Vitamin A (Retinol): Sources, Requirements, Absorption, Deficiency and Functions
Posted on : 25-11-2017 Posted by : Admin

Introduction

Way back in 1912, Frederick Gowland Hopkins discovered that some mysterious elements in milk were essential for growth in rats. He was clear that these factors were not fats or proteins or carbohydrates. Again in 1914, these elements were re-discovered by Elmer McCollum and his group in cod liver oil. Later in 1920 these elements were termed as Vitamin A. Hopkins was awarded the Nobel prize for discovering these substances in 1929.

Vitamin A is found only in foods of animal origin. It is present in almost all species of fish, birds and mammals. The carotenoid compounds like yellow plant pigments (α, β and γ carotenes) and cryptoxanthin are precursors of vitamin A. Human body has the ability to convert these carotenoid compounds present in the diet into vitamin A.

The β-carotene gets oxidized to form two molecules of vitamin A. The other precursors like α and γ form only one molecule of vitamin A. There are two forms of vitamin A:

• Vitamin A1 found in the liver of marine water fish
• Vitamin A2 found in the liver of freshwater fish

Vitamin A which contains alcoholic group in the side chain is called as retinol & vitamin A which contains aldehyde group is known as retinal. Though the two vitamins differ slightly in their chemical structures their physiological functions are the same.

Sources of vitamin A

The liver of animals is a rich source of vitamin A especially fish liver oil is an excellent source of vitamin A. Whole milk, egg yolk, dark green leafy vegetables, deep yellow vegetables and fruits are rich in carotenes, which are converted into vitamin A by the small intestinal wall.

Requirements of vitamin A

Vitamin A is required to maintain normal blood level. The daily requirement of Vitamin A is as follows:

Absorption and storage of vitamin A

Vitamin A and carotene are absorbed from the small intestine into the lymph system. The rate of absorption of vitamin A is more rapid than that of carotene. In the human beings about 95% of vitamin A is stored in the liver and small amounts are also stored in lungs, adipose tissue and kidneys.

Deficiency of vitamin A

The first sign of deficiency is shown as an issue with vision. The eyes loose their green light  sensitivity, later eyes cannot adjust to dim light. This condition finally results in night blindness. Severe deficiency of vitamin A is shown up as ulcers in cornea a condition also known as xerophthalmia or keratomalacia.

Functions of vitamin A

The following are the functions of vitamin A,

• Vitamin A is essential for the growth and metabolism of all body cells.
• It is needed for the formation of rhodopsin pigment, which is formed from retinol and proteins. This pigment is necessary for vision in dim light.
• It helps in the maintenance of healthy skin.
• It forms mucous membrane of the cornea and lines the respiratory tract.

1. Mention the sources of Vitamin A.

2. How can the deficiency of Vitamin A affect human body?

3. List out the functions of Vitamin A.