Vitamin A (Retinol): Nutritional Significance, Source Of Intake And Mechanism Of Action

Vitamins are organic substances which have to be applied in food in minute quantities to maintain the biochemical and structural integrity of many cells and tissues. The human body cannot synthesize them. Most of them are integral parts of certain coenzymes required for biochemical reactions at tissue levels.

The vitamins have been classified into fat-soluble and water-soluble groups. The fat soluble vitamins are A, D, E and K. The water soluble vitamins are C and B complex group consisting of thiamine, niacin, riboflavin, pyridoxine, biotin, cyanocobalamin, folic acid and pantothenic acid.

Vitamin A has a crucial role in normal visual processes and the maintenance of health of epithelial cells. Deficiency of vitamin A is one of the major causes of preventable blindness occurring in many developing countries. This deficiency is widely prevalent in India and about 10% of school children are affected. Its prevalence is higher in the southern states.

Source of intake: Vitamin A is found exclusively in animal foods. Rich sources of animal origin are liver, especially fish liver, butter, ghee, cheese, egg yolk and milk. Vegetable sources contain the precursor of vitamin A, i.e carotene. Rich sources of carotene include dark green leafy vegetables such as spinach and amaranth, yellow like mangoes and papaya. Red palm oil is a very rich source of carotene. In the body, carotene is converted into vitamin A. Six parts of carotene are equivalent to one part of vitamin A. Vitamin A is stable at temperatures below 100⁰C. Vitamin A is stored in the liver as retinyl esters and these stores can last for 6 to 9 months.

Vitamin A aldehyde is present ar rhodopsin in the retina. The changed undergone by rhodopsin form the molecular basis of visual excitation. Vitamin A is required for growth, reproduction and maintenance of life. Synthesis of glyceoprotein, essential for proper epithelial function also may be a major function of vitamin A. Vitamin A is essential for proper metabolism and integrity of epithelial cells in many sites. In the absence of vitamin A, the epithelium undergoes squamous metaplasia and the flattened epithelial cells are heaped upon one another. Dryness of the eyes leads to xerophthalmia. Bitot’s spots develop as whitish scaly plaques on the sclera and they consist of heaped up metaplastic epithelium. The cornea softens, opacifies, ulcerates and necroses and this condition is called keratomalacia. This leads to blindness.