Conceptual, methodological and computational issues concerning the compartmental modeling of a complex biological system: Postprandial inter-organ metabolism of dietary nitrogen in humans

Phenylalanine stable isotope tracer labeling of cow milk and meat and human experimental applications to study dietary protein-derived amino acid availability

BACKGROUND & AIMS

Availability of dietary protein-derived amino acids (AA) is an important determinant for their utilization in metabolism and for protein synthesis. Intrinsic labeling of protein is the only method to directly trace availability and utilization. The purpose of the present study was to produce labeled milk and meat proteins and investigate how dietary protein-derived AA availability is affected by the protein-meal matrix.

METHODS

Four lactating cows were infused with L-[ring-d₅]phenylalanine and one with L-[¹⁵N]phenylalanine for 72 h. Milk was collected, and three of the [d₅]phenylalanine cows were subsequently slaughtered. Two human studies were performed to explore plasma AA availability properties utilizing the labeled proteins. One study compared the intake of whey protein either alone or together with carbohydrates-fat food-matrix. The other study compared the intake of meat hydrolysate with minced beef. Cow blood, milk, meat and human blood samples were collected and analyzed by mass spectrometry.

RESULTS

Whey and caseinate acquired label to 15-20 mol percent excess (MPE), and the meat proteins reached 0.41-0.73 MPE. The [d₅]phenylalanine appeared fast in plasma and peaked 30 min after whey protein alone and meat hydrolysate intake, whereas whey protein with a food-matrix and the meat minced beef postponed the [d₅]phenylalanine peak until 2 and 1 h, respectively.

CONCLUSIONS

Phenylalanine stable isotope-labeled milk and meat were produced and proved a valuable tool to investigate AA absorption characteristics. Dietary protein in food-matrices showed delayed postprandial plasma AA availability as compared to whey protein alone and meat hydrolysate.

Stable Isotope Techniques for the Assessment of Host and Microbiota Response During Gastrointestinal Dysfunction

The International Atomic Energy Agency convened a technical meeting on environmental enteric dysfunction (EED) in Vienna (October 28-30, 2015; https://nucleus.iaea.org/HHW/Nutrition/EED_Technical_Meeting/index.html) to bring together international experts in the fields of EED, nutrition, and stable isotope technologies. Advances in stable isotope-labeling techniques open up new possibilities to improve our understanding of gastrointestinal dysfunction and the role of the microbiota in host health. In the context of EED, little is known about the role gut dysfunction may play in macro- and micronutrient bioavailability and requirements and what the consequences may be for nutritional status and linear growth. Stable isotope labeling techniques have been used to assess intestinal mucosal injury and barrier function, carbohydrate digestion and fermentation, protein-derived amino acid bioavailability and requirements, micronutrient bioavailability and to track microbe-microbe and microbe-host interactions at the single cell level. The noninvasive nature of stable isotope technologies potentially allow for low-hazard, field-deployable tests of gut dysfunction that are applicable across all age groups. The purpose of this review is to assess the state-of-the-art use of stable isotope technologies and to provide a perspective on where these technologies can be exploited to further our understanding of gut dysfunction in EED.

Characterisation of mixing in the proximal duodenum of the rat during longitudinal contractions and comparison with a fluid mechanical model based on spatiotemporal motility data

The understanding of mixing and mass transfers of nutrients and drugs in the small intestine is of prime importance in creating formulations that manipulate absorption and digestibility. We characterised mixing using a dye tracer methodology during spontaneous longitudinal contractions, i.e. pendular activity, in 10 cm segments of living proximal duodenum of the rat maintained ex-vivo. The residence time distribution (RTD) of the tracer was equivalent to that generated by a small number (8) of continuous stirred tank reactors in series. Fluid mechanical modelling, that was based on real sequences of longitudinal contractions, predicted that dispersion should occur mainly in the periphery of the lumen. Comparison with the experimental RTD showed that centriluminal dispersion was accurately simulated whilst peripheral dispersion was underestimated. The results therefore highlighted the potential importance of micro-phenomena such as microfolding of the intestinal mucosa in peripheral mixing. We conclude that macro-scale modeling of intestinal flow is useful in simulating centriluminal mixing, whereas multi-scales strategies must be developed to accurately model mixing and mass transfers at the periphery of the lumen.

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.

Available versus digestible amino acids – new stable isotope methods

The nutritive value of food protein sources is dependent on the amino acid composition and the bioavailability of the nutritionally indispensable amino acids. Traditionally the methods developed to determine amino acid bioavailability have focused on intestinal absorption or digestibility, which is calculated as the percent of amino acid intake that does not appear in digesta or faeces. Traditional digestibility based methods do not always account for gut endogenous amino acid losses or absorbed amino acids which are unavailable due to the effect of heat processing and the presence of anti-nutritional factors, though methods have been developed to address these issues. Furthermore, digestibility based methods require the use of animal models, thus there is a need to developin vivomethods that can be applied directly in human subjects to identify the proportion of dietary amino acids which is bioavailable, or metabolically available to the body for protein synthesis following digestion and absorption. The indicator amino acid oxidation (IAAO) method developed in our laboratory for humans has been systematically applied to determine almost all indispensable amino acid requirements in adult humans. Oxidation of the indicator amino acid is inversely proportional to whole body protein synthesis and responds rapidly to changes in the bioavailability of amino acids for metabolic processes. Using the IAAO concept, we developed a newin vivomethod in growing pigs, pregnant sows and adult humans to identify the metabolic availability of amino acids in foods. The stable isotope based metabolic availability method is suitable for rapid and routine analysis in humans, and can be used to integrate amino acid requirement data with dietary amino acid availability of foods.

Computerized evaluation of mean residence times in multicompartmental linear system and pharmacokinetics

Deriving mean residence times (MRTs) is an important task both in pharmacokinetics and in multicompartmental linear systems. Taking as starting point the analysis of MRTs in open or closed (Garcia-Meseguer et al., Bull Math Biol 2003, 65, 279) multicompartmental linear systems, we implement a versatile software, using the Visual Basic 6.0 language for MS-Windows, that is easy to use and with a user-friendly format for the input of data and the output of results. For any multicompartmental linear system of up to 512 compartments, whether closed or open, with traps or without traps and with zero input in one or more of the compartments, this software allows the user to obtain the symbolic expressions, in the most simplified form, and/or the numerical values of the MRTs in any of its compartments, in the entire system or in a part of the system. As far as we known from the literature, such a software has not been implemented before. The advantage of the present software is that it reduces on the work time needed and minimizes the human errors that are frequent in compartmental systems even those that are relatively staightforward. The software bioCelTer, along with instructions, can be downloaded from http://oretano.iele-ab.uclm.es/~fgarcia/bioCelTer/.

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.

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.