Effects of fat-enriched diet and methionine on insulin sensitivity in lactating cows

The hyperinsulinemic-euglycemic clamp (EGC) technique was used to investigate the effects of calcium salts of long-chain fatty acids (LCFA-Ca) and rumen-protected Met (RPM) on insulin sensitivity in the peripheral tissues of lactating cows. Six multiparous Holstein cows were used in a 3 × 3 Latin square experiment in each 14-d period. Dietary treatments were 0 (RPM0), 20 (RPM20), and 60 (RPM60) g/d of RPM, supplemented with a diet containing 1.5% of LCFA-Ca equal to 110% of the cows' ME requirement. And as a control for the 3 LCFA-Ca-containing diets, a dietary treatment without LCFA-Ca (Con) was also included. After a 10-d adaptation period, milk samples were collected for 4 d, and EGC experiments were performed on d 14 of each treatment period. Insulin solution was infused through a jugular vein catheter at a rate of 0.1, 0.2, 0.3, and 0.4 milliunits·kg BW-1·min-1 for 30 min and then at a rate of 0.5 milliunits·kg BW-1·min-1 for 60 min. Glucose solution was variably infused to maintain plasma glucose at steady state through the same catheter. Blood samples for measurements were taken using the contralateral catheter. Plasma total cholesterol, cholesterol ester, free cholesterol, and phospholipid concentrations in RPM0 and RPM20 were higher than those in Con, whereas the concentrations in RPM60 were low at the same degree of those in RPM0 (P < 0.05). Plasma Met concentration was greatest in RPM60 (P < 0.05). In the EGC experiment, the glucose infusion rate was greater in RPM60 than in RPM0 and RPM20 and an effective concentration of insulin resulting in 50% maximal glucose infusion rate was lower in RPM60 compared with RPM0 (P < 0.05), indicating that insulin sensitivity was intensified in RPM60. Although the insulin sensitivity evaluated from the EGC data in RPM0, RPM20, and RPM60 was not different from Con, a slight decline was observed in RPM0 and insulin sensitivity in RPM60 was higher than Con. Our results from the EGC experiment demonstrated that the feeding RPM lead to increased insulin sensitivity, which suggests that dietary Met affects lipid metabolism via insulin action in lactating dairy cows fed a LCFA-Ca-containing diet.