Net hepatic release of glucose from precursor supply in ruminants: a meta-analysis

For their glucose supply, ruminants are highly dependent on the endogenous synthesis in the liver, but despite the numerous studies that evaluated hepatic glucose production, very few simultaneously measured hepatic glucose production and uptake of all precursors. As a result, the variability of precursor conversion into glucose in the liver is not known. The present study aimed at investigating by meta-analysis the relationships between hepatic glucose net release and uptake of precursors. We used the FLuxes of nutrients across Organs and tissues in Ruminant Animals database, which gathers international results on net nutrient fluxes at splanchnic level measured in catheterized animals. Response equations were developed for intakes up to 41 g DM intake/kg BW per day of diets varying from 0 to 100 g of concentrate/100 g DM in the absence of additives. The net hepatic uptake of propionate, α-amino-N and l-lactate was linearly and better related to their net portal appearance (NPA) than to their afferent hepatic flux. Blood flow data were corrected for lack of deacetylation of the para-aminohippuric acid, and this correction was shown to impact the response equations. To develop response equations between the availability of precursors (portal appearance and hepatic uptake) and net glucose hepatic release, missing data on precursor fluxes were predicted from dietary characteristics using previously developed response equations. Net hepatic release of glucose was curvilinearly related to hepatic supply and uptake of the sum of precursors, suggesting a lower conversion rate of precursors at high precursor supply. Factors of variation were explored for the linear portion of this relationship, which applied to NPA of precursors ranging from 0.99 to 9.60 mmol C/kg BW per h. Hepatic release of glucose was shown to be reduced by the portal absorption of glucose from diets containing bypass starch and to be increased by an increased uptake of β-hydroxybutyrate indicative of higher body tissue mobilization. These relationships were affected by the physiological status of the animals. In conclusion, we established equations that quantify the net release of glucose by the liver from the net availability of precursors. They provide a quantitative overview of factors regulating hepatic glucose synthesis in ruminants. These equations can be linked with the predictions of portal absorption of nutrients from intake and dietary characteristics, and provide indications of glucose synthesis from dietary characteristics.     

Empirical prediction of net splanchnic release of ketogenic nutrients,acetate, butyrate and β-hydroxybutyrate in ruminants: a meta-analysis

For energy feeding systems for ruminants to evolve towards a nutrient-based system, dietary energy supply has to be determined in terms of amount and nature of nutrients. The objective of this study was to establish response equations of the net hepatic flux and net splanchnic release of acetate, butyrate and β-hydroxybutyrate to changes in diet and animal profiles. A meta-analysis was applied on published data compiled from the FLuxes of nutrients across Organs and tissues in Ruminant Animals database, which pools the results from international publications on net splanchnic nutrient fluxes measured in multi-catheterized ruminants. Prediction variables were identified from current knowledge on digestion, hepatic and other tissue metabolism. Subsequently, physiological and other, more integrative, predictors were obtained. Models were established for intakes up to 41 g dry matter per kg BW per day and diets containing up to 70 g concentrate per 100 g dry matter. Models predicted the net hepatic fluxes or net splanchnic release of each nutrient from its net portal appearance and the animal profile. Corrections were applied to account for incomplete hepatic recovery of the blood flow marker, para-aminohippuric acid. Changes in net splanchnic release (mmol/kg BW per hour) could then be predicted by combining the previously published net portal appearance models and the present net hepatic fluxes models. The net splanchnic release of acetate and butyrate were thus predicted from the intake of ruminally fermented organic matter (RfOM) and the nature of RfOM (acetate: residual mean square error (RMSE)=0.18; butyrate: RMSE=0.01). The net splanchnic release of β-hydroxybutyrate was predicted from RfOM intake and the energy balance of the animals (RMSE=0.035), or from the net portal appearance of butyrate and the energy balance of the animals (RMSE=0.050). Models obtained were independent of ruminant species, and presented low interfering factors on the residuals, least square means or individual slopes. The model equations highlighted the importance of considering the physiological state of animals when predicting splanchnic metabolism. This work showed that it is possible to use simple predictors to accurately predict the amount and nature of ketogenic nutrients released towards peripheral tissues in both sheep and cattle at different physiological status. These results provide deeper insight into biological processes and will contribute to the development of improved tools for dietary formulation.     

Effects of forage intake level on nitrogen net flux by portal-drained viscera of mature sheep with abomasal infusion of an amino acid mixture

This study aimed to investigate the pattern of nitrogen (N) metabolites flux across the portal-drained viscera (PDV) of mature sheep over a wide range of forage intake, and to determine the effect of dry matter intake (DMI) on the PDV recovery of an abomasally infused amino acids (AA) mixture. Four Suffolk mature sheep (61.4 ± 3.6 kg BW) surgically fitted with abomasal cannulae and multi-catheters were fed four levels of DMI of lucerne hay cubes ranging from 0.4 to 1.6 fold the metabolizable energy requirements for maintenance. Each period lasted for 17 days: 7 days for diet adaptation, 5 days for measurement of N balance and N metabolites flux under basal pre-infusion conditions (basal phase) and 5 days for determining the recovery of the infused AA (584 mmol/day) across the PDV (infusion phase). Six sets of blood samples were collected on the last day of both basal and infusion phases. Increasing DMI increased portal release of AA and enhanced N retention. At 0.4 M and as a proportion of digested N, there was a marked drop in total AA-N release accompanied by greater ammonia-N release and urea-N uptake across the PDV. The incremental recovery ratio of infused AA across the PDV was altered with increasing DMI accounting for 0.88, 1.12, 1.23 and 1.31 at 0.4, 0.8, 1.2 and 1.6 M, respectively. In addition, across the individual AA, the net portal recovery ratio of infused methionine and valine increased linearly (P < 0.05) while that of phenylalanine, branched-chain AA and total essential AA tended to increase linearly (P < 0.10) with increasing DMI. These results indicated that DMI affects the net portal recovery of AA available in the small intestine of mature sheep.     

Effects of a slow-release urea source on absorption of ammonia and endogenous production of urea by cattle

Three experiments were conducted with Angus or Holstein steers to evaluate effects of dietary urea–calcium (a slow rumen-release urea source) on absorption of ammonia N from the gut and urea N production in the liver. Steers were fed a high-grain diet (Experiment 1) or an all-forage diet (Experiments 2 and 3). Urea or urea–calcium (0.25 g/kg body weight) was dosed into the esophagus (Experiments 1 and 2) or rumen (Experiment 3), and blood samples were serially collected for 180 min. Blood concentrations of ammonia N and urea N were measured in all experiments, and net flux of metabolites across splanchnic tissues was measured in Experiment 3. Compared to urea, urea–calcium reduced (P<0.05) plasma concentrations of ammonia N in steers fed all-forage diets, and tended (P<0.06) to reduce arterial glucose concentrations in Experiment 3. Plasma concentrations of urea N were not affected by treatment in any experiment. Treatment and time post-dosing interactions (P<0.05) in Experiment 3 were due to increased ruminal fluid concentrations of ammonia N, net release of ammonia N by portal-drained viscera and total splanchnic tissues with urea versus urea–calcium treatment shortly after dosing. Similar interactions (P<0.05) indicated that urea caused higher hepatic glucose release and increased l-lactate release by total splanchnic tissues after dosing than urea–calcium. Urea–calcium was effective in mitigating rapid ammonia release in the rumen and subsequent effects on glucose and lactate metabolism.     

Portal-drained viscera and hepatic fluxes of branched-chain amino acids do not account for differences in circulating branched-chain amino acids in rats fed arginine-deficient diets

Summary Concentrations and fluxes of amino acids across the portal-drained viscera (PDV) and liver were assessed in rats fed a meal of one of three arginine-deficient diets containing either alanine or the arginine precursors, ornithine or citrulline. A previous report included findings of seven arginine-related amino acids and indicated that only the citrulline-containing diet protected blood arginine concentrations. In the present report we extend these findings and note that the concentrations and fluxes of the non-arginine-related amino acids showed remarkable consistency across diet groups. However, total branched-chain amino acid (BCAA) concentrations of arterial blood were higher in rats fed the - Arg/+ Ala and the - Arg/+ Orn diets than in rats fed the control (+ Arg) diet. The elevated BCAA correlated with higher circulating concentrations of other essential amino acids but were inversely correlated with arginine concentrations. PDV and hepatic fluxes of BCAA were not different across diet groups, indicating that amino acid absorption and hepatic utilization of BCAA were generally comparable across diet groups. Hepatic concentrations of 14 of 22 measured amino acids, including total BCAA, were correlated with their arterial concentrations. The circulating concentrations and net PDV and hepatic fluxes of rats fed the control diet were comparable to our previous observations in fed rats and illustrate the role of the liver in utilization of diet-derived essential amino acids.Abbreviations PDV portal-drained viscera - BCAA branched-chain amino acids - SSA 5-sulfosalicylic acid - PBF portal blood flow - HBF hepatic blood flow - SELSM pooled standard errors of least squares means - TAA total amino acids - NEAA nonessential amino acids - EAA essential amino acids - LNAA large neutral amino acids Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.     

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-[¹⁵N]-[¹³C] 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. [¹⁵N] and [¹³C] 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.     

Ileal and total tract digestibility and nitrogen utilisation in blue fox (Vulpes lagopus) fed low-protein diets supplemented with DL-methionine and L-histidine

ABSTRACT

To formulate low-protein diets for blue foxes with sufficient amounts of amino acids (AA), AA digestibility and AA requirements of the animals are crucial information. Therefore, a digestibility and nitrogen (N) balance trial was conducted with 20 blue foxes to determine the macronutrient and AA digestibility and N utilisation in low-protein diets supplemented with DL-methionine (Met) and L-histidine (His). In addition, plasma urea and plasma AA were measured. The diets were designated as P24 (control), P20, P20M, P16M and P16MH and contained energy from digestible crude protein (DCP) at 24%, 20% or 16% of total dietary metabolisable energy (ME). The 20% protein level was fed with or without Met and the 16% protein level was fed with Met and with or without His. The apparent total-tract digestibility (ATTD) of crude protein linearly decreased with decreasing dietary protein level. The ATTD of dry matter, organic matter and crude carbohydrates increased when wheat starch was added as a replacement for protein. The apparent ileal digestibility (AID) and ATTD methods were compared to determine the AA digestibility. The decreasing dietary protein supply decreased the ATTD of most of the AA: threonine, tryptophan (Trp), valine, alanine (Ala), aspartic acid (Asp), glutamic acid, glycine (Gly), proline (Pro), serine (Ser) and total AA. The AID of the AA was constant between diets. Diverging AA showed higher or lower digestibility when determined in the AID or ATTD methods. Isoleucine, lysine, Met, Ala and tyrosine showed higher levels of AID. Arginine, His, cysteine (Cys), Trp, Asp, Gly, Pro and Ser showed higher levels of ATTD, which may reflect the net loss of these AA in the large intestine. Met and His supplementation improved the ATTD and AID of the AA in question, respectively, but did not affect the other variables examined. N retention did not differ between diets and renal N excretion decreased with decreasing protein level; thus N utilisation improved. It was concluded that the protein supply and AA composition in low-protein diets with supplemented Met were adequate for adult blue foxes, since the lower protein supply improved N utilisation and did not affect N retention. However, His supplementation failed to reach the designed level and therefore showed no clear results.     

Responses in mammary and splanchnic metabolism to altered lysine supply in dairy cows

Lysine is usually taken up in excess by the mammary gland (MG) relative to milk protein output, allowing for mammary synthesis of non-essential (NE) amino acids (AA) from Lys-N. It is unclear whether this NEAA synthesis from Lys is obligate or whether more efficient use of Lys can be made under limiting conditions. Six multi-catheterized dairy cows received a basal diet low in protein plus an abomasal infusion of AA (560 g/day) with or without Lys (50.3 g/day), in a crossover design with 7-day periods. On day 7, all cows received a 7.5-h jugular infusion of [2-15N]lysine. Six blood samples were collected from arterial, portal, hepatic and mammary vessels at 45 min intervals. In addition, cows were milked at 6 and 7 h with the milk casein plus arterial and mammary plasma collected at 7 h analyzed for AA enrichment. Milk protein concentration and casein yield tended (P < 0.10) to decrease with Lys deletion, while Lys secretion in milk protein was lowered (P < 0.05). The addition of Lys in the AA mixture increased the net portal absorption of Lys by the amount infused, suggesting limited oxidation of this extra supply by the gut. Net liver flux of Lys was unaltered by treatment and, therefore, net splanchnic release of Lys reflected closely the amounts absorbed. For both treatments, however, post-liver supply was greater than mammary uptake, which exceeded milk output. Nonetheless, while Lys deletion decreased mammary uptake by 10.1 mmol/h, Lys in milk protein secretion was reduced by only 3.9 mmol/h. On a net basis, there was no evidence of the additional uptake of any other measured AA during the Lys deletion. The mammary uptake to output ratio of Lys decreased from 1.37 to 1.12, but still showed an excess with Lys deletion. The total amount of 15N in milk protein did not change with treatment but the distribution into AA was altered. In conditions that simulated normal feeding (Lys infused), 83% of the 15N was present as Lys, with Glx, Asx, Ser and Ala harvesting, respectively, 6.8%, 2.4%, 2.1% and 1.0%. With Lys depletion, N-transfers from Lys to other AA within the MG were still present, but rates were considerably lower. This would suggest that part, at least, of Lys catabolism in the MG is either needed or cannot be prevented completely, even at low supply of Lys. Such catabolism will provide N to support the synthesis of NEAA.     

Strategies for optimizing nitrogen use by ruminants

The efficiency of N utilization in ruminants is typically low (around 25%) and highly variable (10% to 40%) compared with the higher efficiency of other production animals. The low efficiency has implications for the production performance and environment. Many efforts have been devoted to improving the efficiency of N utilization in ruminants, and while major improvements in our understanding of N requirements and metabolism have been achieved, the overall efficiency remains low. In general, maximal efficiency of N utilization will only occur at the expense of some losses in production performance. However, optimal production and N utilization may be achieved through the understanding of the key mechanisms involved in the control of N metabolism. Key factors in the rumen include the efficiency of N capture in the rumen (grams of bacterial N per grams of rumen available N) and the modification of protein degradation. Traditionally, protein degradation has been modulated by modifying the feed (physical and chemical treatments). Modifying the rumen microflora involved in peptide degradation and amino acid deamination offers an alternative approach that needs to be addressed. Current evidence indicates that in typical feeding conditions there is limited net recycling of N into the rumen (blood urea-N uptake minus ammonia-N absorption), but understanding the factors controlling urea transport across the rumen wall may reverse the balance to take advantage of the recycling capabilities of ruminants. Finally, there is considerable metabolism of amino acids (AA) in the portal-drained viscera (PDV) and liver. However, most of this process occurs through the uptake of AA from the arterial blood and not during the 'absorptive' process. Therefore, AA are available to the peripheral circulation and to the mammary gland before being used by PDV and the liver. In these conditions, the mammary gland plays a key role in determining the efficiency of N utilization because the PDV and liver will use AA in excess of those required by the mammary gland. Protein synthesis in the mammary gland appears to be tightly regulated by local and systemic signals. The understanding of factors regulating AA supply and absorption in the mammary gland, and the synthesis of milk protein should allow the formulation of diets that increase total AA uptake by the mammary gland and thus reduce AA utilization by PDV and the liver. A better understanding of these key processes should allow the development of strategies to improve the efficiency of N utilization in ruminants.     

Glucagon-like peptide-2 (GLP-2) increases net amino acid utilization by the portal-drained viscera of ruminating calves

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in ruminant calves, but its impact on nutrient metabolism across the portal-drained viscera (PDV) and liver is unknown. Eight Holstein calves with catheters in the carotid artery, mesenteric vein, portal vein and hepatic vein were paired by age and randomly assigned to control (0.5% bovine serum albumin in saline; n = 4) or GLP-2 (100 μg/kg BW per day bovine GLP-2 in bovine serum albumin; n = 4). Treatments were administered subcutaneously every 12 h for 10 days. Blood flow was measured on days 0 and 10 and included 3 periods: baseline (saline infusion), treatment (infusion of bovine serum albumin or 3.76 μg/kg BW per h GLP-2) and recovery (saline infusion). Arterial concentrations and net PDV, hepatic and total splanchnic fluxes of glucose, lactate, glutamate, glutamine, β-hydroxybutyrate and urea-N were measured on days 0 and 10. Arterial concentrations and net fluxes of all amino acids and glucose metabolism using continuous intravenous infusion of [U¹³-C]glucose were measured on day 10 only. A 1-h infusion of GLP-2 increased blood flow in the portal and hepatic veins when administered to calves not previously exposed to exogenous GLP-2, but after a 10-day administration of GLP-2 the blood flow response to the 1-h GLP-2 infusion was substantially attenuated. The 1-h GLP-2 infusion also did not appreciably alter nutrient fluxes on either day 0 or 10. In contrast, long-term GLP-2 administration reduced arterial concentrations and net PDV flux of many essential and non-essential amino acids. Despite the significant alterations in amino acid metabolism, glucose irreversible loss and utilization by PDV and non-PDV tissues were not affected by GLP-2. Fluxes of amino acids across the PDV were generally reduced by GLP-2, potentially by increased small intestinal epithelial growth and thus energy and amino acid requirements of this tissue. Increased PDV extraction of glutamine and alterations in PDV metabolism of arginine, ornithine and citrulline support the concept that GLP-2 influences intestine-specific amino acid metabolism. Alterations in amino acid metabolism but unchanged glucose metabolism suggests that the growth effects induced by GLP-2 in ruminants increase reliance on amino acids preferentially over glucose. Thus, GLP-2 increases PDV utilization of amino acids, but not glucose, concurrent with stimulated growth of the small intestinal epithelium in post-absorptive ruminant calves.     

Linseed plus nitrate in the diet for fattening bulls: effects on methane emission,animal health and residues in offal

The combination of linseed and nitrate is known to decrease enteric methane emission in dairy cows but few studies have been carried out in fattening cattle for animal liveweight gain, enteric methane emission, animal health and presence of residues in beef products. To address this gap, 16 young bulls received a control (C) diet between weaning at 9 months and 14 months, then were split into two groups of eight balanced on feed intake, BW gain and methane emission to receive either the C diet or a diet moderately supplemented with extruded linseed and calcium nitrate (LN) for 2 months before being slaughtered. On a dry matter (DM) basis, the C diet contained 70% baled grass silage and 30% concentrate mainly made of maize, wheat and rapeseed meal. In the LN diet, rapeseed meal and a fraction of cereals were replaced by 35% extruded linseed and 6% calcium nitrate; linseed fatty acids and nitrate supply in the LN diet were 1.9% and 1.0%, respectively. Methane emission was measured continuously using the GreenFeed system. Methaemoglobin was determined every week in peripheral blood from bulls receiving the LN diet. Nitrate and nitrite concentrations were determined in rumen, liver and tongue sampled at slaughter. Dry matter intake tended to be lower for LN diet (P=0.10). Body weight gain was lower for LN diet (P=0.01; 1.60 and 1.26 kg/day for C and LN diet, respectively). Daily methane emission was 9% lower (P<0.001) for LN than C diet (249 and 271 g/day, respectively) but methane yield did not differ between diets (24.1 and 23.2 g/kg DM intake for C and LN diet, respectively, P=0.34). Methaemoglobin was under the limit of detection (<2% of total haemoglobin) for most animals and was always lower than 5.6%, suggesting an absence of risk to animal health. Nitrite and nitrate concentrations in offal did not differ between C and LN diets. In conclusion, a moderate supply of linseed and nitrate in bull feed failed to decrease enteric methane yield and impaired bull liveweight gain but without adverse effects for animal health and food safety.     

The potentiating and protective effects of ascorbate on oxidative stress depend upon the concentration of dietary iron fed C3H mice

Ascorbic acid (AA) is an antioxidant that, in the presence of iron and hydrogen peroxide, increases the production of hydroxyl radicals in vitro. Whether AA has similar pro-oxidant properties in vivo may depend upon the relative balance of iron and AA concentrations. In this study, C3H mice were fed diets supplemented with 100 or 300 mg/kg iron, with or without AA (15 g/kg), for 12 months. Liver AA concentrations were greater in mice fed AA-supplemented diets with either low or high iron (P=.0001), while the high-iron diet was associated with a significantly lower liver AA concentration regardless of AA supplementation (P=.0001). Only mice fed the high-iron diet with AA had a significantly greater liver iron concentration (P=.05). In the high-iron group, AA reduced oxidative stress, as measured by greater activities of glutathione peroxidase, superoxide dismutase (SOD) and catalase and by significantly lower concentrations of 4-hydroxylalkenal (HAE) and malondialdehyde (MDA). In mice fed the low-iron diet, AA was associated with greater concentrations of HAE and MDA and with lower activities of SOD. However, AA did not increase the concentrations of modified DNA bases with the low-iron diet but was associated with significantly lower concentrations of modified DNA bases in mice fed the high-iron diet. In conclusion, dietary AA appears to have mild pro-oxidant properties at low-iron concentrations but has a strong antioxidant effect against oxidative stress and DNA damage induced by dietary iron in mouse liver.     

Contribution of dietary arginine to nitrogen utilisation and excretion in juvenile sea bass (Dicentrarchus labrax) fed diets differing in protein source

The role of dietary arginine in affecting nitrogen utilisation and excretion was studied in juvenile European sea bass (Dicentrarchus labrax) fed for 72 days with diets differing in protein sources (plant protein-based (PM) and fish-meal-based (FM)). Fish growth performance and nitrogen utilisation revealed that dietary Arg surplus was beneficial only in PM diets. Dietary Arg level significantly affected postprandial plasma urea concentrations. Hepatic arginase activity increased (P<0.05) in response to dietary Arg surplus in fish fed plant protein diets; conversely ornithine transcarbamylase activity was very low and inversely related to arginine intake. No hepatic carbamoyl phosphate synthetase III activity was detected. Dietary arginine levels did not affect glutamate dehydrogenase activity. A strong linear relationship was found between liver arginase activity and daily urea-N excretion. Dietary Arg excess reduced the proportion of total ammonia nitrogen excreted and increased the contribution of urea-N over the total N excretion irrespective of dietary protein source. Plasma and excretion data combined with enzyme activities suggest that dietary Arg degradation via hepatic arginase is a major pathway for ureagenesis and that ornithine-urea cycle is not completely functional in juvenile sea bass liver.     

Amino acid utilization and body composition of growing pigs fed processed soybean meal or rapeseed meal with or without amino acid supplementation

Feed ingredients used in swine diets are often processed to improve nutritional value. However, (over-)processing may result in chemical reactions with amino acids (AAs) that decrease their ileal digestibility. This study aimed to determine effects of (over-)processing of soybean meal (SBM) and rapeseed meal (RSM) on post-absorptive utilization of ileal digestible AAs for retention and on body AA composition of growing pigs. Soybean meal and RSM were processed by secondary toasting in the presence of lignosulfonate to obtain processed soybean meal (pSBM) and processed rapeseed meal (pRSM). Four diets contained SBM, pSBM, RSM or pRSM as sole protein source. Two additional diets contained pSBM or pRSM and were supplemented with crystalline AA to similar standardized ileal digestible (SID) AA level as the SBM or RSM diet. These diets were used to verify that processing affected AA retention by affecting ileal AA digestibility rather than post-absorptive AA utilization. The SID AA levels of the protein sources were determined in a previous study. In total, 59 pigs were used (initial BW of 15.6±0.7 kg) of which five were used to determine initial body composition at the start of the experiment. In total, 54 pigs were fed one of six experimental diets and were slaughtered at a BW of 40 kg. The organ fraction (i.e. empty organs plus blood) and carcass were analyzed separately for N and AA content. Post-absorptive AA utilization was calculated from AA retention and SID AA intake. An interaction between diet type, comprising effects of processing and supplementing crystalline AA, and protein source was observed for CP content in the organ fraction, carcass and empty body and for nutrient retention. Processing reduced CP content and nutrient retention more for SBM than for RSM. Moreover, processing reduced (P<0.001) the lysine content in the organ fraction for both protein sources. Supplementing crystalline AA ameliorated the effect of processing on these variables. Thus, the data indicated that processing affected retention by reducing digestibility. Correcting AA retention for SID AA intake was, therefore, expected to result in similar post-absorptive AA utilization which was observed for the RSM diets. However, post-absorptive AA utilization was lower for the pSBM diet than for the SBM diet which might be related to an imbalanced post-absorptive AA supply. In conclusion, processing negatively affected nutrient retention for both protein sources and post-absorptive utilization of SID AA for retention for SBM. Effects of processing were compensated by supplementing crystalline AA.     

Carbohydrate quantitative digestion and absorption in ruminants: from feed starch and fibre to nutrients available for tissues

Carbohydrates are the main source of energy in ruminants. Their site, extent and kinetics of digestion highly impact the amount and profile of nutrients delivered to peripheral tissues, and the responses of the animal, i.e. ingestion, efficiency of production, N and methane excretion, quality of products and welfare. Development of multi-objective feed evaluation systems thus requires a more integrated quantitative knowledge on carbohydrate digestion and yield of terminal products, as well as on their metabolism by splanchnic tissues. The objective of this paper is to review (i) quantitative knowledge on fibre, starch and sugar digestion, volatile fatty acids (VFA) and glucose production and splanchnic metabolism and (ii) modelling approaches which aim at representing and/or predicting nutrient fluxes in the digestive tract, portal and hepatic drainage. It shows that the representation of carbohydrate digestion and VFA yield is relatively homogeneous among models. Although published quantitative comparisons of these models are scarce, they stress that prediction of fibre digestion and VFA yield and composition is still not good enough for use in feed formulation, whereas prediction of microbial N yield and ruminal starch digestion seems to be more satisfactory. Uncertainties on VFA stoichiometric coefficients and absorption rates may partly explain the poor predictions of VFA. Hardly any mechanistic models have been developed on portal-drained viscera (PDV) metabolism whereas a few exist for liver metabolism. A qualitative comparison of these models is presented. Most are focused on dairy cows and their level of aggregation in the representation of nutrient fluxes and metabolism highly differs depending on their objectives. Quantitative comparison of these models is still lacking. However, recent advances have been achieved with the empirical prediction of VFA and glucose production and fluxes through PDV and liver based on the current INRA feed evaluation system. These advances are presented. They illustrate that empirical prediction of ruminal VFA and intestinal glucose production can be evaluated by comparison with measured net portal net fluxes. We also illustrate the potential synergy between empirical and mechanistic modelling. It is concluded that concomitant empirical and mechanistic approach may likely help to progress towards development of multi-objective feed evaluation systems based on nutrient fluxes.     

The effect of jugular or abomasal infusion of amino acids on milk yield in lactating cows fed a protein deficient diet

The influence of first-pass splanchnic metabolism was investigated by comparing the response of 5 lactating cows to an infusion of an amino acid mixture into the abomasum or a jugular vein over 5 d according to a complete block design. The basal diet and the amino acid infusion provided 71% and 14% of crude protein requirements, respectively. The jugular infusion increased (P = 0.01) milk yield by 0.80 kg in comparison to the abomasal infusion, but milk protein yield was not altered. The jugular infusion tended to increase (P = 0.06) the arterial concentration of total essential amino acids by 11% relatively to the abomasal infusion. Mammary plasma flow and net fluxes of amino acids and glucose were not affected by the infusion sites. Variations in essential amino acid concentrations suggest that splanchnic metabolism alters peripheral delivery of amino acids but the recirculation of amino acids within the animal decreased the impact of the first-pass splanchnic metabolism on lactational performances.     

Nitrogen supply in cattle coupled with appropriate supply of utilisable crude protein at the duodenum,a precursor to metabolisable protein

The overall objective of this study was to calculate the amount of nitrogen (N) that cattle feed must contain in order to utilise the potential supply of utilisable crude protein at the duodenum provided by their energy intake without incurring a negative N balance, that is, without having to break down body protein. For this purpose, the literature was screened for measurements of net degradation and renal excretion of urea as well as N balances (N intake, faecal N and urinary N) in ruminants (cattle, sheep and goats) fed diets with varying N concentrations. Irreversible loss of N from the body urea pool increased with increasing N intake, but net degradation of urea as a proportion of irreversible loss decreased concurrently. Faecal N appeared not to be influenced by N intake and exceeded 11 g/kg dry matter intake (DMI) only in 7% of the data sets available. Urinary non-urea-N rarely exceeded 4 g/kg DMI and appeared independent of N intake. Urinary urea-N showed a clear dependence of N intake, and it is concluded that 1 g N/kg DMI is sufficient for compensating inevitable N losses in the form of urinary urea. In conclusion, ruminant rations should contain the following N concentrations (per kg DM) to account for obligatory losses: 11 g for compensating losses as faecal N, 4 g for compensating losses as urinary non-urea-N and 1 g for compensating inevitable losses as urinary urea-N. The derived recommendations should be helpful for limiting N excretion where this is desirable for ecological reasons.     

Hepatic mitochondrial energetics during catch-up fat with high-fat diets rich in lard or safflower oil

We have investigated whether altered hepatic mitochondrial energetics could explain the differential effects of high-fat diets with low or high ω6 polyunsaturated fatty acid content (lard vs. safflower oil) on the efficiency of body fat recovery (catch-up fat) during refeeding after caloric restriction. After 2 weeks of caloric restriction, rats were isocalorically refed with a low-fat diet (LF) or high-fat diets made from either lard or safflower oil for 1 week, and energy balance and body composition changes were assessed. Hepatic mitochondrial energetics were determined from measurements of liver mitochondrial mass, respiratory capacities, and proton leak. Compared to rats refed the LF, the groups refed high-fat diets showed lower energy expenditure and increased efficiency of fat gain; these differences were less marked with high-safflower oil than with high-lard diet. The increase in efficiency of catch-up fat by the high-fat diets could not be attributed to differences in liver mitochondrial activity. By contrast, the lower fat gain with high-safflower oil than with high-lard diet is accompanied by higher mitochondrial proton leak and increased proportion of arachidonic acid in mitochondrial membranes. In conclusion, the higher efficiency for catch-up fat on high-lard diet than on LF cannot be explained by altered hepatic mitochondrial energetics. By contrast, the ability of the high-safflower oil diet to produce a less pronounced increase in the efficiency of catch-up fat may partly reside in increased incorporation of arachidonic acid in hepatic mitochondrial membranes, leading to enhanced proton leak and mitochondrial uncoupling.     

Pathogenesis of the hypertriglyceridemia at early stages of alcoholic liver injury in the baboon

To study the mechanism of alcoholic hypertriglyceridemia, baboons were pair-fed liquid diets containing 50% of energy as ethanol or as additional carbohydrate for 5-16 months. Alcohol-fed animals developed hypertriglyceridemia and early stages of alcoholic injury, namely fatty liver with or without perivenular fibrosis. In the fasting state, the triglyceride content was sixfold higher in very low density (VLDL) and intermediate density (IDL) lipoproteins and twofold higher in low density (LDL) and high density (HDL) lipoproteins. The increase in VLDL was markedly exaggerated in the postprandial state. To investigate the source of these increases, we determined net output or removal of serum triglycerides during circulation through either splanchnic or extrasplanchnic (lower extremities) vascular beds. In the splanchnic territory, there was net output of triglycerides in VLDL and net removal from the other lipoproteins. In alcohol-fed baboons, the output of VLDL-triglycerides into the hepatic (but not into the portal) vein tripled. This increase was mainly due to production of VLDL particles that were larger and had a flotation (Sf greater than 400) different from the Sf 20-400 which predominated in controls. This was associated with increased splanchnic removal of labeled chylomicron- or VLDL-triglycerides. In the lower extremities, there was an arteriovenous difference in VLDL-triglyceride concentration and this was increased in the alcohol-fed animals. Thus, the primary mechanism of the hypertriglyceridemia in alcohol-fed baboons was increased production of large, chylomicron-like VLDL by the liver, whereas both the extrasplanchnic extraction of VLDL-triglycerides and the splanchnic extraction of triglycerides from chylomicron- and VLDL-remnants were secondarily enhanced.     

Comparison of high-protein diets and leucine supplementation in the prevention of metabolic syndrome and related disorders in mice

High-protein diets have been shown to promote weight loss, to improve glucose homeostasis and to increase energy expenditure and fat oxidation. We aimed to study whether leucine supplementation is able to mimic the alleviating effects of high-protein diets on metabolic syndrome parameters in mice fed high-fat diet.Male C57BL/6 mice were fed for 20 weeks with semisynthetic high-fat diets (20% w/w of fat) containing either an adequate (10% protein, AP) or high (50% protein, HP) amount of whey protein, or an AP diet supplemented with l-leucine corresponding to the leucine content of the HP diet (6% leucine, AP+L). Body weight and composition, energy expenditure, glucose tolerance, hepatic triacylglycerols (TG), plasma parameters as well as expression levels of mRNA and proteins in different tissues were measured. HP feeding resulted in decreased body weight, body fat and hepatic TG accumulation, as well as increased insulin sensitivity compared to AP. This was linked to an increased total and resting energy expenditure (REE), decreased feed energy efficiency, increased skeletal muscle (SM) protein synthesis, reduced hepatic lipogenesis and increased white fat lipolysis. Leucine supplementation had effects that were intermediate between HP and AP with regard to body composition, liver TG content, insulin sensitivity, REE and feed energy efficiency, and similar effects as HP on SM protein synthesis. However, neither HP nor AP+L showed an activation of the mammalian target of rapamycin pathway in SM. Leucine supplementation had no effect on liver lipogenesis and white fat lipolysis compared to AP. It is concluded that the essential amino acid leucine is able to mimic part but not all beneficial metabolic effects of HP diets.