α‐Retinol acts as a chemical tracer for the RBP4‐independent distrubution of vitamin A in lactating sows

Abstract

α‐Retinol (αR) is a structural analog of retinol that does not bind to serum retinol‐binding protein (RBP4). In this study, α‐retinyl acetate (αRA) was synthesized and given orally (35 μmol) to vitamin A (VA)‐deficient lactating sows (n = 11) as a tracer for RBP4‐independent retinol transport and tissue uptake. Serum αR concentrations peaked at 2 h (70 ± 23 nmol/L, mean ± SEM) primarily as four distinct α‐retinyl esters. Peak αR milk concentrations were detected at 7.5 h (371 ± 83 nmol/L). Sow livers collected 25 d post‐dose contained a high percentage of the αR dose (52 ± 15%, mean ± SD) likely due to the inability of RBP4 to transport αR from the liver. Nursing piglets (n = 17) killed 3 days after the αRA‐dosing of sows (n = 5) had 2.2 ± 0.7% of the αRA dose in their livers, which correlated to an estimated 15–26% transfer of the αRA dose from individual sows to their litters. Total liver retinol levels for all sows and piglets were <0.1 μmol/g liver confirming their VA‐deficient status. RBP4‐independent mechanisms play a significant role in the uptake of retinol into milk when single, large doses of retinyl acetate are given to VA‐deficient lactating women in developing nations. αR has great potential as a tracer for RBP4‐independent retinoid bioprocesses. Supported by USDA‐NRI 2007‐35200‐17729.