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.
Zinc is needed to release vitamin A stored in the liver and vitamin E acts as a protector and antioxidant for vitamin A.