Vitamin A can be obtained in two forms:
- preformed retinol (retinyl esters) found in animal derived foods
• carotenoids which are mainly plant derived (beta carotene being the most abundant carotenoid), some of which can be converted to retinol in the body; 6mg of beta carotene is equivalent to 1mg of retinol.
The total vitamin A content of the diet (from both animal and plant sources) is normally expressed as retinol equivalents (RE).
Vitamin A is essential to the normal structure and function of the skin and mucous membranes such as in the eyes, lungs and digestive system. Therefore, it is vital for vision, embryonic development, growth and cellular differentiation, and the immune system.
Vitamin A deficiency is a serious public health problem worldwide,. It can lead to night blindness (impaired adaptation to low-intensity light) and an eye condition called xerophthalmia (dryness of the conjunctiva and cornea) and eventually total blindness. Marginal deficiency contributes to childhood susceptibility to infection, and therefore morbidity and mortality, in both developing and developed countries. Vitamin A deficiency is common in many developing countries especially among young children.
In the UK, frank deficiency is rare but low intakes are relatively common. For example, depending on age and sex between 6% and 20% of children have intakes that are unlikely to be adequate (below the Lower Reference Nutrient intake, LRNI). In adults, intakes tend to be higher although 16% of men aged 19-24 have intakes below the LRNI. In the UK, supplements containing 233µg of vitamin A are recommended for infants and young children from age 1 to 5 years (from 6 months for infants that receive breast milk as their main drink).
Excess retinol during pregnancy can increase the risk of birth defects. As a precautionary measure, women who are pregnant, or who might become pregnant, are advised not to consume high dose vitamin A supplements (>2800-3300 μg/day). Liver and liver products may contain a large amount of vitamin A, so these should also be avoided in pregnancy.
Large amounts of retinol can also cause liver and bone damage. To prevent adverse effects on bones, intakes above 1500 µgrams of retinol equivalents from food or supplements should be avoided. The Food Standards Agency advises that, as a precaution, regular consumers of liver (once a week or more) should not increase their intake of liver or take supplements containing retinol (for example, cod liver oil).
Liver, whole milk, cheese, butter and many reduced fat spreads are dietary sources of retinol. Carrots, dark green leafy vegetables and orange-coloured fruits, e.g. mangoes and apricots are dietary sources of carotenoids. Vitamin A is also often voluntarily added to reduced fat spreads, as is vitamin D.