Postprandial modulation of dietary and whole-body nitrogen utilization by carbohydrates in humans

Dietary intake in individuals with phenylketonuria: an integrative review

Introduction

Introduction: the dietary intake of individuals with phenylketonuria (PKU) may vary widely according to different cultural eating habits, lifestyle, access to multidisciplinary team, and metabolic formulas available. Thus, knowing the dietary intake of this population makes it possible to tailor nutritional treatment strategies to impact their health. Objective: to analyze the evidence on the dietary intake of individuals with PKU. Methods: an integrative literature review was conducted on the dietary intake of individuals with PKU in the databases PUBMED, BIREME and Science Direct. Original articles that addressed the energy and macronutrient food intake of children, adolescents and/or adults with PKU were included in the study, without time restriction, in any language. A total of 384 articles were found and 27 articles were selected and analyzed. Results: evidence about the nutritional composition of their diet showed that individuals with PKU consume between 1160-2721 kcal of energy -7.2-17.4 % (32.4-76.9 g) of energy as protein, 45.9-69.2 % of energy as carbohydrates, 16.6-39 % of energy as lipids- and between 7.6 and 20 g of fiber. Conclusion: most individuals with PKU have low energy, protein and fiber intake, adequate lipid intake, and high carbohydrate intake. Metabolic control of the disease is still a challenge in all countries. Nutritional strategies to improve dietary nutritional composition and phenylalanine blood levels in individuals with PKU remain an urgent issue.

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.

Dairy structures and physiological responses: a matter of gastric digestion

Digestion and health properties of food do not solely rely on the sum of nutrients but are also influenced by food structure. Dairy products present an array of structures due to differences in the origin of milk components and the changes induced by processing. Some dairy structures have been observed to induce specific effects on digestion rates and physiological responses. However, the underlying mechanisms are not fully understood. Gastric digestion plays a key role in controlling digestion kinetics. The main objective of this review is to expose the relevance of gastric phase as the link between dairy structures and physiological responses. The focus is on human and animal studies, and physiological relevant in vitro digestion models. Data collected showed that the structure of dairy products have a profound impact on rate of nutrient bioavailability, absorption and physiological responses, suggesting gastric digestion as the main driver. Control of gastric digestion can be a tool for delivering specific rates of nutrient digestion. Therefore, the design of food structure targeting specific gastric behavior could be of great interest for particular population needs e.g. rapid nutrient digestion will benefit elderly, and slow nutrient digestion could help to enhance satiety.

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.

Dietary intervention in the management of phenylketonuria: current perspectives

Phenylketonuria (PKU) is a well-described inborn error of amino acid metabolism that has been treated for >60 years. Enzyme deficiency causes accumulation of phenylalanine (Phe) and if left untreated will lead to profound and irreversible intellectual disability in most children. Traditionally, it has been managed with a low-Phe diet supplemented with a Phe-free protein substitute although newer treatment options mainly in combination with diet are available for some subgroups of patients with PKU, for example, sapropterin, large neutral amino acids, and glycomacropeptide. The diet consists of three parts: 1) severe restriction of dietary Phe; 2) replacement of non-Phe l-amino acids with a protein substitute commonly supplemented with essential fatty acids and other micronutrients; and 3) low-protein foods from fruits, some vegetables, sugars, fats and oil, and special low-protein foods (SLPF). The prescription of diet is challenging for health professionals. The high-carbohydrate diet supplied by a limited range of foods may program food preferences and contribute to obesity in later life. Abnormal tasting and satiety-promoting protein substitutes are administered to coincide with peak appetite times to ensure their consumption, but this practice may impede appetite for other important foods. Intermittent dosing of micronutrients when combined with l-amino acid supplements may lead to their poor bioavailability. Much work is required on the ideal nutritional profiling for special SLPF and Phe-free l-amino acid supplements. Although non-diet treatments are being studied, it is important to continue to fully understand all the consequences of diet therapy as it is likely to remain the foundation of therapy for many years.

A Slow- Compared with a Fast-Release Form of Oral Arginine Increases Its Utilization for Nitric Oxide Synthesis in Overweight Adults with Cardiometabolic Risk Factors in a Randomized Controlled Study

BACKGROUND

Oral l-arginine supplements can have a beneficial effect on nitric oxide (NO)-related functions when subjects have cardiovascular disease risk factors.

OBJECTIVE

The study was designed to determine the utilization for NO synthesis of oral l-arginine as a function of the cardiometabolic risk and the speed of absorption by comparing immediate-release arginine (IR-Arg), as in supplements, and sustained-release arginine (SR-Arg), which mimics the slow release of dietary arginine.

METHODS

In a randomized, single-blind, 2-period crossover, controlled trial (1 wk of treatment, >2 wk of washout), using [(15)N-(15)N-(guanidino)]-arginine for the first morning dose, we compared the bioavailability (secondary outcome) and utilization for NO synthesis (primary outcome) of 1.5 g IR- and SR-Arg 3 times/d in 12 healthy overweight [body mass index (BMI; in kg/m(2)): 25-30] adults with the hypertriglyceridemic waist phenotype [HTW; plasma triglycerides (TGs): >150 mg/dL; waist circumference: >94 cm (men) or >80 cm (women)] and 15 healthy control adults (CON; BMI: 18.5-25; no elevated TGs and waist circumference).

RESULTS

Plasma oral arginine areas under the curve were lower after supplementation with SR-Arg than with IR-Arg (112 ± 52.3 and 142 ± 50.8 μmol ⋅ h/L; P < 0.01). The utilization of oral arginine for NO synthesis was 58% higher in HTW subjects than in CON subjects and higher with SR-Arg than with IR-Arg (P < 0.05 both), particularly in HTW subjects (group-by-treatment interaction, P < 0.05). In HTW subjects administered the SR form, utilization for NO synthesis was 32% higher than with the IR form and 87% higher than in CON subjects who were administered the SR form.

CONCLUSION

In overweight adults with the HTW phenotype, a slow- compared with a fast-release form of oral arginine markedly favors the utilization of arginine for NO synthesis. The utilization of low-dose, slow-release arginine for NO synthesis is higher in overweight adults with the HTW phenotype than in healthy controls, suggesting that the sensitivity of NO synthesis to the dietary arginine supply increases with cardiometabolic risk. The trial was registered at clinicaltrials.gov as NCT02352740.

Protein Requirements Are Elevated in Endurance Athletes after Exercise as Determined by the Indicator Amino Acid Oxidation Method

A higher protein intake has been recommended for endurance athletes compared with healthy non-exercising individuals based primarily on nitrogen balance methodology. The aim of this study was to determine the estimated average protein requirement and recommended protein intake in endurance athletes during an acute 3-d controlled training period using the indicator amino acid oxidation method. After 2-d of controlled diet (1.4 g protein/kg/d) and training (10 and 5km/d, respectively), six male endurance-trained adults (28±4 y of age; Body weight, 64.5±10.0 kg; VO₂peak, 60.3±6.7 ml·kg^-1·min^-1; means±SD) performed an acute bout of endurance exercise (20 km treadmill run) prior to consuming test diets providing variable amounts of protein (0.2–2.8 g·kg^-1·d^-1) and sufficient energy. Protein was provided as a crystalline amino acid mixture based on the composition of egg protein with [1-¹³C]phenylalanine provided to determine whole body phenylalanine flux, ¹³CO₂ excretion, and phenylalanine oxidation. The estimated average protein requirement was determined as the breakpoint after biphasic linear regression analysis with a recommended protein intake defined as the upper 95% confidence interval. Phenylalanine flux (68.8±8.5 μmol·kg^-1·h^-1) was not affected by protein intake. ¹³CO₂ excretion displayed a robust bi-phase linear relationship (R² = 0.86) that resulted in an estimated average requirement and a recommended protein intake of 1.65 and 1.83 g protein·kg^-1·d^-1, respectively, which was similar to values based on phenylalanine oxidation (1.53 and 1.70 g·kg^-1·d^-1, respectively). We report a recommended protein intake that is greater than the RDA (0.8 g·kg^-1·d^-1) and current recommendations for endurance athletes (1.2–1.4 g·kg^-1·d^-1). Our results suggest that the metabolic demand for protein in endurance-trained adults on a higher volume training day is greater than their sedentary peers and current recommendations for athletes based primarily on nitrogen balance methodology.

Trial Registration: ClinicalTrial.gov NCT02478801

High True Ileal Digestibility but Not Postprandial Utilization of Nitrogen from Bovine Meat Protein in Humans Is Moderately Decreased by High-Temperature, Long-Duration Cooking

BACKGROUND

Meat protein digestibility can be impaired because of indigestible protein aggregates that form during cooking. When the aggregates are subsequently fermented by the microbiota, they can generate potentially harmful compounds for the colonic mucosa.

OBJECTIVE

This study evaluated the quantity of bovine meat protein escaping digestion in the human small intestine and the metabolic fate of exogenous nitrogen, depending on cooking processes.

METHODS

Sixteen volunteers (5 women and 11 men; aged 28 ± 8 y) were equipped with a double lumen intestinal tube positioned at the ileal level. They received a test meal exclusively composed of 120 g of intrinsically (15)N-labeled bovine meat, cooked either at 55°C for 5 min (n = 8) or at 90°C for 30 min (n = 8). Ileal effluents and blood and urine samples were collected over an 8-h period after the meal ingestion, and (15)N enrichments were measured to assess the digestibility of meat proteins and the transfer of dietary nitrogen into the metabolic pools.

RESULTS

Proteins tended to be less digestible for the meat cooked at 90°C for 30 min than at 55°C for 5 min (90.1% ± 2.1% vs. 94.1% ± 0.7% of ingested N; P = 0.08). However, the particle number and size in ileal digesta did not differ between groups. The appearance of variable amounts of intact fibers was observed by microscopy. The kinetics of (15)N appearance in plasma proteins, amino acids, and urea were similar between groups. The amount of exogenous nitrogen lost through deamination did not differ between groups (21.2% ± 0.8% of ingested N).

CONCLUSIONS

Cooking bovine meat at a high temperature for a long time can moderately decrease protein digestibility compared with cooking at a lower temperature for a short time and does not affect postprandial exogenous protein metabolism in young adults. The study was registered at www.clinicaltrials.gov as NCT01685307.

Kinetics of the utilization of dietary arginine for nitric oxide and urea synthesis: insight into the arginine-nitric oxide metabolic system in humans

BACKGROUND

The systemic availability of oral/dietary arginine and its utilization for nitric oxide (NO) synthesis remains unknown and may be related to a competitive hydrolysis of arginine into urea in the splanchnic area and systemic circulation.

OBJECTIVES

We investigated the kinetics and dose-dependency of dietary arginine utilization for NO compared with urea synthesis and studied the characteristics of the arginine-NO metabolic system in healthy humans.

DESIGN

We traced the metabolic fate and analyzed the utilization dynamics of dietary arginine after its ingestion at 2 nutritional amounts in healthy humans (n = 9) in a crossover design by using [(15)N-(15)N-(guanido)]-arginine, isotope ratio mass spectrometry techniques, and data analysis with a compartmental modeling approach.

RESULTS

Whatever the amount of dietary arginine, 60 ± 3% (±SEM) was converted to urea, with kinetics indicative of a first-pass splanchnic phenomenon. Despite this dramatic extraction, intact dietary arginine made a major contribution to the postprandial increase in plasma arginine. However, the model identified that the plasma compartment was a very minor (~2%) precursor for the conversion of dietary arginine into NO, which, in any case, was small (<0.1% of the dose). The whole-body and plasma kinetics of arginine metabolism were consistent with the suggested competitive metabolism by the arginase and NO synthase pathways.

CONCLUSIONS

The conversion of oral/dietary arginine into NO is not limited by the systemic availability of arginine but by a tight metabolic compartmentation at the systemic level. We propose an organization of the arginine metabolic system that explains the daily maintenance of NO homeostasis in healthy humans.

A role for PPARα in the regulation of arginine metabolism and nitric oxide synthesis

The pleiotropic effects of PPARα may include the regulation of amino acid metabolism. Nitric oxide (NO) is a key player in vascular homeostasis. NO synthesis may be jeopardized by a differential channeling of arginine toward urea (via arginase) versus NO (via NO synthase, NOS). This was studied in wild-type (WT) and PPARα-null (KO) mice fed diets containing either saturated fatty acids (COCO diet) or 18:3 n-3 (LIN diet). Metabolic markers of arginine metabolism were assayed in urine and plasma. mRNA levels of arginases and NOS were determined in liver. Whole-body NO synthesis and the conversion of systemic arginine into urea were assessed by using (15)N(2)-guanido-arginine and measuring urinary (15)NO(3) and [(15)N]-urea. PPARα deficiency resulted in a markedly lower whole-body NO synthesis, whereas the conversion of systemic arginine into urea remained unaffected. PPARα deficiency also increased plasma arginine and decreased citrulline concentration in plasma. These changes could not be ascribed to a direct effect on hepatic target genes, since NOS mRNA levels were unaffected, and arginase mRNA levels decreased in KO mice. Despite the low level in the diet, the nature of the fatty acids modulated some effects of PPARα deficiency, including plasma arginine and urea, which increased more in KO mice fed the LIN diet than in those fed the COCO diet. In conclusion, PPARα is largely involved in normal whole-body NO synthesis. This warrants further study on the potential of PPARα activation to maintain NO synthesis in the initiation of the metabolic syndrome.

Effects of different dietary protein sources on expression of genes related to protein metabolism in growing rats

Protein metabolism is known to be affected by dietary proteins, but the fundamental mechanisms that underlie the changes in protein metabolism are unclear. The aim of the present study was to test the effects of feeding growing rats with balanced diets containing soya protein isolate, zein and casein as the sole protein source on the expression of genes related to protein metabolism responses in skeletal muscle. The results showed that feeding a zein protein diet to the growing rats induced changes in protein anabolic and catabolic metabolism in their gastrocnemius muscles when compared with those fed either the reference protein casein diet or the soya protein isolate diet. The zein protein diet increased not only the mRNA levels and phosphorylation of mammalian target of rapamycin (mTOR), but also the mRNA expression of muscle atrophy F-box (MAFbx)/atrogin-1 and muscle ring finger 1 (MuRF1), as well as the forkhead box-O (FoxO) transcription factors involved in the induction of the E3 ligases. The amino acid profile of proteins seems to control signalling pathways leading to changes in protein synthesis and proteolysis.

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.

Hydrolyzed dietary casein as compared with the intact protein reduces postprandial peripheral, but not whole-body, uptake of nitrogen in humans

BACKGROUND

Compared with slow proteins, fast proteins are more completely extracted in the splanchnic bed but contribute less to peripheral protein accretion; however, the independent influence of absorption kinetics and the amino acid (AA) pattern of dietary protein on AA anabolism in individual tissues remains unknown.

OBJECTIVE

We aimed to compare the postprandial regional utilization of proteins with similar AA profiles but different absorption kinetics by coupling clinical experiments with compartmental modeling.

DESIGN

Experimental data pertaining to the intestine, blood, and urine for dietary nitrogen kinetics after a 15N-labeled intact (IC) or hydrolyzed (HC) casein meal were obtained in parallel groups of healthy adults (n = 21) and were analyzed by using a 13-compartment model to predict the cascade of dietary nitrogen absorption and regional metabolism.

RESULTS

IC and HC elicited a similar whole-body postprandial retention of dietary nitrogen, but HC was associated with a faster rate of absorption than was IC, resulting in earlier and stronger hyperaminoacidemia and hyperinsulinemia. An enhancement of both catabolic (26%) and anabolic (37%) utilization of dietary nitrogen occurred in the splanchnic bed at the expense of its further peripheral availability, which reached 18% and 11% of ingested nitrogen 8 h after the IC and HC meals, respectively.

CONCLUSIONS

The form of delivery of dietary AAs constituted an independent factor of modulation of their postprandial regional metabolism, with a fast supply favoring the splanchnic dietary nitrogen uptake over its peripheral anabolic use. These results question a possible effect of ingestion of protein hydrolysates on tissue nitrogen metabolism and accretion. This trial was registered at clinicaltrials.gov as NCT00873951.

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.

Ultra high temperature treatment, but not pasteurization, affects the postprandial kinetics of milk proteins in humans

Although the chemical and physical modifications to milk proteins induced by technological treatments have been characterized extensively, their nutritional consequences have rarely been assessed in humans. We measured the effect of 2 technological treatments on the postprandial utilization of milk nitrogen (N), pasteurization (PAST) and ultra high temperature (UHT), compared with microfiltration (MF), using a sensitive method based on the use of milk proteins intrinsically labeled with (15)N. Twenty-five subjects were studied after a 1-wk standardization of their diet. On the day of the investigation, they ingested a single test meal corresponding to 500 mL of either MF, PAST, or UHT defatted milk. Serum amino acid (AA) levels as well as the transfer of (15)N into serum protein and AA, body urea, and urinary urea were determined throughout the 8-h postprandial period. The kinetics of dietary N transfer to serum AA, proteins, and urea did not differ between the MF and PAST groups. The transfer of dietary N to serum AA and protein and to body urea was significantly higher in UHT than in either the PAST or MF group. Postprandial deamination losses from dietary AA represented 25.9 +/- 3.3% of ingested N in the UHT group, 18.5 +/- 3.0% in the MF group, and 18.6 +/- 3.7% in the PAST group (P < 0.0001). The higher anabolic use of dietary N in plasma proteins after UHT ingestion strongly suggests that these differences are due to modifications to digestive kinetics and the further metabolism of dietary proteins subsequent to this particular treatment of milk.

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.

Meal amino acids with varied levels of arginine do not affect postprandial vascular endothelial function in healthy young men

Postprandial endothelial dysfunction is a key event mediating the effects of diet on early atherogenesis. The potential effects of protein intake have been overlooked in the past, although amino acids are precursors for homocysteine and nitric oxide (NO). Our objective was to study the effect of amino acids on postprandial vascular function, in relation to the utilization of meal arginine for NO production. In a crossover design, 9 men ingested 50 g of a complete amino acid mixture, trace-labeled with (13)C-glycine and (15)N(2)-arginine, without (meal A) or with (meal B) 3 g extra arginine. The postprandial utilization of meal arginine for NO production was determined from urinary (15)NO(3). We monitored endothelial function of the brachial artery, the stiffness of the common carotid artery, aortic pulse wave velocity and soluble markers related to endothelial function for 8 h. Meal A did not significantly increase plasma homocysteine and did not alter endothelial function markers. The amount of NO synthesized from meal arginine doubled after meal B (107.1 +/- 16.5% increase vs. meal A, P < 0.01) but was very low (271 +/- 84 ppm vs. 332 +/- 73 ppm, P < 0.05, respectively). After meal B, flow-mediated and nitroglycerine-induced dilation decreased but common carotid artery compliance, pulse wave velocity, plasma soluble intercellular adhesion molecule-1, and von Willebrand factor, and urinary cGMP did not differ when compared with meal A. Together, the data indicate that, in healthy men, meal amino acids do not adversely affect endothelial function, and meal arginine only slightly enters the NO pathway. Unexpectedly, arginine in physiological amounts may acutely lessen smooth muscle cell reactivity to a high dynamic NO release.

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.

Milk ingestion stimulates net muscle protein synthesis following resistance exercise

PURPOSE

Previous studies have examined the response of muscle protein to resistance exercise and nutrient ingestion. Net muscle protein synthesis results from the combination of resistance exercise and amino acid intake. No study has examined the response of muscle protein to ingestion of protein in the context of a food. This study was designed to determine the response of net muscle protein balance following resistance exercise to ingestion of nutrients as components of milk.

METHOD

Three groups of volunteers ingested one of three milk drinks each: 237 g of fat-free milk (FM), 237 g of whole milk (WM), and 393 g of fat-free milk isocaloric with the WM (IM). Milk was ingested 1 h following a leg resistance exercise routine. Net muscle protein balance was determined by measuring amino acid balance across the leg.

RESULTS

Arterial concentrations of representative amino acids increased in response to milk ingestion. Threonine balance and phenylalanine balance were both > 0 following milk ingestion. Net amino acid uptake for threonine was 2.8-fold greater (P < 0.05) for WM than for FM. Mean uptake of phenylalanine was 80 and 85% greater for WM and IM, respectively, than for FM, but not statistically different. Threonine uptake relative to ingested was significantly (P < 0.05) higher for WM (21 +/- 6%) than FM (11 +/- 5%), but not IM (12 +/- 3%). Mean phenylalanine uptake/ingested also was greatest for WM, but not significantly.

CONCLUSIONS

Ingestion of milk following resistance exercise results in phenylalanine and threonine uptake, representative of net muscle protein synthesis. These results suggest that whole milk may have increased utilization of available amino acids for protein synthesis.

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.

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.

Contribution of plasma proteins to splanchnic and total anabolic utilization of dietary nitrogen in humans

Splanchnic tissues are largely involved in the postprandial utilization of dietary amino acids, but little is yet known, particularly in humans, about the relative contributions of different splanchnic protein pools to splanchnic and total postprandial anabolism. Our aim was to develop a compartmental model that could distinguish dietary nitrogen (N) incorporation among splanchnic constitutive, plasma (splanchnic exported), and peripheral proteins after a mixed-protein meal in humans. Eight healthy subjects were fed a single mixed meal containing 15N-labeled soy protein, and dietary N postprandial kinetics were measured in plasma free amino acids, proteins, and urea and urinary urea and ammonia. These experimental data and others previously obtained for dietary N kinetics in ileal effluents under similar experimental conditions were used to develop the compartmental model. Six hours after the mixed-meal ingestion, 31.5, 7.5, and 21% of ingested N were predicted to be incorporated into splanchnic constitutive, splanchnic exported, and peripheral proteins, respectively. The contribution of splanchnic exported proteins to total splanchnic anabolism from dietary N was predicted to be approximately 19% and to remain steady throughout the simulation period. Model behavior and its predictions were strongly in line with current knowledge of the system and the scarce, specific data available in the literature. This model provides the first data concerning the anabolism of splanchnic constitutive proteins in the nonsteady postprandial state in humans. By use of only slightly invasive techniques, this model could help to assess how the splanchnic anabolism is modulated under different nutritional or pathophysiological conditions in humans.

Postprandial kinetics of dietary amino acids are the main determinant of their metabolism after soy or milk protein ingestion in humans

Soy proteins have been shown to result in lower postprandial nitrogen retention than milk proteins, but the mechanisms underlying these differences have not been elucidated. To investigate this question, we measured the postprandial kinetics of the appearance of individual (15)N-amino acids in the serum of healthy adults after the ingestion of either (15)N-soy (n = 8) or (15)N-milk proteins (n = 8) in a mixed single meal (46 kJ/kg). The kinetics of total and dietary amino acids (AA) in the peripheral circulation were characterized by an earlier and higher peak after soy protein ingestion. Dietary AA levels peaked at 2.5 h in the soy group vs. 3.9 h in the milk group (P < 0.02). This time interval difference between groups was associated with a faster transfer of dietary N into urea in the soy group (peak at 3 vs. 4.75 h in the milk group, P < 0.005) and a higher level of incorporation into the serum protein pool from 3 to 8 h after the soy meal. The dietary AA pattern in the peripheral blood closely reflected the dietary protein AA pattern. Postprandial glucose, insulin, and glucagon levels and profiles did not differ between groups. Soy AA were digested more rapidly and were directed toward both deamination pathways and liver protein synthesis more than milk AA. We conclude that differences in the metabolic postprandial fates of soy and milk proteins are due mainly to differences in digestion kinetics; however, the AA composition of dietary proteins may also play a role.

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.

Peripheral and splanchnic metabolism of dietary nitrogen are differently affected by the protein source in humans as assessed by compartmental modeling

We used a previously developed compartmental model to assess the postprandial distribution and metabolism of dietary nitrogen (N) in the splanchnic and peripheral areas after the ingestion of a single mixed meal containing either (15)N-labeled milk or soy purified protein. Although the lower whole-body retention of dietary N from soy protein was measured experimentally, the splanchnic retention of dietary N was predicted by the model not to be affected by the protein source, and its incorporation into splanchnic proteins was predicted to reach approximately 35% of ingested N at 8 h after both meals. However, dietary N intestinal absorption and its appearance in splanchnic free amino acids were predicted to be more rapid from soy protein and were associated with a higher deamination, concomitant with a higher efficiency of incorporation of dietary N into proteins in the splanchnic bed. In contrast, soy protein was predicted to cause a reduction in peripheral dietary N uptake, as a consequence of both similar splanchnic retention and increased oxidation compared with milk protein. In addition, protein synthesis efficiency was reduced in the peripheral area after soy protein intake, leading to dietary N incorporation in peripheral proteins that fell from 26 to 19% of ingested N 8 h after milk and soy protein ingestion, respectively. Such a model thus enables a description of the processes involved in the differential metabolic utilization of dietary proteins and constitutes a valuable tool for further definition of the notion of protein quality during the period of protein gain.

Guar gum does not impair the absorption and utilization of dietary nitrogen but affects early endogenous urea kinetics in humans

BACKGROUND

Viscous gums enhance viscosity in the upper gastrointestinal lumen, quickly disturbing motility and promoting fluid secretion.

OBJECTIVE

We sought to determine whether guar gum could acutely affect the absorption and utilization of dietary nitrogen and whether these luminal effects could also perturb the kinetics of urea.

DESIGN

We studied the short-term effect of adding 1% of highly viscous guar gum to a (15)N-labeled protein meal (30 g soy protein isolate in 500 mL water) during the postprandial phase in humans. The effects on bioavailability were studied by using the [(13)C]glycine breath test (to assess gastric emptying) and (15)N enrichment in plasma amino acids (for systemic amino acid bioavailability). The kinetics of dietary and endogenous urea were assessed in plasma and urine.

RESULTS

Guar gum modulated the gastric emptying kinetics of the liquid phase of the meal slightly (P < 0.05), but had no significant effect on either the systemic appearance of dietary amino acids or plasma and urinary dietary urea kinetics. Without significantly affecting plasma urea concentrations, guar gum reduced by approximately 40% the urinary excretion of endogenous urea for the first 2-h period after the meal (P < 0.01), although endogenous urinary excretion was similar at later stages.

CONCLUSIONS

Guar gum did not significantly affect the bioavailability or utilization of dietary protein. We showed an early effect of guar gum on endogenous urea kinetics, which most probably arose from very early, short-term stimulation of the intestinal disposal of endogenous urea, at the expense of its urinary excretion.

Energy nutrients modulate the splanchnic sequestration of dietary nitrogen in humans: a compartmental analysis

We used a previously developed compartmental model to assess the postprandial distribution and metabolism of dietary nitrogen (N) in the splanchnic and peripheral areas after the ingestion of a single meal containing milk protein either alone (MP) or with additional sucrose (SMP) or fat (FMP). The addition of fat was predicted to enhance splanchnic dietary N anabolism only transiently, without significantly affecting the global kinetics of splanchnic retention and peripheral uptake. In contrast, the addition of sucrose, which induced hyperinsulinemia, was predicted to enhance dietary N retention and anabolism in the splanchnic bed, thus leading to reduced peripheral dietary amino acid availability and anabolism. The incorporation of dietary N into splanchnic proteins was thus predicted to reach 18, 24, and 35% of ingested N 8 h after MP, FMP, and SMP, respectively. Such a model provides insight into the dynamics of the system in the nonsteady postprandial state and constitutes a useful, explanatory tool to determine the region-specific utilization of dietary N under different nutritional conditions.

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

Pulse seed proteins such as those found in peas (Pisum sativum) contain fractions of very dissimilar composition and properties, which may therefore be differently utilized by the human body. To analyze the nutritional value of the soluble protein fractions of pea seed, human volunteers ingested a mixed meal of 30 g of raw purified pea protein either as [15N]-globulins (G, n = 9) or as a mix of [15N]-globulins and [15N]-albumins (GA, n = 7) in their natural proportions (22:8). Dietary and endogenous nitrogen fluxes at the terminal ileum were assessed using a tube perfusion technique with an isotopic dilution method. Systemic dietary amino acid availability and the retention of dietary amino acids were determined using 15N enrichment in plasma amino acids and deamination products in blood and urine for 8 h postprandially. The results showed that the pea albumin fraction had the following effects: 1) significantly lowered the real ileal digestibility of pea protein (94 +/- 2.5% for G vs. 89.9 +/- 4% for GA), probably because of a direct effect of trypsin inhibitors; 2) did not promote acute intestinal losses of endogenous nitrogen; and 3) did not significantly improve the postprandial biological value of pea protein (76.5 +/- 3.9% for G vs. 78.7 +/- 3.6% for GA), despite the fact that it corrected the globulin deficiency in sulfur amino acids. We conclude that both G and GA are of good nutritional value for humans and show that cysteine-rich albumins have a far more modest effect on the efficiency of postprandial dietary protein utilization than would be expected from the amino acid scores.