Influence of supplemental, dietary vitamin A on retinol-binding protein concentrations in the plasma of preruminant calves

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 conce ntrations in plasma are regulated and remain constant except in extremes of vitamin A nutriture or in disease. In the present study, retinol-binding p rotein concentrations in plasma from preruminant calves supplemented with 0 , 1700 (i.e., current NRC requirement), 34,000, or 68,000 IU of vitamin A d aily from birth to 27 d of age (n = 6/treatment) were quantified. Retinol-b inding protein concentrations at birth averaged 21 mug/ml (n = 24) or appro ximately 50% of concentrations in dairy heifers and cows. plasma retinol an d retinol-binding protein concentrations were correlated positively, corrob orating the role of vitamin A nutriture in the regulation of retinol-bindin g protein secretion from the liver. In this regard; dietary vitamin A influ enced positively retinol and retinol-binding protein concentrations and, as a consequence, the degree of saturation of retinol-binding protein with re tinol. At 27 d of age, calves fed greater than or equal to 34,000 IU of vit amin A had substantially higher retinol and retinol-binding protein concent rations than did calves fed less than or equal to 1700 IU of vitamin A, ind icating that dietary vitamin A effects positively vitamin A status. The dat a also suggest that the current NRC requirement may not be sufficient to as sure vitamin A adequacy in preruminant calves. Percent saturation of retion ol-binding protein with retinol in all calves was <35%, much lower than ant icipated and suggests that the retinol requirement of vitamin A-responsive tissues exceeded vitamin A availability.