Differential changes to splanchnic and peripheral protein metabolism during the diet-induced development of metabolic syndrome in rats.

Little is known about the effects of the development of metabolic syndrome (MS) on protein and amino acid (AA) metabolism. During this study, we took advantage of the variability in interindividual susceptibility to high fat diet-induced MS to study the relationships between MS, protein synthesis, and AA catabolism in multiple tissues in rats. After 4 mo of high-fat feeding, an MS score (ZMS) was calculated as the average of the z-scores for individual MS components [weight, adiposities, homeostasis model for the assessment of insulin resistance (HOMA-IR), and triglycerides]. In the small intestine, liver, plasma, kidneys, heart, and muscles, tissue protein synthesis was measured by 2H2O labeling, and we evaluated the proportion of tissue AA catabolism (relative to protein synthesis) and nutrient routing to nonindispensable AAs in tissue proteins using natural nitrogen and carbon isotopic distances between tissue proteins and nutrients (Δ15N and Δ13C), respectively. In the liver, protein mass and synthesis increased, whereas the proportion of AA catabolism decreased with ZMS. By contrast, in muscles, we found no association between ZMS and protein mass, protein synthesis (except for a weak positive association in the gastrocnemius muscle only), and proportion of AA catabolism. The development of MS was also associated with altered metabolic flexibility and fatty acid oxidation, as shown by less routing of dietary lipids to nonindispensable AA synthesis in liver and muscle. In conclusion, MS development is associated with a greater gain of both fat and protein masses, with higher protein anabolism that mainly occurs in the liver, whereas muscles probably develop anabolic resistance due to insulin resistance.

Nitrogen isotopic fractionation as a biomarker for nitrogen use efficiency in ruminants: a meta-analysis.

Animal proteins are naturally 15N enriched relative to the diet and the extent of this difference (Δ15Nanimal-diet or N isotopic fractionation) has been correlated to N use efficiency (NUE; N gain or milk N yield/N intake) in some recent ruminant studies. The present study used meta-analysis to investigate whether Δ15Nanimal-diet can be used as a predictor of NUE across a range of dietary conditions, particularly at the level of between-animal variation. An additional objective was to identify variables related to N partitioning explaining the link between NUE and Δ15Nanimal-diet. Individual values from eight publications reporting both NUE and Δ15Nanimal-diet for domestic ruminants were used to create a database comprising 11 experimental studies, 41 treatments and individual animal values for NUE (n=226) and Δ15Nanimal-diet (n=291). Data were analyzed by mixed-effect regression analysis taking into account experimental factors as random effects on both the intercept and slope of the model. Diets were characterized according to the INRA feeding system in terms of N utilization at the rumen, digestive and metabolic levels. These variables were used in a partial least squares regression analysis to predict separately NUE and Δ15Nanimal-diet variation, with the objective of identifying common variables linking NUE and Δ15Nanimal-diet. For individuals reared under similar conditions (within-study) and at the same time (within-period), the variance of NUE and Δ15Nanimal-diet not explained by dietary treatments (i.e. between-animal variation plus experimental error) was 35% and 55%, respectively. Mixed-effect regression analysis conducted with treatment means showed that Δ15Nanimal-diet was significantly and negatively correlated to NUE variation across diets (NUE=0.415 -0.055×Δ15Nanimal-diet). When using individual values and taking into account the random effects of study, period and diet, the relationship was also significant (NUE=0.358 -0.035×Δ15Nanimal-diet). However, there may be a biased prediction for animals close to zero, or in negative, N balance. When using a novel statistical approach, attempting to regress between-animal variation in NUE on between-animal variation in Δ15Nanimal-diet (without the influence of experimental factors), the negative relationship was still significant, highlighting the ability of Δ15Nanimal-diet to capture individual variability. Among the studied variables related to N utilization, those concerning N efficiency use at the metabolic level contributed most to predict both Δ15Nanimal-diet and NUE variation, with rumen fermentation and digestion contributing to a lesser extent. This study confirmed that on average Δ15Nanimal-diet can predict NUE variation across diets and across individuals reared under similar conditions.

Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?

Animal tissues are naturally 15N enriched relative to their diet and the extent of this difference (Δ15Nanimal-diet) has been correlated to the efficiency of N assimilation in different species. The rationale is that transamination and deamination enzymes, involved in amino acid metabolism are likely to preferentially convert amino groups containing 14N over 15N. However, in ruminants the contribution of rumen bacterial metabolism relative to animal tissues metabolism to naturally enrich animal proteins in terms of 15N has been not assessed yet. The objective of this study was to assess the impact of rumen and digestion processes on the relationship between Δ15Nanimal-diet and efficiency of N utilization for milk protein yield (milk N efficiency (MNE); milk N yield/N intake) as well as the relationship between the 15N natural abundance of rumen bacteria and the efficiency of N use at the rumen level. Solid- and liquid-associated rumen bacteria, duodenal digesta, feces and plasma proteins were obtained (n=16) from four lactating Holstein cows fed four different diets formulated at two metabolizable protein supplies (80% v. 110% of protein requirements) crossed by two different dietary energy source (diets rich in starch v. fiber). We measured the isotopic N fractionation between animal and diet (Δ15Nanimal-diet) in these different body pools. The Δ15Nanimal-diet was negatively correlated with MNE when measured in solid-associated rumen bacteria, duodenal digesta, feces and plasma proteins, with the strongest correlation found for the latter. However, our results showed a very weak 15N enrichment of duodenal digesta (Δ15Nduodenal digesta-diet mean value=0.42) compared with that observed in plasma proteins (Δ15Nplasma protein-diet mean value=2.41). These data support the idea that most of the isotopic N fractionation observed in ruminant proteins (Δ15Nplasma protein-diet) has a metabolic origin with very little direct impact of the overall digestion process on the existing relationship between Δ15Nplasma protein-diet and MNE. The 15N natural abundance of rumen bacteria was not related to either rumen N efficiency (microbial N/available N) or digestive N efficiency (metabolizable protein supply/CP intake), but showing a modest positive correlation with rumen ammonia concentration. When using diets not exceeding recommended protein levels, the contribution of rumen bacteria and digestion to the isotopic N fractionation between animal proteins and diet is low. In our conditions, most of the isotopic N fractionation (Δ15Nplasma protein-diet) could have a metabolic origin, but more studies are warranted to confirm this point with different diets and approaches.

Diet-animal fractionation of nitrogen stable isotopes reflects the efficiency of nitrogen assimilation in ruminants.

The natural abundance of ¹5N in animal proteins (δ¹5Nanimal) is greater than that in the diet consumed by the animals (δ¹5Ndiet), with a discrimination factor (Δ¹5N = δ¹5Nanimal - δ¹5Ndiet) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that Δ¹5N variations depend on the efficiency of nitrogen utilisation (ENU) in growing beef cattle, and to identify some of the physiological mechanisms responsible for this N isotopic fractionation in ruminants. Thus, we performed the regression of the Δ¹5N of plasma proteins obtained from thirty-five finishing beef cattle fed standard and non-conventional diets against different feed efficiency indices, including ENU. We also performed the regression of the Δ¹5N of different ruminant N pools (plasma and milk proteins, urine and faeces) against different splanchnic N fluxes obtained from multi-catheterised lactating dairy cows. The Δ¹5N of plasma proteins was negatively correlated with feed efficiency indices in beef cattle, especially ENU (body protein gain/N intake) and efficiency of metabolisable protein (MP) utilisation (body protein gain/MP intake). Although Δ¹5N obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when Δ¹5N was calculated from plasma proteins. Δ¹5N showed no correlation with urea-N recycling or rumen NH3 absorption, but exhibited a strong correlation with liver urea synthesis and splanchnic amino acid metabolism, which points to a dominant role of splanchnic tissues in the present N isotopic fractionation study.

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.

Compartmental modeling of postprandial dietary nitrogen distribution in humans.

A linear 11-compartment model was developed to describe and simulate the postprandial distribution of dietary nitrogen. The values of its 15 constant diffusion coefficients were estimated from the experimental measurement of (15)N nitrogen kinetics in the intestine, blood, and urine after the oral administration of (15)N-labeled milk protein in humans. Model structure development, parameter estimation, and sensibility analysis were achieved using SAAM II and SIMUSOLV softwares. The model was validated at each stage of its development by testing successively its a priori and a posteriori identifiability. The model predicted that, 8 h after a meal, the dietary nitrogen retained in the body comprised 28% free amino acids and 72% protein, approximately 30% being recovered in the splanchnic bed vs. 70% in the peripheral area. Twelve hours after the meal, these values had decreased to 18 and 23% for the free amino acid fraction and splanchnic nitrogen, respectively. Such a model constitutes a useful, explanatory tool to describe the processes involved in the metabolic utilization of dietary proteins.

Net postprandial utilization of [15N]-labeled milk protein nitrogen is influenced by diet composition in humans.

The aim of this study was to follow the fate of dietary nitrogen to assess the postprandial utilization of purified milk protein and to determine the acute influence of energy nutrients. For this purpose, a [15N]-labeling dietary protein approach was used. Twenty-five subjects swallowed an ileal tube and ingested [15 N]-milk protein alone or supplemented with either milk fat or sucrose. The absorption and postprandial deamination of dietary protein was monitored for 8 h. Sucrose delayed the absorption of protein longer than fat, but the ileal digestibility did not differ among groups (94.5-94.8%). Sucrose, but not fat, significantly reduced the postprandial transfer of [15N]-milk nitrogen to urea. Consequently, the net postprandial protein utilization (NPPU) of milk protein calculated 8 h after meal ingestion was 80% when ingested either alone or supplemented with fat and was significantly greater with sucrose (NPPU = 85%). This study shows that energy nutrients do not affect the nitrogen absorption but modify the metabolic utilization of dietary protein in the phase of nitrogen gain. Our method provides information concerning the deamination kinetics of dietary amino acids and further allows the detection of differences of dietary protein utilization in acute conditions. The diet composition should be carefully considered, and protein quality must be determined under optimal conditions of utilization.