Fig. 1

A The current vitamin A concept (the vitamin A1 concept): Summarized is the metabolic conversion starting from animal derived food products (at the left side with a reddish background), rich in vitamin A derivatives such as all-trans-retinyl esters (ATROL-ES) and all-trans-retinol (ATROL), which can be metabolized to the intermediate all-trans-retinal (ATRAL, a precursor of the visual pigment 11-cis-retinal, which performs important functions in the visual cycle) towards the active all-trans-retinoic acid (ATRA) as the bioactive metabolite of the vitamin A/vitamin A1 pathway. Besides (on the right side with a greenish background), the plant-derived food sources rich in provitamin A(1) are displayed with the double sided vitamin A(1)-precursor all-trans-β,β-carotene (ATBC) and additional carotenoids with a half-sided vitamin A(1) function such as all-trans-α,β-carotene (ACAR), all-trans-β,β-cryptoxanthin (CRYPT), 13-cis-β,β-carotene (13CBC) and 9-cis-β,β-carotene (9CBC), which are metabolized via the intermediate ATRAL to ATRA. In addition, in dashed lines, indicating a high uncertainty of this pathway, starting from 9CBC and ATRA to the potential as well as possibly to the endogenous RXR-agonist 9-cis-retinoic acid (9CRA). B The vitamin A5/X concept: Summarized is the metabolic pathway of the novel vitamin A5/X / provitamin A5/X pathway, starting from nutritional derived retinoids and carotenoids (on the left side with a grey blue background the bioactive metabolites of the vitamin A1 cluster pathway). On the right side (with a yellow background) the vitamin A5/X pathway with 9-cis-13,14-dihydroretinyl esters (9CDHROL-ES), as well as the direct nutritional precursor vitamin A5/X alcohol 9-cis-13,14-dihydroretinol (9CDHROL) and the provitamin A5/X carotenoids 9-cis-13,14-dihydro-β,β-carotene (9CDHBC) and 9-cis-β,β-carotene (9CBC), to vitamin A5/X acid, the lipid hormone, the endogenous RXR-ligand 9-cis-13,14-dihydroretinoic acid (9CDHRA) as the resulting bioactive ligand enabling RXR-mediated signalling