All-trans and 13-cis-retinoic acids are normal constituents of human blood formed from all-trans-retinyl palmitate.

All-trans and 13-cis-retinoic acids are normal constituents of human blood. Blood levels of both compounds increase upon oral ingestion of all-trans-retinyl palmitate, to a greater extent with higher amounts of ingested retinyl palmitate. These data suggest a mechanism whereby levels of dietary vitamin A could influence blood levels of the known cancer chemopreventive retinoic acids.