Vitamin A comes in two main forms within our diet, carotenoids and retinol. Both forms are converted by the body into retinal and retinoic acid. These are the active forms of vitamin A and are named after the retina where they perform a vital role in helping us see.
Vitamin A is one of the fat soluble vitamins. This means that the body can store any excess vitamin A in fatty cells, particularly the liver. Because of this storage ability it can take quite some time before the effects of vitamin A deficiency are noticed.
As well as being used in the retina to help us see in low light (night vision) vitamin A also has important function in maintaining a healthy immune and reproductive system. Vitamin A also helps to control cellular communication which is vital for correct cell differentiation and growth, especially in the development of the major organs such as the heart, lungs and kidneys.
Although rare in developed countries a number of people can still be at risk to vitamin A deficiency due to age, pregnancy or a poor diet. A lack of vitamin A can have very damaging effects on the eyes in particular causing night blindness and a condition called keratomalcia. This is a whitening of the cornea which if left untreated can cause the cornea to become completely opaque. A vitamin A deficiency can also lead to a condition called hyperkeratosis which is a build up of excess skin cells.
Because vitamin A plays such a vital part in the proper development of major organs it is especially important that pregnant and lactating women as well as children receive enough in their diet.
Likewise as vitamin A is hugely important to the proper functioning of the immune system a deficiency can lead to chronic illness and leave vulnerable groups, especially the young and elderly, at risk from infections such as pneumonia and the flu virus.
When working out the daily recommended dietary allowance for vitamin A we use a unit of measurement called Retinol Activity Equivalents (RAE). These convert to International Units (IU) differently depending on the source and type of vitamin A being taken. The more bio-available the vitamin A the more IU you get for your RAE.
As a guide 1 IU of beta-carotene vitamin A supplement will equal approximately 0.15 mcg RAE.
Therefore a supplement that provides 5000 IU of beta-carotene vitamin A will provide approximately 750 mcg RAE.
Vitamin A: Recommended Daily Allowance*:
|*All measurements expressed in mcg RAE|
A number of medicines can potentially interact with vitamin A supplements causing undesired effects. If you are taking any of the following medications, or suffer from any of the conditions mentioned please consult your doctor before taking a Vitamin A supplement.
The weight loss drug Orlistat can cause a decrease in the body's ability to absorb fat-soluble vitamins like vitamin A which can lead to low plasma levels.
Some medications are derived from retinoids, in particular some cancer treatments like bexarotene used to treat T-cell lymphoma and retinoic acid used as a treatmetn for neuroblastoma. Taking a vitamin A supplement while using these treatments could result in hypervitaminosis A.