Micronutrient: Vitamin A
Type of Papers: Reports completed by expert consultations and published by the World Health Organization jointly with the Food and Agriculture Organization of the United Nations (FAO).
Note: Two WHO reports were summarized together as they contain much of the same information. See references below.
- Roles of vitamin A include:
- Required for normal visual functioning
- Involved in growth and development, immune function, and reproduction
- Maintains the growth and integrity of cells (particularly important for epithelial cells)
- Regulates the transcription of hundreds of genes
- Vitamin A deficiency is a serious problem, being both disabling and fatal, for children under the age of 6 years.
Prevalence of Deficiency:
- The actual prevalence of vitamin A deficiency is not known because the subclinical deficiency is difficult to measure. However, the World Health Organization has identified vitamin A deficiency as one of the top three most common deficiencies worldwide
- The World Health Organization estimates that 254 million preschool children are vitamin A deficient.
- Prevalence in the developing world ranges from 15% to 60%, with the highest rates in South-East Asia and Africa, followed by the Western Pacific
Risk Factors for Deficiency:
- Dietary factors:
- Inadequate intake of vitamin A, especially in populations who consume limited amounts of animal source foods, including low intakes of dairy products and eggs.
- It is difficult for children to consume enough vitamin A if their diet is low in animal source foods (e.g., dairy products and eggs).
- Poor nutritional status
- High rate of infection (particularly measles and diarrhea). Infection decreases serum vitamin A.
- The large requirements of vitamin A to support the rapid growth in children under 3 years of age, combined with the transition from breastfeeding to other food sources of vitamin A, increases the risk for vitamin A deficiency.
- Clinical symptoms such as xerophthalmia (dry eye condition) and night blindness can be used to diagnose vitamin A deficiency (however, night blindness is difficult to assess in children under 3 years of age).
- Subclinical vitamin A deficiency (e.g., without xerophthalmia) is difficult to diagnose.
- Low serum or plasma retinol is the preferred biochemical indicator for diagnosing vitamin A deficiency; however there are limitations to using serum retinol. For instance, serum retinol concentrations are lowered by infections, and the prevalence of health consequences do not correspond well with serum retinol levels.
Dietary Sources and Bioavailability:
- There are 2 sources of dietary vitamin A: preformed retinol and provitamin A carotenoids.
- Retinol (pre-formed vitamin A):
- This is the best source of vitamin A, and it is readily used by the body.
- Found almost exclusively in animal source foods, particularly liver and fish liver oil, egg yolk, and dairy products.
- Carotenoids (e.g., beta-carotene)
- Not as good a source as retinol. It is not readily used by the body; it must first be converted to retinol by tissues to be used.
- Found only in fruits and vegetables, particularly green leafy vegetables, yellow vegetables (e.g., squash, carrots), and yellow and orange non-citrus fruits (e.g., mangoes, apricots, papayas). Also found in red palm oil.
- Food preparation methods can improve the absorption of carotenoids. Examples include cooking, grinding, and adding oil.
- Vitamin A is fat-soluble. Fat is required in the diet for intestinal absorption.
Recommended Nutrient Intakes (µg RE/day): There are no recommended nutrient intakes (RNI) for vitamin A due to a lack of adequate data; however, the World Health Organization has estimated mean requirements for vitamin A, and the following values are estimated to be similar to an RNI.
The RNI for vitamin A [µg retinol equivalents (RE)/day]:
Recommended Safe Intake
- High intakes (mainly through supplementation) of retinol, the preformed vitamin A, can have adverse health effects. Symptoms of toxicity include liver damage, bone abnormalities, alopecia, headaches, vomiting, and skin peeling.
- High intakes of the pro-vitamin A carotenoids (beta-carotene) have not been shown to have adverse effects. This is because it is not the active form of the vitamin and is not absorbed as well (however, there may be yellowing of the skin with high intakes).
- In children, large single oral doses do not generally have adverse consequences. Occasionally, there may be temporary diarrhea or vomiting with large doses.
- The tolerable upper limit (UL) of vitamin A (the pre-formed vitamin A only) for children under 3 years of age is 600 µg RE/day. The UL has been set for healthy populations and not for malnourished populations.
- Vitamin A is fat-soluble, and is easily added to fat-based foods.
- Fortification of margarine and sugar have been effective, and vegetable oils and cereals may also be used for fortification
Health Consequences of Deficiency and Benefits of Intervention:
- Health Consequences of Deficiency:
- Vision disorders including night blindness and xerophthalmia (dry eye syndrome)
- Vitamin A deficiency is the number one cause of preventable severe visual impairment and blindness in children.
- Between 250,000 and 500,000 children become blind due to vitamin A deficiency each year, and approximately half die within one year of becoming blind.
- May contribute to slowed growth and development in children
- Increases morbidity in young children, especially increasing the incidence and severity of diarrhea and the severity of measles.
- Increases the risk of death in children (especially from diarrhea and measles). Vitamin A deficiency is a major determinant of childhood survival.
- Increases the risk for iron deficiency and anemia
- Poor reproductive health
- Health Outcomes Associated with Interventions in Vitamin A Deficient Children Include:
- Reduces mortality by 20-30% in children between the ages of 6 months and 6 years
- Prevents and treats eye disorders
- Lowers morbidity risk (especially related to severe diarrhea)
Vitamin A. Vitamin and mineral requirements in human nutrition. Report of a joint FAO/WHO expert consultation on human vitamin and mineral requirements, Bangkok, Thailand, 21–30 September 1998. 2nd ed. Geneva: World Health Organization, 2004:17-44.