Vitamin A – Functions, Deficiency and Daily Needs

What is Vitamin A?

Vitamin A is a fat-soluble vitamin that the body cannot produce on its own and must therefore obtain through the diet. It exists in two main forms: retinol (preformed vitamin A), which is absorbed directly from animal-based foods, and provitamin A (e.g., beta-carotene), found in plant-based foods and converted into active vitamin A in the body.

Biological Functions

Vitamin A performs numerous vital functions in the human body:

• Vision: Vitamin A is a key component of rhodopsin, the visual pigment required for low-light and night vision.
• Immune system: It supports immune cell function and helps maintain the integrity of mucous membranes as a first line of defense against pathogens.
• Cell growth and differentiation: Vitamin A regulates the development and specialization of cells, particularly in the skin, lungs, and intestines.
• Embryonic development: It plays a critical role in normal embryonic development, especially in organ formation.
• Skin health: Vitamin A promotes skin cell renewal and is widely used in dermatology.

Dietary Sources

Animal Sources (Retinol)

• Liver (particularly rich in vitamin A)
• Fish and fish liver oil (e.g., cod liver oil)
• Milk and dairy products
• Eggs
• Butter

Plant Sources (Provitamin A / Beta-Carotene)

• Carrots, sweet potatoes, and pumpkin
• Spinach, kale, and other dark green leafy vegetables
• Mango, papaya, and apricots

Recommended Daily Intake

The recommended daily intake for vitamin A is expressed in micrograms of retinol activity equivalents (µg RAE). According to the World Health Organization (WHO) and national health authorities, the following reference values apply:

• Adult men: approx. 900 µg RAE/day
• Adult women: approx. 700 µg RAE/day
• Pregnant women: approx. 770 µg RAE/day
• Breastfeeding women: approx. 1,300 µg RAE/day
• Children: 300–600 µg RAE/day (depending on age)

Vitamin A Deficiency

Vitamin A deficiency is one of the most common micronutrient deficiencies worldwide, particularly in developing countries. Common causes include insufficient dietary intake, malabsorption syndromes (e.g., celiac disease, inflammatory bowel disease), and increased demand (e.g., during pregnancy).

Symptoms of Deficiency

• Night blindness (one of the earliest signs)
• Dry eyes (xerophthalmia) and, in severe cases, blindness
• Increased susceptibility to infections
• Dry, scaly skin (hyperkeratosis)
• Growth retardation in children

At-Risk Groups

• Children in developing countries
• Pregnant and breastfeeding women
• People with fat malabsorption disorders
• Individuals with chronic alcohol abuse

Overconsumption and Toxicity

Because vitamin A is fat-soluble, it can be stored in the body, primarily in the liver. Chronic overconsumption (hypervitaminosis A) -- typically due to excessive supplementation -- can be toxic. Symptoms of overdose include nausea, headaches, dizziness, hair loss, liver damage, and, in pregnant women, birth defects in the unborn child. The tolerable upper intake level (UL) for adults, as established by the European Food Safety Authority (EFSA), is 3,000 µg of retinol per day. Beta-carotene from plant sources is considered safe, as the body regulates its conversion to vitamin A based on need.

Supplementation

Vitamin A supplements are available in the form of retinol, retinyl acetate, or retinyl palmitate. Supplementation should only be undertaken in cases of confirmed deficiency or on medical advice. In certain parts of the world, the WHO recommends vitamin A supplementation to prevent deficiency disorders and reduce child mortality from measles.

Interactions

Vitamin A can interact with various medications and nutrients:

• Retinoids (e.g., isotretinoin used in acne treatment): Combining with vitamin A supplements can lead to dangerous overdose levels.
• Vitamin E: May help reduce the toxicity associated with high vitamin A intake.
• Zinc: Zinc is required for the transport of vitamin A in the blood; zinc deficiency can impair vitamin A utilization.
• Cholestyramine and mineral oils: Can inhibit the absorption of fat-soluble vitamins, including vitamin A.

References

1. World Health Organization (WHO): Vitamin A supplementation. Geneva, 2022. Available at: https://www.who.int/health-topics/vitamin-a-deficiency
2. National Institutes of Health (NIH), Office of Dietary Supplements: Vitamin A -- Fact Sheet for Health Professionals. Bethesda, 2023. Available at: https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
3. European Food Safety Authority (EFSA): Dietary Reference Values for Vitamin A. EFSA Journal, 2015; 13(3):4028.