pooled over the 48-h experiment.

RESULTS

The ratios of SRe to SRp remained the same for the birds force-fed the NFD, NFD+EHC, and 5% CP-SBM diets in experiment 1 and for the birds fasted and force-fed the NFD diet in experiment 2. The proportions of endogenous Leu to total Leu in excreta were 72.8%, 61.4%, and 57.5% for birds force-fed with the 20% CP-SBM diet in experiment 1 and 20% CP-SBM and 20% CP-CSM diets in experiment 2, respectively. Broilers fed the 5% CP-SBM and 20% CP-SBM diets excreted more (P<0.05) endogenous Leu than those fed the NFD and NFD+EHC diets in experiment 1. Broilers fed the 20% CP-SBM diet excreted more (P<0.05) endogenous Leu than those fed the NFD diet and fasted and the 20% CP-CSM diet was intermediate (P>0.05) in experiment 2.

CONCLUSION

The present study verified the hypothesis that the ratio of SRe to SRp remains constant after a single subcutaneous injection of 3H-Leu into broilers and proposes a new method to determine endogenous amino acid losses of broilers.

Ileal digestibility of dietary protein in the growing pig and adult human

The suitability of the pig as an animal model for predicting protein digestibility in man was evaluated. Healthy adult human subjects (mean body weight 67 kg;n11) and growing pigs (mean body weight 40 kg;n15) were fed semi-synthetic mixed meals containing, as a sole source of N, casein (C), hydrolysed casein (HC) or rapeseed isolate (R). There was no prior adaptation to the test meal. Ileal digesta were sampled through a naso-ileal tube (human subjects) or a post-valve T-caecum cannula (pigs) after ingestion of a bolus meal. The protein sources were15N-labelled. Amino acid (AA) digestibilities were not determined for R. Ileal apparent N digestibility was markedly lower (14–16 %;P < 0·001) in human subjects than in pigs (C, HC, R). Similarly, most apparent ileal AA digestibilities were lower (8 % on average;P < 0·05) in human subjects (C, HC). Ileal true N digestibility was slightly lower (3–5 %;P < 0·001) in human subjects than in pigs (C, HC, R) and most true ileal AA digestibilities were similar (P>0·05) between the species (C, HC). Exceptions were for phenylalanine, tyrosine, lysine, histidine and aspartic acid for which digestibilities were lower (3 % on average;P < 0·001) in human subjects. A similar ranking of the diets was observed for true ileal N digestibility between species. The inter-species correlation for true ileal digestibility was high for N (r0·98 over 3 × 2 data;P = 0·11) and AA (r0·87 over 26 × 2 data;P < 0·0001). Overall, this supports the use of the pig as a model for predicting differences among dietary protein digestibility, especially regarding true ileal N digestibility, in man.

Methods for mucin analysis: a comparative study

The aim was to compare five techniques commonly used to quantify mucin concentrations in ileal digesta collected from three growing pigs that had been fed a diet in which the sole protein was casein. Ileal mucin output was estimated by the periodic acid-Schiff, ethanol precipitation, and phenol-sulfuric acid methods as 25.1, 19.3, and 20.7 g kg-1 of dry matter intake (DMI), respectively. The mucin concentration estimated from sialic acid was only 5.9 g kg-1 of DMI. On the basis of the concentrations of the hexosamines N-acetylglucosamine and N-acetylgalactosamine, mucin output was estimated as 44.9 g kg-1 pf DMI. Of the five assays studied, the ethanol precipitation, periodic acid-Schiff, phenol-sulfuric acid, and sialic acid methods may considerably underestimate mucin in the digesta, which calls into question the accuracy of all of these approaches. In contrast, the gas chromatography method for the determination of hexosamines gave more information on the type and state of the mucin present.

A casein hydrolysate does not enhance gut endogenous protein flows compared with intact casein when fed to growing rats

We studied the effect of dietary free peptides vs. peptides released naturally during digestion on gut endogenous nitrogen (N) flow (ENFL) and amino acid (AA) flow (EAAFL). Semisynthetic diets containing 110 g/kg diet of the same casein, intact (C) or hydrolyzed (HC), were formulated with TiO2 as a dietary marker. Sprague-Dawley rats were fed the diets hourly (0800-1500 h) for 10 min each hour for 7 d. Rats received unlabeled diets for 6 d and 15N-labeled diets on d 7, whereby they were killed and digesta sampled (6 observations per group) along the intestinal tract. EAAFL and ENFL were determined by 15N-isotope dilution (ID) for C or by ID or after centrifugation and ultrafiltration (UF) for HC. Ileal EAAFL and ENFL (ID) were not enhanced with diet HC compared with diet C. The AA compositions (g/16 g N) of ileal ENFL did not differ between rats fed HC and C except for Asp, Phe, Tyr, and Ser, for which contributions were relatively lower (P < 0.05) for rats fed C. Ileal EAAFL and ENFL (HC) were considerably lower (P < 0.05) with ID than with UF, but flows of Gly, Phe, and His were similar. There was no stimulatory effect of dietary peptides from HC on endogenous ileal protein flow compared with C, but the result is tentative given the high degree of dietary N recycled within endogenous protein and which could have occurred at a differential rate between rats fed diets C and HC.

Comparison of the isotope dilution method for determination of the ileal endogenous amino acid losses with labelled diet and labelled pigs

The aims of the present study were first to compare the amino acid dilution method performed using labelled animals with that using labelled diets, and second to determine real digestibilities and total ileal endogenous losses of N and amino acids. Two diets containing pea cultivars (Solara and Amino) and a protein-free diet were compared in a 3 × 3 Latin-square experiment. The three pigs were each prepared with an ileo-rectal anastomosis and were continuously infused with [1-13C]leucine. For each cultivar,15N-labelled and unlabelled diets were formulated. The real digestibility and endogenous losses of leucine were higher when obtained by labelling the pig than by labelling the foodstuff. This was due either to the inadequate estimation of the endogenous protein enrichment in the first case or to the importance of dietary N recycling in the second case. However, in both cases the ileal endogenous losses of N and amino acids were higher than the basal losses determined with the protein-free diet. There were significant differences between the two pea cultivars in terms of phenylalanine and leucine when measured with labelled diets. It is suggested that, although ileal endogenous losses may be underestimated, using labelled feedstuffs is of great interest due to the direct estimation of the individual amounts of amino acids.

Route of tracer administration does not affect ileal endogenous nitrogen recovery measured with the 15N-isotope dilution technique in pigs fed rapidly digestible diets

The (15)N-isotope dilution technique ((15)N-IDT), with either pulse-dose oral administration or continuous i.v. administration of [(15)N]-l-leucine (carotid artery), both at 5 mg/(kg body weight . d), was used to measure ileal (postvalve T-cecum cannula) endogenous nitrogen recovery (ENR) in pigs (9 +/- 0.6 kg). Diets were cornstarch, enzyme-hydrolyzed casein with no (control) or high (4%) content of quebracho extract (Schinopsis spp.) rich in condensed tannins. Blood was sampled from a catheter in the external jugular vein. Mean plasma (15)N-enrichment at d 8-10 was higher (P = 0.0009) after i.v. than after oral administration [0.0356 vs. 0.0379 atom% excess (APE)]. Plasma (15)N-enrichment for i.v. infused pigs was 0.01117 APE higher (P < 0.0001) and for orally dosed pigs 0.0081 APE lower (P < 0.0001) at 11 h postprandial compared with 1 h postprandial. Apparent ileal N digestibility was higher (P < 0.0001) for the control (85.5%) than for the quebracho diet (69.5%). ENR was calculated from the ratio of (15)N-enrichment of plasma and digesta. The ENR for the quebracho diet was approximately 300% higher than for the control diet (6.03 vs. 1.94 g/kg dry matter intake, P < 0.001). The real N digestibility (92.2 +/- 0.4%) was equal for both diets (P = 0.1030) and both tracer methods (P = 0.9730). We concluded that oral administration of [(15)N]leucine provides reasonable estimates of ENR in pigs fed semipurified diets with high or low content of tannins; however, one must be careful in extrapolating this conclusion to studies with other protein sources or feeding frequencies.

Intestinal lysine metabolism is driven by the enteral availability of dietary lysine in piglets fed a bolus meal

Previous steady-state continuous-feeding studies have shown that the gut mucosa removes substantial amounts of both dietary and systemic amino acids. However, enteral nutrition is often given under non-steady-state conditions as a bolus meal, and this has been shown to influence systemic metabolism. Therefore, our aim was to quantify the relative metabolism of dietary and systemic lysine by the portal-drained viscera (PDV) under non-steady-state conditions after a single bolus meal. Five 28-day-old piglets implanted with arterial, venous, and portal catheters and with an ultrasonic portal flow probe were given an oral bolus feeding of a milk formula containing a trace quantity of intrinsically 15N-labeled soy protein and a continuous intravenous infusion of [U-13C]lysine for 8 h. Total lysine use by the PDV was maximal 1 h after the meal (891 micromol x kg(-1) x h(-1)) and was predominantly of dietary origin (89%), paralleling the enteral delivery of dietary lysine. Intestinal lysine use returned to a low level after 4 h postprandially and was derived exclusively from the arterial supply until 8 h. Cumulative systemic appearance of dietary lysine reached 44 and 80% of the ingested amount 4 and 8 h after the meal, respectively, whereas the PDV first-pass use of dietary lysine was 55 and 32% of the intake for these two periods, respectively. We conclude that the first-pass utilization rate of dietary lysine by the PDV is directly increased by the enteral lysine availability and that it is higher with a bolus than with continuous oral feeding.

Comparison of the enzyme-hydrolyzed casein, guanidination, and isotope dilution methods for determining ileal endogenous protein flow in the growing rat and pig1

The objectives of the two studies were to determine whether the guanidination and isotope dilution methods applied both by labeling the animal (15N-infusion method) and by diet (15N-dilution method) give similar estimates of ileal endogenous lysine (EL) and endogenous nitrogen (EN) flows, respectively, to that of the enzyme-hydrolyzed casein (EHC) method in the growing pig and to determine whether the guandination and 15N-dilution methods give similar estimates of EL and EN flows, respectively, to that of the EHC method in the rat. For the first study, the test diet contained guanidinated and enzymatically hydrolyzed casein (molecular weight < 5,000 Da), which was also labeled with 15N. Rats (n = 30; mean BW 178 g) and pigs (n = 6; mean BW 19.2 kg) received a preliminary EHC-based diet for 7 d. The test diet was then given to the rats and pigs on d 8. Digesta were sampled from the terminal 20 cm of ileum of killed animals. The EL flows determined using the guanidination method were lower than those determined using the EHC method (means of 298 vs. 382, and 214 vs. 287 microg/g of DMI, in the rat and pig, respectively; P < 0.05 for the rat and P < 0.01 for the pig). The EN flows determined with the 15N-dilution method were lower than those determined using the EHC method (means of 1,034 vs. 1,942 and 1,011 vs. 1,543 microg/g of DMI, in the rat and pig, respectively, P < 0.001 for the rat and P < 0.05 for the pig). In the second study, pigs (n = 6; mean BW 27 kg) were continuously infused via the jugular vein with 15N-leucine for 11 d. The pigs received an EHC-based diet (molecular weight < 5,000 Da) during this 11-d period, after which digesta were sampled at the terminal ileum under anesthesia. The EN flow determined using the 15N-infusion method (mean of 1,971 microg/g DMI) was higher (P < 0.01) than that determined using the EHC method (mean of 1,233 microg/g DMI). The guanidination method gave a lower estimate of EL flow than did the EHC method in both the pig and rat. The 15N-dilution method also gave a lower estimate of EN flow than the EHC method in the pig and rat, and the 15N-infusion method gave a higher estimate of EN flow than the EHC method in the pig.

Effect of dietary fiber on endogenous nitrogen flows in lactating dairy cows

The effect of dietary fiber on endogenous N secretion was studied using a 15N isotope dilution technique in four fistulated Holstein cows. Two isonitrogenous diets differing only in fiber (NDF and ADF) content were used in a crossover design. One diet (HF) contained 37.4% NDF, while the other (LF) contained 23.3%. A new model was developed to estimate endogenous N secretions and losses for the preintestinal, intestinal, and the total sections of the gastrointestinal tract. Three precursor pools: TCA-soluble fraction of plasma, intestinal mucosa, and milk were compared. Although endogenous losses estimated with the model were numerically different for each precursor pool selected (TCA-soluble fraction > mucosa > milk), treatment effects were similar. As intestinal mucosa is probably closest to the precursor pool, these data are discussed. Non-urea N endogenous secretions contributed 13% of the duodenal N flow but were not affected by the fiber content of the diet. The nonurea N endogenous flow at the duodenum was comprised of approximately equal inputs from endogenous N direct, and that incorporated into the microbial biomass. Total endogenous N flows at the duodenum exceeded, by nearly twofold, estimated inputs of urea-N to microbial biomass. Metabolic fecal output averaged 17% of fecal N and was not affected by level of dietary fiber, but net losses from secretions occurring in the small intestine were higher with the low fiber diet. Overall, endogenous N secretions represented 30% of total digestive tract protein synthesis.

Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements

BACKGROUND & AIMS

Irreversible amino acid losses at the human ileum are not taken into account when tracer-derived amino acid requirements are calculated because the data available are scarce. We have investigated amino acid losses at the ileal level in humans after ingestion of a protein meal.

METHODS

Thirteen volunteers ingested a single meal of 15N milk or soy proteins. The appearance of 15N and 15N amino acids in the ileal effluents collected using an ileal tube was monitored for 8 hours.

RESULTS

In the soy group, higher losses of endogenous nitrogen, especially originating from amino acids, were observed, as well as a higher flow rate of dietary non-amino acid nitrogen. With soy protein, the digestibilities of valine, threonine, histidine, tyrosine, alanine, and proline were significantly lower than with milk. Ileal losses of leucine, valine, and isoleucine amounted to 12, 10, and 7 mg x kg(-1) x day(-1), respectively. Threonine ileal loss (9-12 mg x kg(-1) x day(-1)) was particularly high compared with the current amino acid requirement.

CONCLUSIONS

Amino acid losses at the human terminal ileum are substantial and depend on the type of dietary protein ingested. Although it remains unclear whether intact amino acids are absorbed in the colon, we suggest that ileal losses should be considered an important component of amino acid requirements.

Contribution of microbial amino acids to amino acid homeostasis of the host

Among the reasons suggested for the discrepancy between N balance and tracer-derived indispensable amino acid (IAA) requirement estimates is the possibility that the metabolic requirement is met not only by the diet but also by IAA synthesized de novo by the gastrointestinal microflora, which are then absorbed. It is therefore crucial to better understand and quantify the microbial biosynthesis of amino acids in the human gastrointestinal tract and its potential role in providing IAA to meet human amino acid requirement. Here, the available evidence on the contribution of microbial amino acids to the host's amino acid homeostasis, applying the (15)N labeling paradigm, is summarized. Between 1 and 20% of circulating plasma lysine, urinary lysine and body protein lysine of the host, respectively, is derived from intestinal microbial sources and corresponds to a gross microbial lysine contribution of 11-68 mg. kg(-1). d(-1) in adult humans with an adequate protein intake when fecal or ileal microbial lysine enrichment is used as precursor. Factors affecting estimates of net microbial IAA contribution are discussed. It appears that the small intestine is responsible for a large part of microbial lysine uptake, although some absorption from the large intestine cannot be excluded. Nonoxidative lysine losses from the human gastrointestinal tract, which were found to be between 3.9 to 8.5 mg. kg(-1). d(-1), are necessary to estimate the net contribution of microbial IAA. It is reasonable to assume that microbial amino acid synthesis in the human gastrointestinal tract utilizes a mixture of various nitrogen sources, i.e., endogenous amino acids, urea and ammonia. Microbes in the small intestine may rely more on endogenous amino acids. Deprivation of nutrients, the intake of certain dietary nonstarch oligosaccharides, lipids, as well as protein intake level and source and level of consumption of certain amino acids can affect the composition and metabolic activity of the intestinal microflora and thus its fermentation products potentially available to the host. In conclusion, with the use of the (15)N labeling paradigm, a significant contribution of microbial lysine to the host lysine homeostasis is found. However, to assess the net contribution of microbial IAA and its importance in defining the adult IAA requirement, this is not the ultimately successful experimental strategy because the interpretation of results is complicated by the nitrogen recycling in the gut, the uncertainty of the precursor pool of absorption and the limited data on nonoxidative IAA losses from the human gastrointestinal tract.

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.

Availability of intestinal microbial lysine for whole body lysine homeostasis in human subjects

We have investigated whether there is a net contribution of lysine synthesized de novo by the gastrointestinal microflora to lysine homeostasis in six adults. On two separate occasions an adequate diet was given for a total of 11 days, and a 24-h (12-h fast, 12-h fed) tracer protocol was performed on the last day, in which lysine turnover, oxidation, and splanchnic uptake were measured on the basis of intravenous and oral administration of L-[1-(13)C]lysine and L-[6,6-(2)H(2)]lysine, respectively. [(15)N(2)]urea or (15)NH(4)Cl was ingested daily over the last 6 days to label microbial protein. In addition, seven ileostomates were studied with (15)NH(4)Cl. [(15)N]lysine enrichment in fecal and ileal microbial protein, as precursor for microbial lysine absorption, and in plasma free lysine was measured by gas chromatography-combustion-isotope ratio mass spectrometry. Differences in plasma [(13)C]- and [(2)H(2)]lysine enrichments during the 12-h fed period were observed between the two (15)N tracer studies, although the reason is unclear, and possibly unrelated to the tracer form per se. In the normal adults, after (15)NH(4)Cl and [(15)N(2)]urea intake, respectively, lysine derived from fecal microbial protein accounted for 5 and 9% of the appearance rate of plasma lysine. With ileal microbial lysine enrichment, the contribution of microbial lysine to plasma lysine appearance was 44%. This amounts to a gross microbial lysine contribution to whole body plasma lysine turnover of between 11 and 130 mg. kg(-1). day(-1), depending on the [(15)N]lysine precursor used. However, insofar as microbial amino acid synthesis is accompanied by microbial breakdown of endogenous amino acids or their oxidation by intestinal tissues, this may not reflect a net increase in lysine absorption. Thus we cannot reliably estimate the quantitative contribution of microbial lysine to host lysine homeostasis with the present paradigm. However, the results confirm the significant presence of lysine of microbial origin in the plasma free lysine pool.

The 15N amino acid dilution method allows the determination of the real digestibility and of the ileal endogenous losses of the respective amino acid in pigs

We assessed the influence of sampling site when using the isotope dilution method to determine ileal endogenous N losses. Three growing pigs were prepared with ileorectal anastomoses and fitted with three catheters (portal, jugular and carotid). A 15N-leucine solution was infused for 24 d, alternating between the carotid artery and the jugular vein. Blood was sampled from the portal catheter and from the systemic catheter not used for the infusion. The pigs were fed successively a protein-free diet, an isolated pea protein diet and a hydrolyzed pea protein diet according to a Latin-square design. The 15N was transferred from leucine to isoleucine, valine, alanine, glycine and proline. Free 15N alanine, glycine and valine enrichments were closer to the respective amino acid enrichments in secretory tissues in the portal vein than in the systemic blood. The enrichment of total nitrogen was higher in the trichloroacetic acid-soluble fraction of the plasma than in the ileal digesta of pigs fed the protein-free diet. Lysine enrichment was significantly different from zero in all tissues except muscle, suggesting that essential amino acids can be synthesized by microflora and used for protein synthesis in the host. We conclude that the total nitrogen isotope dilution method is inappropriate to determine the endogenous loss of amino acids. Moreover, the amino acid dilution method should be performed with portal blood sampling. The main limitation of this method is that a number of essential amino acid losses cannot be determined.

The current 15N-leucine infusion technique is not suitable for quantitative measurements of ileal endogenous amino acid flows in pigs

The current 15N-leucine infusion technique may overestimate the ileal endogenous nitrogen losses in pigs. To determine the reason, we infused four cannulated pigs intravenously, fed them a pea-based diet with 15N-leucine, and examined some methodological variables. Neither the blood sampling time nor the choice of precursor pool (total N or amino acid N of deproteinized plasma) or the method of estimation of the isotopic equilibrium level significantly affected the results. On the other hand, the 15N-enrichment of purified mucin, isolated from ileal digesta, was higher than that of the plasma amino acid pool (0.114 vs. 0.077 atom % excess). The endogenous proportion of the labeled amino acids (Ala, Gly, Ile, Leu and Val) in the ileal digesta ranged from 23 (Leu) to 74% (Ala), compared with 70% for total N. The low value of leucine was ascribed to the constant marker infusion condition. In pigs infused with 13C-leucine, a similar endogenous proportion was obtained for lumenal leucine with 13C-leucine and 15N-leucine infusion. However, the 13C-enrichment of the leucine bound to mucin was markedly lower than that of plasma leucine (38%). The endogenous amino acid flows were also estimated by combining the ileal N flow measured with 15N and the endogenous amino acid profile obtained by means of an N-free diet. They were different from those obtained with the 15N-amino acid dilution technique. We conclude that the precursor pool currently used (plasma total N or total alpha-amino acid N pools) is a poor indicator of the enrichment of the secretions and that the infusion of one labeled amino acid is not sufficient to extend the method at the amino acid level.

Nitrogen cycling in the gut

This review examines the involvement of the gastrointestinal tract in the utilization of nitrogen, the identities of the nitrogenous substances entering and leaving the gut, and the significance of this recycling in the overall nitrogen economy of the body. It is concerned with nonruminant mammals, including man.

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

Endogenous nitrogen and amino acid losses are associated with the digestion process. Different methods can be used to distinguish between gut endogenous and exogenous amino acid flows. These methods include feeding N-free diets, the regression approach, the use of enzyme hydrolysed proteins coupled with ultrafiltration and the use of markers (e.g., homoarginine, enzyme hydrolysed casein, 15N-labelled protein, 15N-leucine). All of the methods described have their limitations, but they have allowed major advances to be made in the measurement and understanding of endogenous amino acid secretion, reabsorption, flow and excretion.

[15N]-labeled pea flour protein nitrogen exhibits good ileal digestibility and postprandial retention in humans

The aim of the present study was to evaluate postprandial absorption of pea protein as well as exogenous nitrogen retention in humans. For this purpose, after fasting overnight, seven healthy adults (4 males and 3 females) ingested [15N]-labeled pea protein (195 mmol N). Ileal effluents were collected for 8 h at 30-min intervals using a nasointestinal intubation technique. Urine and plasma samples were collected for 24 h. The [15N]-enrichment was determined in the intestinal samples, in the plasma amino acids and urea as well as in the urinary urea and ammonia fractions. The true gastroileal absorption of pea protein was 89.4 +/- 1.1%. This absorption was correlated with a significant increase (P < 0.05) in [15N]-enrichment in the plasma amino acids and in the nitrogen incorporated into the body urea pool for 1 h following pea ingestion. The enrichment remained significantly higher than the basal values in these pools 24 h after pea ingestion. The recovery of total urinary exogenous nitrogen after 22 h was 31.1 +/- 9.3 mmol N. Moreover, the kinetics of [15N]-labeled pea amino acids deamination reached a plateau of 39 mmol. Under these conditions, pea nitrogen retention represented 78% of the absorbed dietary nitrogen in healthy humans. The present results demonstrate the good true nitrogen digestibility and retention of pea protein in humans.