Short communication: Regulation of milk fat yield and fatty acid composition by insulin

Animal board invited review: The contribution of adipose stores to milk fat: implications on optimal nutritional strategies to increase milk fat synthesis in dairy cows

A wide range of nutritional and non-nutritional factors influence milk fat synthesis and explain the large variation observed in dairy herds. The capacity of the animal to synthesize milk fat will largely depend on the availability of substrates for lipid synthesis, some of which originate directly from the diet, ruminal fermentation or from adipose tissue stores. The mobilization of non-esterified fatty acids from adipose tissues is important to support the energy demands of milk synthesis and will therefore have an impact on the composition of milk lipids, especially during the early lactation period. Such mobilization is tightly controlled by insulin and catecholamines, and in turn, can be affected indirectly by factors that influence these signals, namely diet composition, lactation stage, genetics, endotoxemia, and inflammation. Environmental factors, such as heat stress, also impact adipose tissue mobilization and milk fat synthesis, mainly through endotoxemia and an immune response-related increase in concentrations of plasma insulin. Indeed, as proposed in the present review, the central role of insulin in the control of lipolysis is key to improving our understanding of how nutritional and non-nutritional factors impact milk fat synthesis. This is particularly the case during early lactation, as well as in situations where mammary lipid synthesis is more dependent on adipose-derived fatty acids.

The impact of environmental and nutritional stresses on milk fat synthesis in dairy cows

Stress reduces milk and milk components synthesis and increases maintenance requirements of cows. The major stress-related alterations involve enhanced secretion of glucocorticoids and increased sympathetic nervous system activity, which results in biochemical and physiologic changes. In dairy cows exposed to social (ie housing conditions, overstocking, regrouping, feed delivery), physiological (ie initiation of lactation and parturition), or physical (ie heat or cold stress) stressors, responses involve alterations in energy balance and nutrient partitioning. The capacity of the animal to synthesize milk fat largely depends on the availability of substrates for lipid synthesis from the diet, ruminal fermentation or adipose tissue stores, all of which can be altered under stress conditions. Indeed, milk fat concentration is particularly responsive to diet and environment modifications, where a wide range of nutritional and non-nutritional factors influence milk fat output. Milk fat synthesis is an energy demanding process, and extremely sensitive to stress factors during lactation and the involvement of multiple organs. Recent studies examining social, physical, and physiological stressors have provided important insights into how differences in milk yield and milk components may be associated with biological responses to stress factors in dairy cows. This review focuses primarily on the role of stress sources and indicators to which the dairy cow is exposed in regulating milk fat synthesis. We will review the role of nutritional and non-nutritional factors on milk fat synthesis in dairy cows under stress conditions.

Tumor necrosis factor-α promotes lipolysis and reduces insulin sensitivity by activating nuclear factor kappa B and c-Jun N-terminal kinase in primary bovine adipocytes

Sustained lipolysis and insulin resistance increase the risk of metabolic dysfunction in dairy cows during the transition period. Proinflammatory cytokines are key regulators of adipose tissue metabolism in nonruminants, but biological functions of these molecules in ruminants are not well known. Thus, the objective of this study was to investigate whether tumor necrosis factor-α (TNF-α) could affect insulin sensitivity and lipolysis in bovine adipocytes as well as the underlying mechanisms. Bovine adipocytes (obtained from the omental and mesenteric adipose depots) isolated from 5 Holstein female calves (1 d old) with similar body weight (median: 36.9 kg, range: 35.5-41.2 kg) were differentiated and used for (1) treatment with different concentrations of TNF-α (0, 0.1, 1, or 10 ng/mL) for 12 h; (2) pretreatment with 10 μM lipolytic agonist isoproterenol (ISO) for 3 h, followed by treatment with or without 10 ng/mL TNF-α for 12 h; and (3) pretreatment with the c-Jun N-terminal kinase (JNK) inhibitor SP600125 (20 μM for 2 h) and nuclear factor kappa B (NF-κB) inhibitor BAY 11-7082 (10 μM for 1 h) followed by treatment with or without 10 ng/mL TNF-α for 12 h. The TNF-α increased glycerol content in supernatant, decreased triglyceride content and insulin-stimulated phosphorylation of protein kinase B suggesting activation of lipolysis and impairment of insulin sensitivity. The TNF-α reduced cell viability, upregulated mRNA abundance of Caspase 3 (CASP3), an apoptosis marker, and increased activity of Caspase 3. In addition, increased phosphorylation of NF-κB and JNK, upregulation of mRNA abundance of interleukin-6 (IL-6), TNFA, and suppressor of cytokine signaling 3 (SOCS3) suggested that TNF-α activated NF-κB and JNK signaling pathways. Furthermore, ISO plus TNF-α-activated NF-κB and JNK signaling pathway to a greater extent than TNF-α alone. Combining TNF-α and ISO aggravated TNF-α-induced apoptosis, insulin insensitivity and lipolysis. In the absence of TNF-α, inhibition of NF-κB and JNK did not alter glycerol content in supernatant, triglyceride content or insulin-stimulated phosphorylation of protein kinase B. In the presence of TNF-α, inhibition of NF-κB and JNK alleviated TNF-α-induced apoptosis, insulin insensitivity and lipolysis. Overall, TNF-α impairs insulin sensitivity and induces lipolysis and apoptosis in bovine adipocytes, which may be partly mediated by activation of NF-κB and JNK. Thus, the data suggested that NF-κB and JNK are potential therapeutic targets for alleviating lipolysis dysregulation and insulin resistance in adipocytes.

The Processes of Nutrition and Metabolism Affecting the Biosynthesis of Milk Components and Vitality of Cows with High- and Low-Fat Milk

In order to clarify the mechanism of the depression of milk fat formation and preserve the health of animals, the aim of the research was to study the characteristics of rumen digestion, energy metabolism, and milk composition in high-producing dairy cows with high and low levels of milk fat that are fed the same diet. Two groups of cows with normal milk fat content (3.94 ± 0.12; n = 10) and low milk fat content (2.95 ± 0.14, n = 10) contained in the same diet were identified. Gas exchange (O2 uptake and CO2 output) was studied in cows and blood samples, rumen contents (pH, NH3-N), and VFA and milk (fat, protein, and fatty acid composition) were collected and analyzed. It was determined that cows with low fat milk are more efficient at using the metabolized energy of their diets due to the tendency to have a decrease in the proportion of heat production (by 6.2 MJ; p = 0.055) and an earlier start of a positive energy balance. At the same time, the fat content in milk did not depend on the level of hormones in the blood or on the formation of acetate in the rumen. An analysis of the duration of the productive use of cows on this farm (n = 650) showed that the number of lactations was inversely correlated with the level of fat in milk (r = −0.68; p < 0.05, n = 1300). These results indicate the advantages of cows that can reduce the fat content of their milk in the first months of lactation.

Modulation of Plasma and Milk Sphingolipids in Dairy Cows Fed High-Starch Diets

Bovine milk is a significant source of sphingolipids, dietary compounds that can exert anti-inflammatory actions, and which can modulate the host’s microbiome. Because sphingolipid synthesis can be modified by diet, we hypothesized that dietary conditions which reduced FFA availability may result in reduced sphingolipid synthesis. Twelve ruminally cannulated cows (120 ± 52 DIM; 35.5 ± 8.9 kg of milk/d; mean ± SD) were randomly assigned to treatment in a crossover design with 21-d periods. Treatments were (1) High starch (HS), (2) Control. The HS diet contained 29% starch, 24% NDF, and 2.8% fatty acids (FA), whereas the Control diet contained 20% starch, 31% NDF, and 2.3% FA. Plasma and milk samples were obtained on d 21 of each period and sphingolipids were quantified using targeted metabolomics. Univariate and multivariate analyses of generalized log-transformed and Pareto-scaled data included ANOVA (fixed effects of treatment) and discriminant analysis. The lipidomics analysis detected 71 sphingolipids across plasma and milk fat, including sphinganines (n = 3), dihydro-ceramides (n = 8), ceramides (Cer; n = 15), sphingomyelins (SM; n = 17), and glycosylated ceramides (n = 28). Followed by Cer, SM were the most abundant sphingolipids detected in milk and plasma, with a preponderance of 16:0-, 23:0-, and 24:0-carbon sidechains. Although no effects of HS diets were observed on plasma sphingolipids, we detected consistent reductions in the concentrations of several milk Cer (e.g., 22:0- and 24:0-Cer) and SM (17:0- and 23:0-SM) in response to HS. Discriminant analysis revealed distinct metabolite separation of HS and Control groups, with several Cer and SM being distinctively predictive of dietary treatment. We conclude that HS diets can reduce the secretion of milk Cer and SM, even in the absence of changes in circulating sphingolipids.

Impact of varied time of feeding on the lactation and growth performance of West African Dwarf goat

This study was conducted to evaluate the effect of time of feeding on production performance of West African Dwarf (WAD) goats. Two experiments involving twenty-seven goats (15 bucks and 12 gravid does) were conducted. In Experiment I, the bucks were randomly allocated into three treatments of five replicates and fed for 115 days. In experiment II, pregnant goat-does were randomly allocated into three experimental treatments of four replicates per treatment  1 month to kidding. Goats were either fed in the morning (06:00 h), afternoon (12:00 h), or evening (18:00 h) respectively under natural light cycles. In experiment I, dry matter intake, weight gain, carcass characteristic, and meat chemical composition were not affected (P > 0.05) by time of feeding. In experiment II, data collection on feed intake, feed efficiency (FE), and milk yield spanned 6 weeks and were analyzed. From the results, milk yield in morning-fed goat-does was higher than evening-fed does while afternoon-fed does have the lowest yield at P < 0.001. Energy corrected milk in evening-fed does was higher (P = 0.006) than other treatments. Feed efficiency for milk yield and energy corrected milk yield (P < 0.001) in evening-fed does were higher than morning and afternoon-fed does. Milk component yield (g/day) for lactose (P = 0.002) was the highest in morning-fed does; evening-fed does had the highest (P = 0.001) crude fat while afternoon-fed does had the lowest yield for all milk component parameters. Conclusively, feeding in the morning and evening is recommended for milk production and component yield per day. However, for improved energy corrected milk yield and feed efficiency in lactating goats, feeding in the evening is recommended. It is recommended that further studies should be conducted on improving milk productivity in evening-fed goats due to the advantage it had over other feeding regimes. Thus, altering time of feeding could be a suitable alternative feeding strategy in a changing climate with a potential to improve production efficiency, especially in the tropics.

Effects of supplementing Holstein cows with soybean oil compared with palmitic acid-enriched triglycerides on milk production and nutrient partitioning

Both insulin and trans-10,cis-12 C18:2 (t10c12CLA) can be increased by high-starch diets; thus, it is difficult to determine whether insulin or t10c12CLA mediates nutrient partitioning toward body tissues during milk fat depression. To minimize insulin secretion while manipulating t10c12CLA levels, diets supplemented with palmitic acid-enriched triglycerides and soybean oil were fed to cows. Thirty-two Holstein cows (93 ± 35 d in milk) were included in the crossover experiment with each treatment period being 28 d. Treatment diets contained 25% neutral detergent fiber, 32% starch, 18% crude protein, and 4.6% fatty acids (dry matter basis). Treatment diets contained either palmitic acid-enriched triglycerides (2.5% dry matter, BergaFat T-300, Berg + Schmidt America LLC, Libertyville, IL; PAT) or soybean oil (2.5% dry matter; SBO). Cows were blocked by milk yield, body weight, and parity, and then randomly assigned to 1 of 2 treatment sequences (PAT-SBO or SBO-PAT). Cows fed PAT produced milk with only 3.1% fat, indicating milk fat depression; SBO decreased fat content further to only 2.4%. No effect of treatment was observed on dry matter intake, apparent net energy intake, milk yield, body condition score, or fat thickness over the rump and rib. However, compared with PAT, SBO decreased fat-corrected milk yield, energy-corrected milk yield, milk fat yield, de novo fatty acids, and 16-carbon fatty acid yield, whereas SBO increased body weight gain. Neutral detergent fiber digestibility tended to be lower in SBO, whereas fatty acid digestibility was higher. Additionally, the concentration of plasma insulin, nonesterified fatty acids, and triglycerides, and milk metabolites (trans-10 C18:1 and t10c12CLA) were all higher in SBO. In conclusion, with similar dietary starch content, the diet containing palmitic acid-enriched triglycerides partitioned more energy toward milk synthesis, whereas the diet containing soybean oil partitioned more energy toward body tissue gain.

Transient reductions in milk fat synthesis and their association with the ruminal and metabolic profile in dairy cows fed high-starch, low-fat diets

Sub-acute ruminal acidosis (SARA) is sometimes observed along with reduced milk fat synthesis. Inconsistent responses may be explained by dietary fat levels. Twelve ruminally cannulated cows were used in a Latin square design investigating the timing of metabolic and milk fat changes during Induction and Recovery from SARA by altering starch levels in low-fat diets. Treatments were (1) SARA Induction, (2) Recovery and (3) Control. Sub-acute ruminal acidosis was induced by feeding a diet containing 29.4% starch, 24.0% NDF and 2.8% fatty acids (FAs), whereas the Recovery and Control diets contained 19.9% starch, 31.0% NDF and 2.6% FA. Relative to Control, DM intake (DMI) and milk yield were higher in SARA from days 14 to 21 and from days 10 to 21, respectively (P < 0.05). Milk fat content was reduced from days 3 to 14 in SARA (P < 0.05) compared with Control, while greater protein and lactose contents were observed from days 14 to 21 and 3 to 21, respectively (P < 0.05). Milk fat yield was reduced by SARA on day 3 (P < 0.05), whereas both protein and lactose yields were higher on days 14 and 21 (P < 0.05). The ruminal acetate-to-propionate ratio was lower, and the concentrations of propionate and lactate were higher in the SARA treatment compared with Control on day 21 (P < 0.05). Plasma insulin increased during SARA, whereas plasma non-esterified fatty acids and milk β-hydroxybutyrate decreased (P < 0.05). Similarly to fat yield, the yield of milk preformed FA (>16C) was lower on day 3 (P < 0.05) and tended to be lower on day 7 in SARA cows (P < 0.10), whereas yield of de novo FA (<16C) was higher on day 21 (P < 0.01) in the SARA group relative to Control. The t10- to t11-18:1 ratio increased during the SARA Induction period (P < 0.05), but the concentration of t10-18:1 remained below 0.5% of milk fat, and t10,c12 conjugated linoleic acid remained below detection levels. Odd-chain FA increased, whereas branched-chain FA was reduced during SARA Induction from days 3 to 21 (P < 0.05). Sub-acute ruminal acidosis reduced milk fat synthesis transiently. Such reduction was not associated with ruminal biohydrogenation intermediates but rather with a transient reduction in supply of preformed FA. Subsequent rescue of milk fat synthesis may be associated with higher availability of substrates due to increased DMI during SARA.

Methionine supply during the periparturient period enhances insulin signaling, amino acid transporters, and mechanistic target of rapamycin pathway proteins in adipose tissue of Holstein cows

Enhanced postruminal supply of Met during the periparturient period increases dry matter intake and milk yield. In nonruminants, adipose tissue is responsive to AA supply, and can use AA as fuels or for protein synthesis regulated in part via insulin and mechanistic target of rapamycin (mTOR) signaling. Whether enhancing supply of Met has an effect on insulin and mTOR pathways in adipose tissue in peripartal cows is unknown. Multiparous Holstein cows were assigned from -28 to 60 d relative to parturition to a basal diet (control; 1.47 Mcal/kg of dry matter and 15.3% crude protein prepartum; 1.67 Mcal/kg and 17.7% crude protein postpartum) or the control plus ethyl-cellulose rumen-protected Met (RPM). The RPM was fed individually at a rate of 0.09% of dry matter intake prepartum and 0.10% postpartum. Subcutaneous adipose tissue harvested at -10, 10, and 30 d relative to parturition (days in milk) was used for quantitative PCR and Western blotting. A glucose tolerance test was performed at -12 and 12 d in milk to evaluate insulin sensitivity. Area under the curve for glucose in the pre- and postpartum tended to be smaller in cows fed Met. Enhanced Met supply led to greater overall mRNA abundance of Gln (SLC38A1), Glu (SLC1A1), l-type AA (Met, Leu, Val, Phe; SLC3A2), small zwitterionic α-AA (SLC36A1), and neutral AA (SLC1A5) transporters. Abundance of AKT1, RPS6KB1, and EIF4EBP1 was also upregulated in response to Met. A diet × day interaction was observed for protein abundance of insulin receptor due to Met cows having lower values at 30 d postpartum compared with controls. The diet × day interaction was significant for hormone-sensitive lipase due to Met cows having greater abundance at 10 d postpartum compared with controls. Enhanced Met supply upregulated protein abundance of insulin-responsive proteins phosphorylated (p)-AKT, peroxisome proliferator-activated receptor gamma, and fatty acid synthase. Overall abundance of solute carrier family 2 member 4 tended to be greater in cows fed Met. A diet × day interaction was observed for mTOR protein abundance due to greater values for RPM cows at 30 d postpartum compared with controls. Enhanced RPM supply upregulated overall protein abundance of solute carrier family 1 member 3, p-mTOR, and ribosomal protein S6. Overall, data indicate that mTOR and insulin signaling pathways in adipose tissue adapt to the change in physiologic state during the periparturient period. Further studies should be done to clarify whether the activation of p-AKT or increased availability of AA leads to the activation of mTOR.

Symposium review: Milk fat globule size: Practical implications and metabolic regulation

Milk fat globule (MFG) size ranges over 3 orders of magnitude, from less than 200 nm to over 15 µm. The significance of MFG size derives from its tight association with its lipidome and proteome. More specifically, small MFG have relatively higher content of membrane compared with large globules, and this membrane exerts diverse positive health effects, as reported in human and animal studies. In addition, MFG size has industrial significance, as it affects the physicochemical and sensory characteristics of dairy products. Studies on the size regulation of MFG are scarce, mainly because various confounders indirectly affect MFG size. Because MFG size is determined before and during its secretion from mammary epithelial cells, studies on the size regulation of its precursors, the intracellular lipid droplets (LD), have been used as a proxy for understanding the mechanisms controlling MFG size. In this review, we provide evidence for 2 distinct mechanisms regulating LD size in mammary epithelial cells: co-regulation of fat content and triglyceride-synthesis capacity of the cells, and fusion between LD. The latter is controlled by the membrane's polar lipid composition and involves mitochondrial enzymes. Accordingly, this review also discusses MFG size regulation in the in vivo metabolic context, as MFG morphometric features are often modulated under conditions that involve animals' altered energy status.

Changes in lipid droplets morphometric features in mammary epithelial cells upon exposure to non-esterified free fatty acids compared with VLDL

The effects of the macrostructure of long chain fatty acids on the lipid metabolism and biosynthesis of lipid droplets (LD) was studied in mammary epithelial cells (MEC). MEC were exposed to similar compositions and concentrations of fatty acids in the form of either triglycerides (Tg), as part of the very-low-density lipids (VLDL) isolated from lactating cow plasma, or as non-esterified- free fatty acids (FFA). Exposing MEC to FFA resulted in two distinct processes; each independently could increase LD size: an elevation in Tg production and alterations in phospholipid (PL) composition. In particular, the lower PC/PE ratio in the FFA treatment indicated membrane destabilization, which was concomitant with the biosynthesis of larger LD. In addition, 6 fold increase in the cellular concentration of the exogenously added linoleic acid (C18:2) was found in MEC treated with FFA, implying that long chain fatty acids administrated as FFA have higher availability to MEC, enabling greater PL synthesis, more material for the LD envelope, thereby enhancing LD formation. Availability of long chain fatty acids administrated as VLDL-Tg, is dependent on LPL which its activity can be inhibited by the hydrolysis products. Therefore, we used increasing concentrations of albumin, to reduce the allosteric inhibition on LPL by the hydrolysis products. Indeed, a combined treatment of VLDL and albumin, increased LD size and number, similar to the phenotype found in the FFA treatment. These results reveal the role played by the macrostructure of long chain fatty acids in the regulation of LD size in MEC which determine the size of the secreted MFG.

Biological underpinnings of breastfeeding challenges: the role of genetics, diet, and environment on lactation physiology

Lactation is a dynamic process that has evolved to produce a complex biological fluid that provides nutritive and nonnutritive factors to the nursing offspring. It has long been assumed that once lactation is successfully initiated, the primary factor regulating milk production is infant demand. Thus, most interventions have focused on improving breastfeeding education and early lactation support. However, in addition to infant demand, increasing evidence from studies conducted in experimental animal models, production animals, and breastfeeding women suggests that a diverse array of maternal factors may also affect milk production and composition. In this review, we provide an overview of our current understanding of the role of maternal genetics and modifiable factors, such as diet and environmental exposures, on reproductive endocrinology, lactation physiology, and the ability to successfully produce milk. To identify factors that may affect lactation in women, we highlight some information gleaned from studies in experimental animal models and production animals. Finally, we highlight the gaps in current knowledge and provide commentary on future research opportunities aimed at improving lactation outcomes in breastfeeding women to improve the health of mothers and their infants.

Insulin signaling, inflammation, and lipolysis in subcutaneous adipose tissue of transition dairy cows either overfed energy during the prepartum period or fed a controlled-energy diet

Adipose tissue mobilization is a hallmark of the transition period in dairy cows. Cows overfed energy during the dry period have higher concentrations of nonesterified fatty acids (NEFA) and β-hydroxybutyrate (BHB) compared with cows fed a controlled-energy diet prepartum. The reason for an increase in blood NEFA concentrations at the level of adipose tissue in cows overfed energy has not been fully elucidated. One hypothesis is that cows with high BHB concentrations suffer from adipose tissue-specific insulin resistance, leading to higher rates of adipose tissue mobilization in the postpartum period. To test this hypothesis, subcutaneous adipose tissue biopsies of cows overfed energy in excess of predicted requirements by 50% in the dry period, and that had high concentrations of blood BHB postpartum (group H; n=12), were used. Findings were compared with results of biopsies from cows fed a controlled-energy diet and with low BHB concentrations postpartum (group C; n=12) to create the biggest contrast in BHB concentrations. Subcutaneous adipose tissue biopsies were obtained before and 60 min after an intravenous glucose challenge (0.25 g/kg of glucose) at 28 and 10 d before expected calving as well as on d 4 and 21 postpartum. Phosphorylation of protein kinase B, extracellular signal-regulated kinase, and hormone-sensitive lipase was determined before and after glucose infusion by Western blot. Western blot was also used to assess the baseline protein abundance of peroxisome proliferator-activated receptor gamma and insulin receptor β-subunit. In addition, gene expression of fatty acid synthase, adiponectin, monocyte chemoattractant protein 1, and tumor necrosis factor α was determined by real-time quantitative reverse-transcription PCR. Backfat thickness was determined in the thurl area by ultrasonography. Cows in group H showed a greater degree of lipogenesis prepartum, but no differences were found in lipolytic enzyme activity postpartum compared with cows in group C. Baseline plasma insulin concentrations were decreased and serum NEFA concentrations increased postpartum in group H. Insulin signaling through protein kinase B, quantity of insulin receptor, markers of inflammation, and peroxisome proliferator-activated receptor gamma in adipose tissue were not different between the groups, but expression of adiponectin was increased in adipose tissue of cows in group H during the immediate peripartum period. In conclusion, differences in serum concentrations of NEFA between cows overfed energy prepartum and high blood concentrations of BHB are likely due to greater negative energy balance postpartum reflected in lower circulating concentrations of glucose and insulin and an increase in the total amount of mobilized adipose tissue mass rather than due to changes in adipose tissue insulin signaling.

The demands of lactation promote differential regulation of lipid stores in fasting elephant seals

Fasting animals must ration stored reserves appropriately for metabolic demands. Animals that experience fasting concomitant with other metabolically demanding activities are presented with conflicting demands of energy conservation and expenditure. Our objective was to understand how fasting northern elephant seals regulate the mobilization of lipid reserves and subsequently milk lipid content during lactation. We sampled 36 females early and 39 at the end of lactation. To determine the separate influences of lactation from fasting, we also sampled fasting but non-lactating females early and late (8 and 6 seals, respectively) in their molting fasting period. Mass and adiposity were measured, as well as circulating non-esterified fatty acid (NEFA), triacylglycerol (TAG), cortisol, insulin and growth hormone levels. Milk was collected from lactating females. Milk lipid content increased from 31% in early to 51% in late lactation. In lactating females plasma NEFA was positively related to cortisol and negatively related to insulin, but in molting seals, only variation in cortisol was related to NEFA. Milk lipid content varied with mass, adiposity, NEFA, TAG, cortisol and insulin. Surprisingly, growth hormone concentration was not related to lipid metabolites or milk lipid. Suppression of insulin release appears to be the differential regulator of lipolysis in lactating versus molting seals, facilitating mobilization of stored lipids and maintenance of high NEFA concentrations for milk synthesis. Milk lipid was strongly impacted by the supply of substrate to the mammary gland, indicating regulation at the level of mobilization of lipid reserves.

Adipose Triglyceride Lipase, Not Hormone-Sensitive Lipase, Is the Primary Lipolytic Enzyme in Fasting Elephant Seals (Mirounga angustirostris)

Little is known about the mechanisms that allow capital breeders to rapidly mobilize large amounts of body reserves. Northern elephant seals (Mirounga angustirostris) utilize fat reserves for maternal metabolism and to create high fat milk for the pup. Hormone-sensitive lipase (HSL) has been hypothesized to be an important lipolytic enzyme in fasting seals, but the activity of HSL and adipose triglyceride lipase (ATGL) has not been quantified in fasting adult seals, nor has their relationship to milk lipid content been assessed. Blubber and milk samples were obtained from 18 early lactation and 19 late lactation females, as well as blubber from five early and five late molting female seals. Blubber lipolytic activity was assessed with radiometric assays. HSL activity was negligible in seal blubber at all fasting stages. Total triglyceride lipase activity was stable among early and late lactation and early molt but increased in late molting seals. Relative abundance of ATGL protein increased across fasting, but neither activity nor relative protein levels were related to circulating nonesterified fatty acids or milk lipid content, suggesting the possibility of other regulatory pathways between lipolytic activity and milk lipid content. These results demonstrate that HSL is not the primary lipolytic enzyme in fasting adult female seals and that ATGL contributes more to lipolysis than HSL.

Conjoint regulation of glucagon concentrations via plasma insulin and glucose in dairy cows

Insulin and glucagon are glucoregulatory hormones that contribute to glucose homeostasis. Plasma insulin is elevated during normoglycemia or hyperglycemia and acts as a suppressor of glucagon secretion. We have investigated if and how insulin and glucose contribute to the regulation of glucagon secretion through long term (48 h) elevated insulin concentrations during simultaneous hypoglycemia or euglycemia in mid-lactating dairy cows. Nineteen Holstein dairy cows were randomly assigned to 3 treatment groups: an intravenous insulin infusion (HypoG, n = 5) to decrease plasma glucose concentrations (2.5 mmol/L), a hyperinsulinemic-euglycemic clamp to study effects of insulin at simultaneously normal glucose concentrations (EuG, n = 6) and a 0.9% saline infusion (NaCl, n = 8). Plasma glucose was measured at 5-min intervals, and insulin and glucose infusion rates were adjusted accordingly. Area under the curve of hourly glucose, insulin, and glucagon concentrations on day 2 of infusion was evaluated by analysis of variance with treatments as fixed effect. Insulin infusion caused an increase of plasma insulin area under the curve (AUC)/h in HypoG (41.9 ± 8.1 mU/L) and EuG (57.8 ± 7.8 mU/L) compared with NaCl (13.9 ± 1.1 mU/L; P < 0.01). Induced hyperinsulinemia caused a decline of plasma glucose AUC/h to 2.3 ± 0.1 mmol/L in HypoG (P < 0.01), whereas plasma glucose AUC/h remained unchanged in EuG (3.8 ± 0.2 mmol/L) and NaCl (4.1 ± 0.1 mmol/L). Plasma glucagon AUC/h was lower in EuG (84.0 ± 6.3 pg/mL; P < 0.05) and elevated in HypoG (129.0 ± 7.0 pg/mL; P < 0.01) as compared with NaCl (106.1 ± 5.4 pg/mL). The results show that intravenous insulin infusion induces elevated glucagon concentrations during hypoglycemia, although the same insulin infusion reduces glucagon concentrations at simultaneously normal glucose concentrations. Thus, insulin does not generally have an inhibitory effect on glucagon concentrations. If simultaneously glucose is low and insulin is high, glucagon is upregulated to increase glucose availability. Therefore, insulin and glucose are conjoint regulatory factors of glucagon concentrations in dairy cows, and the plasma glucose status is the key factor to decide if its concentrations are increased or decreased. This regulatory effect can be important for the maintenance of glucose homeostasis if insulin secretion is upregulated by other factors than high glucose such as high plasma lipid and protein concentrations at simultaneously low glucose.

Adiponectin and Insulin in Gray Seals during Suckling and Fasting: Relationship with Nutritional State and Body Mass during Nursing in Mothers and Pups

Animals that fast during breeding and/or development, such as phocids, must regulate energy balance carefully to maximize reproductive fitness and survival probability. Adiponectin, produced by adipose tissue, contributes to metabolic regulation by modulating sensitivity to insulin, increasing fatty acid oxidation by liver and muscle, and promoting adipogenesis and lipid storage in fat tissue. We tested the hypotheses that (1) circulating adiponectin, insulin, or relative adiponectin gene expression is related to nutritional state, body mass, and mass gain in wild gray seal pups; (2) plasma adiponectin or insulin is related to maternal lactation duration, body mass, percentage milk fat, or free fatty acid (FFA) concentration; and (3) plasma adiponectin and insulin are correlated with circulating FFA in females and pups. In pups, plasma adiponectin decreased during suckling (linear mixed-effects model [LME]: T = 4.49; P < 0.001) and the early postweaning fast (LME: T = 3.39; P = 0.004). In contrast, their blubber adiponectin gene expression was higher during the early postweaning fast than early in suckling (LME: T = 2.11; P = 0.046). Insulin levels were significantly higher in early (LME: T = 3.52; P = 0.004) and late (LME: T = 6.99; P < 0.001) suckling than in fasting and, given the effect of nutritional state, were also positively related to body mass (LME: T = 3.58; P = 0.004). Adiponectin and insulin levels did not change during lactation and were unrelated to milk FFA or percentage milk fat in adult females. Our data suggest that adiponectin, in conjunction with insulin, may facilitate fat storage in seals and is likely to be particularly important in the development of blubber reserves in pups.

Functional Role of PPARs in Ruminants: Potential Targets for Fine-Tuning Metabolism during Growth and Lactation

Characterization and biological roles of the peroxisome proliferator-activated receptor (PPAR) isotypes are well known in monogastrics, but not in ruminants. However, a wealth of information has accumulated in little more than a decade on ruminant PPARs including isotype tissue distribution, response to synthetic and natural agonists, gene targets, and factors affecting their expression. Functional characterization demonstrated that, as in monogastrics, the PPAR isotypes control expression of genes involved in lipid metabolism, anti-inflammatory response, development, and growth. Contrary to mouse, however, the PPARγ gene network appears to controls milk fat synthesis in lactating ruminants. As in monogastrics, PPAR isotypes in ruminants are activated by long-chain fatty acids, therefore, making them ideal candidates for fine-tuning metabolism in this species via nutrients. In this regard, using information accumulated in ruminants and monogastrics, we propose a model of PPAR isotype-driven biological functions encompassing key tissues during the peripartal period in dairy cattle.

Lactation and neonatal nutrition: defining and refining the critical questions

This paper resulted from a conference entitled "Lactation and Milk: Defining and refining the critical questions" held at the University of Colorado School of Medicine from January 18-20, 2012. The mission of the conference was to identify unresolved questions and set future goals for research into human milk composition, mammary development and lactation. We first outline the unanswered questions regarding the composition of human milk (Section I) and the mechanisms by which milk components affect neonatal development, growth and health and recommend models for future research. Emerging questions about how milk components affect cognitive development and behavioral phenotype of the offspring are presented in Section II. In Section III we outline the important unanswered questions about regulation of mammary gland development, the heritability of defects, the effects of maternal nutrition, disease, metabolic status, and therapeutic drugs upon the subsequent lactation. Questions surrounding breastfeeding practice are also highlighted. In Section IV we describe the specific nutritional challenges faced by three different populations, namely preterm infants, infants born to obese mothers who may or may not have gestational diabetes, and infants born to undernourished mothers. The recognition that multidisciplinary training is critical to advancing the field led us to formulate specific training recommendations in Section V. Our recommendations for research emphasis are summarized in Section VI. In sum, we present a roadmap for multidisciplinary research into all aspects of human lactation, milk and its role in infant nutrition for the next decade and beyond.

Morning vs. evening feed delivery for lactating dairy cows

Nikkhah, A., Furedi, C. J., Kennedy, A. D., Scott, S. L., Wittenberg, K. M. Crow, G. H. and Plaizier, J. C. 2011. Morning vs. evening feed delivery for lactating dairy cows. Can. J. Anim. Sci. 91: 113–122. A study was conducted to investigate the effects of changing feed delivery time from morning (0900) to evening (2100) on feed intake, diurnal feed intake patterns, digestion, rumen fermentation, milk production and composition, as well as on several blood metabolites. Eight (four ruminally cannulated) lactating Holstein cows, consisting of four primiparous and four multiparous cows, were used in the study. The change in the time of feed delivery increased the proportion of feed consumed within 3 h of feed delivery in all cows (54.7 vs. 46.3±4.1%, means±SE, P=0.05), but increased daily dry matter intake only in primiparous cows (18.5 vs. 20.4±0.7 kg d−1, P=0.05). Average daily concentrations of blood glucose, lactate, urea and β-hydroxybutyrate (BHBA) as well as rumen pH and concentrations of volatile fatty acids (VFA) were not affected by feed delivery time. However, rumen pH was lower (P<0.05), at 5 and 6 h after feed delivery, while total rumen VFA and acetate were higher (P<0.05) only at 6 h after feed delivery in cows fed in the evening. Also, compared with feed delivery at 0900, feeding cows at 2100 resulted in lower (P<0.05) blood glucose and urea at 2 h after feed delivery, and increased BHBA (P<0.05) at 4 and 6 h after feed delivery. Cows fed at 2100 tended to produce more milk fat than those fed at 0900 (1.11 vs. 0.97±0.07 kg d−1, P=0.08). Milk yield and milk protein were not affected by the time of feed delivery. Changing the feed delivery time from morning to evening increased apparent total tract digestibility of dry matter, nitrogen, and neutral detergent fibre. The results of this study suggest that changing time of feed delivery from 0900 to 2100 increases feed consumption within 3 h after this delivery and rumen fermentation within 6 h after this delivery, changes diurnal patterns of glucose, urea and BHBA in peripheral blood, tends to increase milk fat, and increases nutrient digestibilities.

The use of nicotinic acid to induce sustained low plasma nonesterified fatty acids in feed-restricted Holstein cows

The objectives were to determine the effects of nicotinic acid (NA) on blood metabolites (experiment 1) and whether successive doses of NA could induce sustained reductions of plasma nonesterified fatty acids (NEFA; experiment 2) in feed-restricted, nonlactating Holstein cows. Experiment 1 was a single 4 x 4 Latin square with 1-wk periods. Each period consisted of 2.5 d of feed restriction to increase plasma NEFA and 4.5 d of ad libitum feeding. Treatments were abomasal administration of 0, 6, 30, or 60 mg of NA/kg of body weight (BW), given as a single bolus 48 h after initiation of feed restriction. Plasma NEFA concentration decreased from 546 microEq/L to 208 +/- 141 microEq/L at 1 h after the infusion of 6 mg of NA/kg of BW, and to less than 100 +/- 148 microEq/L at 3 h after the abomasal infusion of the 2 highest doses of NA. A rebound occurred after the initial decrease of plasma NEFA concentration. The rebound lasted up to 9 h for the 30-mg dose of NA, and up to 6 h for the 6-mg dose. Experiment 2 was a randomized complete block design with 3 treatments and 6 cows. Starting at 48 h of feed restriction, cows received 9 hourly abomasal infusions of 0, 6, or 10 mg of NA/kg of BW. Plasma NEFA concentrations decreased from 553 microEq/L +/- 24 immediately before the initiation of treatments to <100 microEq/L during hourly infusions of 6 or 10 mg of NA/kg. Data suggest that the maximal antilipolytic response was achieved with the lowest dose of NA. A rebound of NEFA started 2 to 3 h after NA infusions were terminated. In both experiments, the NEFA rebound period coincided with increases in insulin and no change or increased glucose concentrations, suggesting a state of insulin resistance induced by elevated NEFA. This model for reducing plasma NEFA concentration by abomasal infusions of NA can be used to study the metabolic ramifications of elevated vs. reduced NEFA concentrations. The data demonstrate potential benefits and pitfalls of using NA to regulate plasma NEFA and prevent lipid-related metabolic disorders.

Selection of barley grain affects ruminal fermentation, starch digestibility, and productivity of lactating dairy cows

The objective of this study was to evaluate the effects of 2 lots of barley grain cultivars differing in expected ruminal starch degradation on dry matter (DM) intake, ruminal fermentation, ruminal and total tract digestibility, and milk production of dairy cows when provided at 2 concentrations in the diet. Four primiparous ruminally cannulated (123 +/- 69 d in milk; mean +/- SD) and 4 multiparous ruminally and duodenally cannulated (46 +/- 14 d in milk) cows were used in a 4 x 4 Latin Square design with a 2 x 2 factorial arrangement of treatments with 16-d periods. Primiparous and multiparous cows were assigned to different squares. Treatments were 2 dietary starch concentrations (30 vs. 23% of dietary DM) and 2 lots of barley grain cultivars (Xena vs. Dillon) differing in expected ruminal starch degradation. Xena had higher starch concentration (58.7 vs. 50.0%) and greater in vitro 6-h starch digestibility (78.0 vs. 73.5%) compared with Dillon. All experimental diets were formulated to supply 18.3% crude protein and 20.0% forage neutral detergent fiber. Dry matter intake and milk yield were not affected by treatment. Milk fat concentration (3.55 vs. 3.29%) was greater for cows fed Dillon compared with Xena, but was not affected by dietary starch concentration. Ruminal starch digestion was greater for cows fed high-starch diets compared with those fed low-starch diets (4.55 vs. 2.49 kg/d), and tended to be greater for cows fed Xena compared with those fed Dillon (3.85 vs. 3.19 kg/d). Ruminal acetate concentration was lower, and propionate concentration was greater, for cows fed Xena or high-starch diets compared with cows fed Dillon or low-starch diets, respectively. Furthermore, cows fed Xena or high-starch diets had longer duration that ruminal pH was below 5.8 (6.6 vs. 4.0 and 6.4 vs. 4.2 h/d) and greater total tract starch digestibility (94.3 vs. 93.0 and 94.3 vs. 93.0%) compared with cows fed Dillon or low-starch diets, respectively. These results demonstrate that selection of barley grain can affect milk fat production and rumen fermentation to an extent at least as great as changes in dietary starch concentration.

Technical Note: Estimation of Milk Fatty Acid Yield from Milk Fat Data

Most publications reporting milk fatty acid (FA) yields estimate these yields from milk fat yields and a coefficient estimating the proportion of FA in milk fat. The most widely used coefficient is, in fact, the proportion of fatty acyl radicals (i.e., FA from which the OH group has been removed) in milk triglycerides, equivalent to a mean proportion of 88% FA in milk fat. From a metabolic point of view, because the digestive flows and mammary FA uptake consist of nonesterified FA, it is more logical to estimate the FA proportion rather than the fatty acyl proportion in milk fat. From 588 milk FA profiles, we estimated a mean proportion of 94.4% FA in milk triglycerides. Moreover, when the other milk lipid classes (phospholipids, diglycerides, etc.) were taken into account, the proportion of FA in milk total lipids was estimated at 93.3%, almost independently of the milk FA profile. The use of this coefficient to estimate the secretion of milk FA on the basis of milk fat yield data is more physiologically relevant for milk FA secretion studies.