Vitamin A

Vitamin A refers to a group of fat-soluble compounds—including retinol, retinal, retinoic acid, and provitamin A carotenoids (e.g., β-carotene)—that are essential for vision, cellular differentiation, and immune function. In the body it acts primarily as a regulator of gene expression, enabling the body to maintain healthy epithelial tissues, support visual phototransduction, and modulate immune responses.

• Vitamin A is a family of C₁₈ isoprenoid compounds.
• The most biologically active form, retinol, has the molecular formula C₂₀H₃₀O and an IUPAC name (2E,4E,6E,8E)-3,7-dimethyl-2,4,6,8,10‑dodecapentaen-1‑ol.
• Retinal (C₂₀H₂₈O) differs by a terminal aldehyde group, while retinoic acid (C₂₀H₂₈O₂) contains a carboxylic acid.
• Key structural features: a β-ionone ring, a polyunsaturated hydrocarbon chain with conjugated double bonds (conjugated triene system) essential for light absorption, and a terminal hydroxyl (retinol) or aldehyde (retinal) or acid (retinoic acid) functional group that determines its biochemical role.
• The molecule is lipophilic (log P ≈ 6), poorly soluble in water, and stable under low-light, neutral-pH conditions, but susceptible to oxidation and isomerization upon exposure to light, heat, or oxygen.

• Natural vitamin A is sourced from animal liver (beef, cod liver) and dairy (butter, cheese) as retinyl esters; plant sources (carrots, sweet potatoes, leafy greens) provide provitamin A carotenoids such as β-carotene, α-carotene, and β-cryptoxanthin.
• Commercially, retinyl acetate/palmitate is synthesized chemically from β-ionone through a series of condensation and reduction steps, yielding high-purity retinoids for pharmaceutical use.
• Carotenoid extracts are obtained by supercritical CO₂ or solvent extraction from carrots or algae, then saponified to yield β-carotene.
• Quality considerations include verification of RAE conversion factor (β-carotene: 2 µg RAE = 1 µg retinol), absence of oxidation products (e.g., retinyl esters > 90 % purity), and compliance with GMP and USP/Ph. Eur. standards.
• For clinical use, pharmaceutical-grade retinyl acetate is preferred for its stability and predictable bioavailability.