Vitamin A is probably best known for its role in eye health and promoting good vision and the legend that eating carrots is helpful for night vision, in particular, is based on the high levels of betacarotene that they contain. It is indeed true that retinol, one of the products of beta carotene within the body, is essential for the production of adequate amounts of rhodopsin, a substance also known as "visual purple". Adequate amounts of visual purple in the light receptor cells of the retina are vital for good night vision. Deficiency of retinol vitamin A is therefore commonly associated with the condition known as night blindness which is in fact the first symptom of the deficiency. If deficiency persists it may ultimately lead to damage to the cornea and even blindness; sadly still a major cause of blindness in the developing world.



More generally, vitamin A provides very good examples of the holistic functioning of the body's countless systems; in particular the way in which various nutrients depend upon each other if they're to operate effectively. A deficiency of the essential mineral, zinc, for example, has an inhibiting effect on the process by which vitamin A is metabolised and activated for use within the body.

Deficiency of vitamin A, on the other hand, is known to contribute to the anaemia caused by iron deficiency. It appears that vitamin A is essential to make iron available for the production of oxygen carrying red blood cells and supplementation with vitamin A has therefore been shown to help in the alleviation of anaemia when combined with the supplements of iron which are of course also necessary.

Not surprisingly, therefore, vitamin A is also required for the proper functioning of the immune system and in particular for the development of the white blood cells which are vital for the body's effective immune response. Deficiency in vitamin A has been shown to lead to an increase in the incidence and severity of various infectious diseases, including HIV and measles, which remain a major cause of mortality in the developing world, particularly amongst children.

Vitamin A is also known as a powerful anti-oxidant which operates with vitamins C and E, and the minerals selenium and zinc, to destroy both fat and water soluble free radicals. So important is this anti-oxidant role of vitamin A, that some research has suggested it may play a part in combatting certain common cancers, although this issue remains controversial. There are two types of vitamin A of which to be aware; retinol, also known as preformed vitamin A, and the provitamin A carotenoids, of which betacarotene is the most important and best known, which may be converted to retinol within the body. Rich food sources of retinol vitamin A are meat, especially offal such as liver, oily fish and fish liver oil, and dairy produce. Betacarotene and other carotenoids are principally derived from fruits and vegetables.

The US Recommended Dietary Allowance (RDA) for vitamin A is 3,000 IU (900 mcg) per day for adolescents over 14 and adults. In Europe the recommended figures are slightly lower at 2,664 IU (800 mcg). Both these figures are supposed to be sufficient to obtain the many health benefits of the vitamin, but these are so numerous and important that it is probably wise to regard the RDA as the minimum necessary for the avoidance of deficiency. Supplementing to a total intake of 5,000 IU should ensure optimum benefits and levels of up to 10,000 should do no harm in most cases.

The one very important exception to this is pregnant women and those seeking to become pregnant, for whom intakes of 5,000 IU and above may increase the risk of birth defects. Women in these categories should supplement only with the much less potent betacarotene, if at all, and should also avoid the high retinol foods identified above.

Some caution is required for all people, however, because being fat soluble, vitamin A is stored in the liver and can in rare instances build up to levels which may give rise to problems. Of course this characteristic of the vitamin is not confined to the human liver, and writers on this subject are fond of pointing out by way of example, apparently in all seriousness, that polar bear liver is likely to contain a concentration of vitamin A which is toxic to humans, and should therefore be avoided as a foodstuff.

At the risk of stating the obvious, that's unlikely to present any significant practical difficulties for most of us. And with the exception of pregnancy, the potentially serious consequences of outright vitamin A toxicity seem generally to have arisen only from very large doses.

So exercise a little common sense, and if you can just manage to steer clear of that polar bear liver you should be able to enjoy the benefits of vitamin A without any problems.