Dietary fat effects on blood insulin, glucose utilization, and milk protein content of lactating cows.

Relationships among dietary fat, glucose and insulin in blood plasma, and milk protein were investigated to determine mechanisms by which high fat diets depress milk protein percentage of lactating cows. Glucose clearance, determined by intravenous glucose infusion tolerance tests, of cows fed high fat diets was lower and insulin release higher than of control cows. Negative relationship between glucose utilization rate and insulin release was linear (correlation -.85), suggesting that fat feeding induced insulin resistance. Feeding lipid in a protected supplement did not change production of milk or milk protein but depressed milk protein percentage and increased quantity and concentration of milk fat. Concentrations of total lipids in blood plasma were increased whereas glucose and insulin were reduced by protected lipid supplement. Dietary fat may impair amino acid transport into the mammary gland and milk protein synthesis by inducing insulin resistance.