Vitamin A is a vitamin which is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal. This molecule is absolutely necessary for both scotopic and color vision. Vitamin A 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.

Vitamin A is a fat-soluble vitamin. This means you don't need it every day because any of the vitamin your body doesn't need immediately is stored for future use. It helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes, and skin. One of the most important functions of this vitamin is that it maintains the health and well being of the epithelial tissues of the body. These are generally the tissues that line the openings, skin and mucous membranes.

Vitamin A helps in the growth and repair of body tissues, especially bones, and the formation and maintenance of tooth enamel and gums, it helps strengthen immunity from infections. Vitamin A prompts the secretion of gastric juices necessary for proper digestion of proteins. Night vision and the general maintenance of the eye are a function of Vitamin A, along with the proper health of the sex glands and uterus.

The eyes are obvious indicators of Vitamin A deficiency. One of the first symptoms is night blindness. Other eye indicators include dry, itchy and inflamed eyeballs. Susceptibility to colds, flu bacterial and viral infections, especially of the respiratory and urinary tract, is indicators of Vitamin A deficiency. Acne, rough, dry, scaly premature aged skins are all deficiency signs.

Sources of Vitamin A include beef, liver, carrots, apricots, sweet potatoes, dandelion greens, spinach, cantaloupe, oat flakes, swordfish, butter, raison bran, cheese, eggs, oily fish (such as mackerel), skimmed milk, fortified margarine and yoghurt. However, all of these sources -- except for the fruits and vegetables and skimmed milk that has been fortified with Vitamin A are high in saturated fat and cholesterol.

The best way to get the daily requirement of essential vitamins is to eat a balanced diet. The Food and Nutrition Board at the Institute of Medicine recommends the following daily intake of Vitamin A:

Infants
0 - 6 months: 400 micrograms per day (mcg/day)
7 - 12 months: 500 mcg/day

Children
1 - 3 years: 300 mcg/day
4 - 8 years: 400 mcg/day
9 - 13 years: 600 mcg/day

Adolescents and Adults
Males age 14 and older: 900 mcg/day
Females age 14 and older: 700 mcg/day

Specific recommendations depend on age, gender, and other factors (such as pregnancy). Women who are pregnant or producing breast milk (lactating) need higher amounts. Ask your doctor what dose is best for you. If you don't get enough Vitamin A, you are more susceptible to infectious diseases and vision problems.

Some research suggests that having more than an average of 1.5mg per day of Vitamin A over many years may affect your bones and make them more likely to fracture when you're older. Older people, particularly women, are already at risk of osteoporosis. This is where bone density reduces and so the risk of fractures increases. If you eat liver or liver products such as pâté once a week, you are likely to be having, on average, 1.5mg of Vitamin A per day.

If you aren't getting enough Vitamin D, you might be more at risk of the harmful effects of too much Vitamin A. People who may be particularly short of Vitamin D include women of Asian origin who always cover up their skin when they're outside and older people who rarely get outdoors. So if you're short of this vitamin it might be a good idea to boost the amount of Vitamin D you're getting. Good sources of Vitamin D include oily fish and eggs. The best source of Vitamin D is summer sunlight – but remember, if you're out in the sun, take care not to burn.

Many multivitamins contain Vitamin A. Other supplements, such as fish liver oil, are also high in Vitamin A. So if you take supplements containing Vitamin A, make sure you don't have more than a total of 1.5mg per day from your food and supplements. If you eat liver every week, you should avoid taking any supplements that contain Vitamin A.

If you're pregnant, having large amounts of Vitamin A can harm your unborn baby. Therefore, if you are pregnant or thinking of having a baby, you should avoid eating liver or liver products such as pâté because these are very high in Vitamin A. You should also avoid taking supplements that contain Vitamin A. Ask your GP or midwife if you would like more information.

If you get too much Vitamin A, you can become sick. Large doses of Vitamin A can also cause birth defects. Acute Vitamin A poisoning usually occurs when an adult takes several hundred thousand IU (In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on measured biological activity or effect. ). Symptoms of chronic Vitamin A poisoning may occur in adults who regularly take more than 25,000 IU a day. Babies and children are more sensitive and can become sick after taking smaller doses of Vitamin A or Vitamin A-containing products such as retinol (found in skin creams).

The conclusion that can be drawn from the newer research is that fruits and vegetables are not as useful for obtaining Vitamin A as was once thought; in other words, the IU's that these foods were reported to contain were worth much less than the same number of IU's of fat-dissolved oils and (to some extent) supplements. This is important for vegetarians. (Night blindness is prevalent in countries where little meat or Vitamin A-fortified foods are available.)