Vitamin A is a yellow viscous oil

Vitamin A is a yellow viscous oil. Chemically, it is related to carotene, the common yellow pigment of plants. The generally accepted formula follows:

When carotene is eaten, it is converted to vitamin A. Because of this, it is sometimes called a provitamin. Another provitamin of vitamin A is kitol, a dihydric alcohol found in whale liver oil. The conversion of carotene to vitamin A occurs in the intestines of higher animals. The vitamin is then stored in the liver. The livers of polar bears and Arctic foxes are so rich in vitamin A that they are toxic. The toxicity is an example of what is known as hypervitaminosis.

Vitamin A has been synthesized in several different laboratories. There are really two forms of the vitamin. One form (A1) constitutes most of the vitamin content of the livers of salt water fishes. In the livers of fresh water fishes vitamin A 2 occurs. The 2 substances give slightly different colors with antimony trichloride and can be distinguished spectroscopically.

In higher animals, vitamin A is important for growth. When rats are deprived of vitamin A, the cornea of the eye becomes horny. This condition is known as xerophthalmia. It can occur also in children. Not only the corneal epithelium, but other types of epithelia are also adversely affected in vitamin A deficiency; this may lead to lessened resistance to infection. Vitamin A has a relation to vision. In man, vitamin A deficiency causes night blindness, but the effect is not as readily produced as formerly supposed.

In so far as they have been investigated, invertebrates, with the apparent exception of the snail Helix, do not seem to require vitamin A. Aside from its relation to vision, little is known as to the reason why vitamin A is necessary. Its presence in cells can be recognized with a fluorescence microscope because of the fact that in ultraviolet light it gives a green fluorescence.