• • maintaining the integrity of skin and mucosal barriers that protect from pathogen invasion.
  • supporting the innate (general) immune system (e.g. regulating Natural Killer (NK) cell production, supporting phagocytic activity of macrophages).
  • supporting the adaptive (specialised) immune system (e.g. development and differentiation of Th1 and Th2 cells which direct the destruction of invading cells, B cell mediated antibody responses to antigen) ^{1, 2}.

There is an approved European Commission health claim for vitamin A, stating that it “contributes to the normal function of the immune system”, and is available to foods that meet defined criteria within the EU ³.

Recommended Intakes

Vitamin A recommendations for adults vary by region:

• • China: the Chinese Nutrition Society Reference Nutrient intake (RNI) is 660mg per day for adult women and 770mg per day for adult men up to 50 years ⁴.
  • Europe: the European Food Safety Authority (EFSA) population reference daily intakes (PRI) are 650 micrograms for women and 750 micrograms for men ⁵.
  • United States: the Institute of Medicine (IOM) recommended dietary allowance (RDA) is 700 micrograms per day for women and 900 micrograms per day for men ⁶.

Dietary Sources

Vitamin A in the diet comes from two sources: preformed vitamin A (retinol and retinyl esters) and provitamin A (carotenoids).  Preformed vitamin A is found in foods from animal sources, while provitamin A  are plant pigments that include beta-carotene, alpha-carotene, and beta-cryptoxanthin.  These provitamin A carotenoids are converted into vitamin A in the body, although conversion efficiency shows considerable variation and is influenced by the food source, an individual’s vitamin A levels, and the amount eaten ⁷.

Preformed Vitamin A or retinol is found in animal products mainly including liver, fish and eggs while provitamin A sources are generally found in colourful vegetables like carrots, sweet potato and peppers (See Table 1).

Some countries such as the US routinely add vitamin A to milk and margarine while some ready-to-eat cereals are also voluntarily fortified with vitamin A.  For this reason, it is important to use local information when calculating dietary intakes.

In Western diets, retinol accounts for nearly 65% of total vitamin A intake with carotenoids making up 35% of the total ⁸ but the contribution of carotenoids is higher in countries such as Southeast Asia and Africa where it can make up to 80% of the vitamin A intake ⁹.  Recent data shows that in China, vegetables are the greatest contributor to total vitamin A intakes ¹⁰.

Table 1. Food sources of Dietary Vitamin A ⁷

Deficiency

Vitamin A deficiency is a public health problem in more than half of all countries especially those in Africa and South-East Asia ¹¹.  The most severe effects of vitamin A deficiency are seen in young children and pregnant women in low-income countries, ranging from preventable blindness to a weakened ability to fight infections.  Vitamin A deficiency is a double‑edged cycle in which illnesses like diarrhoea and measles further deplete vitamin A levels in the body.

In areas of deficiency, routine vitamin A supplementation is recommended in infants and children up to 5 years of age ¹².  Other strategies include dietary based approaches, biofortification, and food fortification.  Even in developed countries, the importance of vitamin A in the very young is recognised, e.g. it is recommended that children in the UK aged 6 months to 5 years take a vitamin supplement containing vitamins A, C and D every day ¹³.

Excess Intakes

As vitamin A is fat-soluble, it can be stored in the body, particularly the liver and excessive intakes can cause harm.  The US IOM set an upper limit of 3,000mg per day of pre-formed vitamin A for adult men and women including pregnant adults ⁴.  The EFSA have set the same upper limit for adults including women of child-bearing age, pregnant and lactating women and post-menopausal women.  Lower limits are recommended for younger groups ¹⁴.

In terms of the provitamin, beta-carotene, there is no indication that intakes from dietary sources are linked to adverse health effects.  However, smokers have been recommended to avoid consuming food supplements containing beta-carotene, and their use by the general population should be limited to the purpose of meeting vitamin A requirements ¹⁴.

Vitamin A Supplementation

Vitamin A deficiency affects not only the growth and development of children but also increases susceptibility to infectious diseases including respiratory and gastrointestinal infections ⁸.

Across Asia, India and Africa, vitamin A supplementation has been associated with a lower incidence of diarrhoea and measles among children (low quality evidence) while all-cause mortality was also reduced with supplementation (high quality evidence) ¹⁵.

A 2024 Cochrane review showed that vitamin A supplementation did not prevent or reduce the duration of acute upper respiratory infections (URTIs) in children up to seven years of age in low to middle income countries ¹⁶.  However, this was based on a limited number of studies and more research is needed.