Vitamin A is only found in animal foods, mainly as retinol. Plants provide a source of vitamin A for animals in the form of orange yellow pigments called the carotenoids. The chief source in human nutrition is beta carotene, which the body converts to vitamin A in the intestinal mucosa during absorption.
Vitamin A has a number of different functions in the body. It is necessary for normal growth and development. If the intake of vitamin A is not sufficient for normal growth, the bones will stop growing, before the soft tissues are affected. This may result in overcrowding of the brain and nervous system, cranial pressure and consequent brain and nerve injury. Vitamin A deficiency may sometimes cause degeneration of nervous tissue also.
Function in vision: Vitamin A occurs in the retina of the eye and is required in the process of vision to adjust to light of varying intensity. It occurs in the light receptor cells in the retina in combination of protein (rhodopsin). If more vitamin A is not available, ability to adjust to changes in intensity of light is impaired. Night blindness occurs in severe vitamin A deficiency; it indicates the inability of a person to see at night when the amount of light is far too little to permit adequate vision.
Health of epithelial tissues: These tissues covers the outer surface of the body, line the major cavities and all the tubular systems in the body. Inadequate supply of vitamin A results in suppression of the normal secretions and produces a keratinized (dry) type of epithelium. The skin may become excessively dry and mucous membranes may fail to secrete normally and hence be prone to bacterial invasion.
Recommended Daily Intake
The daily requirement of an adult for vitamin A is of the order of 750mcg of retinol or 3000 mcg of beta carotene per day derived from foods of either animal or vegetable origin. The allowance for infants is 300mcg to 400mcg and the need increases gradually as the child grows to adolescence. No increased allowance during pregnancy is recommended but the allowance is increased to 1380 mcg during lactation.
Vitamin A is present in animal foods only and liver is the richest source of vitamin A. Other sources include butter, ghee, curd, egg yolk and vegetable oils if fortified with vitamin A. Vitamin A is not present in vegetable foods but these foods contain the pigment, beta carotene, which is the precursor of vitamin A and is therefore also known as provitamin A .
One molecule of beta carotene should be able to yield two molecules of vitamin A. In the body, however, the absorption and conversion of beta carotene is not perfect and may range from 25-50 percent. Leafy vegetables such as spinach, amaranth, coriander, drumstick leaves as well as ripe fruits such a mango, papaya, yellow pumpkin are good sources of beta carotene. Generally, dark green leafy vegetables contain greater amounts of carotene than those which are light in colour. Large intake of vitamin A in times of plenty are stored in the liver and help the body to tide over periods of shortage. Similarly an occasional intake of a concentrated source also helps to meet the requirements for this vitamin.
An over dosage of vitamin A may cause serious injury to health and self administration of highly potent concentrates is likely to cause serious health conditions. It is always advisable to full fill your nutrient requirements from natural sources and consult your doctor before starting any type of supplements.