Quantitative secretion and maximal secretion capacity of retinol, β-carotene and α-tocopherol into cows' milk

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Abstract
Changes in the contents of retinol, α-tocopherol and β-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 α-tocopherol and β-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, α-tocopherol and β-carotene differed with stage of lactation; in addition, retinol and β-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, α-tocopherol and β-carotene into milk (expressed as mg/d) differed with sire and stage of lactation. The quantitative secretion of α-tocopherol and β-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 Km for both α-tocopherol and β-carotene varied according to sire. Overall means for Vmax for α-tocopherol and β-carotene were 32·4 and 27·5 mg/d. Thus, the daily secretion of α-tocopherol and β-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.

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