Vitamin A
Vitamin A is a group of fat-soluble compounds essential for vision (retinal in the rhodopsin cycle), epithelial cell differentiation, immune function, and reproduction. It exists as preformed retinol in animal foods and as carotenoid precursors in plants — chiefly beta-carotene, which the body converts to retinol as needed. It has the smallest safety margin of any vitamin: chronic excess is teratogenic and hepatotoxic.
Daily requirements (retinol activity equivalents)
- Adult men: 900 µg RAE/day (~3,000 IU).
- Adult women: 700 µg RAE/day (~2,333 IU).
- Pregnancy: 770 µg RAE/day.
- Lactation: 1,300 µg RAE/day.
- Tolerable Upper Intake Level: 3,000 µg RAE/day (10,000 IU) — pertains to preformed retinol only; beta-carotene has no UL.
Deficiency
Rare in the developed world; still a leading cause of childhood blindness in low-income countries. Early sign is night blindness (nyctalopia) — impaired dark adaptation. Progressive damage: xerophthalmia, Bitot's spots, corneal ulceration, permanent blindness. Immunodeficiency and increased infection risk (particularly measles) accompany deficiency.
Toxicity — the safety margin is narrow
- Acute: single doses > 200,000 IU cause nausea, vomiting, headache.
- Chronic: intakes > 25,000 IU/day for months cause liver damage, bone pain, alopecia, dry skin, and hypercalcaemia.
- Teratogenic: > 10,000 IU/day preformed retinol in pregnancy causes birth defects. Isotretinoin (a retinoid drug) is strictly contraindicated in pregnancy.
- Bone fracture risk: observational data suggest chronic intakes > 3,000 µg RAE/day are associated with reduced bone mineral density.
Beta-carotene from food does not cause vitamin A toxicity because the conversion to retinol is regulated. However, high-dose beta-carotene supplements (> 20 mg/day) increase lung cancer risk in smokers (CARET and ATBC trials) — avoid supplemental beta-carotene if you smoke.
Food sources
Preformed retinol: beef liver (a single 3-oz serving contains ~9× the
RDA — do not eat regularly), fish liver oils, egg yolk, dairy fat.
Beta-carotene: sweet potato, carrots, dark leafy greens (spinach,
kale), butternut squash, cantaloupe. Cooking with a little fat improves absorption.
Supplementation
- Most multivitamins provide 100% of the RDA (750–900 µg RAE) — appropriate for most adults.
- Don't add a stand-alone vitamin A supplement unless prescribed for a documented deficiency.
- Do not take cod liver oil AND a vitamin A supplement — cod liver oil can contain 4,500 IU per teaspoon.
- Pregnancy: prenatal vitamins typically use beta-carotene rather than retinol for safety.