Vitamin A (Retinol)  does not occur as such in plant tissues but rather as its precursor carotene. Carotene, or provitamin A, which occurs in yellow fruits, carrots and particularly in dandelion and beet greens, can be converted by the body into the active vitamin. The vitamin A itself is found only in animal tissues in which the provitamin forms have already been metabolised into the vitamin. Liver, the organ in which vitamin A is stored, is the richest source of vitamin A.The liver oils of cod, whale and also halibut were administered in order to avoid a deficiency. As vitamin A has a very labile reaction to oxidation, however, and thes oils tend to oxidize easily and therefore put the animal’s metabolism under extreme strain, they are hardly used today. Vitamin A is also known as the anti-infective vitamin as it protects the tissues of the epithelial cells of the respiratory, alimentary and genito-urinary tracts enabling them to maintain their ability to secrete mucus and thus their resistance to infections. Vitamin A is stored in the liver but the liver of young bird is less able to store it than the adults’ can. Therefore a vitamin A deficiency in the diet leads to illness and disease in the young faster than in adult birds. Retarded growth, skin lesions, abnormal bone growth and low resistance to infective organisms are the deficiency symptoms of vitamin A. One of the vitamin A’s most important physiological functions is the maintneance of the visual sense organs. Insufficient vitamin A in the diet interferes with the regeneraion of visual purple in the eye, causing night blindness and leads to degenerative changes in the eye epithelium. The eye dries out. The PROVITAMIN A (Beta-Carotene) is a precursor of vitamin A and can be converted into the active vitamin by the body. Beta-Carotene is found in the green parts of plants. Also carrots have a paricularly high concentration of this substance. If the colour of the fruit or vegetable more intensive, the higher the Beta-Carotene content.
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