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Vitamin A
Description
Vitamin A (retinol) only occurs in its preformed state in
animal foods. One microgram of retinol is known as one "Retinol
Equivalent" and the latter is also a term by which plant
precursors of vitamin A are measured:
1 R.E. = 1 µg (3.33 i.u.) retinol = 6 µg Beta
Carotene = 12 µg other provitamin A Carotenoids
STABILITY
Vitamin A is one of the
fat-soluble vitamins. The vitamin is sensitive to oxygen,
acids and ultraviolet light but is slightly more stable in
its "ester" form (i.e. as vitamin A palmitate or
acetate).
Functions
Eyes
The primary role of vitamin A is in vision. The vitamin is
essential for the formation of visual purple, an eye pigment
involved in night vision.
Cell differentiation
Vitamin A also has an important role in cell differentiation
and so is necessary for the formation of new cells.
Skin and mucus membranes
Vitamin A has a role in maintaining the health and integrity
of the Skin and mucus membranes, helping to prevent against
Infection of the nose, throat, lungs, urinary tract etc.
Other Properties
Vitamin A also functions in foetal and bone development, appetite,
taste, hearing, growth and the production of sperm.
Deficiency
Severe vitamin A deficiency
leads to various physical changes in the eye and eventually
leads to blindness. A marginal vitamin A deficiency will lead
to increased susceptibility to respiratory tract Infections
and skin problems.
Requirements
Upper safe level for daily
supplementation = 2300µg (800µg in Pregnancy)
Recommended Daily Allowance
= 800µg
Supplemental Uses
Vitamin A can be taken by
anyone who is worried that they may be at risk of marginal
vitamin A deficiency. This could include:
Vegetarians
Slimmers
Those with fat malabsorption syndrome
Vitamin A has also been used successfully in the treatment
of certain Skin conditions, e.g. Acne and Psoriasis (1,2).
In the developing world
where vitamin A deficiency is very severe, mega doSage s of
around 300,000 i.u. are given to children on a yearly or six
monthly basis. Sadly, despite this, there are about 6-7 million
new cases of blindness each year as a result of vitamin A
deficiency (3).
Safety
Vitamin A is one of the
vitamins that if taken in excess can lead to toxicity because
it is stored in the liver. However it still has a higher safety
margin in that regular daily intakes generally have to exceed
7500 µg (25000 i.u.) in women and 9000 µg (30000
i.u.) in men before toxic side effects are experienced.
Because very high intakes
of vitamin A have been reported to cause birth defects (4)
(no such cases have been reported in the UK), the Chief Medical
Officer advises that pregnant women should not eat liver.
Also, supplements of more than 800 µg vitamin A have
to carry a warning for pregnant women not to take them.
The effects of vitamin A
excess would take the form of Skin scaling, joint pains, liver
enlargement and nausea (5). Vitamin A toxicity is usually
fully reversible.
Interactions and Contra-Indications
Zinc
A deficiency of the mineral Zinc can affect the function of
vitamin A and vice versa.
Certain drugs can intereact
with vitamin A and so anyone using prescribed medication should
consult a medical practitioner before supplementing with the
vitamin.
Adverse reactions have been
reported between vitamin A and cholesterol lowering drugs.
Some patients have experienced
Headaches and double vision after combining vitamin A with
the tetracycline antibiotic Minocycline.
Side effects have been reported
after using vitamin A in combination with certain drugs prescribed
for the treatment of Acne.
Food Sources
Food (µg/100g) (i.u./100g)
Halibut liver oil 3000000 900000
Lamb’s liver 66333 19900
Cod liver oil 60000 18000
Carrots (old) 6667 12000
Butter 3283 985
Margarine 2667 800
Cheese, cheddar 1210 363
Eggs 633 190
Pig’s kidney 533 160
Milk 187 56
Mackerel 150 45
Beef 33 10
Sardines, canned 23 7
References
1. Proc Nutr Soc. 2002 Aug;61(3):397-400.
2. Fleischer AB Jr et al. Alternative therapy is commonly
used within a population of patients with Psoriasis. Cutis,
58;3:216-220, 1996.
3. "Human Nutrition and Dietetics", J S Garrow &
W P T James, Churchill Livingstove, 1993.
4. Rothman KJ et al. Teratogenicity of high vitamin A intake.
N Engl J Med, 333;21:1369-1373, 1995.
5. "Dietary Reference Values for Food, Energy and Nutrients
for the United Kingdom" Dept of Health, HMSO,1991. | 
