658. Factors influencing the vitamin content of milk fat: IV. The source of milk fat vitamin A and carotene

Abstract

With 2 Figures)'During the course of investigations into factors affecting the vitamin content of New Zealand milk fats(i, 2,3) the need for further information about the source of the milk fat vitamin A and carotene became apparent. Experiments were therefore carried out to investigate the relationship between the levels of carotene and vitamin A ester in the blood plasma and milk fat, and to assess the relative contributions of vitamin A of hepatic origin (through the blood plasma vitamin A alcohol) and vitamin A of immediate dietary origin (through the blood plasma vitamin A ester) to the vitamin A content of the milk fat. It is well recognized that for the production of milk fat of high vitamin A potency, cows are more dependent on a daily carotene or vitamin A intake than on liver reserves of vitamin A (e.g. (4)). For other species, relationships between dietary and milk fat vitamin A have been studied (e.g. (5)) and aspects of the problem have recently been investigated in cows by Chanda and co-workers(6,7,8). It seemed, however, that further information obtained under New Zealand conditions would enable a more precise interpretation of the seasonal variations already reported (3,9,10,11) in the levels of vitamin A and carotenoids in the milk fat and blood plasma of pasture-fed cows in this country. EXPERIMENTALThe cows used were predominantly Jersey from the herds of The Dairy Research Institute (N.Z.) and Massey Agricultural College. Unless otherwise stated, all cows were milked normally twice daily, milk samples being collected at the evening milking.In the first experiment, carried out in December, two cows, which had been fed normally on pasture, were brought indoors and fed, for 11 days, a carotene-free diet consisting of linseed meal, crushed oats and bran in equal quantities and straw ad lib. as roughage. At the end of this period they were returned to pasture. Milk and blood samples were taken throughout the experiment.In the second experiment, carried out in August, the effect on the milk fat levels of a single dose of vitamin A was investigated. The cows were on pasture throughout the experiment and were dosed orally with varying levels of vitamin A.* Milk samples were collected at intervals and, in the case of one cow, simultaneous milk and blood samples were taken at frequent intervals following the dose of vitamin A. * The vitamin A was administered as an emulsion supplied by Nicholas Products Ltd., Wellington, and containing approximately 8 mg. vitamin A per gram.

659. Factors influencing the vitamin content of milk fat: V. Changes in vitamin A and carotenoid content of the blood plasma of cows and heifers near parturition in relation to secretion of these substances in the milk fat

“…For vitamin A secretion, however, the picture is complicated by the presence of both vitamin A alcohol and ester in the blood, and the above arguments presuppose esterification of vitamin A alcohol in the mammary gland. While plasma vitamin A ester may be the main source of milk fat ester (12), there seems ample evidence of the ability of the mammary gland to esterify plasma alcohol fairly completely (13). If esterification also occurs in the synthesis of colostrum, the process must be remarkably efficient, since very little vitamin A alcohol appears in colostral fat, the levels found in this investigation ranging from a maximum of l-6fig./g.…”

The Influence of Pasture on the Vitamin A and Carotene in the Milk of Cows

“…The plasma vitamin A is taken up as the alcohol form but is secreted in the colostral fat mainly as the ester (Thompson & McGillivray, 1957). In experiments in which cows were transferred from pasture to a carotenefree diet, McGillivray (1957b) showed that there was no immediate change in levels of carotene in the plasma, yet the milk-fat carotene decreased from 7 -2 to 3.0 pglg in 7 days. It seems reasonable to suggest therefore, on analogy with plasma vitamin A alcohol and ester, that there may be two distinct forms of carotene in the blood plasma.…”