Vitamin A or Retinol

Vitamin A is a fat-soluble vitamin that occurs in two forms in nature.
It is found in its true form (also called retinol) in animal foods such as fish oils and liver. The body readily uses this form.

Vitamin A can be found in vegetables in the form of beta-carotene or provitamin A. This form is found in plants and is the precursor of the actual vitamin. Beta-carotene has to be converted in the body in order to be used by it. Fat and bile are needed for the conversion.

The liver regulates the blood level of vitamin A. It needs a special protein carrier to be transported throughout the body.
An adequate protein and fat intake is required for a good absorption of vitamin A.

Roles
It is an anti-oxidant, a compound that may protect against disease by neutralizing unstable oxygen molecules, called free radicals, within the body.
This vitamin is involved in the night vision, growth, cell differentiation and reproduction. It also maintains the health of the skin (prevents acne and dermatitis) and surface tissues especially those with mucous linings. These linings are the body first defense against infection that is why vitamin A helps fight colds and infections, particularly in the mucous membranes of the eyes, ear, nose, throat, lungs and bladder.
It may also reduce breast cancer.

Units of measurement
Vitamin and mineral amounts are usually expressed in milligrams (mg) or micrograms (mcg).
Vitamin A values are expressed in different ways.
The nutriment was originally measured in I.U (international unit) but in 1974 the United States began using a measurement called Retinol Equivalents (REs).