The influence of the albumin fraction on the bioavailability and postprandial utilization of pea protein given selectively to humans

The role of fiber in modulating plant protein-induced metabolic responses

The rising consumption of plant protein foods and the emergence of meat alternatives have prompted interest in the health benefits of such products, which contain fiber in addition to protein. This review investigates the effect of fiber on plant-based protein metabolism and evaluates its contribution to gut-derived health impacts. Plant proteins, which often come with added fiber, can have varying health outcomes. Factors such as processing and the presence of fiber and starch influence the digestibility of plant proteins, potentially leading to increased proteolytic fermentation in the gut and the production of harmful metabolites. However, fermentable fiber can counteract this effect by serving as a primary substrate for gut microbes, decreasing proteolytic activity. The increased amount of fiber, rather than the protein source itself, plays a significant role in the observed health benefits of plant-based diets in human studies. Differences between extrinsic and intrinsic fiber in the food matrix further impact protein fermentation and digestibility. Thus, in novel protein products without naturally occurring fiber, the health impact may differ from conventional plant protein sources. The influence of various fibers on plant-based protein metabolism throughout the gastrointestinal tract is not fully understood, necessitating further research.

True Ileal Amino Acid Digestibility and Protein Quality of 15N-Labeled Faba Bean in Healthy Humans

BACKGROUND

The recommended transition toward more plant-based diets, particularly containing legumes, requires a wider knowledge of plant protein bioavailability. Faba beans are cultivated at different latitudes and are used increasingly in human nutrition.

OBJECTIVES

We aimed to assess the nutritional quality of faba bean protein in healthy volunteers equipped with an intestinal tube to implement the ileal 15N balance method.

METHODS

Nine volunteers completed the study (7 males, 2 females, aged 33 ± 10 y, BMI: 24.7 ± 2.6 kg/m2). They were equipped with a nasoileal tube. After fasting overnight, they ingested a test meal consisting of cooked mash of dehulled faba bean seeds (20 g protein per serving of approximately 250 g) intrinsically labeled with 15N. Samples of ileal contents, plasma, and urine were collected over an 8-h postprandial period. Undigested nitrogen (N) and amino acids (AAs) were determined using isotopic MS, and subsequently, ileal digestibility and digestible indispensable amino acid score (DIAAS) were calculated. The measurement of postprandial deamination allowed calculation of the net postprandial protein utilization (NPPU).

RESULTS

The ileal N digestibility was 84.1% ± 7.7%. Postprandial deamination represented 19.2% ± 3.6% of ingested N, and the NPPU was 64.7% ± 9.7%. The ileal digestibility of individual AAs varied from 85.1% ± 13.7% for histidine to 94.2% ± 3.6% for glutamine + glutamate. The mean AA digestibility was ∼6 percentage points higher than the digestibility of N, reaching 89.8% ± 5.9%, whereas indispensable AA digestibility was 88.0% ± 7.3%. Histidine and tryptophan were the first limiting AAs [DIAAS = 0.77 (calculated by legume-specific N-to-protein conversion factor 5.4); 0.67 (by default factor 6.25)]. Sulfur AAs were limiting to a lesser extent [DIAA ratio = 0.94 (N × 5.4); 0.81 (N × 6.25)].

CONCLUSIONS

Protein ileal digestibility of cooked, dehulled faba beans in humans was moderate (<85%), but that of AAs was close to 90%. Overall protein quality was restricted by the limited histidine and tryptophan content. This trial was registered at clinicaltrials.gov as NCT05047757.

Values for the Digestibility of Pea Protein Isolate or Casein Amino Acids Determined using the Dual Isotope Method Are Not Similar to Those Derived with the Standard Ileal Balance Method in Healthy Volunteers

BACKGROUND

The measurement of ileal amino acid (AA) digestibility is invasive and inappropriate when applied to vulnerable populations. The dual isotope method has been developed over the past 5 y as an alternative method.

OBJECTIVE

The aim of this work was to compare the indispensable amino acid (IAA) digestibility values of 2 different proteins obtained using the dual isotope and the standard ileal balance methods in the same subjects.

METHODS

Fifteen healthy adults completed the study. Over 4 h, they ingested 9 successive portions of mashed potatoes containing the test protein (pea protein or casein) labeled intrinsically with 15N and 2H, and a 13C-free AA mixture as a reference for the dual isotope method. Plasma was sampled regularly over the 8-h postprandial period, whereas the ileal digesta was collected continuously via a naso-ileal tube. Isotopic enrichments (15N and 13C) were measured in the digesta for the direct determination of ileal IAA digestibility, whereas plasma enrichments (2H and 13C) were measured to determine IAA digestibility using the dual isotope method.

RESULTS

The 4-h repeated meal procedure enabled the almost complete digestion of test proteins at 8 h and the attainment of a plasma isotopic plateau between 2.5 and 4 h. These conditions were necessary to perform the ileal balance and dual isotope methods simultaneously. For pea protein, the mean IAA digestibility was similar between the 2 methods, but significant differences (from 10% to 20%) were observed for individual IAA values. For casein, IAA digestibility was significantly lower with the dual isotope method for all the IAA analyzed.

CONCLUSIONS

Under our experimental conditions, the degree of agreement between the dual isotope and ileal balance methods varied among AAs and depended on the protein source. Further research is needed to validate the dual isotope method. This study was registered at clinicaltrials.gov as NCT04072770.

Impact of ultrasonication on the contents, profiles and biofunctional properties of free and bound phenolics from white desert truffle (Tirmania nivea) and its protein fractions

The molecular and biofunctional properties of protein and phenolic fractions in edible truffles remain largely unknown. This study examined the effect of ultrasonication on the contents, profiles, and bioactive properties of free and bound phenolics (FP and BP) from desert truffle (Tirmania nivea) and its protein fractions. Protein fractions from the Osborne extraction scheme were biochemically and structurally characterized. The albumin fraction showed the highest abundance (16.8%) and yield (35.8%). Total phenolic contents were the highest in non-sonicated samples (3.5-34.1 mg/g), particularly in the albumin fraction and in whole truffle. FP extracted at 30 °C (FP-30 °C) accounted for the largest proportion of total phenolics in all protein fractions, whereas BP-30 °C and FP-60 °C were predominant in non-sonicated and sonicated truffle, respectively. The highest antioxidant activity was obtained with FP-30 °C extracts from non-sonicated albumins, globulins and truffle (91.9, 72.7 and 30.0%), followed by BP-30 °C from non-sonicated albumins (25.4%) and FP-60 °C from sonicated glutelins-1 (24.2%). High inhibition of α-amylase was evidenced in several extracts, including FP-30 °C from non-sonicated glutelins-1 (99.2%) and FP-30 °C from sonicated globulins (72.4%). Several extracts also displayed high inhibition of angiotensin I-converting enzyme (ACE), including FP-60 °C from non-sonicated glutelins-1 (65.1%) and sonicated glutelins-1 (71.1%) and globulins (64.7%). Most extracts were rich in epicatechin, gallic acid, chlorogenic acid and catechin. Correlations between phenolic content, antioxidant activity, anti-α-amylase and anti-ACE activities were influenced by sonication. Sonication reduced the particle size of the proteins and modified their structural characteristics. These findings demonstrate that white desert truffle proteins co-occur with bioactive phenolics whose functionalities can be tailored by protein fractionation and sonication.

Ingestion of mycoprotein, pea protein, and their blend support comparable postexercise myofibrillar protein synthesis rates in resistance-trained individuals

Pea protein is an attractive nonanimal-derived protein source to support dietary protein requirements. However, although high in leucine, a low methionine content has been suggested to limit its anabolic potential. Mycoprotein has a complete amino acid profile which, at least in part, may explain its ability to robustly stimulate myofibrillar protein synthesis (MyoPS) rates. We hypothesized that an inferior postexercise MyoPS response would be seen following ingestion of pea protein compared with mycoprotein, which would be (partially) rescued by blending the two sources. Thirty-three healthy, young [age: 21 ± 1 yr, body mass index (BMI): 24 ± 1 kg·m-2] and resistance-trained participants received primed, continuous infusions of l-[ring-2H5]phenylalanine and completed a bout of whole body resistance exercise before ingesting 25 g of protein from mycoprotein (MYC, n = 11), pea protein (PEA, n = 11), or a blend (39% MYC, 61% PEA) of the two (BLEND, n = 11). Blood and muscle samples were taken pre-, 2 h, and 4 h postexercise/protein ingestion to assess postabsorptive and postprandial postexercise myofibrillar protein fractional synthetic rates (FSRs). Protein ingestion increased plasma essential amino acid and leucine concentrations (time effect; P < 0.0001), but more rapidly in BLEND and PEA compared with MYC (time × condition interaction; P < 0.0001). From similar postabsorptive values (MYC, 0.026 ± 0.008%·h-1; PEA, 0.028 ± 0.007%·h-1; BLEND, 0.026 ± 0.006%·h-1), resistance exercise and protein ingestion increased myofibrillar FSRs (time effect; P < 0.0001) over a 4-h postprandial period (MYC, 0.076 ± 0.004%·h-1; PEA, 0.087 ± 0.01%·h-1; BLEND, 0.085 ± 0.01%·h-1), with no differences between groups (all; P > 0.05). These data show that all three nonanimal-derived protein sources have utility in supporting postexercise muscle reconditioning.NEW & NOTEWORTHY This study provides evidence that pea protein (PEA), mycoprotein (MYC), and their blend (BLEND) can support postexercise myofibrillar protein synthesis rates following a bout of whole body resistance exercise. Furthermore, these data suggest that a methionine deficiency in pea may not limit its capacity to stimulate an acute increase in muscle protein synthesis (MPS).

Digestive and metabolic bioavailability in healthy humans of 15N-labeled rapeseed and flaxseed protein incorporated in biscuits

BACKGROUND

In the search to diversify protein sources for humans, oilseeds are good candidates due to the high protein content of their coproducts after oil extraction. Among them, rapeseed presents a well-balanced amino acid (AA) profile. Flaxseed is an emerging source but the nutritional value of its protein is not yet documented.

OBJECTIVES

This study aimed to determine the nitrogen (N) and AA bioavailability of these protein sources.

METHODS

Nineteen healthy volunteers were intubated with a naso-ileal tube. They ingested 156 g biscuits containing intrinsically labeled ¹⁵N rapeseed (n = 10) or flaxseed (n = 9) protein over a 4-h period. Ileal digesta, blood, and urine were sampled over 8 h after the first meal ingestion. N and ¹⁵N enrichment and AAs were measured to determine digestive and deamination losses. Ileal digestibility, the digestible indispensable AA score (DIAAS) and net postprandial protein utilization (NPPU) were calculated.

RESULTS

Real ileal digestibility was 80.7 ± 6.5% for rapeseed protein and 92.2 ± 2.0% for flaxseed protein (P = 0.0002). Mean indispensable AA (IAA) digestibility reached 84.1 ± 6.9% and 93.3 ± 6.7% for rapeseed and flaxseed, respectively, lysine being the lowest digestible IAA for both sources. Despite moderate digestibility, the DIAAS was 1.1 for rapeseed but only 0.6 for flaxseed due to lysine insufficiency. Deamination losses accounted for 20.0 ± 6.5% of dietary N for flaxseed and 11.0 ± 2.8% for rapeseed (P = 0.002). The NPPU did not differ between the protein sources, with 71.3 ± 6.5% for flaxseed and 69.7 ± 7.6% for rapeseed.

CONCLUSIONS

Despite good digestibility, flaxseed protein cooked in biscuits was penalized by both lysine insufficiency and poor lysine digestibility that decreased its DIAAS and increased deamination. By contrast, rapeseed was moderately digestible but presented no limiting IAA, resulting in an excellent DIAAS and low deamination. This study was registered at clinicaltrials.gov as NCT04024605.

Measurement of True Indispensable Amino Acid Digestibility by the Dual Isotope Tracer Technique: A Methodological Review

The digestible indispensable amino acid score uses ileal digestibility of each indispensable amino acid (IAA) of a dietary protein to calculate its protein quality. However, true ileal digestibility, which is the exclusive sum of digestion and absorption of a dietary protein up to the terminal ileum, is difficult to measure in humans. It is traditionally measured using invasive oro-ileal balance methods but can be confounded by endogenous secreted protein in the intestinal lumen, although the use of intrinsically labeled protein corrects for this. A recent, minimally invasive dual isotope tracer technique is now available to measure true IAA digestibility of dietary protein sources. This method involves simultaneous ingestion of 2 intrinsically but differently (stable) isotopically labeled proteins, a (²H or ¹⁵N-labeled) test protein and (¹³C-labeled) reference protein whose true IAA digestibility is known. Using a plateau-feeding protocol, the true IAA digestibility is determined by comparing the steady state ratio of blood to meal test protein IAA enrichment to the similar reference protein IAA ratio. The use of intrinsically labeled protein also distinguishes between IAA of endogenous and dietary origin. The collection of blood samples makes this method minimally invasive. As the α-¹⁵N and α-²H atoms of AAs of the intrinsically labeled protein are prone to label loss because of transamination, underestimation of digestibility, appropriate correction factors need to be employed when using ¹⁵N or ²H labeled test protein. The true IAA digestibility values of highly digestible animal protein by the dual isotope tracer technique are comparable to that measured by direct oro-ileal balance measurements, but no data are yet available for proteins with lower digestibility. A major advantage is that the minimally invasive method allows for true IAA digestibility measurement in humans across different age groups and physiological conditions.

Recovery and Utilization of Pea Albumins as Acidic Emulsion Stabilizer by Complexation with Dextran Sulfate

In this work, pea albumins (PAs) were efficiently recovered by complexation with dextran sulfate (DS), and the emulsifying ability and stability of PA/DS complexes were studied. The largest amounts of PAs (81.25%) were recovered at r = 5:1 and pH_max (pH 3.41) by forming insoluble complexes; and only soluble complexes were formed at r = 2:1 and over the whole pH range (2.0-7.0). The emulsions stabilized by PA/DS soluble complexes remained stable under acidic conditions due to the highly negatively charge (from -45.10 ± 0.40 to -57.23 ± 0.66 mV) and small particle size (0.168 ± 0.010-0.448 ± 0.004 μm), while emulsions stabilized by PAs alone generated a strong creaming and serum separation at pH 5 and 6. In terms of emulsifying stability, all PA emulsions and unheated PA/DS emulsions became unstable with different creaming index after 14 days storage. SDS-PAGE results showed that the interface adsorption proteins of unheated emulsions mainly consisted of PA1a, which was unfavorable to the stability of the interface. On the contrary, heat treatment (95 °C, 30 min) and complexation (PA/DS = 2:1) enhanced the adsorption of PA2 and lectin at the interface, inhibiting the aggregation of PA2 and lectin. This resulted in long-term stability of the PA/DS emulsions under acidic conditions.

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

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

The True Amino Acid Digestibility of 15N-Labelled Sunflower Biscuits Determined with Ileal Balance and Dual Isotope Methods in Healthy Humans

BACKGROUND

Sunflower is a promising protein source but data on amino acid (AA) digestibility are lacking in humans. Classically, the determination of AA digestibility requires ileal digesta sampling. The dual isotope method is minimally invasive but has not been compared to the conventional approach.

OBJECTIVES

This study aimed to determine the true ileal digestibility of sunflower AAs in healthy volunteers who ate biscuits containing 15nitrogen (N) protein isolate, in comparison with the dual isotope method.

METHODS

Twelve healthy volunteers (men and women; 40.4 ± 10.5 years old; BMI, 23.7 ± 2.9 kg/m2) were equipped with a naso-ileal tube. For 4 hours, they consumed 9 repeated meals comprising 15N-sunflower protein biscuits together with 13carbon (C)-AAs, carried either in chocolate (SUN + Ch; n = 7) or apple puree (SUN + P; n = 5). Ileal digesta and blood were sampled throughout 8 hours after ingestion of the first meal. The 15N and 13C AA enrichments were measured in digesta to determine ileal digestibility directly and in plasma to determine lysine and threonine digestibility using the dual isotope method. Differences between methods and between vector groups were analyzed using paired and unpaired t-tests, respectively.

RESULTS

The ileal digestibility of sunflower indispensable AAs (IAA) was 89% ± 5.3%, with threonine and lysine having the lowest digestibility. In the SUN + Ch meal, IAA digestibility was 3% below that of SUN + P (P < 0.05). The mean free 13C-AA ileal digestibility was 98.1% ± 0.9%. No matter which matrix was used to carry 13C-AAs, plasma 15N and 13C-AA kinetics displayed a 1-hour offset. Digestibility obtained with the dual isotope method (70.4% ± 6.0% for threonine and 75.9% ± 22.3% for lysine) was below the target values.

CONCLUSIONS

The ileal digestibility of IAAs from a sunflower isolate incorporated in a biscuit was close to 90% in healthy adults. Under our experimental conditions, the dual isotope method provided lower values than the usual method. Further protocol developments are needed to validate the equivalence between both methods. This trial was registered at clinicaltrials.gov as NCT04024605.

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.

Using naso- and oro-intestinal catheters in physiological research for intestinal delivery and sampling in vivo: practical and technical aspects to be considered

Intestinal catheters have been used for decades in human nutrition, physiology, pharmacokinetics, and gut microbiome research, facilitating the delivery of compounds directly into the intestinal lumen or the aspiration of intestinal fluids in human subjects. Such research provides insights about (local) dynamic metabolic and other intestinal luminal processes, but working with catheters might pose challenges to biomedical researchers and clinicians. Here, we provide an overview of practical and technical aspects of applying naso- and oro-intestinal catheters for delivery of compounds and sampling luminal fluids from the jejunum, ileum, and colon in vivo. The recent literature was extensively reviewed, and combined with experiences and insights we gained through our own clinical trials. We included 60 studies that involved a total of 720 healthy subjects and 42 patients. Most of the studies investigated multiple intestinal regions (24 studies), followed by studies investigating only the jejunum (21 studies), ileum (13 studies), or colon (2 studies). The ileum and colon used to be relatively inaccessible regions in vivo. Custom-made state-of-the-art catheters are available with numerous options for the design, such as multiple lumina, side holes, and inflatable balloons for catheter progression or isolation of intestinal segments. These allow for multiple controlled sampling and compound delivery options in different intestinal regions. Intestinal catheters were often used for delivery (23 studies), sampling (10 studies), or both (27 studies). Sampling speed decreased with increasing distance from the sampling syringe to the specific intestinal segment (i.e., speed highest in duodenum, lowest in ileum/colon). No serious adverse events were reported in the literature, and a dropout rate of around 10% was found for these types of studies. This review is highly relevant for researchers who are active in various research areas and want to expand their research with the use of intestinal catheters in humans in vivo.

Multi-criteria assessment of pea protein quality in rats: a comparison between casein, gluten and pea protein alone or supplemented with methionine

The objective of this study was to assess the nutritional quality of pea protein isolate in rats and to evaluate the impact of methionine (Met) supplementation. Several protein diets were studied: pea protein, casein, gluten, pea protein-gluten combination and pea protein supplemented with Met. Study 1: Young male Wistar rats (n 8/group) were fed the test diets ad libitum for 28 d. The protein efficiency ratio (PER) was measured. Study 2: Adult male Wistar rats (n 9/group) were fed the test diets for 10 d. A protein-free diet group was used to determine endogenous losses of N. The rats were placed in metabolism cages for 3 d to assess N balance, true faecal N digestibility and to calculate the Protein Digestible-Corrected Amino Acid Score (PDCAAS). They were then given a calibrated meal and euthanised 6 h later for collection of digestive contents. The true caecal amino acid (AA) digestibility was determined, and the Digestible Indispensable Amino Acid Score (DIAAS) was calculated. Met supplementation increased the PER of pea protein (2·52 v. 1·14, P < 0·001) up to the PER of casein (2·55). Mean true caecal AA digestibility was 94 % for pea protein. The DIAAS was 0·88 for pea protein and 1·10 with Met supplementation, 1·29 for casein and 0·25 for gluten. Pea protein was highly digestible in rats under our experimental conditions, and Met supplementation enabled generation of a mixture that had a protein quality that was not different from that of casein.

True ileal amino acid digestibility and digestible indispensable amino acid scores (DIAASs) of plant-based protein foods

Plant-based protein foods are increasingly common, but data on their nutritional protein quality are scarce. This study evaluated it for seitan (wheat-based food), tofu (soya-based food), soya milk, and a pea emulsion. The true ileal digestibility (TID) of their amino acids was determined in minipigs, to calculate the digestible indispensable amino acid score (DIAAS). The TID of the proteins was high and not significantly different between the foods tested: 97% for seitan, 95% for tofu, 92% for soya milk and 94% for pea emulsion. There were only minor differences in individual amino acid TIDs. DIAAS ranking was thus essentially driven by the amino acid composition of the food: soya-based food > pea emulsion > seitan. Nevertheless, the lower TID of sulphur-containing amino acids in tofu than in soya milk induced a significant decrease in DIAAS (from 117% to 97%), highlighting the importance of the matrix effect on nutritional protein quality.

Very low ileal nitrogen and amino acid digestibility of zein compared to whey protein isolate in healthy volunteers

BACKGROUND

Whey protein and zein are of nutritional interest due to their high leucine content, but little data are available on their amino acid (AA) ileal digestibility.

OBJECTIVE

This study aimed to determine ileal digestibility of whey protein isolate (WPI) and zein in healthy volunteers by use of the naso-ileal intubation method, which allows continuous collection of postprandial ileal digesta.

METHODS

Twenty-two healthy volunteers were intubated with a naso-ileal sampling device positioned at the terminal ileum level. They received a single meal of protein-free biscuits and a drink containing zein (n = 8), WPI (n = 7), or no protein (protein free, n = 7). Ileal effluents and plasma samples were collected over a 9-h postprandial period. Total nitrogen and AA contents were quantified in effluents. True ileal digestibility was calculated after correction for endogenous losses evaluated in the protein-free group.

RESULTS

True ileal nitrogen digestibility of zein was markedly lower than WPI (60.2 ± 4.5% and 91.2 ± 2.6%, respectively, P = 0.0003). True ileal digestibility of AAs ranged from 87.4 ± 2.7% for threonine to 98.4 ± 1.0% for methionine in the WPI group, and from 59.3 ± 5.6% for methionine to 69.0 ± 5.8% for arginine in the zein group. The digestible indispensable AA (IAA) score was 1.03 (histidine) for WPI and close to 0 for zein, owing to its negligible lysine content. Plasma IAA concentration significantly increased after WPI intake (P = 0.0319), whereas no effect of zein on aminoacidemia was observed, including plasma leucine, despite its high leucine content.

CONCLUSIONS

Our findings provide data on ileal digestibility of WPI and zein AAs in healthy humans and, in contrast to WPI, zein is poorly digestible. This study was registered at clinicaltrials.gov as NCT03279211.

Dietary Protein and Amino Acids in Vegetarian Diets-A Review

While animal products are rich in protein, the adequacy of dietary protein intake from vegetarian/vegan diets has long been controversial. In this review, we examine the protein and amino acid intakes from vegetarian diets followed by adults in western countries and gather information in terms of adequacy for protein and amino acids requirements, using indirect and direct data to estimate nutritional status. We point out that protein-rich foods, such as traditional legumes, nuts and seeds, are sufficient to achieve full protein adequacy in adults consuming vegetarian/vegan diets, while the question of any amino acid deficiency has been substantially overstated. Our review addresses the adequacy in changes to protein patterns in people newly transitioning to vegetarian diets. We also specifically address this in older adults, where the issues linked to the protein adequacy of vegetarian diets are more complex. This contrasts with the situation in children where there are no specific concerns regarding protein adequacy because of their very high energy requirements compared to those of protein. Given the growing shifts in recommendations from nutrition health professionals for people to transition to more plant-based, whole-food diets, additional scientific evidence-based communications confirming the protein adequacy of vegetarian and vegan diets is warranted.

GutSelf: Interindividual Variability in the Processing of Dietary Compounds by the Human Gastrointestinal Tract

Nutritional research is currently entering the field of personalized nutrition, to a large extent driven by major technological breakthroughs in analytical sciences and biocomputing. An efficient launching of the personalized approach depends on the ability of researchers to comprehensively monitor and characterize interindividual variability in the activity of the human gastrointestinal tract. This information is currently not available in such a form. This review therefore aims at identifying and discussing published data, providing evidence on interindividual variability in the processing of the major nutrients, i.e., protein, fat, carbohydrates, vitamins, and minerals, along the gastrointestinal tract, including oral processing, intestinal digestion, and absorption. Although interindividual variability is not a primary endpoint of most studies identified, a significant number of publications provides a wealth of information on this topic for each category of nutrients. This knowledge remains fragmented, however, and understanding the clinical relevance of most of the interindividual responses to food ingestion described in this review remains unclear. In that regard, this review has identified a gap and sets the base for future research addressing the issue of the interindividual variability in the response of the human organism to the ingestion of foods.

True ileal digestibility of legumes determined by dual-isotope tracer method in Indian adults

BACKGROUND

Good-quality plant protein sources are important for protein adequacy in a balanced diet. Legumes are known to be a source of good quality plant protein, but the true ileal digestibility of indispensable amino acids (IAAs) of commonly consumed legumes is not known in humans.

OBJECTIVES

In this study we measured the true ileal IAA digestibility of 2H-intrinsically labeled chickpea, yellow pea, and mung bean (hulled and dehulled) protein, using the dual-isotope tracer technique referenced to a standard protein ([U-13C] spirulina). The study also aimed to validate the use of [U-13C] spirulina as a reference protein in this method.

METHODS

2H-intrinsically labeled legumes, obtained by watering plants with deuterium oxide (2H2O), were administered in a plateau feeding method to healthy Indian adults to measure their true ileal IAA digestibility with the dual-isotope tracer technique, using [U-13C] spirulina protein or a 13C-algal IAA mixture as the standard.

RESULT

The true ileal IAA digestibilities (mean ± SD) of chickpea, yellow pea, and mung bean were 74.6 ± 0.8%, 71.6 ± 1.3%, and 63.2 ± 1.5%, respectively. The true mean ileal IAA digestibility of mung bean when referenced to [U-13C] spirulina protein or a 13C-algal IAA mixture did not differ significantly (63.2 ± 1.5% versus 64.0 ± 2.4%, P > 0.05). The true ileal IAA digestibility of mung bean improved to 70.9 ± 2.1% after dehulling.

CONCLUSIONS

The true mean ileal IAA digestibility of legumes in healthy Indian adults was lower than expected. Traditional processing techniques such as dehulling improve protein digestibility by about 8%. This study was registered in the Clinical Trials Registry of India (CTRI): CTRI/2017/11/010468 (http://ctri.nic.in, accessed on 28/03/2019).

Food and nutritional security requires adequate protein as well as energy, delivered from whole-year crop production

Human food security requires the production of sufficient quantities of both high-quality protein and dietary energy. In a series of case-studies from New Zealand, we show that while production of food ingredients from crops on arable land can meet human dietary energy requirements effectively, requirements for high-quality protein are met more efficiently by animal production from such land. We present a model that can be used to assess dietary energy and quality-corrected protein production from various crop and crop/animal production systems, and demonstrate its utility. We extend our analysis with an accompanying economic analysis of commercially-available, pre-prepared or simply-cooked foods that can be produced from our case-study crop and animal products. We calculate the per-person, per-day cost of both quality-corrected protein and dietary energy as provided in the processed foods. We conclude that mixed dairy/cropping systems provide the greatest quantity of high-quality protein per unit price to the consumer, have the highest food energy production and can support the dietary requirements of the highest number of people, when assessed as all-year-round production systems. Global food and nutritional security will largely be an outcome of national or regional agroeconomies addressing their own food needs. We hope that our model will be used for similar analyses of food production systems in other countries, agroecological zones and economies.

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.

Re-print of "Intestinal luminal nitrogen metabolism: role of the gut microbiota and consequences for the host"

Alimentary and endogenous proteins are mixed in the small intestinal lumen with the microbiota. Although experimental evidences suggest that the intestinal microbiota is able to incorporate and degrade some of the available amino acids, it appears that the microbiota is also able to synthesize amino acids raising the view that amino acid exchange between the microbiota and host can proceed in both directions. Although the net result of such exchanges remains to be determined, it is likely that a significant part of the amino acids recovered from the alimentary proteins are used by the microbiota. In the large intestine, where the density of bacteria is much higher than in the small intestine and the transit time much longer, the residual undigested luminal proteins and peptides can be degraded in amino acids by the microbiota. These amino acids cannot be absorbed to a significant extent by the colonic epithelium, but are precursors for the synthesis of numerous metabolic end products in reactions made by the microbiota. Among these products, some like short-chain fatty acids and organic acids are energy substrates for the colonic mucosa and several peripheral tissues while others like sulfide and ammonia can affect the energy metabolism of colonic epithelial cells. More work is needed to clarify the overall effects of the intestinal microbiota on nitrogenous compound metabolism and consequences on gut and more generally host health.

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

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

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

BACKGROUND

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

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

BACKGROUND

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

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.

Dietary protein absorption of the small intestine in human neonates

BACKGROUND

The intestine plays a key role in the absorption of dietary proteins, which determines growth of human neonates. Bowel resection in the neonatal period brings loss of absorptive and protective surface and may consequently lead to malabsorption of dietary nutrients. However, there are no data on net dietary protein absorption of the small intestine in the period after intestinal surgery in human neonates. We therefore evaluated dietary feeding tolerance and quantified net dietary protein absorption capacity of the small intestine in human neonates in whom a temporary jejunostomy or ileostomy was created.

METHODS

Seventeen patients were included in the study. We collected small intestinal outflow fluid at the level of the enterostomy weekly for 24-48 hours during weeks 3 through 6 postoperatively. Protein levels in the intestinal outflow fluid were determined by bicinchoninic acid (BCA) assay.

RESULTS

In 14 patients, an enteral intake of >100 mL/kg/d was reached at a median of 17 days (range, 8-32 days) postoperatively. Three patients did not reach this level within the study period. Overall, the net dietary protein absorption capacity was 70%-90% of the total enteral protein intake.

CONCLUSIONS

This study demonstrates that the dietary protein absorption capacity of the small intestine is intact in most human neonates after intestinal surgery in a very critical period of their lives. Furthermore, our results do not support the use of hydrolyzed or elemental formula in newborns with an enterostomy to improve amino acid uptake.

The bioavailability and postprandial utilisation of sweet lupin (Lupinus albus)-flour protein is similar to that of purified soyabean protein in human subjects: a study using intrinsically15N-labelled proteins

Sweet lupin (Lupinus albus), a protein-rich legume devoid of anti-nutritional factors, is considered to have a high potential for protein nutrition in man. Results concerning the nutritional value of lupin protein are, however, conflicting in animals and very scarce in human subjects. Furthermore, where fibre-rich protein sources are concerned, the long-term nutritional results are often obscured, particularly since fibre-promoted colonic fermentation may bias the energy supply and redistribute N flux. We therefore studied, during the postprandial phase, the bioavailability and utilisation of lupin-flour protein in nine healthy men who had ingested a mixed meal containing intrinsically15N-labelled lupin flour as the protein source (Expt 1). The real ileal digestibility (RID) and ileal endogenous N losses (IENL) were assessed using a perfusion technique at the terminal ileum, and the N content and15N enrichment of ileal samples. Lupin flour exhibited a high RID of 91 (SD 3) % AND LOW IENL (5·4 (sd 1·3) mmol N/h). Postprandial dietary deamination was also assessed from body dietary urea and urinary dietary N excretion, and compared with results in nine healthy men following an isoenergetic meal containing a15N-soyabean-protein isolate with a similar RID, as a control (Expt 2). Postprandial dietary deamination was similar after lupin and soyabean meals (17 (sd 2) and 18 (sd 4) % ingested N respectively). We therefore conclude that lupin protein is highly bioavailable, even if included in fibre-rich flour, and that it can be used with the same efficiency as soyabean protein to achieve postprandial protein gain in healthy human subjects.

The poor digestibility of rapeseed protein is balanced by its very high metabolic utilization in humans

Rapeseed protein (RP, Brassica napus) is used in only animal feed despite its high nutritional potential for human nutrition. We sought to assess the nutritional quality of rapeseed by measuring its real ileal digestibility (RID) and net postprandial protein utilization (NPPU) in humans fed (15)N-RP. Volunteers equipped with an intestinal tube at the jejunal (n = 5) or ileal level (n = 7) ingested a mixed meal containing 27.3 g (15)N-RP and a total energy content of 700 kcal (2.93 MJ). Dietary N kinetics was quantified in intestinal fluid, urine, and blood sampled at regular intervals during the postprandial period. The RID of RP was 84.0 +/- 8.8%. Dietary N at the ileal level was mostly in the form of undigested protein from both 12S and 2S rapeseed fractions. Aminoacidemia was not significantly increased by meal ingestion. The postprandial distribution of dietary N was 5.4 +/- 1.8% in urinary urea and ammonia, 8.2 +/- 3.4% in body urea, and 7.7 +/- 2.0% in plasma protein 8 h after the meal. The NPPU of RP amounted to 70.5 +/- 9.6% and the postprandial biological value (PBV) was high at 83.8 +/- 4.6%. RP has a low RID in humans compared with other plant proteins but also exhibits a very low deamination rate. Thus, the PBV of RP is excellent in humans, being as high as that of milk protein. We conclude that RP has a high nutritional potential for human nutrition.

Effects of amino acid-derived luminal metabolites on the colonic epithelium and physiopathological consequences

Depending on the amount of alimentary proteins, between 6 and 18 g nitrogenous material per day enter the large intestine lumen through the ileocaecal junction. This material is used as substrates by the flora resulting eventually in the presence of a complex mixture of metabolites including ammonia, hydrogen sulfide, short and branched-chain fatty acids, amines; phenolic, indolic and N-nitroso compounds. The beneficial versus deleterious effects of these compounds on the colonic epithelium depend on parameters such as their luminal concentrations, the duration of the colonic stasis, the detoxication capacity of epithelial cells in response to increase of metabolite concentrations, the cellular metabolic utilization of these metabolites as well as their effects on colonocyte intermediary and oxidative metabolism. Furthermore, the effects of metabolites on electrolyte movements through the colonic epithelium must as well be taken into consideration for such an evaluation. The situation is further complicated by the fact that other non-nitrogenous compounds are believed to interfere with these various phenomenons. Finally, the pathological consequences of the presence of excessive concentrations of these compounds are related to the short- and, most important, long-term effects of these compounds on the rapid colonic epithelium renewing and homeostasis.

Identification of dietary and endogenous ileal protein losses in pigs by immunoblotting and mass spectrometry

Ileal flows and the endogenous or dietary origin of soluble proteins present in ileal digesta were determined in pigs fed diets containing different pea cultivars (Solara, Madria and Eiffel) and micro-ground peas (c.v. Solara). Ileal digesta proteins were analyzed by electrophoresis and densitometry analysis and were identified by LC-MS-MS spectrometry and immunoblotting. The ileal flows of proteins differed (P < 0.1) among the 3 pea cultivars; the flow in pigs fed the Madria-containing diet was higher than that of pigs fed the Eiffel- and Solara-containing diets. The flow was reduced by micro-grinding the peas. The true digestibility of pea proteins and the endogenous losses were not correlated. However, at this intestinal level, protein losses were essentially of endogenous origin (enzymes, antibodies), and from the partly digested pea albumin fraction. Pea lectin and albumin PA1b were totally resistant to gastric and small intestinal digestion and a minor resistant peptide from pea albumin PA2 was detectable. In contrast, the storage proteins, legumin and vicilin, were not detectable by antibodies or by LC-MS-MS.

Postprandial metabolic utilization of wheat protein in humans

BACKGROUND

The quality of cereal protein has been little studied in humans despite its quantitative importance in the diet, particularly in developing countries.

OBJECTIVE

The objective of this study was to determine the nutritional value of wheat protein in humans as assessed by the measurement of their real ileal digestibility and postprandial retention.

DESIGN

Healthy young adults (n = 14) were fitted with an intestinal tube to allow the collection of intestinal fluid in the duodenum or terminal ileum. Subjects received a mixed meal of 136 g wheat toast that contained 24.6 g uniformly and intrinsically [(15)N]-labeled wheat protein. Intestinal fluid, blood, and urine were collected for 8 h postprandially.

RESULTS

The real ileal digestibility of dietary wheat nitrogen amounted to 90.3 +/- 4.3%. The cumulative amount of dietary nitrogen transferred to the deamination pools reached a plateau at 8 h of 24.7 +/- 6.8% of the amount ingested. The urinary excretion of dietary nitrogen in ammonia was high (0.8 +/- 0.3% of ingested dose). The incorporation of dietary nitrogen into serum protein reached 7.0 +/- 1.9% of the meal. Postprandial wheat protein retention was 66.1 +/- 5.8%.

CONCLUSIONS

Our results show that wheat proteins had the same true ileal digestibility as did most of the plant proteins already studied in humans, but also that they had a lower postprandial nitrogen retention value. However, this low value was higher than that predicted from the calculation of indispensable amino acid scores, ie, 89% rather than 30-40% of the nutritional value of milk proteins.

Protein and amino acid requirements and the composition of complementary foods

In this paper, factorial models of the dietary requirements for protein, nitrogen and individual indispensable amino acids are developed from published information on the relationship between age and protein deposition and between protein (amino acid) intake and nitrogen balance. The results are used to develop recommendations on the protein-energy ratio and the amino acid pattern of the diet. As part of the development of the models, factors affecting dietary protein digestibility, bioavailability and efficiency of utilization are discussed. Over the age range of 6-24 mo the models predict a fall in the weight-specific protein and amino acid requirement that results almost entirely from the changes in the growth rate of the children. It is also concluded that the requirement for the maintenance of body protein equilibrium (so-called maintenance) changes little with age. This contrasts markedly with the relationship between age and energy requirements. The amino acid modeling implies that the optimum pattern of individual essential amino acids also changes only marginally across the age range considered in the report. The calculations of the dietary requirement for whole protein imply that achieving a minimum protein-energy ratio of 6.3% is desirable. The amount of protein needed from complementary foods for breast-fed children is discussed.

Increasing habitual protein intake accentuates differences in postprandial dietary nitrogen utilization between protein sources in humans

It is appropriate to characterize the nutritional value of dietary proteins in humans through the specific study of dietary nitrogen metabolism during the postprandial period. However, the influence of the habitual protein intake on this variable has not been studied. We aimed to describe the influence of prior protein intake on the specific metabolic utilization of dietary nitrogen in humans. Healthy men and women were adapted for 7 d to two diets with a normal [NP, 1 g/(kg x d)] and high protein content [HP, 2 g/(kg x d)]. After each period, they were studied for an 8-h postmeal period after ingesting a single (15)N-labeled mixed meal (0.41 g/kg protein) containing either milk (n = 12) or soy protein (n = 8). The HP diet reduced the peak of dietary N incorporation into free serum amino acids in the soy group but had no effect in the milk group. The incorporation of dietary N into plasma protein was higher after soy than after milk protein, but habitual protein level had no effect. The postprandial retention of milk protein was reduced by the HP diet compared with the NP diet by only 5% and that of soy protein was diminished by 13% (protein source: P < 0.0001, protein level: P < 0.0001, interaction: P < 0.001). In conclusion, the efficiency of the meal N postprandial retention was lower after HP adaptation, but this decrease was much more pronounced for soy than for milk protein, indicating that increasing the habitual protein intake accentuates differences in metabolic utilization among dietary proteins.

Approaches to quantifying protein metabolism in response to nutrient ingestion

The investigation of protein metabolism under various nutritional and physiological conditions has been made possible by the use of indirect, principally tracer-based methods. Most studies were conducted at the whole-body level, mainly using steady-state isotopic techniques and equations based on simple two-pool models, in which amino acids are either free or protein bound. Because whole-body methods disregard regional contributions to protein metabolism, some regional approaches have tried to distinguish the distribution of protein kinetics in the different tissues. The organ-balance tracer technique, involving the arteriovenous catheterization of regions or organs with concomitant isotopic tracer infusion, distinguishes between amino acid uptake and release in the net amino acid balance and measures protein synthesis and degradation under steady-state conditions. Last, the importance has become clear of the difference in dietary and endogenous amino acids recycled from proteolysis for anabolic and catabolic pathways. In humans, the dual tracer technique, which consists of the simultaneous oral/enteral administration and intravenous infusion of different tracers of the same amino acid, allows an estimate of the splanchnic uptake of amino acids administered. Furthermore, the whole-body retention of labeled dietary nitrogen after the ingestion of a single protein meal has enabled a clearer understanding of the metabolic fate of dietary amino acids. Based on such data, a newly developed compartmental model provides a simulation of the regional distribution and metabolism of ingested nitrogen in the fed state by determining its dynamic fate through free and protein-bound amino acids in both the splanchnic and peripheral areas in humans.

Plasma glucagon and insulin responses depend on the rate of appearance of amino acids after ingestion of different protein solutions in humans

To find out whether the hormonal response to feeding with protein solutions is influenced by the nature and degree of protein fractionation, we examined insulin and glucagon responses after intake of protein solutions containing the same amount of nitrogen (2.9 g each) in three men and three women. Four test meals (600 mL) [glucose (419 kJ/L), pea (PPH) and whey peptide hydrolysates (WPH) (921 and 963 kJ/L, respectively) and a cow's milk solution (MS) containing complete milk proteins (2763 kJ/L)] were tested. Peptide hydrolysates elicited a faster increase in venous plasma amino acids than did MS (P < 0.05). Despite the higher carbohydrate content of the MS, the peptide hydrolysates elicited a peak insulin response that was two and four times greater than that evoked by the MS and glucose solutions, respectively (P < 0.05). The insulin response was closely related to the increase in plasma amino acids, especially leucine, isoleucine, valine, phenylalanine and arginine, regardless of the rate of gastric emptying. The three protein solutions elicited similar increases of plasma glucagon; however, the response was fastest for both peptide hydrolysates (P < 0.05) and more prolonged for the MS (P < 0.05). The glucagon response was linearly related to the increase in plasma amino acids, regardless of the rate of gastric emptying or meal composition (r = 0.93, r = 0.96 and r = 0.78, all P < 0.05, for the PPH, WPH and MS). Among the plasma amino acids, tyrosine (r = 0.82-0.98, P < 0.05) and methionine (r = 0.98, P < 0.001) were most closely related to the plasma glucagon response. This study shows that the glucagon response to feeding with protein solutions depends on the increase in plasma amino acid concentrations. The combined administration of glucose and peptide hydrolysates stimulates a synergistic release of insulin, regardless of the protein source.

Isotopic studies of protein and amino acid requirements

The quantification of protein and amino acid requirements in health and disease is still an incompletely resolved issue, despite its importance to our knowledge of nutrition, to the clinical management of most health disorders and to food policy. However, the dynamic and adaptive features of protein metabolism render this determination difficult. The first nitrogen balance studies performed have demonstrated their limitations in providing accurate protein and amino acid requirements. Isotopic methods developed over the past 15 years have considerably enhanced the quantification of amino acid and protein requirements and our knowledge of the physiological phenomena underlying these needs. These methods are consistently being improved and producing new estimates for protein and amino acid requirements, together with a clearer understanding of this complex issue.

Methods for assessing intestinal absorptive function in relation to enteral nutrition

The success of nasoenteral nutrition support can be limited by intestinal impairment. In particular, reduced absorptive area, mucosal atrophy and abnormal motility may reduce absorption of macronutrients and micronutrients, and diarrhoea remains a commonly encountered complication. We review how basic physiological techniques can be used to investigate such pathophysiology. Lumenal nutrients control mucosal growth, expression of mucosal transporters and regional gut motility. Cell biology techniques now complement classical intestinal perfusion methods in determining the 'safety factor' of excess absorptive capacity. The controversial role of the sodium-glucose linked transporter in dietary glucose assimilation is described in terms of its control, its true function and its role in uptake of other solutes. Techniques that involve brush-border membrane vesicles, Caco-2 cells, mucosal immunohistochemistry and gene expression probes are described. Together, these techniques describe a picture of an organ with remarkable ability to maintain digestive and absorptive function in response to a wide variety of nutritional intakes, often in the face of inflammatory illness.