What is Vitamin A?
Vitamin A occurs naturally in two forms:
- Vitamin A as retinol, or retinyl compounds, derived from animal sources. This is sometimes called Preformed vitamin A.
- Provitamin A, or Beta-carotene, derived from vegetable sources, which forms part of the yellow or orange pigment present in many fruits and vegetables. The human body can readily convert beta-carotene into vitamin A.
Vitamin A is a fat-soluble substance, which means that it is transported with fats in the body and stored in the fatty tissues. It is not washed out of the system via the urinary tract, as is the case with water-soluble vitamins. Fat-soluble vitamins are accumulated in the body, to be used when needed.
Vitamin A is measured in micrograms (i.e. weight) and international units (i.e. biological activity).
One microgram (mcg or µg) = 3.33 international units (iu). Although most people think of vitamin A in terms of international units, EU legislation requires it to be declared on food labels in weight as micrograms.
Rich natural sources of vitamin A (retinol) are:
- Dairy produce – butter, margarine, cheese, eggs, milk and cream.
- Offal – liver (calf, lamb, ox, pig), kidneys.
- Oily fish – herring, mackerel, salmon.
- Fish liver – cod, halibut, shark.
Beta-carotene, or provitamin A, is found in:
- Vegetables such as carrots, parsley, spinach, broccoli, endives, spring greens, watercress, lettuce, sweet potatoes, squashes and beans.
- Fruits such as melons, prunes, plums, apricots, peaches, redcurrants, cherries, blackcurrants, bananas, tangerines and tomatoes.
When eaten in the diet vitamin A, like all fats and oils, is first emulsified in the upper intestinal tract. This takes approximately 3 to 5 hours. After emulsification the esters – combinations of acid and alcohol – are split by digestive enzymes releasing free retinol. Absorption can then begin. With the aid of emulsifying agents such as bile, retinol is carried in the blood to the liver and kidneys, where it is stored to be released as and when it is required.
Absorption can be impaired by conditions that cause fat malabsorption, including low protein intake or impaired liver or pancreatic functions, which would need to be medically diagnosed. About 80% of the vitamin A content of food is fully absorbed and of this, between 30 and 50% is stored in the liver. A further quantity finds its way to the intestines and a small amount will pass through the system and be excreted as the active metabolites of the vitamin, which are water soluble.
Like most vitamins, vitamin A can be destroyed by overcooking, particularly by frying in hot fats and oils. Some drugs prevent absorption and using liquid paraffin based laxatives can reduce vitamin A activity.
Vegetables should be lightly steamed or boiled or eaten raw wherever possible. Leaving chopped or grated vegetables in the air and light will cause provitamin A loss. In canned green vegetables, levels of provitamin A are reduced by 15-20%. Drying fruit and vegetables under mild conditions will cause 20% loss, while traditional open-air drying methods destroy the vitamin content completely.
The recorded effects of vitamin A deficiency vary greatly but generally, symptoms include loss of appetite, inhibited growth in children, night blindness and recurring infections.
The first tissue to be affected and show signs of deficiency would be the skin, which would tend to dry and scale. The scalp would be affected, with dandruff and poor hair condition caused by flaking of the cells.
As well as impairing night vision vitamin A deficiency can affect the tissues of the eyes causing painful, itchy and burning eyeballs and possibly ulceration or dryness.
Since vitamin A is fat-soluble and can be stored in the body and is also so widely available in foods there should be no danger of deficiency in the western world. However, deficiencies do occur and it is important to be aware of the vitamin A content of foods to ensure that the diet is supplying adequate levels of the nutrient.
Zinc is needed to release vitamin A stored in the liver and vitamin E acts as a protector and antioxidant for vitamin A.