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WELLNESS AND NUTRITION
The
Crucial Benefits Of Vitamin A
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
October 2007
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