From the biochemical pieces to the nutritional puzzle: using meta-reactions in teaching and research

We now live in an era where metabolic data are increasingly accessible and available. Analysis of this data can be done using novel techniques (e.g., machine learning and artificial intelligence) but this does not alleviate scientists to use "human intelligence". The objective of this paper is to combine the information of a large database of biochemical reactions with a method and tool to make nutritional biochemistry more accessible to nutritionists. A script was developed to extract information from a database with more than 16 000 biochemical reactions so that it can be used for "biochemical bookkeeping". A system of more than 300 meta-reactions (i.e., the outcome reaction of a series of connected individual reactions) was constructed covering a wide range of metabolic pathways for macro- and micronutrients. Meta-reactions were constructed by identifying metabolic nodes, which are inputs or outputs of a metabolic system or that serve as connection points between meta-reactions. Complete metabolic pathways can be constructed by combining and balancing the meta-reactions using a simple Excel tool. To illustrate the use of meta-reactions and the tool in the teaching of nutritional biochemistry, examples are given to illustrate how much ATP can be synthesized from glucose, either directly or indirectly (i.e., via storage and mobilization or via transfer of intermediate metabolites between tissues and generations). To illustrate how meta-reactions and the tool can be used in research, nutrient balance data of the mammary gland of a dairy cow were used to construct a plausible pathway of nutrient metabolism of the whole mammary gland. The balance data included 34 metabolites taken up or exported by the mammary gland and 39 meta-reactions were used to construct a metabolic pathway that accounted for the uptake and output of metabolites. The results highlighted the importance of the synthesis of proline from arginine and the concomitant synthesis of urea by the mammary gland. It also raised the question of whether the availability of metabolic pathways or glucose uptake would be the more limiting factor for the synthesis of NADPH required for fatty acid synthesis. The availability of an open database with biochemical reactions, the concept of meta-reactions, and the provision of a tool allow users to construct metabolic pathways, which helps acquiring a more comprehensive and integrated view of metabolism and may raise issues that may be difficult to identify otherwise.

Synthesis of milk components involves different mammary metabolism adaptations in response to net energy and protein supplies in dairy cows

Net energy for lactation (NEL) and metabolizable protein (MP) are the 2 main nutritional forces that drive synthesis of milk components. This study investigated mammary-gland metabolism in dairy cows in response to variations in the supply of NEL and MP. Four Holstein dairy cows were randomly assigned to a 4 × 4 Latin square design, in which each experimental period consisted of 14 d of dietary treatment. The diets provided 2 levels of NEL (low energy, 25.0 Mcal/d vs. high energy, 32.5 Mcal/d) and 2 levels of MP (low protein, 1,266 g/d vs. high protein, 2,254 g/d of protein digestible in the intestine) in a 2 × 2 factorial arrangement. Performance and dry matter intake (DMI) were measured during the last 5 d of each period, and the mammary net balance was measured on d 13 by collecting 6 sets of blood samples from the left carotid artery and left mammary vein. Mammary plasma flow was measured according to the Fick principle for Phe and Tyr. The mammary net balance of carbon equaled the uptake of nutrients expressed as carbon minus the output of lactose, fatty acids (FA) synthesized in the mammary gland, AA of milk protein, and glycerol-3P from triglyceride on d 13. Milk, lactose, fat, and protein yields increased when NEL and MP supplies increased. However, increasing the NEL supply increased FA synthesis more than increasing the protein supply did. In addition, FA secretion increased more than lactose secretion when the NEL supply increased. Increasing the NEL supply increased the left half-udder uptake of all major energy-yielding nutrients by increasing mammary plasma flow. However, nutrient uptake increased more than milk output did, which in turn increased carbon dioxide output. This increase in nutrient oxidation by the mammary gland decreased the mammary efficiency of nutrients utilization when the NEL supply increased. Increasing MP supply tended to increase glucose uptake through mammary clearance and increased mammary AA uptake with no change in mammary plasma flow. In addition, the protein supply did not change the mammary uptake of acetate or β-hydroxybutyrate. The increase in milk-component secretions in response to either NEL or MP supplies occurred through different metabolic adaptations (increase in mammary plasma flow vs. clearances, respectively). These results suggest that the nutrient use by the mammary gland is highly flexible, which helps in maintaining milk and milk-component yields even with limiting nutrient supplies.

Metabolic and physiological adaptations to first and second lactation in Holstein dairy cows: Postprandial patterns

Dairy cows during their first and second lactation have different milk yield, body development, feed intake, and metabolic and endocrine statuses. However, large diurnal variations can also exist in terms of biomarkers and hormones related to feeding behavior and energy metabolism. Thus, we investigated the diurnal patterns of the main metabolic plasma analytes and hormones in the same cows during their first and second lactations in different stages of the lactation cycle. Eight Holstein dairy cows were monitored during their first and second lactation, during which they were reared under the same conditions. Blood samples were collected before the morning feeding (0 h) and after 1, 2, 3, 4.5, 6, 9, and 12 h on scheduled days between -21 d relative to calving (DRC) and 120 DRC for the assessment of some metabolic biomarkers and hormones. Data were analyzed using the GLIMMIX procedure of SAS (SAS Institute Inc.). Regardless of parity and stage of lactation, glucose, urea, β-hydroxybutyrate, and insulin peaked a few hours after the morning feeding, whereas nonesterified fatty acids decreased. The insulin peak was attenuated during the first month of lactation, whereas postpartum growth hormone spiked on average 1 h after the first meal in cows during their first lactation. This peak occurred earlier than during the second lactation. Most of the differences in diurnal trends between lactations were observed in the postpartum period (and in some cases even in early lactation). Glucose and insulin were higher during the first lactation throughout the day, and the differences increased 9 h after feeding. Conversely, nonesterified fatty acids and β-hydroxybutyrate showed the opposite trend, and their plasma concentrations at 9 and 12 h after feeding differed between lactations. These results confirmed the differences observed between the first 2 lactations in prefeeding metabolic marker concentrations. Furthermore, plasma concentrations of investigated analytes showed high variability during the day, and thus we advise caution when interpreting metabolic biomarker data in dairy cows, especially during the periods close to calving.

Influence of Parturition on Rumen Bacteria and SCFAs in Holstein Cows Based on 16S rRNA Sequencing and Targeted Metabolomics

The rumen fluids from ten cows at Day 3~5 before calving and Day 0 after calving were collected to analyze the composition and quantity of bacterial communities and concentrations of SCFAs. The results showed that the relative abundances of unidentified Lachnospiraceae, Acetitomaculum, Methanobrevibacter, Olsenella, Syntrophococcus, Lachnospira, and Lactobacillus genera were significant increased (p < 0.05), while that of unidentified-Prevotellaceae was notably decreased after calving (p < 0.05). In addition, the concentrations of acetic acid, propionic acid, butyric acid, and caproic acid obviously decreased after calving (p < 0.01). Our findings show that parturition altered the rumen microbiota and their fermentation ability in dairy cows. This study defines a rumen bacteria and metabolic profile of SCFAs associated with parturition in dairy cows.

Essential amino acid profile of supplemental metabolizable protein affects mammary gland metabolism and whole-body glucose kinetics in dairy cattle

This study investigated mammary gland metabolism and whole-body (WB) rate of appearance (Ra) of glucose in dairy cattle in response to a constant supplemental level of metabolizable protein (MP) composed of different essential AA (EAA) profiles. Five multiparous rumen-fistulated Holstein-Friesian dairy cows (2.8 ± 0.4 lactations; 81 ± 11 d in milk; mean ± standard deviation) were abomasally infused according to a 5 × 5 Latin square design with saline (SAL) or 562 g/d of EAA delivered in different profiles where individual AA content corresponded to their relative content in casein. The profiles consisted of (1) a complete EAA mixture (EAAC), (2) Ile, Leu, and Val (ILV), (3) His, Ile, Leu, Met, Phe, Trp, Val (GR1+ILV), and (4) Arg, His, Lys, Met, Phe, Thr, Trp (GR1+ALT). A total mixed ration (58% corn silage, 16% alfalfa hay, and 26% concentrate on a dry matter basis) was formulated to meet 100 and 83% of net energy and MP requirements, respectively, and was fed at 90% of ad libitum intake on an individual cow basis. Each experimental period consisted of 5 d of continuous abomasal infusion followed by 2 d of no infusion. Arterial and venous blood samples were collected on d 4 of each period for determination of mammary gland AA and glucose metabolism. On d 5 of each period, D-[U-¹³C]glucose (13 mmol priming dose; continuous 3.5 mmol/h for 520 min) was infused into a jugular vein and arterial blood samples were collected before and during infusion to determine WB Ra of glucose. Milk protein yield did not differ between EAAC, GR1+ILV, and GR1+ALT, or between SAL and ILV, and increased over SAL and ILV with EAAC and GR1+ILV. Mammary plasma flow increased with ILV infusion compared with EAAC and GR1+ILV. Infusion of EAAC tended to increase mammary gland net uptake of total EAA and decreased the mammary uptake to milk protein output ratio (U:O) of non-EAA compared with SAL. Infusion of ILV increased mammary net uptake and U:O of Ile, Leu, and Val markedly over all treatments. The U:O of total Ile, Leu, and Val increased numerically (25%) with GR1+ILV infusion compared with EAAC, and the U:O of total Arg, Lys, and Thr tended to decrease, primarily from decreased U:O of Lys. During GR1+ALT infusion, U:O of total Arg, Lys, and Thr was greater than that during EAAC infusion, whereas U:O of Ile, Leu, and Val did not differ from EAAC. Glucose WB Ra increased 16% with GR1+ALT over SAL, and increased numerically 8 and 12% over SAL with EAAC and GR1+ILV, respectively. The average proportion of lactose yield relative to glucose WB Ra did not differ across treatments and averaged 0.53. On average, 28% of milk galactose arose from nonglucose precursors, regardless of treatment. In conclusion, intramammary catabolism of group 2 AA increased to support milk component synthesis when the EAA profile of MP was incomplete with respect to casein. Further, WB and mammary gland glucose metabolism was flexible in support of milk component synthesis, regardless of absorptive EAA profile.

Effect of timing of paddock allocation in tropical grass on performance, nitrogen excretion, and enteric methane emissions from dairy cows

The objective of this study was to investigate the influence of timing of paddock allocation (AM or PM) in tropical grass on nutritive value of the herbage, dry matter intake (DMI), milk yield and composition, ruminal fermentation, nitrogen excretion, and enteric CH₄ emissions of dairy cows. Twenty cows were grouped in pairs and randomly distributed within pair to one of two treatments. PM herbage had greater contents of dry matter (DM), soluble carbohydrates, starch, and nonfibrous carbohydrate to protein ratio and lower contents of neutral detergent fiber and acid detergent fiber. There was no treatment effect on DMI, N excretion, milk yield, and CH₄ emissions. However, milk protein and casein yields tended to be greater for PM than AM, while milk urea nitrogen concentration was lower for PM than AM. The increase in nutritive value of the afternoon relative to the morning herbage within the framework of this study was not large enough to increase DMI and milk yield or to decrease CH₄ emission intensity by the dairy cows as hypothesized. The findings indicate that PM treatment can be a simple and useful grazing strategy that results in an herbage harvest with greater nutritional value and in lower excretion of urea N into milk.

Plasma essential amino acid concentration and profile are associated with performance of lactating dairy cows as revealed through meta-analysis and hierarchical clustering

Our aim was to explore whether changes in plasma essential AA (EAA) concentration ([EAA]_p) or profile (defined here as the molar proportion of individual [EAA]_p relative to the total [EAA]_p) may serve as an indicator of the EAA status of a cow. We undertook a meta-analysis with the objectives to determine if different plasma EAA profiles exist among cows and to explore the association of [EAA]_p or the profile of EAA with lactating cow performance and measures of N utilization. We hypothesized the existence of differences in [EAA]_p and different plasma EAA profile for cows with greater milk output, feed efficiency, and greater N use efficiency (NUE; milk true protein-N:N intake) compared with cows with lower milk output, feed efficiency, and lower NUE. The data set included 22 feeding trials and 96 dietary treatments. First, a mixed-effect model analysis was used to predict [EAA]_p in response to the categorical fixed effect of EAA, continuous fixed effect of National Research Council model-predicted metabolizable protein (MP) supply, continuous fixed effect of body weight, the fixed effect of EAA and MP supply interaction, the fixed effect of EAA and body weight interaction, and the random effect of study. Then, residuals of the model were standardized based on Z-score and clustered using the hierarchical method (Euclidean distance and Ward's minimum variance method) resulting in 2 clusters. Finally, a fixed-effect model was used to evaluate the significance with which clusters were associated with [EAA]_p, cow performance, feed efficiency, and NUE. The total concentration of [EAA]_p was lower (784 vs. 983 µM) and the concentration of each EAA was on average 22 µM lower for cows in cluster 1 compared with cluster 2 with the smallest and greatest difference found for Met (4 µM) and Val (59 µM), respectively. The percentage difference in [EAA]_p was the smallest for Thr (-5.3%) and the greatest for Leu (-37.1%). There was no difference between clusters for Arg, His, and Met molar proportions; however, cows in cluster 1 had a lower molar proportion of Leu and a tendency for lower molar proportion of Val compared with cows in cluster 2. Additionally, cows in cluster 1 had greater molar proportions of Ile, Lys, and Thr and a tendency for greater molar proportion of Phe compared with cows in cluster 2. The fixed-effect model analysis indicated that cows in cluster 1 had higher milk energy output (+3.2 Mcal/d), true protein yield (+87 g/d) and fat yield (+236 g/d), feed efficiency (milk Mcal:dry matter intake; +8% unit), and a tendency for greater MP efficiency (Milk true protein/MP supply; +2.3% unit) than cows in cluster 2. These results suggested greater use of EAA by the mammary gland (as reflected by greater milk protein synthesis) and lower hepatic catabolism of AA (as reflected by a tendency to greater MP efficiency) in cows of cluster 1 compared with cluster 2. Our findings should be evaluated further, including whether the relative molar proportions of plasma EAA might serve as a holistic indicator of the EAA status of cows as related to their productivity, feed efficiency and N utilization.

Effects of branched-chain amino acids on glucose uptake and lactose synthesis rates in bovine mammary epithelial cells and lactating mammary tissue slices

Even though supplementations of essential AA (EAA) are often related to increased lactose yields in dairy cows, underlying mechanisms connecting EAA availability to the mammary glands and lactose synthesis are poorly understood. The objective of this study was to examine the effects of branched-chain AA (BCAA) including Leu, Ile, and Val on (1) glucose transporter (GLUT1) abundance and glucose uptake, (2) the abundance of proteins regulating lactose synthesis pathway, and (3) fractional synthesis rates of lactose (FSR) using bovine mammary epithelial cells (BMEC) and mammary tissues slices (MTS). The BMEC (n = 4) were allocated randomly to regular Dulbecco's Modified Eagle Medium with Ham's F12 (DMEM/F12) media (+EAA) or +EAA deficient (by 90%) in all EAA (-EAA), all BCAA (-BCAA), only Leu (-Leu), only Ile (-Ile) or only Val (-Val). Western immunoblotting analyses, depletion of glucose in media, and a proteomic analysis were performed to determine the abundance of GLUT1 in the cell membrane, net glucose uptake, and the abundance of enzymes involved in lactose synthesis pathway in BMEC, respectively. The MTS (n = 6) were allocated randomly to DMEM/F12 media having all EAA and ¹³C-glucose at concentrations similar to plasma concentrations of cows (+EAA_p), and +EAA_p deprived of all BCAA (-BCAA_p) or only Leu (-Leu_p) for 3 h. The ¹³C enrichments of free glucose pool in MTS (E_Glu-free) and the enrichments of glucose incorporated into lactose in MTS and media [E_{Lactose-bound (T&M)}] were determined and used in calculating FSR. In BMEC, -BCAA increased the fraction of total GLUT1 translocated to the cell membrane and the fraction that was potentially glycosylated compared with +EAA. Among individual BCAA, only -Leu was associated with a 63% increase in GLUT1 translocated to the cell membrane and a 40% increase in glucose uptake of BMEC. The -BCAA tended to be related to a 75% increase in the abundance of hexokinase in BMEC. Deprivation of Leu tended to increase glucose uptake of MTS but did not affect E_Glu-free, E_{Lactose-bound (T&M)}, or FSR relative to +EAA_p. On the other hand, -BCAAp did not affect glucose uptake of MTS but was related to lower E_{Lactose-bound (T&M)}, or FSR relative to +EAA_p. Considering together, decreasing Leu supply to mammary tissues enhances GLUT1 and thus glucose uptake, which, however, does not affect lactose synthesis rates. Moreover, the deficiency of other BCAA, Ile, and Val alone or together with the deficiency of Leu seemed to decrease lactose synthesis rates without affecting glucose uptake. The data also emphasize the importance of addressing the effect of the supply of other nutrients to the mammary glands than the precursor supply in describing the synthesis of a milk component.

Regulation of Milk Protein Synthesis by Free and Peptide-Bound Amino Acids in Dairy Cows

Milk protein (MP) synthesis in the mammary gland of dairy cows is a complex biological process. As the substrates for protein synthesis, amino acids (AAs) are the most important nutrients for milk synthesis. Free AAs (FAAs) are the main precursors of MP synthesis, and their supplies are supplemented by peptide-bound AAs (PBAAs) in the blood. Utilization of AAs in the mammary gland of dairy cows has attracted the great interest of researchers because of the goal of increasing MP yield. Supplying sufficient and balanced AAs is critical to improve MP concentration and yield in dairy cows. Great progress has been made in understanding limiting AAs and their requirements for MP synthesis in dairy cows. This review focuses on the effects of FAA and PBAA supply on MP synthesis and their underlying mechanisms. Advances in our knowledge in the field can help us to develop more accurate models to predict dietary protein requirements for dairy cows MP synthesis, which will ultimately improve the nitrogen utilization efficiency and lactation performance of dairy cows.

Research on the Applications of Calcium Propionate in Dairy Cows: A Review

Calcium propionate is a safe and reliable food and feed additive. It can be metabolized and absorbed by humans and animals as a precursor for glucose synthesis. In addition, calcium propionate provides essential calcium to mammals. In the perinatal period of dairy cows, many cows cannot adjust to the tremendous metabolic, endocrine, and physiological changes, resulting in ketosis and fatty liver due to a negative energy balance (NEB) or milk fever induced by hypocalcemia. On hot weather days, cow feed (TMR or silage) is susceptible to mildew, which produces mycotoxins. These two issues are closely related to dairy health and performance. Perinatal period metabolic disease significantly reduces cow production and increases the elimination rate because it causes major glucose and calcium deficiencies. Feeding a diet contaminated with mycotoxin leads to rumen metabolic disorders, a reduced reproductive rate (increased abortion rate), an increased number of milk somatic cells, and decreased milk production, as well as an increased occurrence of mastitis and hoof disease. Propionic acid is the primary gluconeogenic precursor in dairy cows and one of the safest mold inhibitors. Therefore, calcium propionate, which can be hydrolyzed into propionic acid and Ca2+ in the rumen, may be a good feed additive for alleviating NEB and milk fever in the perinatal period of dairy cows. It can also be used to inhibit TMR or silage deterioration in hot weather and regulate rumen development in calves. This paper reviews the application of calcium propionate in dairy cows.

Post-ruminal supplies of glucose and casein, but not acetate, stimulate milk protein synthesis in dairy cows through differential effects on mammary metabolism

Amino acids and glucose have been shown to regulate protein synthesis in the mammary gland through their effects on cellular signaling pathways. Acetate might also have an effect on protein synthesis via the AMP-activated kinase signaling pathway, because it is the main energy source for the mammary secretory cell. Thus, the objective of this experiment was to evaluate the effects of casein and energy-yielding nutrients (acetate and glucose), and their combination, on performance and mammary metabolism. Six multiparous Holstein cows, averaging 49 kg of milk/d, were used in a 6 × 6 Latin square design with 14-d periods. Cows were fed to 100% National Research Council requirements for metabolizable protein (MP) and energy (ME) for 9 d, after which they were feed-restricted for 5 d to 85% of their individual ad libitum intake and then abomasally infused with 1 of 6 treatments. Treatments were acetate (A), glucose (G), each at 5% of ad libitum ME intake, casein (C) at 15% of ad libitum MP intake, A + C, G + C, or a saline solution (negative control). Casein infused alone increased milk protein yield numerically, with 25% recovery of the infused casein in milk protein. Glucose infused alone increased milk and milk protein yield and promoted the highest efficiency of nitrogen utilization (37%), with an efficiency of MP use for milk protein of 58%. We discovered no effect of treatment on mammary plasma flow, and the increase in milk protein yield with glucose infusion was brought about by greater mammary AA clearance rate. Infusion of casein and glucose together further increased milk protein yield in an additive fashion, and 47% of the infused casein was recovered in milk protein. Acetate infused alone had no effect on milk protein yield but increased milk fat yield numerically, suggesting that the greater amount of acetate taken up by the mammary gland was used for milk fat synthesis. Infusion of acetate and casein together yielded responses similar to those of casein alone. In conclusion, glucose has a major effect on stimulating milk protein synthesis, and the mammary gland has the ability to increase its supply of nutrients to match its synthetic capacity.

Metagenomic Insights into Effects of Thiamine Supplementation on Carbohydrate-Active Enzymes' Profile in Dairy Cows Fed High-Concentrate Diets

As the co-enzyme of pyruvate formate-lyase under ruminal anaerobic condition, thiamine plays a critical role in carbohydrate metabolism in dairy cows. The objective of this study was to investigate the impacts of thiamine supplementation on ruminal carbohydrate-active enzymes. Twelve Holstein dairy cows were randomly assigned into three dietary treatments: control diet (CON; 20% starch, dry matter (DM) basis), high-concentrate diet (HC; 33.2% starch, DM basis) and a high-concentrate diet supplemented with 180 mg thiamine/kg DM (HCT; 33.2% starch, DM basis). Dry matter intake and milk production were recorded for 21 days. Rumen fluid samples were collected, and ruminal pH and volatile fatty acids (VFAs) were measured. The metagenome sequencing technique was used to detect the genes in ruminal microorganisms and identify putative carbohydrate-active enzymes. The total abundances of carbohydrate-active enzymes and fiber-degrading enzymes were both reduced by HC with no effect on starch-degrading enzymes compared with CON. However, the fiber-degrading enzymes and starch-degrading enzymes were both increased after thiamine supplementation. These results indicated that 180 mg thiamine /kg DM might effectively improve rumen carbohydrate metabolism through increasing the abundance of ruminal carbohydrate-active enzymes and consequently balanced the rumen volatile fatty acids and rumen pH, providing a practical strategy in preventing subacute ruminal acidosis in cows offered HC.

Temporal effects of ruminal infusion of propionic acid on hepatic metabolism in cows in the postpartum period

A faster rate of infusion of propionic acid into the rumen of cows in the postpartum period increased meal size compared with a slower rate of infusion in a previous experiment. Because propionate is anaplerotic and stimulates oxidation of acetyl coenzyme A (CoA) in the liver, and hepatic oxidation has been linked to satiety, this result was opposite to our expected response. We then hypothesized that the faster rate of infusion might have saturated the pathway for propionate metabolism in hepatocytes resulting in lower first-pass extraction by the liver. Because we were measuring feeding behavior, we could not sample blood and liver tissue over time in that experiment. Therefore, to determine the temporal effects of propionic acid (PA) infusion on hepatic metabolism and plasma metabolites over the time course of a meal, we infused 1.25 mol of PA (2.5 L of 0.5M PA) over 5 min (FST) or 15 min (SLW) into the rumen. We evaluated response to PA infusions both before feeding, when ruminal PA production by rumen microbes is lower and hepatic acetyl CoA concentration is greater, and 4 h after feeding, when PA production is greater and hepatic acetyl CoA concentration is lower. Blood and liver samples were collected before, and after 5, 15, and 30 min of infusion. Contrary to our hypothesis, the rate of PA infusion into the rumen did not affect plasma propionate concentration, indicating the FST effects on feeding behavior were not because of a limitation on propionate uptake by the liver. However, FST increased plasma glucose and insulin concentrations faster than SLW, resulting in a reduction in plasma nonesterified fatty acid concentration during the time frame of meals. Decreased plasma nonesterified fatty acid concentration during infusion likely decreased the supply of acetyl CoA for oxidation in the liver. The FST treatment also increased fumarate concentration at 5 min after the initiation of infusion but did not affect oxaloacetate concentration compared with SLW, consistent with a limitation to propionate metabolism at that reaction. A metabolic bottleneck at the malate dehydrogenase reaction for FST compared with SLW would further contribute to a reduction in hepatic oxidation within the time frame of a meal, allowing greater meal size, consistent with the hepatic oxidation theory and our previous results.

Effects of jugular infusions of isoleucine, leucine, methionine, threonine, and other amino acids on insulin and glucagon concentrations, mammalian target of rapamycin (mTOR) signaling, and lactational performance in goats

The supply and profile of absorbed AA may affect milk protein synthesis through hormonal changes and mammalian target of rapamycin (mTOR) signaling pathways; and Ile, Leu, Met, and Thr (ILMT) are the 4 AA that have been reported to have the greatest effect on mammary mTOR signaling. The extent to which ILMT and the other remaining AA (RAA) differ in their effects on milk protein synthesis needs to be systematically investigated. In this study, 5 lactating goats, averaging 120 ± 10 d in milk, fitted with jugular vein and carotid artery catheters, were fasted for 24 h, followed by intravenous infusions of a mixture containing AA and glucose for 8 h in a 5 × 5 Latin square design. The AA mixtures were formulated according to the profile of casein. The amounts of AA infused were calculated based on supplies of AA when metabolizable protein (MP) was at requirement (MR). Treatments were an infusate containing glucose without AA (NTAA); an infusate containing 3 × the MR of Ile, Leu, Met and Thr (3F0R); and infusates containing 3F0R plus 1, 2, or 3 × MR of RAA (3F1R, 3F2R, and 3F3R, respectively) according to amounts provided when fed to meet MP requirements for maintenance and lactation for each goat. Milk, arterial blood, and mammary tissue samples were collected immediately after halting the infusion. Relative to NTAA, supplementation of ILMT tended to increase milk protein production and plasma glucose concentrations, and increased milk and lactose production, but had no effects on production or content of milk fat. Graded supplementation of RAA tended to quadratically affect production of milk and lactose. Arterial glucose and glucagon concentrations decreased linearly, and plasma insulin concentrations decreased quadratically with increased RAA. Mammary p70-S6K1 phosphorylation was decreased by addition of ILMT compared with NTAA but increased linearly with increased RAA infusion. Furthermore, EIF4EBP1 gene expression was much lower for 3F-treated goats than for the NTAA treatment. Both MTOR and RPS6KB1 gene expressions were decreased quadratically with increased RAA supply. These results suggested that short-term milk protein yield tended to be increased by elevated ILMT availability, and this trend was not explained by variations in mammary mTOR signaling or pancreatic hormone secretions, whereas graded increase of RAA in combination with ILMT appeared to regulate the efficiency of conversion of glucose to lactose in a manner not involving milk protein production.

Mammary gland metabolite utilization in response to exogenous glucose or long-chain fatty acids at low and high metabolizable protein levels

We investigated mammary gland metabolism in lactating dairy cattle in response to energy from glucogenic (glucose; GG) or lipogenic (palm olein; LG) substrates at low (LMP) and high (HMP) metabolizable protein levels. According to a 6 × 6 Latin square design, 6 rumen-fistulated second-lactation Holstein-Friesian dairy cows (97 ± 13 d in milk) were abomasally infused with saline (LMP-C); isoenergetic infusions (digestible energy basis) of 1,319 g/d glucose (LMP-GG), 676 g/d palm olein (LMP-LG), or 844 g/d essential AA (EAA; HMP-C); or isoenergetic infusions of 1,319 g/d glucose + 844 g/d EAA (HMP-GG) or 676 g/d palm olein + 844 g/d EAA (HMP-LG). Each experimental period consisted of 5 d of continuous infusion followed by 2 d of rest. A total mixed ration (42% corn silage, 31% grass silage, and 27% concentrate on a dry matter basis) formulated to meet 100 and 83% of net energy and metabolizable protein requirements, respectively, was fed at 90% of ad libitum intake by individual cow. Arterial and venous blood samples were collected on d 5 of each period. Infusing GG or LG at the HMP level did not affect milk yield or composition differently than at the LMP level. Neither GG nor LG infusion stimulated milk protein or lactose yield, but fat yield tended to decrease with GG and tended to increase with LG. Infusion of GG increased arterial plasma concentrations of glucose and insulin and decreased concentrations of β-hydroxybutyrate (BHB), nonesterified fatty acids, long-chain fatty acids (LCFA), total AA, EAA, and group 2 AA. Infusion of LG increased arterial triacylglycerides (TAG) and LCFA but did not affect EAA concentrations. Compared with the LMP level, the HMP level increased arterial concentrations of BHB, urea, and all EAA groups and decreased the concentration of total non-EAA. Mammary plasma flow increased with GG and was not affected by LG or protein level. Uptake and clearance of total EAA and group 2 AA were affected or tended to be affected by GG × AA interactions, with their uptakes being lower and their clearances higher with GG, but only at the LMP level. Infusion of LG did not affect uptake or clearance of any AA group. The HMP level increased uptake and decreased clearance of all EAA groups and decreased non-EAA uptake. Infusion of GG tended to increase mammary glucose uptake, and tended to decrease BHB uptake only at the LMP level. Infusion of LG increased mammary uptake of TAG and LCFA and increased or tended to increase clearance of TAG and LCFA. We suspect GG increased mammary plasma flow to maintain intramammary energy and AA balance and stimulated lipogenesis in adipose, accounting for depressed arterial BHB and group 2 AA concentrations. Mammary glucose uptake did not cover estimated requirements for lactose and fat synthesis at the HMP level, except during HMP-GG infusion. Results of this study illustrate flexibility in mammary metabolite utilization when absorptive supply of glucogenic, lipogenic, and aminogenic substrate is increased.

Mammary gland utilization of amino acids and energy metabolites differs when dairy cow rations are isoenergetically supplemented with protein and fat

Mammary gland utilization of AA and other metabolites in response to supplemental energy from protein (PT) and supplemental energy from fat (FT) was tested in a 2 × 2 factorial arrangement using a randomized complete block design. Fifty-six Holstein-Friesian dairy cows were adapted during a 28-d control period to a basal total mixed ration consisting of 34% grass silage, 33% corn silage, 5% grass hay, and 28% concentrate on a dry matter (DM) basis. Experimental rations were fed for 28 d immediately following the control period and consisted of (1) low protein, low fat (LP/LF), (2) high protein, low fat (HP/LF), (3) low protein, high fat (LP/HF), and (4) high protein, high fat (HP/HF). To obtain the high-protein (HP) and high-fat (HF) diets, intake of the basal ration was restricted and supplemented isoenergetically [net energy (MJ/d) basis] with 2.0 kg/d rumen-protected protein (soybean + rapeseed, 50:50 mixture on a DM basis) and 0.68 kg/d hydrogenated palm fatty acids on a DM basis. Arterial and venous blood samples were collected on d 28 of both periods. Isoenergetic supplements (MJ/d) of protein and fat independently and additively increased milk yield, PT increased protein yield, and FT increased fat yield. A PT × FT interaction affected arterial concentration of all essential AA (EAA) groups, where they increased in response to PT by a greater magnitude at the LF level (on average 35%) compared with the HF level (on average 14%). Mammary gland plasma flow was unaffected by PT or FT. Supplementation with PT tended to decrease mammary clearance of total EAA and decreased group 1 AA clearance by 19%. In response to PT, mammary uptake of total EAA and group 2 AA increased 12 and 14%, respectively, with significantly higher uptake of Arg, Ile, and Leu. Energy from fat had no effect on mammary clearance or uptake of any AA group. The mammary gland uptake:milk protein output ratio was not affected by FT, whereas PT increased this ratio for EAA and group 2 AA. Arterial plasma insulin concentration decreased in response to FT, in particular on the HP/HF diet, as indicated by a PT × FT interaction. Arterial concentrations of nonesterified fatty acids, triacylglycerol, and long-chain fatty acids increased in response to FT, and concentrations of β-hydroxybutyrate and acetate decreased in response to FT only at the HP level. Mammary clearance and uptake of triacylglycerol and long-chain fatty acids increased in response to FT. Energy from PT and FT increased lactose yield despite no change in arterial glucose concentration or mammary glucose uptake. Mammary-sequestered glucose with PT or FT was used in the same amount for lactose synthesis, and a positive net mammary glucose balance was found across all treatments. Results presented here illustrate metabolic flexibility of the mammary gland in its use of aminogenic versus lipogenic substrates for milk synthesis.

Local Mammary Glucose Supply Regulates Availability and Intracellular Metabolic Pathways of Glucose in the Mammary Gland of Lactating Dairy Goats Under Malnutrition of Energy

As glucose is the regulator of both the milk yield and mammary oxidative status, glucose supply is considered to play important nutritional and physiological role on mammary gland (MG) metabolism. However, inconsistent results were observed from different infusion methods to evaluate the effect of glucose on MG glucose metabolism. Thus, precise method should be developed to learn how availability and intracellular metabolic pathways of glucose in the MG are altered by the direct mammary glucose supply. In addition, limited information is available on the role of mammary glucose supply in milk synthesis in lactating ruminants under an energy-deficient diet. Direct glucose supply to the MG was implemented in the current study through the external pudendal artery infusion under an energy-deficient diet. Six doses of glucose (0, 20, 40, 60, 80, and 100 g/d) were infused through the external pudendal arteries, which is the main artery to the MG, to six lactating goats fed with basal diet meeting 81% energy requirement in a 6 × 6 Latin square design. Milk and lactose yields were both quadratically increased with increased glucose infusion, whereas the milk yield changed inconsistently with the increased energy balance (EB), indicating local glucose supply, rather than EB, improved milk production. Glucose fluxes in the MG were significantly increased and correlated with mammary plasma flow. However, the ratio of lactose yield to glucose absorbed by the MG was significantly decreased. The increased glucose fluxes in the MG and changed glucose-related metabolites in milk indicated that the glucose availability and intracellular metabolic pathways was regulated by local mammary glucose. Acute glycolysis consumed the superfluous glucose and induced accumulation of oxygen radicals in the MG during over-supplied glucose conditions. The present study provided insight to optimal glucose supply to the MG during the lactation.

Effect of replacing calcium salts of palm oil with camelina seed at 2 dietary ether extract levels on digestion, ruminal fermentation, and nutrient flow in a dual-flow continuous culture system

Camelina is a drought- and salt-tolerant oil seed, which in total ether extract (EE) contains up to 74% polyunsaturated fatty acids. The objective of this study was to assess the effects of replacing calcium salts of palm oil (Megalac, Church & Dwight Co. Inc., Princeton, NJ) with camelina seed (CS) on ruminal fermentation, digestion, and flows of fatty acids (FA) and AA in a dual-flow continuous culture system when supplemented at 5 or 8% dietary EE. Diets were randomly assigned to 8 fermentors in a 2 × 2 factorial arrangement of treatments in a replicated 4 × 4 Latin square design, with four 10-d experimental periods consisting of 7 d for diet adaptation and 3 d for sample collection. Treatments were (1) calcium salts of palm oil supplementation at 5% EE (MEG5); (2) calcium salts of palm oil supplementation at 8% EE (MEG8); (3) 7.7% CS supplementation at 5% EE (CS5); and (4) 17.7% CS supplementation at 8% EE (CS8). Diets contained 55% orchardgrass hay, and fermentors were fed 72 g of dry matter/d. On d 8, 9, and 10 of each period, digesta effluent samples were taken for ruminal NH₃, volatile fatty acids, nitrogen metabolism analysis, and long-chain FA and AA flows. Statistical analysis was performed using the MIXED procedure (SAS Institute Inc., Cary, NC). We detected an interaction between FA source and dietary EE level for acetate, where MEG8 had the greatest molar proportion of acetate. Molar proportions of propionate were greater and total volatile fatty acids were lower on CS diets. Supplementation of CS decreased overall ruminal nutrient true digestibility, but dietary EE level did not affect it. Diets containing CS had greater biohydrogenation of 18:2 and 18:3; however, biohydrogenation of 18:1 was greater in MEG diets. Additionally, CS diets had greater ruminal concentrations of trans-10/11 18:1 and cis-9,trans-11 conjugated linoleic acid. Dietary EE level at 8% negatively affected flows of NH₃-N (g/d), nonammonia N, and bacterial N as well as the overall AA outflow. However, treatments had minor effects on individual ruminal AA digestibility. The shift from acetate to propionate observed on diets containing CS may be advantageous from an energetic standpoint. Moreover, CS diets had greater ruminal outflow of trans-10/11 18:1 and cis-9,trans-11 conjugated linoleic acid than MEG diets, suggesting a better FA profile available for postruminal absorption. However, dietary EE at 8% was deleterious to overall N metabolism and AA outflow, indicating that CS can be fed at 5% EE without compromising N metabolism.

Plasma and Pancreas Islet Hormone Concentrations in Lactating Rats Are Associated with Dietary Protein Amounts

Background

Circulating amino acid (AA) and nitric oxide (NO) concentrations and hepatic gluconeogenesis are affected by previous protein intake. However, information about their relations and islet hormone responses is limited.

Objective

This study investigated the associations between islet hormone concentrations with circulating AA and NO concentrations as well as with hepatic gluconeogenesis in lactating rats.

Methods

At delivery, 18 Wistar rats aged 14 wk were assigned either to low-protein (LP; 9% protein), standard-protein (SP; 21% protein), or high-protein (HP; 35% protein) diets for 15 d in groups of 6 pups/dam. Circulating AA and NO concentrations, circulating and pancreas islet hormone concentrations, and the activities and gene expressions of hepatic phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) were measured at the end of treatment.

Results

Circulating insulin and glucagon concentrations were greater in the HP than in the LP (25% and 17%, respectively) and SP (37% and 31%) diet groups, whereas compared with the SP group, pancreatic concentrations were lower in the LP (32% and 49%) and HP (34% and 46%) groups (P < 0.01). Hepatic PEPCK and G6Pase activities in the HP group were greater than those in the SP (15% and 15%) and LP (8% and 19%) groups (P < 0.05). In all groups, plasma NO concentrations were correlated negatively to circulating insulin (r = -0.77, P = 0.0003) and positively to pancreas insulin and glucagon concentrations and the insulin-to-glucagon ratio (r = 0.50-0.63; P < 0.05). Some circulating AAs correlated positively to circulating insulin and pancreas insulin and glucagon (r = 0.50-0.82, P < 0.05) but negatively to circulating glucagon (r = -0.53-0.68, P < 0.05).

Conclusion

Variations in circulating AA and NO concentrations and hepatic gluconeogenic enzyme activities are likely intermediary responses involved in the effects of dietary protein amounts on the synthesis and secretion of islet hormones in lactating rats.

The effect of long term under- and overfeeding on the expression of genes related to glucose metabolism in the mammary tissue of goats

The glucose metabolism in the mammary tissue (MT) is controlled by several genes whose nutritional regulation is poorly documented. In this study, the effect of long-term under- and overfeeding on the expression of glucose transporter 1 (GLUT1), glucose transporter 3 (GLUT3), sodium glucose contransporter 1 (SGLT1), two isoforms of β- (1, 4) galactosyltransferase [β- (1, 4) GAT1 and β- (1, 4) GAT3], and α-lactalbumin (LALBA), related to glucose metabolism in goat MT, was examined by using real-time RT-PCR. Twenty- four lactating goats were divided into three homogenous subgroups and fed the same ration in quantities covering 70% (underfeeding), 100% (control) and 130% (overfeeding) of their energy and crude protein requirements, respectively. The results showed that the feeding level did not affect the GLUT1, GLUT3 and SGLT1 genes expression in goats MT. On the contrary, a significant reduction and a trend for reduction on mRNA of β- (1, 4) GAT1 and on β- (1, 4) GAT3, respectively, in the MT of underfed goats, compared with the overfed ones, was observed. Moreover, a significant decrease in the LALBA mRNA accumulation in the MT of underfed goats compared with the overfed was found. In conclusion, the MT of goats, unlike cows, adapt to changes in glucose or energy supply from different levels of feeding by changing the utilisation of glucose for the synthesis of lactose.

Abomasal amino acid infusion in postpartum dairy cows: Effect on whole-body, splanchnic, and mammary glucose metabolism

Nine Holstein cows fitted with rumen cannulas and indwelling catheters in splanchnic blood vessels were used to study the effects of supplementing AA on milk lactose secretion, whole-body rate of appearance (WB-Ra) of glucose, and tissue metabolism of glucose, lactate, glycerol, and β-OH-butyrate (BHBA) in postpartum dairy cows according to a generalized randomized incomplete block design with repeated measures in time. At calving, cows were blocked according to parity (second and third or greater) and were allocated to 2 treatments: abomasal infusion of water (n=4) or abomasal infusion of free AA with casein profile (AA-CN; n=5) in addition to the same basal diet. The AA-CN infusion started with half the maximal dose at 1 d in milk (DIM) and then steadily decreased from 791 to 226 g/d from DIM 2 to 29 to cover the estimated essential AA deficit. On DIM 5, 15, and 29, D[6,6-(2)H2]-glucose (23.7 mmol/h) was infused into a jugular vein for 5h, and 6 blood samples were taken from arterial, portal, hepatic, and mammary sources at 45-min intervals, starting 1h after the initiation of the D[6,6-(2)H2]glucose infusion. Trans-organ fluxes were calculated as veno-arterial differences times plasma flow (splanchnic: downstream dilution of deacetylated para-aminohippurate; mammary: Fick principle using Phe+Tyr). Energy-corrected milk and lactose yields increased on average with AA-CN by 6.4 kg/d and 353 g/d, respectively, with no DIM × treatment interaction. Despite increased AA supply and increased demand for lactose secretion with AA-CN, net hepatic release of glucose remained unchanged, but WB-Ra of glucose tended to increase with AA-CN. Portal true flux of glucose increased with AA-CN and represented, on average, 17% of WB-Ra. Splanchnic true flux of glucose was unaltered by treatments and was numerically equivalent to WB-Ra, averaging 729 and 741 mmol/h, respectively. Mammary glucose utilization increased with AA-CN infusion, averaging 78% of WB-Ra, and increased gradually as lactation advanced. Net portal, hepatic, splanchnic, and mammary fluxes of lactate, glycerol, and BHBA were not affected by AA infusion. Increasing the supply of AA in postpartum dairy cows elevated the WB-Ra of glucose without affecting the true liver glucose release. The greater WB-Ra of glucose with abomasal AA infusion seemed to originate mainly from greater true portal-drained viscera release of glucose. Glucose utilization by the portal-drained viscera was unaffected by abomasal AA infusion, but the exact mechanism behind the greater true portal glucose release could not be assessed in the current study. The increased mammary glucose uptake was in line with the increased milk lactose yield. In early postpartum lactation, the demand for AA seems to be so high that even with increased AA supply, cows have metabolic priorities for AA other than hepatic gluconeogenesis.

Changes in mammary metabolism in response to the provision of an ideal amino acid profile at 2 levels of metabolizable protein supply in dairy cows: Consequences on efficiency

The aim of this study was to compare the modifications in mammary gland metabolism by supplying an ideal versus an imbalanced essential AA (EAA) profile at low and high metabolizable protein (or PDIE, its equivalent in the INRA feeding system). Four lactating, multiparous Holstein cows received 4 treatments composed of 2 basal diets containing 2 levels of PDIE (LP or HP) and 2 different infusions of AA mixtures (AA- or AA+) in the duodenum. The AA+ mixture contained Lys, Met, Leu, His, Ile, Val, Phe, Arg, Trp, and Glu, whereas the AA- mixture contained Glu, Pro, and Ser. The infusion mixtures were iso-PDIE. The diet plus infusions provided 13.9 versus 15.8% of crude protein that corresponded to 102 versus 118g/kg of dry matter of PDIE in LP and HP treatments, respectively. The treatments were designed as a 2×2 crossover design of 2 levels of PDIE supply (LP vs. HP) with 28-d periods. Infusions of AA in the duodenum (AA- vs. AA+) were superimposed to diet within each 28-d period according to 2×2 crossover designs with 14-d subperiods. Increasing the PDIE supply tended to increase milk protein yield; however, the efficiency of PDIE utilization decreased and the plasma urea concentration increased, indicating a higher catabolism of AA. The AA+ treatments increased milk protein yield and content similarly at both levels of protein supply. This was explained by an increase in the mammary uptake of all EAA except His and Trp. The mammary uptake of non-EAA (NEAA) was altered to the increase in EAA uptake so that the total AA uptake was almost equal to milk protein output on a nitrogen basis. The ratio between NEAA to total AA uptake decreased from 46% in LPAA- to 40% in LPAA+, HPAA-, and HPAA+ treatments. The PDIE efficiency tended to increase in the AA+ versus the AA- treatments because the NEAA supply and the amount of NEAA not used by the mammary both decreased. Nevertheless, our AA+ treatments seemed not to be the ideal profile: the mammary uptake-to-output ratio for Thr was higher than 1 in LPAA-, but it decreased to 1 in all the other treatments, suggesting that Thr was deficient in these treatments. Conversely, an excess of His was indicated because its uptake was similar in AA+ and AA- treatments. In conclusion, balancing the EAA profile increased milk protein yield and metabolizable protein efficiency at both levels of protein supply by increasing the mammary uptake of EAA and altering the NEAA uptake, leading to less AA available for catabolism.

Effects of dry period length and dietary energy source on metabolic status and hepatic gene expression of dairy cows in early lactation

In a prior study, we observed that cows with a 0-d dry period had greater energy balance and lower milk production compared with cows with a 30- or 60-d dry period in early lactation. The objective of the current study was to evaluate the influence of dry period length on metabolic status and hepatic gene expression in cows fed a lipogenic or glucogenic diet in early lactation. Holstein-Friesian dairy cows (n=167) were assigned randomly to 3×2 factorial design with 3 dry period lengths (n=56, 55, and 56 for 0-, 30-, and 60-d dry, respectively) and 2 early lactation diets (n=84 and 83 for glucogenic and lipogenic diet, respectively). Cows were fed a glucogenic or lipogenic diet from 10d before the expected calving date and onward. The main ingredient for a glucogenic concentrate was corn, and the main ingredients for a lipogenic concentrate were sugar beet pulp, palm kernel, and rumen-protected palm oil. Blood was sampled weekly from 95 cows from wk 3 precalving to wk 8 postcalving. Liver samples were collected from 76 cows in wk -2, 2, and 4 relative to calving. Liver samples were analyzed for triacylglycerol concentrations and mRNA expression of 12 candidate genes. Precalving, cows with a 0-d dry period had greater plasma β-hydroxybutyrate, urea, and insulin concentrations compared with cows with a 30- or 60-d dry period. Postcalving, cows with a 0-d dry period had lower liver triacylglycerol and plasma nonesterified fatty acids concentrations (0.20, 0.32, and 0.36mmol/L for 0-, 30-, and 60-d dry period, respectively), greater plasma glucose, insulin-like growth factor-I, and insulin (24.38, 14.02, and 11.08µIU/mL for 0-, 30-, and 60-d dry period, respectively) concentrations, and lower hepatic mRNA expression of pyruvate carboxylase, compared with cows with a 30- or 60-d dry period. Plasma urea and β-hydroxybutyrate concentrations were greater in cows fed a lipogenic diet compared with cows fed a glucogenic diet. In conclusion, cows with a 0-d dry period had an improved metabolic status in early lactation, indicated by lower plasma concentrations of nonesterified fatty acids, greater plasma concentrations of glucose, insulin-like growth factor-I, and insulin, and lower mRNA expression of pyruvate carboxylase in the liver, compared with cows with a 30- or 60-d dry period. Independent of dry period length, the glucogenic diet also improved the metabolic status compared with the lipogenic diet.

Exploring metabolism flexibility in complex organisms through quantitative study of precursor sets for system outputs

Background

When studying metabolism at the organ level, a major challenge is to understand the matter exchanges between the input and output components of the system. For example, in nutrition, biochemical models have been developed to study the metabolism of the mammary gland in relation to the synthesis of milk components. These models were designed to account for the quantitative constraints observed on inputs and outputs of the system. In these models, a compatible flux distribution is first selected. Alternatively, an infinite family of compatible set of flux rates may have to be studied when the constraints raised by observations are insufficient to identify a single flux distribution. The precursors of output nutrients are traced back with analyses similar to the computation of yield rates. However, the computation of the quantitative contributions of precursors may lack precision, mainly because some precursors are involved in the composition of several nutrients and because some metabolites are cycled in loops.

Results

We formally modeled the quantitative allocation of input nutrients among the branches of the metabolic network (AIO). It corresponds to yield information which, if standardized across all the outputs of the system, allows a precise quantitative understanding of their precursors. By solving nonlinear optimization problems, we introduced a method to study the variability of AIO coefficients when parsing the space of flux distributions that are compatible with both model stoichiometry and experimental data. Applied to a model of the metabolism of the mammary gland, our method made it possible to distinguish the effects of different nutritional treatments, although it cannot be proved that the mammary gland optimizes a specific linear combination of flux variables, including those based on energy. Altogether, our study indicated that the mammary gland possesses considerable metabolic flexibility.

Conclusion

Our method enables to study the variability of a metabolic network with respect to efficiency (i.e. yield rates). It allows a quantitative comparison of the respective contributions of precursors to the production of a set of nutrients by a metabolic network, regardless of the choice of the flux distribution within the different branches of the network.

Insulin resistance in dairy cows

Glucose is the molecule that drives milk production, and insulin plays a pivotal role in the glucose metabolism of dairy cows. The effect of insulin on the glucose metabolism is regulated by the secretion of insulin by the pancreas and the insulin sensitivity of the skeletal muscles, the adipose tissue, and the liver. Insulin resistance may develop as part of physiologic (pregnancy and lactation) and pathologic processes, which may manifest as decreased insulin sensitivity or decreased insulin responsiveness. A good knowledge of the normal physiology of insulin is needed to measure the in vivo insulin resistance of dairy cows.

A comparative study at two different altitudes with two dietary nutrition levels on rumen fermentation and energy metabolism in Chinese Holstein cows

The object of this study was to investigate the effect of two altitudes (1600 vs. 3600 m) with two nutritional levels [5.88 MJ/kg dry matter (DM) vs. 7.56 MJ/kg DM] on apparent total tract digestibility, rumen fermentation, energy metabolism, milk yield and milk composition in Chinese Holstein cows. Sixteen Chinese Holstein cows in their third lactation with close body weights, days in milk and milk yield were randomly divided into four groups, of which two were directly transferred from Lanzhou (altitude of 1600 m) to Lhasa (altitude of 3600 m). Four treatments (high plateau and high nutrition level, HA-HN; high plateau and low nutrition level, HA-LN; low plateau and high nutrition level, LA-HN; and low plateau and low nutrition level, LA-LN) were randomly arranged in a 2 × 2 factorial experimental design. Results indicated that the apparent total tract digestibility of a diet's DM, organic matter, crude protein, neutral detergent fibre and acid detergent fibre and DM intake were not affected by either altitude or nutrition level (p > 0.05). Milk protein percentage was higher for the diet with the high level of nutrition than for the diet with low nutrition level irrespective of altitude (p < 0.05). Percentages of milk fat and milk lactose were not affected by either altitude or nutrition level (p > 0.05). The metabolizable energy used for milk energy output was decreased by high altitude in comparison with that at low altitude (p < 0.05). No differences were observed in the live body weight or body condition score (BCS) of Chinese Holstein cows among all of the four treatments (p > 0.05).

The effect of marine algae in the ration of high-yielding dairy cows during transition on metabolic parameters in serum and follicular fluid around parturition

Sixteen Holstein cows were assigned to 2 groups to evaluate the caloric and metabolic effect of feeding marine algae (ALG) from 3 wk prepartum until 12 wk postpartum. Milk production characteristics and the profiles of hormones and metabolites in the serum were monitored from -7 to 46 d in milk (DIM) and in follicular fluid (FF) from 14 to 46 DIM. All cows received a corn- and grass silage-based partially mixed ration supplemented with concentrate and protein supplement. In the diet of the ALG group, 2 kg of the concentrate was replaced by a concentrate containing ALG (44 g/d of docosahexaenoic acid). Diets were isocaloric (net energy basis) and equal in intestinal digestible protein. The ALG diet increased milk yield (41.2 vs. 38.2 kg/d) and decreased milk fat yield (1.181 vs. 1.493 kg/d) and milk fat content (31.6 vs. 40.7 g/kg). Protein yield (1.336 vs. 1.301 kg/d) was not affected but a tendency toward decreased milk protein content (32.8 vs. 34.7 g/kg) was observed. Marine algae supplementation increased the β-hydroxybutyric acid (BHBA) concentration in FF of the ALG cows compared with that in the controls (0.992 vs. 0.718 mmol/L). The total protein concentration in FF was decreased in ALG (62.9 vs. 67.6 g/L). Plasma and serum metabolites did not significantly differ between treatments except for a tendency toward a lower concentration of urea in the serum of the control compared with ALG (4.69 vs. 5.13 mmol/L). Based on metabolizable energy calculations, a daily energy-sparing effect of 3.48 Mcal was obtained due to milk fat depression (MFD). The concomitant increase in milk yield suggests that at least part of this spared energy is used to stimulate milk production. Theoretically, 3.48 Mcal of ME could lead to an increase in milk yield of 7.43 kg/d, which is higher than the observed 3 kg/d. However, when evaluating nutrient requirements during MFD in early lactation, we calculated that increased milk production is caused by a propionate-saving effect of 2.71 mol in the udder when milk fat is depressed. Concurrent increased BHBA concentrations in FF in the ALG group cannot be attributed to a worsened energy status of the animals because all other indicators contradict any change in energy balance, indicating that BHBA might not be an appropriate metabolic parameter to estimate the energy balance in early lactating dairy cows during MFD.

Gluconeogenesis in dairy cows: the secret of making sweet milk from sour dough

Gluconeogenesis is a crucial process to support glucose homeostasis when nutritional supply with glucose is insufficient. Because ingested carbohydrates are efficiently fermented to short-chain fatty acids in the rumen, ruminants are required to meet the largest part of their glucose demand by de novo genesis after weaning. The qualitative difference to nonruminant species is that propionate originating from ruminal metabolism is the major substrate for gluconeogenesis. Disposal of propionate into gluconeogenesis via propionyl-CoA carboxylase, methylmalonyl-CoA mutase, and the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK) has a high metabolic priority and continues even if glucose is exogenously supplied. Gluconeogenesis is regulated at the transcriptional and several posttranscriptional levels and is under hormonal control (primarily insulin, glucagon, and growth hormone). Transcriptional regulation is relevant for regulating precursor entry into gluconeogenesis (propionate, alanine and other amino acids, lactate, and glycerol). Promoters of the bovine pyruvate carboxylase (PC) and PEPCK genes are directly controlled by metabolic products. The final steps decisive for glucose release (fructose 1,6-bisphosphatase and glucose 6-phosphatase) appear to be highly dependent on posttranscriptional regulation according to actual glucose status. Glucogenic precursor entry, together with hepatic glycogen dynamics, is mostly sufficient to meet the needs for hepatic glucose output except in high-producing dairy cows during the transition from the dry period to peak lactation. Lactating cows adapt to the increased glucose requirement for lactose production by mobilization of endogenous glucogenic substrates and increased hepatic PC expression. If these adaptations fail, lipid metabolism may be altered leading to fatty liver and ketosis. Increasing feed intake and provision of glucogenic precursors from the diet are important to ameliorate these disturbances. An improved understanding of the complex mechanisms underlying gluconeogenesis may further improve our options to enhance the postpartum health status of dairy cows.