Vitamin A (Retinol)

Mechanism of Action

Retinol is converted to retinal (for vision, rhodopsin cycle in rod photoreceptors) and retinoic acid (for gene regulation). Retinoic acid binds RAR/RXR nuclear receptors, regulating >500 genes involved in cell differentiation, immune function, and embryonic development. It is essential for mucosal barrier integrity (gut, respiratory, skin epithelial cell turnover), T-cell differentiation (promotes Treg and Th2 over Th1/Th17), IgA secretion, and natural killer cell function. Works synergistically with Vitamin D — both share the RXR receptor as a heterodimer partner.

Dosing Protocol

Notes

The A-D-K2 triad should be considered as a unit — these fat-soluble vitamins work synergistically and their receptors interact. Vitamin D toxicity symptoms often resolve with vitamin A supplementation (and vice versa) due to shared receptor competition. Beta-carotene conversion to retinol is impaired in ~45% of the population due to BCMO1 gene polymorphisms — these individuals NEED preformed retinol. Pregnancy warning is critical: keep below 10,000 IU preformed retinol. Zinc deficiency impairs retinol transport (retinol-binding protein requires zinc).

Stacking

• Vitamin D3
• Vitamin K2
• Zinc (essential for retinol-binding protein synthesis)
• Iron

Interactions

• Retinoid medications (isotretinoin, tretinoin) [HIGH] — Additive vitamin A toxicity risk — do not supplement retinol during retinoid therapy
• Tetracycline antibiotics [HIGH] — Combined use increases intracranial pressure (pseudotumor cerebri)
• Warfarin [MEDIUM] — High-dose vitamin A may enhance anticoagulant effect
• Orlistat [LOW] — Reduces fat-soluble vitamin absorption

Contraindications

• Pregnancy (>10,000 IU/day is teratogenic)
• Liver disease (hepatic storage and toxicity)
• Hypervitaminosis A
• Concurrent retinoid medication use

Side Effects

• Headache (chronic high dose)
• Dry skin/lips
• Hepatotoxicity (chronic excess)
• Hypercalcemia
• Hair loss (toxicity)
• Teratogenicity

Key Papers

• 10.1016/j.nut.2011.05.015
• 10.1016/j.jaci.2011.09.036
• 10.1093/ajcn/nqy180

Source Quality

Preformed retinol (retinyl palmitate or retinyl acetate) is preferred for therapeutic supplementation over beta-carotene (conversion is highly variable, 3-28:1 ratio). Cod liver oil provides natural A + D balance. Brands: Thorne Vitamin A, Carlson Vitamin A, Nordic Naturals Arctic Cod Liver Oil. Desiccated liver capsules provide food-form vitamin A.

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