A high-protein meal exceeds anabolic and catabolic capacities in rats adapted to a normal protein diet

Differential effects of milk proteins on amino acid digestibility, post-prandial nitrogen utilization and intestinal peptide profiles in rats

OBJECTIVE

The aim of this study was to analyze the protein digestibility and postprandial metabolism in rats of milk protein matrices obtained by different industrial processes.

MATERIAL AND METHODS

The study was conducted on Wistar rats that consumed a meal containing different ¹⁵N-labeled milk proteins. Four milk matrices were tested: native micellar caseins (C1), caseins low in calcium (C2 low Ca²⁺), a matrix containing a ratio 63:37 of caseins and whey proteins (CW2) and whey proteins alone (W). Blood and urine were collected during the postprandial period and rats were euthanized 6 h after meal intake to collect digestive contents and organs.

RESULTS

Orocaecal digestibility values of amino acids ranged between 96.0 ± 0.2% and 96.6 ± 0.4% for C1-, C2 low Ca²⁺- and W-matrices, while this value was significantly lower for CW2 matrix (92.4 ± 0.5%). More dietary nitrogen was sequestered in the splanchnic area (intestinal mucosa and liver) as well as in plasma proteins after ingestion of W matrix, especially compared to the C1- and C2 low Ca²⁺-matrices. Peptidomic analysis showed that more milk protein-derived peptides were identified in the caecum of rats after the ingestion of the matrices containing caseins compared to W matrix.

CONCLUSION

We found that demineralization of micellar caseins did not modify its digestibility and postprandial metabolism. The low digestibility of the modified casein-to-whey ratio matrix may be ascribed to a lower accessibility of the protein to digestive enzymes due to changes in the protein structure, while the higher nitrogen splanchnic retention after ingestion of whey was probably due to the fast assimilation of its protein content. Finally, our results showed that industrial processes that modify the structure and/or composition of milk proteins influence protein digestion and utilization.

Role of liver AMPK and GCN2 kinases in the control of postprandial protein metabolism in response to mid-term high or low protein intake in mice

PURPOSE

Protein synthesis and proteolysis are known to be controlled through mammalian target of rapamycin, AMP-activated kinase (AMPK) and general control non-derepressible 2 (GCN2) pathways, depending on the nutritional condition. This study aimed at investigating the contribution of liver AMPK and GCN2 on the adaptation to high variations in protein intake.

METHODS

To evaluate the answer of protein pathways to high- or low-protein diet, male wild-type mice and genetically modified mice from C57BL/6 background with liver-specific AMPK- or GCN2-knockout were fed from day 25 diets differing in their protein level as energy: LP (5%), NP (14%) and HP (54%). Two hours after a 1 g test meal, protein synthesis rate was measured after a ¹³C valine flooding dose. The gene expression of key enzymes involved in proteolysis and GNC2 signaling pathway were quantified.

RESULTS

The HP diet but not the LP diet was associated with a decrease in fractional synthesis rate by 29% in the liver compared to NP diet. The expression of mRNA encoding ubiquitin and Cathepsin D was not sensitive to the protein content. The deletion of AMPK or GCN2 in the liver did not affect nor protein synthesis rates and neither proteolysis markers in the liver or in the muscle, whatever the protein intake. In the postprandial state, protein level alters protein synthesis in the liver but not in the muscle.

CONCLUSIONS

Taken together, these results suggest that liver AMPK and GCN2 are not involved in this adaptation to high- and low-protein diet observed in the postprandial period.

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

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

The Food Energy/Protein Ratio Regulates the Rat Urea Cycle but Not Total Nitrogen Losses

Nitrogen balance studies have shown that a portion of the N ingested but not excreted is not accounted for. We compared several diets (standard, high-fat, high-protein, and self-selected cafeteria) to determine how diet-dependent energy sources affect nitrogen handling, i.e., the liver urea cycle. Diet components and rat homogenates were used for nitrogen, lipid, and energy analyses. Plasma urea and individual amino acids, as well as liver urea cycle enzyme activities, were determined. Despite ample differences in N intake, circulating amino acids remained practically unchanged in contrast to marked changes in plasma urea. The finding of significant correlations between circulating urea and arginine-succinate synthase and lyase activities supported their regulatory role of urea synthesis, the main N excretion pathway. The cycle operation also correlated with the food protein/energy ratio, in contraposition to total nitrogen losses and estimated balance essentially independent of dietary energy load. The different regulation mechanisms observed have potentially important nutritional consequences, hinting at nitrogen disposal mechanisms able to eliminate excess nitrogen under conditions of high availability of both energy and proteins. Their operation reduces urea synthesis to allow for a safe (albeit unknown) mechanism of N/energy excess accommodation.

IDH1 deficiency attenuates gluconeogenesis in mouse liver by impairing amino acid utilization

Although the enzymatic activity of isocitrate dehydrogenase 1 (IDH1) was defined decades ago, its functions in vivo are not yet fully understood. Cytosolic IDH1 converts isocitrate to α-ketoglutarate (α-KG), a key metabolite regulating nitrogen homeostasis in catabolic pathways. It was thought that IDH1 might enhance lipid biosynthesis in liver or adipose tissue by generating NADPH, but we show here that lipid contents are relatively unchanged in both IDH1-null mouse liver and IDH1-deficient HepG2 cells generated using the CRISPR-Cas9 system. Instead, we found that IDH1 is critical for liver amino acid (AA) utilization. Body weights of IDH1-null mice fed a high-protein diet (HPD) were abnormally low. After prolonged fasting, IDH1-null mice exhibited decreased blood glucose but elevated blood alanine and glycine compared with wild-type (WT) controls. Similarly, in IDH1-deficient HepG2 cells, glucose consumption was increased, but alanine utilization and levels of intracellular α-KG and glutamate were reduced. In IDH1-deficient primary hepatocytes, gluconeogenesis as well as production of ammonia and urea were decreased. In IDH1-deficient whole livers, expression levels of genes involved in AA metabolism were reduced, whereas those involved in gluconeogenesis were up-regulated. Thus, IDH1 is critical for AA utilization in vivo and its deficiency attenuates gluconeogenesis primarily by impairing α-KG-dependent transamination of glucogenic AAs such as alanine.

Adaptation to a high-protein diet progressively increases the postprandial accumulation of carbon skeletons from dietary amino acids in rats

We aimed to determine whether oxidative pathways adapt to the overproduction of carbon skeletons resulting from the progressive activation of amino acid (AA) deamination and ureagenesis under a high-protein (HP) diet. Ninety-four male Wistar rats, of which 54 were implanted with a permanent jugular catheter, were fed a normal protein diet for 1 wk and were then switched to an HP diet for 1, 3, 6, or 14 days. On the experimental day, they were given their meal containing a mixture of 20 U-[¹⁵N]-[¹³C] AA, whose metabolic fate was followed for 4 h. Gastric emptying tended to be slower during the first 3 days of adaptation. ¹⁵N excretion in urine increased progressively during the first 6 days, reaching 29% of ingested protein. ¹³CO₂ excretion was maximal, as early as the first day, and represented only 16% of the ingested proteins. Consequently, the amount of carbon skeletons remaining in the metabolic pools 4 h after the meal ingestion progressively increased to 42% of the deaminated dietary AA after 6 days of HP diet. In contrast, ¹³C enrichment of plasma glucose tended to increase from 1 to 14 days of the HP diet. We conclude that there is no oxidative adaptation in the early postprandial period to an excess of carbon skeletons resulting from AA deamination in HP diets. This leads to an increase in the postprandial accumulation of carbon skeletons throughout the adaptation to an HP diet, which can contribute to the sustainable satiating effect of this diet.

Central Amino Acid Sensing in the Control of Feeding Behavior

Dietary protein quantity and quality greatly impact metabolic health via evolutionary-conserved mechanisms that ensure avoidance of amino acid imbalanced food sources, promote hyperphagia when dietary protein density is low, and conversely produce satiety when dietary protein density is high. Growing evidence supports the emerging concept of protein homeostasis in mammals, where protein intake is maintained within a tight range independently of energy intake to reach a target protein intake. The behavioral and neuroendocrine mechanisms underlying these adaptations are unclear. While peripheral factors are able to signal amino acid deficiency and abundance to the brain, the brain itself is exposed to and can detect changes in amino acid concentrations, and subsequently engages acute and chronic responses modulating feeding behavior and food preferences. In this review, we will examine the literature describing the mechanisms by which the brain senses changes in amino acids concentrations, and how these changes modulate feeding behavior.

Effects of two different levels of dietary protein on body composition and protein nutritional status of growing rats

This study aimed to investigate the effect of a high-protein diet on growth, body composition, and protein nutritional status of young rats. Newly-weaned Wistar rats, weighing 45–50 g, were distributed in two experimental groups, according to their diets, which contained 12% (G12) or 26% protein (G26), over a period of 3 weeks. The animals were euthanized at the end of this period and the following analyses were performed: chemical composition of the carcass, proteoglycan synthesis, IGF-I concentration (serum, muscle and cartilage), total tissue RNA, protein concentration (muscle and cartilage) and protein synthesis (muscle and cartilage). The high-protein diet was found to result in a higher fat-free mass and lower fat mass in the carcass, with no difference in growth or protein nutritional status.

Indirect calorimetry in laboratory mice and rats: principles, practical considerations, interpretation and perspectives

In this article, we review some fundamentals of indirect calorimetry in mice and rats, and open the discussion on several debated aspects of the configuration and tuning of indirect calorimeters. On the particularly contested issue of adjustment of energy expenditure values for body size and body composition, we discuss several of the most used methods and their results when tested on a previously published set of data. We conclude that neither body weight (BW), exponents of BW, nor lean body mass (LBM) are sufficient. The best method involves fitting both LBM and fat mass (FM) as independent variables; for low sample sizes, the model LBM + 0.2 FM can be very effective. We also question the common calorimetry design that consists of measuring respiratory exchanges under free-feeding conditions in several cages simultaneously. This imposes large intervals between measures, and generally limits data analysis to mean 24 h or day-night values of energy expenditure. These are then generally compared with energy intake. However, we consider that, among other limitations, the measurements of Vo(2), Vco(2), and food intake are not precise enough to allow calculation of energy balance in the small 2-5% range that can induce significant long-term alterations of energy balance. In contrast, we suggest that it is necessary to work under conditions in which temperature is set at thermoneutrality, food intake totally controlled, activity precisely measured, and data acquisition performed at very high frequency to give access to the part of the respiratory exchanges that are due to activity. In these conditions, it is possible to quantify basal energy expenditure, energy expenditure associated with muscular work, and response to feeding or to any other metabolic challenge. This reveals defects in the control of energy metabolism that cannot be observed from measurements of total energy expenditure in free feeding individuals.

Nutritional sensing and its utility in treating obesity

Obesity remains a major worldwide health problem, with current medical treatments being poorly effective. Nutrient sensing allows organs such as the GI tract and the brain to recognize and respond to fuel substrates such as carbohydrates, protein and fats. Specialized neural and hormonal pathways exist to facilitate and regulate these chemosensory mechanisms. Manipulation of factors involved in either central or peripheral chemosensory pathways may provide possible targets for the manipulation of appetite. However, further research is required to assess the utility of this approach to developing novel anti-obesity agents.

The nature of the dietary protein impacts the tissue-to-diet 15N discrimination factors in laboratory rats

Due to the existence of isotope effects on some metabolic pathways of amino acid and protein metabolism, animal tissues are ¹⁵N-enriched relative to their dietary nitrogen sources and this ¹⁵N enrichment varies among different tissues and metabolic pools. The magnitude of the tissue-to-diet discrimination (Δ¹⁵N) has also been shown to depend on dietary factors. Since dietary protein sources affect amino acid and protein metabolism, we hypothesized that they would impact this discrimination factor, with selective effects at the tissue level. To test this hypothesis, we investigated in rats the influence of a milk or soy protein-based diet on Δ¹⁵N in various nitrogen fractions (urea, protein and non-protein fractions) of blood and tissues, focusing on visceral tissues. Regardless of the diet, the different protein fractions of blood and tissues were generally ¹⁵N-enriched relative to their non-protein fraction and to the diet (Δ¹⁵N>0), with large variations in the Δ¹⁵N between tissue proteins. Δ¹⁵N values were markedly lower in tissue proteins of rats fed milk proteins compared to those fed soy proteins, in all sampled tissues except in the intestine, and the amplitude of Δ¹⁵N differences between diets differed between tissues. Both between-tissue and between-diet Δ¹⁵N differences are probably related to modulations of the relative orientation of dietary and endogenous amino acids in the different metabolic pathways. More specifically, the smaller Δ¹⁵N values observed in tissue proteins with milk than soy dietary protein may be due to a slightly more direct channeling of dietary amino acids for tissue protein renewal and to a lower recycling of amino acids through fractionating pathways. In conclusion, the present data indicate that natural Δ¹⁵N of tissue are sensitive markers of the specific subtle regional modifications of the protein and amino acid metabolism induced by the protein dietary source.

Soy protein- and casein-based weaning diets differ in effects on food intake and blood glucose regulation in male Wistar rats

The effect of weaning male Wistar rats to AIN-93G diets based on casein (C) and soy protein (S) on blood glucose and food intake (FI) regulation was determined. In experiment 1, male Wistar rats (n = 21 per group) received either C or S AIN-93G diets for 7 weeks. In experiment 2, 3 groups of rats were formed (n = 21 per group). The C followed by the S diet group (CS) was weaned to the C diet for 6 weeks followed by the S diet for another 7 weeks. Diet sequence was the reverse for the S followed by the C diet group (SC). The control group (CC) received the C diet throughout 13 weeks. Body weight and cumulative FI were not affected by diet in either experiment. In experiment 1, in fasted rats, S preloads reduced FI for 1 hour more in the C diet group (P < .05), but response to C preloads was not affected by diet. A cholecystokinin A receptor blocker prevented FI reduction by S in rats fed C but not S diet (P < .05). At week 7, rats fed the S diet had higher plasma insulin (67%) (P < .005), glucose (30%) (P < .05) and homeostatic model assessment of insulin resistance index (75%) (P < .005). In experiment 2, FI at weeks 6 and 12 was, again, suppressed most strongly by S preloads in rats fed the C diet (P < .05). At week 13, S and C preloads increased insulin and the insulin/glucose ratio (P < .05), but no differences were found due to preload or diet composition. In conclusion, differences in the effects of first diet exposure to the AIN-93G diets on blood glucose did not persist through either diet change or time. In contrast, protein composition of the most recent diet, but not time, affected FI regulation in response to protein preloads.

Postprandial oxidative losses of free and protein-bound amino acids in the diet: interactions and adaptation

Postprandial oxidation of dietary free amino acids or egg white protein was studied using the [13CO2] breath test in rats, as well as in humans. Thirty-eight male rats were assigned to four dietary test groups. Two diets only differed in their protein fraction. Diet I contained 21% egg white protein. For the breath test egg white protein, intrinsically labelled with [1-13C]-leucine, was used as a substrate. Diet II contained the same amino acids as diet I, though not as egg white protein but in free form. Free [1-13C]-leucine was used to label this diet. In addition, two 1:1 mixtures of both diets were used. During the breath test either the free amino acid or the protein fraction was labelled as in diets I or II. The animals were breath-tested following short-term (day 5) and long-term adaptation (day 20) to their experimental diet. For all diets, including the mixed diets, postprandial oxidative losses on day 5 were significantly higher for the free leucine compared with the protein-derived leucine. Differences between free and protein-derived leucine oxidation had, however, largely disappeared on day 20. The human subjects were breath-tested without any adaptation period to the diets. The oxidative losses of free leucine were also higher than those of protein-derived leucine. None of the studies showed any indication for an interaction between the oxidation of protein-derived amino acids and free amino acids. It is concluded that free and protein-derived amino acids in the diet are mainly metabolized independently.

The postprandial use of dietary amino acids as an energy substrate is delayed after the deamination process in rats adapted for 2 weeks to a high protein diet

The aim of this study was to determine the contribution of dietary amino acids (AA) to energy metabolism under high protein (HP) diets, using a double tracer method to follow simultaneously the metabolic fate of α-amino groups and carbon skeletons. Sixty-seven male Wistar rats were fed a normal (NP) or HP diet for 14 days. Fifteen of them were equipped with a permanent catheter. On day 15, after fasting overnight, they received a 4-g meal extrinsically labeled with a mixture of 20 U-[(15)N]-[(13)C] AA. Energy metabolism, dietary AA deamination and oxidation and their transfer to plasma glucose were measured kinetically for 4 h in the catheterized rats. The transfer of dietary AA to liver glycogen was determined at 4 h. The digestive kinetics of dietary AA, their transfer into liver AA and proteins and the liver glycogen content were measured in the 52 other rats that were killed sequentially hourly over a 4-h period. [(15)N] and [(13)C] kinetics in the splanchnic protein pools were perfectly similar. Deamination increased fivefold in HP rats compared to NP rats. In the latter, all deaminated AA were oxidized. In HP rats, the oxidation rate was slower than deamination, so that half of the deaminated AA was non-oxidized within 4 h. Non-oxidized carbon skeletons were poorly sequestrated in glycogen, although there was a significant postprandial production of hepatic glycogen. Our results strongly suggest that excess dietary AA-derived carbon skeletons above the ATP production capacity, are temporarily retained in intermediate metabolic pools until the oxidative capacities of the liver are no longer overwhelmed by an excess of substrates.

Three-dimensional macronutrient-associated Fos expression patterns in the mouse brainstem

Background

The caudal brainstem plays an important role in short-term satiation and in the control of meal termination. Meal-related stimuli sensed by the gastrointestinal (GI) tract are transmitted to the area postrema (AP) via the bloodstream, or to the nucleus tractus solitarii (NTS) via the vagus nerve. Little is known about the encoding of macronutrient-specific signals in the caudal brainstem. We hypothesized that sucrose and casein peptone activate spatially distinct sub-populations of NTS neurons and thus characterized the latter using statistical three-dimensional modeling.

Methodology/Principal Findings

Using immunolabeling of the proto-oncogene Fos as a marker of neuronal activity, in combination with a statistical three-dimensional modeling approach, we have shown that NTS neurons activated by sucrose or peptone gavage occupy distinct, although partially overlapping, positions. Specifically, when compared to their homologues in peptone-treated mice, three-dimensional models calculated from neuronal density maps following sucrose gavage showed that Fos-positive neurons occupy a more lateral position at the rostral end of the NTS, and a more dorsal position at the caudal end.

Conclusion/Significance

To our knowledge, this is the first time that subpopulations of NTS neurons have be distinguished according to the spatial organization of their functional response. Such neuronal activity patterns may be of particular relevance to understanding the mechanisms that support the central encoding of signals related to the presence of macronutrients in the GI tract during digestion. Finally, this finding also illustrates the usefulness of statistical three-dimensional modeling to functional neuroanatomical studies.

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.

Parameter estimation for linear compartmental models--a sensitivity analysis approach

Linear compartmental models are useful, explanatory tools, that have been widely used to represent the dynamic behavior of complex biological systems. This paper addresses the problem of the numerical identification of such models, i.e., the estimation of the parameter values that will generate predictions closest to experimental observations. Traditional local optimization techniques find it difficult to arrive at satisfactory solutions to such a parameter estimation problem, especially when the number of parameters is large and/or few data are available from experiments. We present herewith a method based on a prior sensitivity analysis, which enables division of a large optimization problem into several smaller and simpler subproblems, on which only sensitive parameters are estimated, before the whole optimization problem is tackled from starting points that are already close to the optimum values. This method has been applied successfully to a linear 13-compartment, 21-parameter model describing the postprandial metabolism of dietary nitrogen in humans. The effectiveness of the method has been demonstrated using simulated and real data obtained in the intestine, blood and urine of healthy humans after the ingestion of a [(15)N]-labeled protein meal.

Protein, amino acids and the control of food intake

The influence of protein and amino acid on the control of food intake and the specific control of protein and amino acid intakes remains incompletely understood. The most commonly accepted conclusions are: (1) the existence of an aversive response to diets deficient in or devoid of protein or deficient in at least one essential amino acid; (2) the existence of a mechanism that enables attainment of the minimum requirement for N and essential amino acids by increasing intake of a low-protein diet; (3) a decrease in the intake of a high-protein diet is associated with different processes, including the high satiating effect of protein. Ingested proteins are believed to generate pre- and post-absorptive signals that contribute to the control of gastric kinetics, pancreatic secretion and food intake. At the brain level, two major afferent pathways are involved in protein and amino acid monitoring: the indirect neuro-mediated (mainly vagus-mediated) pathway and the direct blood pathway. The neuro-mediated pathway transfers pre-absorptive and visceral information. This information is for the main part transferred through the vagus nerve that innervates part of the oro-sensory zone: the stomach, the duodenum and the liver. Other information is directly monitored in the blood. It is likely that the system responds precisely when protein and essential amino acid intake is inadequate, but in contrast allows a large range of adaptive capacities through amino acid degradation and substrate interconversion.

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

A multi-compartmental model has been developed to describe dietary nitrogen (N) postprandial distribution and metabolism in humans. This paper details the entire process of model development, including the successive steps of its construction, parameter estimation and validation. The model was built using experimental data on dietary N kinetics in certain accessible pools of the intestine, blood and urine in healthy adults fed a [15N]-labeled protein meal. A 13-compartment, 21-parameter model was selected from candidate models of increasing order as being the minimum structure able to properly fit experimental data for all sampled compartments. Problems of theoretical identifiability and numerical identification of the model both constituted mathematical challenges that were difficult to solve because of the large number of unknown parameters and the few experimental data available. For this reason, new robust and reliable methods were applied, which enabled (i) a check that all model parameters could theoretically uniquely be determined and (ii) an estimation of their numerical values with satisfactory precision from the experimental data. Finally, model validation was completed by first verifying its a posteriori identifiability and then carrying out external validation.

A reduced carbohydrate, increased protein diet stabilizes glycemic control and minimizes adipose tissue glucose disposal in rats

The dietary reference intakes (DRIs) established an acceptable macronutrient distribution range (AMDR); however, few studies have evaluated differences in metabolic regulations across the DRI range. This study examined differences in glycemic regulations associated with specific ratios of carbohydrate and protein. Male rats ( approximately 200 g) were fed either a high-carbohydrate diet (CHO group: 60% of energy as carbohydrates, 12% protein, 28% fat) or a reduced-carbohydrate diet [PRO (protein) group: 42% carbohydrates, 30% protein, 28% fat]. Rats consumed 3 meals/d with energy distributed as 16, 42, and 42%. On d 25, blood and tissues were obtained after 12 h of food deprivation and at 30 and 90 min after the first meal. Before the meal, the CHO group had lower plasma glucose and insulin, reduced liver glycogen, lower expression of hepatic phosphoenolpyruvate carboxylase (PEPCK), and increased fatty acid synthase (FAS) in adipose tissue. After the meal, the CHO group had greater increases in plasma glucose and insulin, producing increased skeletal muscle phosphatidylinositol 3-kinase (PI3-kinase) activity, glucose uptake, and glycogen content, and increased adipose PI3-kinase activity, glucose uptake, and FAS. In contrast, the PRO group had limited postprandial changes in plasma glucose and insulin with reduced muscle PI3-kinase activity and glucose uptake, and no postprandial changes in adipose PI3-kinase activity or FAS. This study demonstrates that changes in carbohydrate and protein intakes within the AMDR produce fundamental shifts in glycemic regulation from high-CHO diets that require insulin-mediated peripheral glucose disposal to high-PRO diets that increase hepatic regulation of glucose appearance into the blood.

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

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

Casein and Soy Protein Meals Differentially Affect Whole-Body and Splanchnic Protein Metabolism in Healthy Humans

Dietary protein quality is considered to be dependent on the degree and velocity with which protein is digested, absorbed as amino acids, and retained in the gut as newly synthesized protein. Metabolic animal studies suggest that the quality of soy protein is inferior to that of casein protein, but confirmatory studies in humans are lacking. The study objective was to assess the quality of casein and soy protein by comparing their metabolic effects in healthy human subjects. Whole-body protein kinetics, splanchnic leucine extraction, and urea production rates were measured in the postabsorptive state and during 8-h enteral intakes of isonitrogenous [0.42 g protein/(kg body weight . 8 h)] protein-based test meals, which contained either casein (CAPM; n = 12) or soy protein (SOPM; n = 10) in 2 separate groups. Stable isotope techniques were used to study metabolic effects. With enteral food intake, protein metabolism changed from net protein breakdown to net protein synthesis. Net protein synthesis was greater in the CAPM group than in the SOPM group [52 +/- 14 and 17 +/- 14 nmol/(kg fat-free mass (FFM) . min), respectively; P < 0.02]. Urea synthesis rates decreased during consumption of both enteral meals, but the decrease tended to be greater in the subjects that consumed CAPM (P = 0.07). Absolute splanchnic extraction of leucine was higher in the subjects that consumed CAPM [306 +/- 31 nmol/(kg FFM . min)] vs. those that consumed SOPM [235 +/- 29 nmol/(kg FFM . min); P < 0.01]. In conclusion, a significantly larger portion of soy protein is degraded to urea, whereas casein protein likely contributes to splanchnic utilization (probably protein synthesis) to a greater extent. The biological value of soy protein must be considered inferior to that of casein protein in humans.

A long-term high-protein diet markedly reduces adipose tissue without major side effects in Wistar male rats

Although there is a considerable interest of high-protein, low-carbohydrate diets to manage weight control, their safety is still the subject of considerable debate. They are suspected to be detrimental to the renal and hepatic functions, calcium balance, and insulin sensitivity. However, the long-term effects of a high-protein diet on a broad range of parameters have not been investigated. We studied the effects of a high-protein diet in rats over a period of 6 mo. Forty-eight Wistar male rats received either a normal-protein (NP: 14% protein) or high-protein (HP: 50% protein) diet. Detailed body composition, plasma hormones and nutrients, liver and kidney histopathology, hepatic markers of oxidative stress and detoxification, and the calcium balance were investigated. No major alterations of the liver and kidneys were found in HP rats, whereas NP rats exhibited massive hepatic steatosis. The calcium balance was unchanged, and detoxification markers (GSH and GST) were enhanced moderately in the HP group. In contrast, HP rats showed a sharp reduction in white adipose tissue and lower basal concentrations of triglycerides, glucose, leptin, and insulin. Our study suggests that the long-term consumption of an HP diet in male rats has no deleterious effects and could prevent metabolic syndrome.

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.

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.

A CLA mixture prevents body triglyceride accumulation without affecting energy expenditure in Syrian hamsters

We examined the effects of feeding conjugated linoleic acids (CLA) to adult male hamsters on several components of energy metabolism and body composition. Hamsters (n = 54) were assigned for 6-8 wk to one of three diets: 1) a standard diet (in percentage energy: lipids, 33, carbohydrates, 49, and proteins, 18); 2) to the standard diet augmented with the 9c,11t-isomer of CLA to 1.6% of energy (R group); or 3) the standard diet augmented with the 9c,11t-isomer and the 10t,12c-CLA isomer to 3.2 (1.6 + 1.6) % of energy (CLA mix group). (15)N uniformly labeled milk-protein was included in the diet to measure the incorporation of dietary protein into liver and muscle. Basal metabolic rate, thermogenic response to feeding and energy expenditure during spontaneous activity or during an exercise at approximately 60% of VO(2max) were measured. Carnitine palmitoyltransferase-I (CPT-I), leptin, insulin and triiodothyronine concentrations, as well as the in vivo overall adiposity changes were also determined. After 6 wk, the whole-body triglyceride content determined in vivo by NMR was significantly higher in the R group than in the control and CLA mix groups. The CLA mix group differed from the others in the lack of body triglyceride accumulation between d 21 and d 45 of the study, and the appearance of a slight insulin-resistance (homeostatic model assessment index, P < 0.05). Paradoxically, the lack of effect on whole-body lipid oxidation was associated with a greater CPT-I-specific activity in tissues of both CLA-fed groups (P < 0.05). No other major effects of CLA feeding were detected. In conclusion, CLA supplementation in hamsters did not affect adipose weight or the components of energy expenditure despite a theoretically higher capacity of red muscle to oxidize lipids. Only a CLA mixture prevented whole-body triglyceride accumulation over time.

Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements

BACKGROUND & AIMS

Irreversible amino acid losses at the human ileum are not taken into account when tracer-derived amino acid requirements are calculated because the data available are scarce. We have investigated amino acid losses at the ileal level in humans after ingestion of a protein meal.

METHODS

Thirteen volunteers ingested a single meal of 15N milk or soy proteins. The appearance of 15N and 15N amino acids in the ileal effluents collected using an ileal tube was monitored for 8 hours.

RESULTS

In the soy group, higher losses of endogenous nitrogen, especially originating from amino acids, were observed, as well as a higher flow rate of dietary non-amino acid nitrogen. With soy protein, the digestibilities of valine, threonine, histidine, tyrosine, alanine, and proline were significantly lower than with milk. Ileal losses of leucine, valine, and isoleucine amounted to 12, 10, and 7 mg x kg(-1) x day(-1), respectively. Threonine ileal loss (9-12 mg x kg(-1) x day(-1)) was particularly high compared with the current amino acid requirement.

CONCLUSIONS

Amino acid losses at the human terminal ileum are substantial and depend on the type of dietary protein ingested. Although it remains unclear whether intact amino acids are absorbed in the colon, we suggest that ileal losses should be considered an important component of amino acid requirements.

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.

The pattern of intestinal substrate oxidation is altered by protein restriction in pigs

BACKGROUND & AIMS

Previous studies indicate that amino acids and glucose are the major oxidative substrates for intestinal energy generation. We hypothesized that low protein feeding would lower the contribution of amino acids to energy metabolism, thereby increasing the contribution of glucose.

METHODS

Piglets, implanted with portal, arterial, and duodenal catheters and a portal flow probe, were fed isocaloric diets of either a high protein (0.9 g/[kg/h] protein, 1.8 g/[kg/h] carbohydrate, and 0.4 g/[kg/h] lipid) or a low protein (0.4 g/[kg/h] protein, 2.2 g/[kg/h] carbohydrate, and 0.5 g/[kg/h] lipid) content. They received enteral or intravenous infusions of [1-13C]leucine (n = 17), [U-13C]glucose (n = 15), or enteral [U-13C]glutamate (n = 8).

RESULTS

CO2 production by the splanchnic bed was not affected by the diet. The oxidation of leucine, glutamate, and glucose accounted for 82% of the total CO2 production in high protein-fed pigs. Visceral amino acid oxidation was substantially suppressed during a low protein intake. Although glucose oxidation increased to 50% of the total visceral CO2 production during a low protein diet, this increase did not compensate entirely for the fall in amino acid oxidation.

CONCLUSIONS

Although low protein feeding increases the contribution of enteral glucose oxidation to total CO2 production, this adaptation is insufficient. To compensate for the fall in amino acid oxidation, other substrates become increasingly important to intestinal energy generation.

Daily delivery of dietary nitrogen to the periphery is stable in rats adapted to increased protein intake

Dietary nitrogen was traced in rats adapted to a 50% protein diet and given a meal containing 1.50 g (15)N-labeled protein (HP-50 group). This group was compared with rats usually consuming a 14% protein diet and fed a meal containing either 0.42 g (AP-14 group) or 1.50 g (AP-50 group) of (15)N-labeled protein. In the HP group, the muscle nonprotein nitrogen pool was doubled when compared with the AP group. The main adaptation was the enhancement of dietary nitrogen transferred to urea (2.2 +/- 0.5 vs. 1.3 +/- 0.1 mmol N/100 g body wt in the HP-50 and AP-50 groups, respectively). All amino acids reaching the periphery except arginine and the branched-chain amino acids were depressed. Consequently, dietary nitrogen incorporation into muscle protein was paradoxically reduced in the HP-50 group, whereas more dietary nitrogen was accumulated in the free nitrogen pool. These results underline the important role played by splanchnic catabolism in adaptation to a high-protein diet, in contrast to muscle tissue. Digestive kinetics and splanchnic anabolism participate to a lesser extent in the regulation processes.

Metabolic evidence for adaptation to a high protein diet in rats

This study was designed to assess the effects of long-term adaptation to a high protein diet on energy intake, body weight gain, body composition and splanchnic metabolic indicators in rats. For this purpose, adult male Wistar rats were fed either a 50 g/100 g dry matter (DM) protein diet (P50 group) or a 14 g/100 g DM protein diet (P14 group) for 21 d. These two groups were compared with a P14 pair-fed (P14-pf) group that consumed the same daily energy as the P50 group. The energy intake of the P50 group was 16 +/- 1% less than that of the P14 group (P < 0.05), and the P50 group had significantly lower body weight. The P50 group had significantly less adipose tissue compared with both P14 and P14-pf rats. The activities of the brush border membrane enzymes, neutral aminopeptidase and gamma-glutamyl transferase, were significantly higher in the P50 group than in the P14 rats. Similarly, the activities of alanine aminotransferase, arginase and serine dehydratase were significantly higher in the liver of P50 rats compared with P14 rats. Both amino acid transporter system A and X(A,G-) activities, measured in freshly isolated hepatocytes, were significantly higher in the P50 group (8- and 1.5-fold, P < 0.05, respectively) compared with the P14 group. The 1.5-fold increase in the steady-state activity of X(A,G-) was accompanied by a doubling of EAAT2 mRNA, involved in the system X(A,G-). This study provides confirmation that specific biochemical and molecular adaptive processes of the splanchnic area are involved in the response to variations in the protein content of the diet.