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

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.

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.

Human gut endogenous proteins as a potential source of angiotensin-I-converting enzyme (ACE-I)-, renin inhibitory and antioxidant peptides

It is well known that endogenous bioactive proteins and peptides play a substantial role in the body's first line of immunological defence, immune-regulation and normal body functioning. Further, the peptides derived from the luminal digestion of proteins are also important for body function. For example, within the peptide database BIOPEP (http://www.uwm.edu.pl/biochemia/index.php/en/biopep) 12 endogenous antimicrobial and 64 angiotensin-I-converting enzyme (ACE-I) inhibitory peptides derived from human milk and plasma proteins are listed. The antimicrobial peptide database (http://aps.unmc.edu/AP/main.php) lists over 111 human host-defence peptides. Several endogenous proteins are secreted in the gut and are subject to the same gastrointestinal digestion processes as food proteins derived from the diet. The human gut endogenous proteins (GEP) include mucins, serum albumin, digestive enzymes, hormones, and proteins from sloughed off epithelial cells and gut microbiota, and numerous other secreted proteins. To date, much work has been carried out regarding the health altering effects of food-derived bioactive peptides but little attention has been paid to the possibility that GEP may also be a source of bioactive peptides. In this review, we discuss the potential of GEP to constitute a gut cryptome from which bioactive peptides such as ACE-I inhibitory, renin inhibitory and antioxidant peptides may be derived.

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.

Gut luminal endogenous protein: Implications for the determination of ileal amino acid digestibility in humans

The true ileal digestibility assay provides the most informative measure of digestibility to assess bioavailability of amino acids in foods for humans. To determine ‘true’ estimates of ileal amino acid digestibility, requires that endogenous amino acids present in digesta at the terminal ileum be quantified. The amounts of endogenous amino acids in ileal digesta can be determined after feeding an animal or human a protein-free diet (traditional approach) or by various methods after giving a protein-containing diet. When the protein-free method has been applied with adult human subjects an overall mean value (three separate studies) for endogenous ileal nitrogen flow of 800 mg N/d has been reported. This value is considerably lower than a comparable value obtained after feeding protein of 1852 mg N/d (mean of four separate studies), and thus endogenous ileal N and amino acids should be measured under conditions of protein alimentation. There is some confusion concerning the terminology used to define digestibility, with the term “true” digestibility having different adopted meanings. Here, true amino acid digestibility is defined as apparent amino acid digestibility corrected for the basal amino acid losses determined after giving either a protein-free or a protein-containing diet. Basal losses should be determined at a defined dry-matter and protein intake. The protein-free diet approach to determining endogenous amino acids is considered unphysiological and basal losses refer to ileal endogenous amino acid flows associated with digesta dry-matter flow, and not including “specific” effects of dietary factors such as non starch polysaccharides and anti nutritional factors. Arguments are advanced that the enzyme hydrolysed protein/ultra filtration method may be suitable for routine application with a cannulated pig model, to obtain physiologically-valid basal estimates of ileal endogenous amino acids to allow calculation of true ileal amino acid digestibility in the pig, and then prediction (via statistical relationships) of true coefficients of amino acid digestibility in humans.

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.

The production of intrinsically labeled milk and meat protein is feasible and provides functional tools for human nutrition research

Administration of labeled, free amino acids does not allow direct assessment of in vivo dietary protein digestion and absorption kinetics. Consequently, dietary protein sources with labeled amino acids incorporated within their protein matrix are required. The aim of the present study was to produce intrinsically L-[1-(13)C]phenylalanine-labeled milk and meat protein that would permit in vivo assessment of postprandial protein digestion and absorption kinetics in humans. One lactating dairy cow was continuously infused with 420 μmol of L-[1-(13)C]phenylalanine/min for 96 h, with plasma and milk being collected before, during, and after isotope infusion. Twenty-four hours after infusion, the cow was slaughtered to produce intrinsically labeled meat. Levels of L-[1-(13)C]phenylalanine enrichment as high as 40 mole percent excess (MPE) in milk and 1.5 MPE in meat protein were achieved. In a subsequent human proof-of-principle experiment, 2 healthy young males (25±1 yr; 66.2±5.2 kg) each ingested 135 g of L-[1-(13)C]phenylalanine intrinsically labeled minced beef, after which plasma samples were collected at regular time intervals. Plasma L-[1-(13)C]phenylalanine enrichments increased during the first 90 min following beef ingestion, reaching peak plasma enrichment levels of 0.61±0.04 MPE. Whole-body net protein balance, assessed by continuous infusion of L-[ring-(2)H(5)]phenylalanine and L-[ring-(2)H(2)]tyrosine, was higher in the postprandial period compared with basal values (6.4±0.1 vs. -4.5±0.1 μmol/kg per h). In conclusion, the production of intrinsically L-[1-(13)C]phenylalanine-labeled milk and meat protein is feasible and provides functional tools to investigate in vivo protein digestion and absorption kinetics in humans.

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.

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.