Vitamin A

Vitamin A is a fat-soluble vitamin which can be stored in the liver. It occurs in two main forms: pre-formed vitamin A (retinol, retinal and retinoid acid) which is only found in animal foods and carotenoids which are only found in plant sources.

There are over 600 carotenoids, of which only around 50 can be converted into vitamin A. The most important of these is betacarotene. Betacarotene is made up of two molecules of vitamin A joined together. It can be split in the body to produce vitamin A when needed. This is a useful protective mechanism as excess vitamin A is toxic, especially during pregnancy. On average, around half of ingested betacarotene is converted into vitamin A in the cells lining the small intestine and in the liver. It is estimated that 6 mcg betacarotene is equivalent to around 1 mcg of preformed retinol but this conversion is not always efficient, especially where intakes of other micronutrients such as zinc are low.

Why you need it

Vitamin A is a powerful antioxidant and also has a hormone-like action in the body. It binds to special receptors inside cells and regulates the way genes are read to produce enzymes and other proteins. A large number of genes are controlled by vitamin A which is essential for normal growth and development, sexual health and fertility. It maintains healthy skin, teeth, bones and mucous membranes such as those lining the eyes, nose, throat and gums.

In the eye, vitamin A is converted into a pigment known as visual purple (rhodopsin). When this pigment is exposed to light it changes and interacts with nerve endings in the back of the eye to trigger messages that are relayed to the brain. These are interpreted by the visual cortex into a series of images.

Vitamin A is important for healthy immunity and is needed in higher amounts during some infections such as measles.

Vitamin A is now known to have a major factor in keeping lungs healthy.

Research

As vitamin A and betacarotene are antioxidants, they should in theory help to protect against heart attack and cancer. Major trials have been disappointing however. A large study looking into whether taking high dose supplements containing 30mg betacarotene and 25,000 IU (7,500 mcg) vitamin A could protect against lung cancer found no benefit in non-smokers and an apparent 46% increased risk in smokers.

High dose betacarotene and vitamin A supplements are therefore not recommended for smokers, especially those who also drink alcohol. Alcohol seems to reduce the level at which vitamin A becomes toxic.

Natural intake of betacarotene and vitamin A from the diet do seem to protective against coronary heart disease and some cancers, however. This may be because they are also associated with an increased intake of other beneficial plant substances known as phytochemicals. At present, the best way to ensure a good intake of natural betacarotene is to eat at least 5 servings of fruit and vegetables - especially dark green leafy ones - daily. Supplements containing natural betacarotene along with other fruit and vegetable extracts are also available.

How much you need

  • 1 International Unit (IU) Vitamin A = 0.3 mcg retinol
  • Conversely, 1mcg vitamin A = 3.33 IU
  • The EC RDA for vitamin A is 800 mcg (2,664 IU)
  • There is no RDA for carotenoids or betacarotene

If vitamin A supplements are used, they should ideally be limited to less than 5,000 IU (1,500 mcg) per day although intakes of up to 10,000 IU (3,000 mcg) are considered safe.

Children should ideally take a supplement providing vitamins A, D and E (at doses appropriate for their age) up until at least the age of 5 unless their diet is known to provide sufficient amounts.

Symptoms that may be due to vitamin A deficiency

If vitamin A is lacking, vision deteriorates until intakes are replenished. One of the first signs of lack of vitamin A is loss of sensitivity to green light. This is followed by difficulty adapting to dim light (night blindness). More severe deficiency leads to hardening of the cornea (the transparent part of the front of the eye) followed by ulceration - a condition known as xerophthalmia. Lack of vitamin A also increases the risk of cataracts. Vitamin A deficiency is uncommon in developed countries. In under-developed countries it is common however. Sadly, it is estimated that as many as half a million people go blind from vitamin A deficiency each year.

Other symptoms of vitamin A deficiency include:

  • Increased susceptibility to infection
  • Scaly skin with raised, pimply hair follicles
  • Flaking scalp
  • Brittle, dull hair
  • Poor eyesight and night vision
  • Loss of appetite
  • Dry, burning, itchy eyes
  • Eye ulceration
  • Kidney stones
  • Inflamed gums and mucous membranes
  • Symptoms of vitamin A poisoning

Never exceed the stated dose on any supplements containing vitamin A. It has a narrow therapeutic window and intakes that are just twice as high as the recommended daily amount.
may cause problems - especially during pregnancy. Vitamin A enters the central nervous system more easily than other vitamins and in excess can cause symptoms of poisoning that include:

  • Headache
  • Blurred vision
  • Nausea
  • Hair loss
  • Itchy eyes
  • Aches and pains
  • Skin sores

Around 10 - 15 cases of vitamin A toxicity are reported in the US each year, usually from taking vitamin A supplements at doses above 30,000 mcg (100,000 IU) daily. Toxicity has also occurred in arctic explorers who ate polar bear liver - which contains such a high level of vitamin A that eating just 100g can be fatal.

In a study of over 22,700 pregnant women, those obtaining intakes of more than 15,000 IU (4,500mcg) of preformed vitamin A from both food and supplements had a three to four fold increased risk of delivering a baby with congenital defects compared with intakes of 5,000IU (1,500mcg) daily. For vitamin A from supplements alone, those obtaining 10,000IU (3,000mcg) daily had a four to five fold increase risk of having a baby with congenital abnormalities. The most dangerous time to take excess vitamin A during pregnancy seems to be within the first seven weeks of pregnancy. The researchers concluded that, of the women taking more than 10,000 IU vitamin A per day, one infant in 57 had abnormalities due to the supplement.

In contrast, a study investigating women who had had a baby with congenital abnormalities found no difference in risk between those taking vitamin A supplements and those with intakes above 10,000 IU per day.

To be on the safe side however, only take supplements during pregnancy that are specifically designed for pregnant women. These will either contain no vitamin A, or sensible levels of vitamin A as deficiency can also cause problems. You should also avoid eating liver and liver products during pregnancy because of their high vitamin A content.

Excess betacarotene (e.g. from drinking large quantities of carrot juice) causes a yellow-orange pigmentation of the skin similar to cheap fake tan. This is probably not harmful and quickly fades once intakes are reduced.

Foods containing preformed vitamin A (retinol) include

  • animal and fish liver
  • kidneys
  • eggs
  • milk
  • cheese
  • yogurt
  • butter
  • oily fish
  • meat
  • margarine - which is fortified by law to contain as much vitamin A as that found in butter.

Foods containing betacarotene include

Dark green leafy vegetables and yellow-orange fruits e.g.

  • carrots
  • sweet potatoes
  • spinach
  • broccoli
  • parsley
  • spinach
  • watercress
  • spring greens
  • cantaloupe melons
  • oranges
  • apricots
  • peaches
  • mango
  • red-yellow peppers
  • tomatoes
  • sweet corn

Vitamin A is easily destroyed by exposure to light. Betacarotene is destroyed by heat and overcooking.


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