Quantitative secretion and maximal secretion capacity of retinol, beta-carotene and alpha-tocopherol into cows' milk.

Changes in the contents of retinol, alpha-tocopherol and beta-carotene in plasma, milk and milk fat from 38 Holstein-Friesian cows were followed during their first lactation, and the quantitative and kinetic relationships for secretion of alpha-tocopherol and beta-carotene from blood into milk were determined. The cows were assigned to three groups such that all cows in the same group had the same sire. Milk yield and milk fat content differed with stage of lactation, but not according to sire. The plasma concentrations of retinol, alpha-tocopherol and beta-carotene differed with stage of lactation; in addition, retinol and beta-carotene concentration also differed according to sire. The concentrations of all three vitamins in milk and milk fat differed according to sire and stage of lactation. Furthermore, the total secretion of retinol, alpha-tocopherol and beta-carotene into milk (expressed as mg/d) differed with sire and stage of lactation. The quantitative secretion of alpha-tocopherol and beta-carotene from blood into milk followed Michaelis-Menten kinetics for active transport across membranes. Values of maximum secretory capacity Vmax and the half-rate constant K(m) for both alpha-tocopherol and beta-carotene varied according to sire. Overall means for Vmax for alpha-tocopherol and beta-carotene were 32.4 and 2.5 mg/d. Thus, the daily secretion of alpha-tocopherol and beta-carotene is limited in quantity, and is independent of the yields of milk and milk fat. It follows that continuing breeding and management systems that focus solely on increasing milk and milk fat yield will result in a steady dilution in the milk fat of these vitamins and antioxidants important for the immune defence of the cows and oxidative stability of milk products. The genetic variation found offers the possibility of utilizing these variations in breeding systems.