Vitamin A Names and Forms
Vitamin A encompasses retinol (the precursor), its active metabolites (retinal and retinoic acid), provitamin A carotenoids (like β-carotene), and synthetic therapeutic retinoids used in clinical practice.
Natural Vitamin A Forms (Retinoids)
Based on the ESPEN micronutrient guideline 1, vitamin A exists in several natural forms:
Precursor and Active Metabolites
- Retinol: The precursor form that circulates bound to retinol binding protein (RBP) in plasma
- Retinal: Active metabolite responsible for vision and reproductive function
- Retinoic acid: Controls cellular growth and differentiation, exists as:
- All-trans retinoic acid
- 9-cis retinoic acid
These active metabolites function as ligands for nuclear receptors (RAR, RXR, PPARs) and activate gene expression in over 500 target genes 1.
Storage Forms
- Retinyl esters: The primary storage form in the liver, representing up to 90% of whole-body vitamin A 1
Provitamin A Carotenoids
Carotenoids are plant-derived precursors that convert to retinol in the body 2, 3:
- β-carotene (beta-carotene): The most important provitamin A carotenoid
- α-carotene (alpha-carotene): Produces both retinaldehyde and α-retinaldehyde 4
- β-cryptoxanthin: Another provitamin A carotenoid that undergoes enzymatic conversion 4
The guideline notes that carotenoid absorption varies greatly depending on food matrix and carotenoid type, with 75-100% absorption for retinoids but variable absorption for carotenoids 1.
Synthetic Therapeutic Retinoids
Multiple synthetic derivatives have been developed for dermatologic and oncologic use 5:
Oral Retinoids
- Isotretinoin (13-cis-retinoic acid)
- Acitretin
- Alitretinoin
- Bexarotene
Topical Retinoids
- Tretinoin (all-trans-retinoic acid)
- Isotretinoin
- Adapalene
- Tazarotene
- Trifarotene
Historical Context
Research from 1999 6 identifies additional natural retinoids including didehydroretinoic acid and retinaldehyde, which takes an intermediate position with ability to convert to both retinol and retinoic acid.
Key Metabolic Distinctions
The guideline emphasizes that vitamin A functions as a prohormone 1. Retinol uptake occurs via saturable carrier-mediated process at physiological doses but switches to passive diffusion at pharmacological doses 1.
Important caveat: The term "vitamin A" can refer either to retinol and its synthetic analogues OR to provitamin A carotenoids, which creates potential confusion in clinical and research contexts 3. When discussing dietary sources, most "vitamin A" intake in populations actually represents β-carotene and other carotenoids from vegetables and fruits rather than preformed retinol 3.