The modified-relative-dose-response values in serum and milk are positively correlated over time in lactating sows with adequate vitamin A status.

The modified-relative-dose-response (MRDR) test, which requires a blood sample after dosing with 3,4-didehydroretinyl acetate (DRA), has been used to determine vitamin A (VA) status of individuals and groups worldwide. Less invasive methods using milk are in development in a swine model. Swine are a good choice for studying VA metabolism because their gastrointestinal anatomy, morphology, physiology, and VA requirements are similar to those of humans. In this study, DRA was used as a VA tracer in lactating sows to follow the metabolism of newly ingested VA. Lactating sows (n = 6) were administered 35 micromol DRA after overnight food deprivation. Blood and milk were collected at 0, 1.5, 3, 5, 7, 9, 24, and 48 h; livers were obtained at the time of killing. Samples were analyzed for didehydroretinol (DR), retinol (R), and didehydroretinyl esters (DRE). Serum DR:R was compared with that in milk and other VA indicators. DRE rapidly increased in serum, corresponding to chylomicra, whereas DR increased at a slower rate corresponding to the holo-DR:retinol-binding protein complex released from the liver. An estimated 10-20% of the dose was irreversibly lost in milk over 48 h. The mean MRDR value was 0.018 +/- 0.013 at 5 h and the mean liver VA was 0.73 +/- 0.21 micromol/g, both signifying sufficient stores. Milk and serum DR:R values were directly correlated (r = 0.64, P < 0.0001). Thus, DR:R values in milk may be a potential alternative to serum in determining VA status in lactating women. Future work is required in VA-deficient sows and women of varying VA status to determine DR trafficking and to compare DR:R values in milk with those in serum.