Vitamin A is the name for a group of fat-soluble
compounds that have a similar structure. This group includes retinol,
retinal, retinyl esters, retinoic acid, and carotene. The most easily
used type of vitamin A is retinol, often called preformed vitamin
A as it is the active form in the body.
Vitamin A is a very versatile nutrient which
serves a wide range functions. It is best known for being essential
for good eye health. It also aids in the formation and maintainenance
of healthy teeth and bones, body tissues including skin, and mucous
membranes. It may play an important role in guarding against damage
caused by oxidation (free radicals) which can contribute to premature
aging, arthritis, cancer, cardiovascular disease, and infection,
among other things.. Vitamin A is vital for normal cell growth and
cell differentiation. It also serves necessary functions related
to reproduction. Read more about the benefits
and functions of vitamin A and signs of vitamin
Vitamin A is naturally found in two forms -
preformed vitamin A and provitamin A, also known as carotene. Sources
can be separted into two broad categories - animal sources and vegetable
sources. Animal sources including things like eggs and meat. Vitamin
A, in a form called retinyl palmitate, can be obtained from beef,
calf, and chicken liver; and fish liver oils as well as many dairy
products. Vegetable sources, which come in the form of beta-carotene,
generally contain neither fat nor cholesterol. Read more about sources
of vitamin A and the daily
requirement of vitamin A.
An overdose of vitamin A can be damaging to
the bones and thin skin, resulting in weakness and brittleness.
It can also result in tiredness and vomiting. Read more about vitamin
The precursors of vitamin A (retinol) are carotenoids
(usually beta-carotene). Retinol, retinal, and retinoic acid are
called retinoids. Retinal converted in the body to retinoic acid,
a type of vitamin A which affects gene transcription. Beta-carotene,
along with the other carotenoids that can be changed by the body
into retinol, are called provitamin A carotenoids.
The structure of vitamin A is similar to carotene.
Carotene is changed into vitamin A by the liver. It is then carried
throughout the body with fat. This form of the vitamin can then
be stored in fat tissues.
The body can get vitamin A via two methods.
The first is by producing it from carotene, a vitamin precursor
obtained from vegetables like carrots, squash, and spinach. The
second is by absorbing vitamin A from animal sources. During digestion,
retinol is inserted into chylomicrons as the ester form, and then
these particles facilitate transportation to the liver. Storage
of vitamin A in liver cells takes place through the ester derivative,
however when retinol is required by other body tissues, it is de-esterifed
and released into the bloodstream as an alcohol.
Retinol then connects to a serum carrier, retinol-binding
protein, to be carried to relevant body tissues. A binding protein
located within cells, cellular retinoic acid binding protein, functions
to store and transport retinoic acid within cells.