Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2000
Publication Date: N/A
Citation: Interpretive Summary: The vitamin A status of newborn calves is considered marginal when compared to that of adult cattle. During the first weeks of life, it is essential that calves receive sufficient vitamin A to ensure optimal growth and disease resistance. More than 50% of calves produced in the United States are fed milk replacers. The vitamin A content of many milk replacers exceeds current recommendations of the National Research Council by 10- to 20-fold. This study evaluated the impact of dietary vitamin A on the levels of a specific protein, referred to as retinol-binding protein (RBP), in the blood of milk replacer-fed calves. This protein transports exclusively vitamin A (i.e. retinol) from the liver (a storage depot for vitamin A) to vitamin A-responsive tissues and as a consequence is important in the metabolism of vitamin A. RBP concentrations at birth were approximately 50% of those in adult cattle, reflective of a sub- optimal vitamin A status. As shown in other species, dietary vitamin A influenced positively RBP levels and as a consequence the vitamin A status of the young calf. Ratios between retinol and RBP levels in all calves were much lower than anticipated, suggesting that the vitamin A requirement of vitamin-responsive tissues exceeded the availability of vitamin A. In conclusion, supplementing young calves with vitamin A in excess of current NRC recommendations may be necessary to optimize vitamin A status. The results of this study will greatly benefit the dairy industry worldwide.
Technical Abstract: Transport of retinol (vitamin A alcohol) from retinoid stores in the liver to target tissues is accomplished exclusively by a specific plasma protein, retinol-binding protein. Within individuals, retinol-binding protein concentrations in plasma are regulated and remain constant except in extremes of vitamin A nutriture or in disease. In the present study, retinol-binding protein concentrations in plasma from preruminant calves supplemented with 0, 1700 (i.e. current NRC requirement), 34,000 or 68,000 IU of vitamin A daily from birth to 27 d of age (n=6/treatment) were quantified. Retinol-binding protein concentrations at birth averaged 21 ug/ml (n=24) or approximately 50% of concentrations in dairy heifers and cows. Plasma retinol and retinol-binding protein concentrations were correlated positively, corroborating the role of vitamin A nutriture in the regulation of retinol-binding protein secretion from the liver. In this regard, dietary vitamin A influenced positively retinol and retinol- binding protein concentrations and as a consequence the degree of saturation of retinol-binding protein with retinol. At 27 d of age, calves fed greater than/equal to 34,000 IU of vitamin A had substantially higher retinol and retinol-binding protein concentrations than did calves fed less than/equal to 1700 IU of vitamin A, indicating that dietary vitamin A effects positively vitamin A status. The data also suggest that the current NRC requirement may not be sufficient to assure vitamin A adequacy in preruminant calves. Percent saturation of RBP with retinol in all calves was less than 35%, much lower than anticipated and suggests that the retinol requirement of vitamin A-responsive tissues exceeded vitamin A availability.