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.
----------------------------------------------------
Steve Smith is a freelance copywriter specialising in
direct marketing and with a particular interest in health
products. Find out more at
http://www.sisyphuspublicationsonline.com/LiquidNutrition/In
formation.htm
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