- Animal foods: Foods rich in retinol are liver, eggs, butter, cheese, milk, fish and meat. Fish liver oils are the richest natural sources of retinal, but they are generally used as nutritional supplements rather than as food sources.
- Plant foods: The cheapest source of vitamin A is green leafy vegetables such as spinach and amaranth, which are found in great abundance in nature throughout the year. The darker the green leaves the higher the carotene content. Vitamin A also occurs in most green and yellow fruits and vegetables (e.g. papaya, mango, pumpkin) and in some roots (e.g. carrots). The most important carotenoid is beta-carotene, which has the highest vitamin A. carotenes, are converted to vitamin A in small intestine. Vegetable sources of beta-carotene are free of fat and cholesterol.
Vitamin A content of some foods is given below in the table
Retinal equivalents (RE)
Halibut liver oil
Functions of vitamin A
Vitamin A is essential for growth. Vitamin a helps in the normal growth of bones and teeth, particularly in children and young people. It is a component of rhodopsin, hence essential for night vision. Vitamin A is the “beauty vitamin”. It helps to keep the skin smooth and soft. It is also needed for the mucous membranes lining the nose, throat, and bronchial tubes, as well as the entire digestive tract, the bladder, kidneys, and pelvic organs. Prevents infection. It also plays some part in protein synthesis. It controls the action of the bone cells, so that normal growth and shape of bone are maintained. It also helps in keeping the normal fertility. It prevents a condition known as urolithiasis where urinary calculi in the form of calcium phosphate is present. Vitamin A also maintains the integrity of the epithelial tissues. A moist surface is maintained and this helps to prevent against hyperplasia, metaplasia and keratinization.
It is believed that free radicals are associated with many degenerative changes with aging. However, it is yet been confirmed. For example, on study it is found that in smokers who took high doses of beta-carotene has increased risk of cancers. With vitamin A, as with many other vitamins, a certain amount is needed, but too much is harmful for the health.
Synthesis: In the body it is synthesized from carotene. One molecule of beta carotene gives two molecule of vitamin A. hence carotene is called provitamin A. in man the liver is believed to be the only organ which performs this conversion.
The daily requirement of vitamin A in adults is 5000 IU for men and 4000 IU in women
For growing children and during puberty, lactation and pregnancy 6000-8000 IU. Foods fortified with vitamin A such as cereals, juices, dairy products can be important sources of vitamin A.
Deficiency symptoms of vitamin A
Lack of vitamin A causes night blindness or inability to see in the dim light. The mother herself can detect this condition when her child cannot see in the late evenings or find her in a darkened room. Conjunctival xerosis is a clinical sign produced by the deficiency of vitamin A. the conjunctiva becomes dry and non-wettable. Instead of looking smooth and shiny, it appears muddy and wrinkled. When vitamin A is lacking the skin becomes thick and rough. An infection appears in various parts of the body. Kidney and bladder stones are more common in people whose diets are deficient in vitamin A. deficiency of vitamin A also results in the abnormal bone growth in certain parts of vertebral column and skull. It is believed that effects on nervous system are partly due to abnormal type of growth giving pressure effect on nerves. Deficiency also interferes with the normal process of ovulation although it is not proved in man. Deficiency of vitamin A also increases the risk of cancer. The deficiency also causes defective enamel formation of the teeth and pyorrhea alveolaris.
Vitamin A deficiency
They include night blindness, conjunctival xerosis, Bitot’s spot, corneal xerosis and keratomalacia
Over dosage signs of vitamin A
Large doses of vitamin A causes drowsiness, sluggishness, severe headache, vomiting, peeling of skin, hair loss, fatigue, ulcerations in the eyes, spontaneous fracture and haemorrhages. The haemorrhagic manifestations associated with hypervitaminosis may be prevented by simultaneous administration of vitamin K. the antioxidant action of tocopherols is probably responsible for its sparing action on vitamin A and carotene. This protective effect of vitamin E is increased by other antioxidants such as ascorbic acid, etc.
How to store the vitamin A
Heat and moisture may alter the vitamin. Refrigeration is advised.