Glucose metabolism in dairy cows supplemented conjugated linoleic acid

Conjugated linoleic acid (CLA), particularly the trans-10, cis-12 CLA isomer, is well known for its milk fat reducing effect, but CLA effects on glucose metabolism are not clear. The objective of the study was to investigate glucose metabolism, especially endogenous glucose production (eGP) and glucose oxidation (GOx) as well as genes involved in metabolic pathways of hepatic eGP. Cows were fed ad libitum TMR supplemented with either 50 g/d Lutrell pure (BASF, Ludwigshafen, Germany) containing 9.3 % trans-10, cis-12 and 10.3 % cis-9, trans-11 CLA isomers (CLA; n=10), or 50 g/d control fat (BASF; Ludwigshafen, Germany; control; n=10). Both fat types were provided in a rumen-protected form. Supplementation of CLA lasted from 2 weeks prior to projected calving to 9 weeks in milk. From 10 weeks to 12 weeks after calving fat supplementation was removed from the ration in both groups. The experiment was terminated after 12 weeks of lactation. Cows were investigated in 5 blocks consisting of 4 cows (2 CLA and 2 control cows) in each block. Individual feed intake was recorded daily and feed samples were collected weekly. Cows were milked twice daily, milk yield was measured daily, and milk samples were taken once weekly for measurement of fat, lactose, and protein in milk. Fatty acid composition in milk fat was measured 3 and 9 weeks after parturition. Body weight, body condition score, and back fat thickness were measured every week. Blood samples were taken once weekly after the morning milking and before cows were allowed to eat by venipuncture. Concentrations of glucose, triglycerides, non-esterified fatty acids (NEFA), beta-hydroxybutyrate (BHBA), cholesterol, urea, insulin, and glucagon were analyzed in blood plasma. An intravenous glucose tolerance test (GTT; 1 g/kg0.75) was performed 3 and 9 weeks after parturition and frequent blood samples were taken after glucose administration to measure pancreatic insulin response. Endogenous glucose production and GOx were measured three days after GTT, respectively. After overnight food withdrawal, a prime injection initiated U-13C-glucose infusion of 4 hours and blood samples were taken frequently. Endogenous glucose production was calculated from U-13C-glucose enrichment in blood plasma, measured by gas chromatography - mass spectrometry. Glucose oxidation was measured in whole blood by isolation of CO2 and determination of the 13C/12C ratio by mass spectrometry. Liver biopsies were taken in week 3 and week 9 after parturition to measure total fat and glycogen concentrations and mRNA abundance of pyruvate carboxylase (PC), cytosolic phosphoenolpyruvate carboxykinase (PEPCKc), glucose-6-phosphatase (G6Pase), and carnitine palmitoyl-transferase-1 (CPT-1). Glyceraldehyde-3-phosphate dehydrogenase was used as reference gene. Conjugated linoleic acid reduced milk fat concentration (P < 0.01) and milk fat yield (P < 0.01) during supplementation by 14 % and 13 %. After finishing CLA supplementation both, milk fat concentration and yield increased (P < 0.01) in CLA cows, and milk fat concentration, but not yield was still lower (P < 0.05) in CLA than in control. In milk fat, concentrations of most of the short- and medium-chain fatty acids (C4:0–C16:0) were lower (P < 0.05 or less) in CLA-fed than control cows. Stearic and oleic acid in milk fat were higher (P < 0.05) in CLA-fed than control cows. Milk yield increased more in CLA than control group after onset of lactation, but after five weeks of lactation milk yield was higher in CLA than control cows (interaction P < 0.01). Lactose concentration and yield increased (P < 0.01) with onset of lactation and lactose yield was higher (P < 0.05) during CLA supplementation and tended to be higher (P < 0.1) after CLA supplementation in CLA than control cows. Dry matter intake increased (P < 0.01) with time, and tended to be lower (P = 0.15) in supplemented cows. Body weight, back fat thickness, and body condition score decreased (P < 0.01) in both groups with onset of lactation. Energy balance decreased (P < 0.05) after parturition, but increased (P < 0.01) after 2 weeks of lactation until the end of the study in both groups. Energy balance was higher (P < 0.05) during CLA supplementation in control than CLA cows. Cows supplemented CLA showed elevated concentrations of glucose in blood plasma from first to fifth week after parturition (interaction P < 0.01), whereas plasma glucose concentrations increased during depletion more in control than in CLA fed cows (interaction P < 0.08). Plasma concentration of NEFA, BHBA, cholesterol, and glucagon and the glucagon to insulin ratio increased (P < 0.01), whereas plasma concentrations of triglyceride and insulin decreased (P < 0.01) after parturition. Plasma glucose concentrations increased after glucose infusion during GTT in 3rd and 9th week after parturition. (P < 0.01), but there were no differences of plasma glucose and insulin concentrations between CLA and control fed cows. Endogenous glucose production and GOx increased (P < 0.01) during lactation and in 3rd week of lactation eGP was lower (P < 0.05) in cows fed CLA than control group. Total fat concentrations in liver decreased (P < 0.05) with time, but did not differ between groups. Hepatic glycogen concentrations were not affected by time or treatment. Gene expression of PEPCKc and G6Pase tended to be lower (P < 0.1 and P < 0.15) in CLA supplemented than control cows. Gene expression of CPT-1 mRNA was not affected by time or treatment. We suppose milk fat depression especially by inhibition of fatty acid synthesis in the mammary gland and a glucose sparing effect due to CLA supplementation. Even if CLA supplementation did not result in an improved energy status and although more glucose was extracted into the milk, less glucose was utilized in the mammary gland due to reduced milk fat synthesis. Because reduction in eGP of CLA-fed cows was greater than the reduction of glucose utilization in the mammary gland, there was obviously reduced glucose utilization in other tissues as well. The decreased glucose use was counterbalanced by reduced eGP to retain glucose homeostasis and enable a more efficient milk production.