Vitamin A deficiency is common in poor countries and extremely rare in developed nations. Sufferers of night blindness - people who cannot see well in dim light - are more likely to have a vitamin A deficiency.

Night blindness is one of the most common signs of vitamin a deficiency. According to the World Health Organization, night blindness among pregnant women in developing nations is worryingly high.

Furthermore, pregnant women with vitamin A deficiency are more likely to die during pregnancy and childbirth, and may have problems with lactation.

People with vitamin A deficiency can also develop xerophthalmia  - dry eyes, and even complete blindness.

Between 250,000 and 500,000 malnourished children worldwide lose their eyesight each year because they do not have enough vitamin A. Half of them die within twelve months of becoming blind. A child with not enough vitamin A has a higher risk of dying from some infectious diseases, such as measles.

Low vitamin A levels make children more susceptible to diarrhoea  slow bone development, and respiratory infections. Approximately one third of all children globally are thought to be affected by vitamin A deficiency - 670,000 of whom die within their first five years of life.

It is possible to have too much vitamin A, which can lead to anorexia, irritability, abdominal pain, weakness, drowsiness, headaches, hair loss, irritability, dry skin, insomnia, weight loss, bone fractures, diarrhoea and anaemia  However, intake would have to be extremely high.

What is Vitamin A

Put simply, vitamin A is a fat-soluble vitamin. It is also known as retinol because it produces pigments in the eye's retina. The eye needs a specific metabolite - retinal - a light-absorbing substance that is crucial for low-light vision.

Vitamin A is also important for healthy teeth, skeletal tissue, soft tissue, the skin, and mucous membranes.

It also functions in a very different role, as an irreversibly oxidized form retinoic acid, which is an important hormone-like growth factor for epithelial and other cells.

In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to an alcohol - known as retinol - in the small intestine. This retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.

The associated acid - retinoic acid, a metabolite which can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina or some essential parts of the reproductive system.

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a 'retinyl group'. This structure is essential for vitamin activity.

The orange pigment of carrots - beta-carotene - can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels.

Alpha-carotene and gamma-carotene also have a single retinyl group which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.

Vitamin A comes from two main types of foods:

Retinol - a yellow, fat-soluble substance. It is the form of vitamin A absorbed when eating animal food sources. Sources include cod liver oil, butter, margarine, liver, eggs, cheese and milk.

Carotenes - such as alpha-carotene, beta-carotene, gamma-carotene, and xanthophyl beta- cryptoxanthin.

Carotene is an orange photosynthetic pigment crucial for plant photosynthesis. The orange colours of carrots, sweet potatoes and cantaloupe melons come from its carotene content. Lower carotene concentrations are what give the yellowish colouration to butter and milk-fat. Some omnivores have yellow-coloured body fat, such as chickens and humans.

Good food sources of vitamin A include Apricots, Butter, Broccoli leaf, Cantaloupe, Carrots, Cheddar cheese, Cod liver oil, Collard greens, Eggs, Fortified cereals, Kale, Liver, Mangos, Milk, Papayas, Peaches, Peas, Pumpkin, Spinach, and Sweet potatoes.

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