Vitamin A

The Fat Soluble Vitamins · Vitamin A · Vitamin D · Vitamin E · Vitamin K The Water Soluble Vitamins · The B-Group Vitamins · Thiamine - Vitamin B1 · Riboflavin-Vitamin B2 · Pantothenic Acid - Vitamin B3 · Biotin - Vitamin B4 · Nicotinic Acid (Niacin) - Vitamin B5 · Pyridoxine - Vitamin B6 · Cyanocobalamin - Vitamin B12 · Ascorbic Acid - Vitamin C

Vitamin A is one of the most versatile vitamins with roles in such diverse functions as vision, maintenance of body linings and skin, bone growth and reproduction.

It is also the most colourful, the active form RETINOL is yellow and the precursor form CAROTENE is bright orange making its presence in foods quite obvious at times.

When carotene is converted to retinol in the body, losses occur, so rather than express the amount of excretions in foods, we use the retinol equivalent (RE) - the amount of retinol the body actually receives from foods after conversion.

The body can make about one unit of retinol from about three units of carotene.  Vitamin A is found only in foods of animal origin, its precursor or provitamin Beta carotene, is the pigment most commonly seen in vegetables such as carrots.

Physiological Action
Vitamin A is essential for vision in dim light.  The viability of the eye to see in dim light is dependent on the presence of the retinal pigment, RHODOPSIN, (VISUAL PURPLE) which is bleached in the presence of light and in the process acts as a stimulus to the rods of the retina. The bleaching of visual purple is the means whereby the human eye can see at night.

Deficiency
People suffering with Vitamin A deficiency are often troubled with night blindness.  The mineral zinc is also required to make rhodopsin.  A more severe deficiency of Vitamin A can result in the accumulation of a protein, keratin, within the cornea of the eye and eventually lead to a condition known as xerophthalmia, which results in blindness.  It can be reversed if caught early enough.

Vitamin A is needed for all epithelial tissues (external skin and internal linings).  The skin and all of the protective linings of the lungs, intestines, vagina, urinary tract and bladder serve as barriers to infection from bacteria, or to damage from other sources.

If Vitamin A is deficient, some of the cells in these areas are displaced by cells that secrete keratin, which is normally produced only in the hair and fingernails.  Keratin makes the surface dry, hard and vulnerable to infection.  The cells then cannot perform their jobs, they die and accumulate on the surface where they become hosts to bacterial infection.

Adequate Vitamin A has been shown to be important in the prevention of some cancers. Healthy skin and internal linings are able to interrupt the process by which cancers begin, but Vitamin A deficiency handicaps this defence; skin, lining, bladder and larynx cancers become more likely when Vitamin A or carotene is lacking.

Vitamin A also assists in bone growth.  In children, failure to grow is one of the first signs of poor Vitamin A status.  Vitamin A is also required for normal reproduction, the body's response to stress, normal metabolism, the nervous system, immunity and in blood making.

Vitamin A deficiency is a vast problem in underdeveloped countries, where many children suffer from stunted growth, poor appetite, impaired immunity to infections.

Toxicity
Toxicity is likely for people who take excess Vitamin A in capsule form.  It can cause many symptoms, including hair loss, joint pain, stunted growth, bone and muscle soreness, amenorrhoea, nausea, diarrhoea, rashes and enlargement of the liver and spleen.  Taken during pregnancy, Vitamin A megadoses can cause major birth defects.  Carotene or Vitamin A in food, rarely cause toxic effects if eaten in large quantities.  The best way to ensure a safe Vitamin A intake is to eat foods which contain it, on a regular basis.

Dietary Sources
Foods from animal sources that include some fat contain the active Vitamin, retinol.  Among the richest sources are liver, cod-liver oil, butter, egg yolks, whole milk, cream and cheese made from whole milk or cream.  Plant sources which contain carotene are carrots, wheat, potatoes, pumpkin, cantaloupe, apricots and peaches.

Other good plant sources are dark green leafy vegetables such as broccoli and spinach, which owe their colour to the presence of chlorophyll and carotene.

Recommended Dietary Intakes
The amount of Vitamin A a person needs is proportional to body weight.  Being practically insoluble in water any dietary excesses are not excreted in the urine.  Intakes above the body's daily requirements are stored mainly by the liver where it is constantly available for use.

The RDI's for Vitamin A are expressed as retinol equivalent as previously discussed.  An average Australian male or female requires a minimum of 750 micrograms retinol equivalents daily.