What Is Vitamin A?
Vitamin A is a fat-soluble micronutrient that your body cannot synthesize, so you must obtain it from food or supplements. It exists in two distinct forms: preformed vitamin A (retinol and retinyl esters found in animal products) and provitamin A carotenoids (β-carotene, α-carotene, and β-cryptoxanthin from plants). Because it dissolves in fat rather than water, excess vitamin A accumulates in liver tissue and adipose stores, making both deficiency and toxicity possible.
Functions and Health Roles
Vitamin A underpins several critical physiological processes:
- Vision: It forms rhodopsin in the retina, enabling sight in low light and colour discrimination. Night blindness is often the first sign of deficiency.
- Immune function: It supports differentiation of immune cells and maintains mucosal barriers against pathogens.
- Growth and development: Essential during pregnancy, infancy, and childhood for skeletal and organ formation.
- Gene expression: Acts as a transcription factor for hundreds of genes throughout the body.
- Skin health: Regulates cell turnover and is used therapeutically (as isotretinoin) for severe acne.
Converting Vitamin A Units
Food labels and supplement packaging traditionally used international units (IU), but regulatory bodies now require retinol activity equivalents (RAE) because different forms have different absorption rates. The conversion factors account for bioavailability in the digestive tract:
RAE from retinol = IU retinol × 0.3 μg
RAE from supplemental β-carotene = IU supplemental β-carotene × 0.3 μg
RAE from dietary β-carotene = IU dietary β-carotene × 0.05 μg
RAE from dietary α-carotene or β-cryptoxanthin = IU × 0.025 μg
Total RAE = RAEretinol + RAEsupplemental β-carotene + RAEdietary β-carotene + RAEcarotenoids
IU— International units of vitamin A from the source (retinol, dietary carotenoids, or supplements)RAE— Retinol activity equivalents in micrograms; accounts for absorption efficiencyBioavailability factor— Multiplier reflecting how much of the ingested form your body actually absorbs and converts
Vitamin A Deficiency and Toxicity
Vitamin A deficiency remains a serious public health concern in low-income countries. Early signs include night blindness, dry skin, and recurrent infections. Prolonged deficiency can cause corneal scarring and irreversible blindness. Pregnant women and young children are at highest risk because their requirements are elevated relative to body size.
Conversely, because vitamin A accumulates in fat tissue, excessive intake—particularly from supplements or liver—can trigger hypervitaminosis A. Symptoms include nausea, headaches, bone pain, hair loss, and blurred vision. Severe or chronic overdose may cause liver cirrhosis. Carotenoid supplements are safer: excess beta-carotene is not significantly toxic because conversion to retinol is self-limiting and the body excretes it.
Practical Considerations
Understanding vitamin A requirements helps you avoid both deficiency and toxicity.
- Distinguish dietary from supplemental carotenoids — The conversion factor for supplemental beta-carotene (0.3 μg RAE per IU) is six times higher than for dietary sources (0.05 μg RAE per IU). This reflects that supplements deliver concentrated, isolated compounds whereas whole foods contain fibre and other factors that reduce absorption. Always check the label to see whether carotenoids came from food or synthesis.
- Account for individual variation — Conversion efficiency varies with genetic polymorphisms, digestive health, fat intake, and inflammation status. People with cystic fibrosis, coeliac disease, or bile duct obstruction absorb carotenoids poorly. If you have malabsorption or are taking fat-blocking medications, request higher vitamin A intake thresholds.
- Preformed vitamin A accumulates; carotenoids do not — Excess retinol stores in the liver and poses toxicity risk at intakes above 3000 μg/day for adults long-term. Conversely, your body tightly regulates conversion of dietary carotenoids to retinol, so plant-based sources are inherently safer. However, supplemental isolates bypass normal regulation, so use them cautiously.
- Check age and sex recommendations — Dietary Reference Intakes for vitamin A range from 400 μg/day for infants to 900 μg/day for adult men and 700 μg/day for adult women. Pregnancy increases needs to 770 μg/day. Never assume one recommendation fits everyone; adjust for life stage.