The rat as a model animal for the growing pig in determining ileal amino acid digestibility in soya and milk proteins

Using the dual isotope method to assess cecal amino acid absorption of goat whey protein in rats, a pilot study

Measurement of ileal amino acids (AA) bioavailability is recommended to evaluate protein quality. A dual isotope tracer method, based on plasma isotopic enrichment ratios, has been proposed to determine true digestibility in humans. In a pilot study, we aimed to evaluate whether this method could be implemented in rats to determine AA bioavailability based on isotopic enrichment ratios measured in cecal digesta or plasma samples. Goat milk proteins were intrinsically labeled with ¹⁵N and ²H. Wistar rats were fed a meal containing the doubly labeled goat whey proteins and a tracer dose of ¹³C-spirulina. Blood samples were collected 0, 1 h and 3 h after meal ingestion from the tail vein. The rats were euthanized 4 h (n = 6) or 6 h (n = 6) after meal to collect plasma and intestinal contents. True orocecal protein digestibility and AA bioavailability were assessed by means of ¹⁵N and ²H enrichment in cecum content and compared with absorption indexes determined at the plasma or cecum level using isotopic ratios. Plasma kinetics of isotopic enrichment could not be completed due to the limited quantity of plasma obtained with sequential blood collection. However, the absorption indexes determined from cecal ¹⁵N or ²H/¹³C ratios gave coherent values with true orocecal AA bioavailability. This dual isotope approach with measurements of isotopic ratios in digestive content could be an interesting strategy to determine true AA bioavailability in ileal digesta of rats.

15N and ²H Intrinsic Labeling Demonstrate That Real Digestibility in Rats of Proteins and Amino Acids from Sunflower Protein Isolate Is Almost as High as That of Goat Whey

BACKGROUND

In the context of developing plant protein sources for humans, sunflower is a good candidate in its form as an oilseed coproduct.

OBJECTIVES

We aimed to compare the real digestibility in rats of a sunflower isolate to that of goat whey protein. We also studied the efficiency of 15N and 2H intrinsic labeling in this assessment.

METHODS

Sunflower seeds and goat milk were labeled with 15N and 2H. Male Wistar rats (10 wk old) were fed a meal containing 12% of either sunflower isolate (n = 8) or whey (n = 8). Six hours after meal ingestion, protein and amino acid digestibility were assessed by measuring nitrogen, hydrogen, and amino acids in the digesta, as well as isotope enrichments in the bulk and individual amino acids. The differences between groups and isotopes were respectively tested with an unpaired and a paired t test.

RESULTS

Protein isolate purity was 87% for whey and 94% for sunflower. 2H and 15N enrichments were, respectively, 0.12 atom % (AP) and 1.06 AP in sunflower isolate and 0.18 AP and 0.95 AP in whey. Fecal 15N protein digestibility was 97.2 ± 0.2% for whey and 95.1 ± 0.5% for sunflower isolate. The use of 2H resulted in a lower digestibility estimate than 15N for whey (96.9 ± 0.2%, P < 0.05) and sunflower (94.2 ± 0.5%, P < 0.01). For both isotopes, protein digestibility was about 2% higher for whey than for sunflower isolate. Mean 15N amino acid caecal digestibility was 97.5 ± 0.2% for whey and 96.3 ± 0.2% for sunflower isolate. The values obtained with 15N and 2H resulted in significant differences ranging from -0.1% to 3.5%. The DIAAS was >1.0 for whey and 0.84 for sunflower (lysine).

CONCLUSIONS

The protein and amino acid digestibility of sunflower isolate was high but its DIAAS reflected a moderate lysine imbalance. Despite slight differences with 15N, deuterium produced comparable results, making it suitable for in vivo digestion studies.

Digestible indispensable amino acid scores of nine cooked cereal grains

True ileal digestibility (TID) values of amino acid (AA) obtained using growing rats are often used for the characterisation of protein quality in different foods and acquisition of digestible indispensable amino acid scores (DIAAS) in adult humans. Here, we conducted an experiment to determine the TID values of AA obtained from nine cooked cereal grains (brown rice, polished rice, buckwheat, oats, proso millet, foxtail millet, tartary buckwheat, adlay and whole wheat) fed to growing Sprague–Dawley male rats. All rats were fed a standard basal diet for 7 d and then received each diet for 7 d. Ileal contents were collected from the terminal 20 cm of ileum. Among the TID values obtained, whole wheat had the highest values (P<0·05), and polished rice, proso millet and tartary buckwheat had relatively low values. The TID indispensable AA concentrations in whole wheat were greater than those of brown rice or polished rice (P<0·05), and polished rice was the lowest total TID concentrations among the other cereal grains. The DIAAS was 68 for buckwheat, 47 for tartary buckwheat, 43 for oats, 42 for brown rice, 37 for polished rice, 20 for whole wheat, 13 for adlay, 10 for foxtail millet and 7 for proso millet. In this study, the TID values of the nine cooked cereal grains commonly consumed in China were used for the creation of a DIAAS database and thus gained public health outcomes.

Invited review: Further progress is needed in procedures for the biological evaluation of dietary protein quality in pig and poultry feeds

Abstract. Recently, biological procedures for feed protein evaluation in pig and poultry diets have been based on the amino acid composition of feed ingredients considering the animal's losses during processes of digestion or total protein utilization in a different manner. Such a development towards individual amino acids (AAs) was inevitable according to the disadvantage of traditional protein quality measures, like biological value (BV) or net protein utilization (NPU), to be non-additive in complex animal diets. In consequence, such measures are generally not suitable for predicting the final protein quality of protein mixtures from the individual protein value of feed ingredients. Otherwise, recent measures of AA disappearance from the small intestine up to the end of the ileum (ileal AA digestibility) also do not provide a true reflection of the biological availability of individual feed AAs independent of the extent of taking into account endogenous AA losses during digestion processes. Sophisticated procedures for protein evaluation are needed considering the AA losses, both during absorption and utilization after absorption. Advantages and limitations of important developments in procedures are discussed. Accordingly, the development of an exponential modelling approach is described (the Göttingen approach), which overcomes some of the traditional disadvantages by measuring the individual AA efficiency. Connecting feed protein evaluation, the modelling of quantitative AA requirements, and improved ideal protein concepts offers different fields of application. In addition, as demonstrated by example, the modelling of nitrogen losses per unit protein deposition and the minimizing of this parameter yields a further interesting tool for lowering the nitrogen burden from protein utilization processes. Finally, it is pointed out that traditional laboratory procedures also need to be updated, adapted to current knowledge, and validated according to the increasing hurdles for animal studies from the viewpoint of animal welfare. Modelling is a procedure with the potential to reduce the number of experimental animals significantly. This development needs more attention, higher acceptance, and wider application in the future of protein evaluation.

Alternative prediction methods of protein and energy evaluation of pig feeds

Precise knowledge of the actual nutritional value of individual feedstuffs and complete diets for pigs is important for efficient livestock production. Methods of assessment of protein and energy values in pig feeds have been briefly described. In vivo determination of protein and energy values of feeds in pigs are time-consuming, expensive and very often require the use of surgically-modified animals. There is a need for more simple, rapid, inexpensive and reproducible methods for routine feed evaluation. Protein and energy values of pig feeds can be estimated using the following alternative methods: 1) prediction equations based on chemical composition; 2) animal models as rats, cockerels and growing pigs for adult animals; 3) rapid methods, such as the mobile nylon bag technique and in vitro methods. Alternative methods developed for predicting the total tract and ileal digestibility of nutrients including amino acids in feedstuffs and diets for pigs have been reviewed. This article focuses on two in vitro methods that can be used for the routine evaluation of amino acid ileal digestibility and energy value of pig feeds and on factors affecting digestibility determined in vivo in pigs and by alternative methods. Validation of alternative methods has been carried out by comparing the results obtained using these methods with those acquired in vivo in pigs. In conclusion, energy and protein values of pig feeds may be estimated with satisfactory precision in rats and by the two- or three-step in vitro methods providing equations for the calculation of standardized ileal digestibility of amino acids and metabolizable energy content. The use of alternative methods of feed evaluation is an important way for reduction of stressful animal experiments.

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.

Effect of oxidation of dietary proteins with performic acid on true ileal amino acid digestibility as determined in the growing rat

The study examined the impact of oxidation with performic acid on the true ileal amino acid digestibility (TIAAD) of seven dietary protein sources. TIAAD of both the unoxidized and oxidized protein sources was determined in the growing rat and compared. After oxidation, TIAAD was 30 and 58% higher across amino acids (P < 0.001) for the more poorly digestible protein sources, zein and blood meal respectively, 6-16% lower (P < 0.05) for the more highly digestible protein sources (casein, soy protein isolate, and beef muscle protein), and generally unchanged (<2% difference) for the more moderately digested protein sources (lactalbumin and gelatin). Overall, the change in TIAAD after oxidation was negatively correlated (P < 0.001, r = -0.90) with the TIAAD prior to oxidation, most likely as a result of the balance between two processes, protein denaturation resulting in increased digestion and indigestible limit peptide formation resulting in decreased digestion.

Validation of a dual in vivo-in vitro assay for predicting the digestibility of nutrients in humans

BACKGROUND

The validation of a dual in vivo-in vitro digestibility assay ('dual digestibility assay') for separately predicting the upper-tract, hindgut and total tract digestibility of nutrients in humans, as estimated using organic matter digestibility (OMD), is described. Human upper-tract OMD was predicted using an animal (rat) model with digesta from the terminal ileum collected from rats fed one of four complete human diets (wheat bran diet, pectin diet, mixed low-fibre diet, mixed high-fibre diet). Large intestinal OMD was predicted using an in vitro hindgut fermentation assay employing a human faecal inoculum and with the rat ileal digesta as the substrate.

RESULTS

A comparison of total tract OMD of the four diets from a human balance study (OMDhuman ) with that predicted using the dual digestibility assay (OMDdual ) showed no significant differences (P > 0.05). OMDhuman and OMDdual were highly correlated (r = 0.953, P = 0.047).

CONCLUSION

The dual digestibility assay accurately predicts the uptake of dietary nutrients (as grams of organic matter) in humans over the total tract. The assay is able to separately quantify the digestibility of nutrients in the upper and lower digestive tracts. The validation of the dual digestibility assay needs to be extended to a wider range of human diets.

Available lysine and digestible amino acid contents of proteinaceous foods of India

Cereals and legumes are staple foods in India and are limiting in lysine and sulphur amino acids, respectively. Available lysine loss, due to Maillard-type reactions that may occur during food preparation, exacerbates the problem of lysine deficiency particularly in cereals. Consequently, determining the contents of digestible essential amino acids, particularly lysine, is important. True ileal digestibilities of most amino acids (including total and reactive lysine) were determined for ten food ingredients and eleven foods commonly consumed in India. Semi-synthetic diets each containing either an ingredient or the prepared food as the sole protein source were formulated to contain 100 g kg(-1) protein (75 g kg(-1) for rice-based diets) and fed to growing rats. Titanium dioxide was included as an indigestible marker. Digesta were collected and the amino acid content (including reactive lysine) of diets and ileal digesta determined. Available (digestible reactive) lysine content ranged from 1·9-15·4 g kg(-1) and 1·8-12·7 g kg(-1) across the ingredients and prepared foods respectively. True ileal amino acid digestibility varied widely both across ingredients and prepared foods for each amino acid (on average 60-92 %) and across amino acids within each ingredient and prepared food (overall digestibility 31-96 %). Amino acid digestibility was low for many of the ingredients and prepared foods and consequently digestibility must be considered when assessing the protein quality of poorer quality foods. Given commonly encountered daily energy intakes for members of the Indian population, it is estimated that lysine is limiting for adults in many Indian diets.

Optimisation of inoculum concentration and incubation duration for an in vitro hindgut dry matter digestibility assay

The aim was to optimise inoculum concentration and incubation duration for a published in vitro hindgut digestibility assay using ileal digesta (sampled from the chicken or rat) pertaining to a mixed human diet as the substrate. The study also sought to investigate the digestibility of the inoculum itself and the importance of correcting for this in the in vitro hindgut digestion assay. For two assays, hindgut dry matter digestibility (DMD) generally increased with inoculum concentration. A sharp increase in DMD observed at high inoculum concentrations may have been related to problems with filtering the inoculum. An inoculum concentration of 160 g/L was considered optimal based on close agreement of observed values with previously published in vivo hindgut dry matter digestibility for similar diets. One of the methods was chosen for optimisation of the duration of incubation. Ileal substrate organic matter digestibility (OMD) increased with increasing time of incubation for all diets. An incubation duration of 18 h using a mean inoculum digestibility value for calculation purposes was considered optimal based on observed in vivo hindgut DMD values in humans, but there was little difference in estimated in vitro hindgut DMD between 18 and 24h incubation durations. Although considerably lower than the OM digestibility of the substrate (no less than 51% after 48 h), the OM digestibility of the inoculum (13% after 48 h) itself was of significance in calculating estimated digestibility. The optimised assay gave realistic hindgut OMD values ranging from 55% to 79% (Wheat Bran Diet and Pectin Diet, respectively) using an 18-h incubation duration.

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.

Predicted apparent digestion of energy-yielding nutrients differs between the upper and lower digestive tracts in rats and humans

The apparent digestibility of energy-yielding nutrients (carbohydrate, protein, and fat) was predicted in the human upper digestive tract and large bowel separately for 4 diverse diets containing either a single dietary fiber source [wheat bran and pectin (PE) diets] or mixed fiber sources [low-fiber (LF) and high-fiber (HF) diets). A human balance study was undertaken to determine fecal energy and nutrient excretion and a rat model was used to predict human ileal energy and nutrient excretion. Total tract energy digestibility ranged from 92 (HF diet) to 96% (PE diet and LF diet), while at the ileal level it ranged from 79 to 86% for the HF diet to the LF diet. The predicted upper-tract digestion of starch, sugars, and fat was high, with ileal digestibilities exceeding 90% for all diets. Nonstarch polysaccharides were poorly digested in the upper tract for all diets except in the PE diet. The daily quantity of protein excreted at the ileal level was between 2 (HF diet) and 5 (PE diet) times higher than that at the fecal level. The large differences between fecal and ileal nutrient loss highlight that fecal digestibility data alone provide incomplete information on nutrient loss. There is a need to be able to routinely determine the uptake of energy in the upper and lower digestive tracts separately.

Predicting metabolisable energy in commercial rat diets: physiological fuel values may be misleading

Knowledge about metabolisable energy (ME) intake is crucial for various experimental settings in rodent studies. ME considers faecal and renal energy losses. In particular, faecal energy excretion can vary considerably between differentially composed diets. Thus determination of faecal energy losses, i.e. apparent energy digestibility, is the most important experimental approach to determine ME. Predictive equations for ME such as Atwater factors or an equation for pigs, which are frequently employed for rodent feed, consider an average energy digestibility for nutrients and average renal losses for protein. Both equations, however, were never validated for rat feed. We therefore determined experimentally the digestibility of energy (experimentally determined digestible energy − 5·2 kJ/g digestible protein) and nutrients of eleven natural and five purified rat diets and compared the present results with the predicted values. Compared with natural diets, digestibility of gross energy (GE) and nutrients was higher by about 20 % in the purified diets (P < 0·0001). Mean GE digestibility in natural diets amounted to 71·4 % (range 53·3–83·5 %;n11). Atwater factors predicted ME with satisfactory accuracy in purified diets. In contrast, for natural diets, only the equation for pig feed gave acceptable estimates of ME. Choosing an inappropriate predictive equation for ME resulted in considerable error. For prediction of ME in mixed rat feed, we propose to use the equation for pig feed for natural diets and Atwater factors for purified diets. If the equation for pig feed cannot be applied we suggest using the lower modified Atwater factors instead of the ‘original’ Atwater factors to estimate the ME of a diet.

Feeding dietary peptides to growing rats enhances gut endogenous protein flows compared with feeding protein-free or free amino acid-based diets

The effect of dietary peptides on gut endogenous nitrogen (N) flow (ENFL) and amino acid (AA) flow (EAAFL) was studied. Semisynthetic diets containing enzyme-hydrolyzed casein (HC; 11%) or a free AA mixture devoid of Asp and Ser (A1) or Gly and Ala (A2) were formulated to have similar AA compositions except for the excluded AA and similar dietary electrolyte balances (Na(+)+K(+)-Cl(-)). A protein-free diet (PF) served as a control. Sprague-Dawley rats were given the diets 8 times/d for 10 min each hour for 7 d. Rats were killed and digesta were sampled (6 observations within each group) along the intestinal tract 6 h after the first meal on d 7. EAAFL and ENFL, estimated with reference to the dietary marker TiO(2), were determined directly (PF, A1, and A2) or after centrifugation and ultrafiltration of the digesta (HC). Endogenous flows of Asp and Ser or Gly and Ala did not differ (P > 0.05) in any of the intestinal sections between rats fed PF and A1 or PF and A2, respectively, except in the stomach where Ser flow was greater for rats fed A1. Ileal endogenous flows for most of the AA and for N were higher (P < 0.05) for rats fed the HC diet compared with those for rats fed the PF, A1, or A2 diets, except for Phe, Tyr, Lys, which did not differ among the groups. Ileal EAAFL and ENFL were not influenced by body N balance per se but were affected by the presence in the gut of dietary peptides derived from casein.

Heat markers and quality indexes of industrially heat-treated [15N] milk protein measured in rats

To determine the bioavailability of industrially heat-treated milk proteins, male Wistar rats were given [15N]-labeled meals containing either nonheated-micellar casein (CAS), milk soluble protein isolate (MSPI), and microfiltered milk (MF)-or heated products-"high temperature short time" pasteurized (HTST), "higher temperature, shorter time" pasteurized (HHST), ultrahigh temperature-treated (UHT), and spray-dried (SPRAY) milks. The postprandial distribution of dietary nitrogen was measured in the splanchnic area and urea. Digestibility was around 96% except for SPRAY (94%) and MSPI (98%). Ingested nitrogen recovered in the splanchnic bed was 19.3% for SPRAY, 16.7% for MF, and around 14-15% for other products. Deamination of dietary nitrogen reached 21.2, 20.6, and 18.2% of ingested nitrogen for MSPI, SPRAY, and CAS, respectively, and around 14-16% for other products. In our model, only spray drying led to a significant increase of splanchnic extraction. Moreover, the biological value of purified protein fractions appeared to be lower than that seen in products containing total milk protein.