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Welcome to Old Dog Paws
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Old Dog Resources
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Vitamin AVitamin A is a fat-soluble vitamin consisting of three biologically-active compounds, retinol (the most active form), retinal, and retinoic acid. Vitamin A is derived from yellow and orange pigments called carotenoids, which are synthesized by plant cells. Vegetables such as carrots and sweet potatoes, as well as dark green vegetables, contain large amounts of carotenoids, especially beta-carotene, the most active form as well as the most plentiful. Animal sources do not contain carotenoids but can be a direct source of vitamin A. Foods such as milk, eggs yolks, and liver (especially fish-liver oils) are especially rich in vitamin A. Most animals can convert vitamin A precursors such as carotenoids into active vitamin A in the lining of their gut. Cats, however, either lack the enzyme that converts carotenoids into Vitamin A or they may be deficient in it. For this reason, cats need preformed vitamin A in their diets, usually from fish liver oils and animal livers. Vitamin A plays several vital roles in the body, including vision, tissue health, skeletal and tooth development, and reproduction. In the retina, Vitamin A is involved in the formation of a light sensitive pigment that allows the eyes to adapt to variations in light intensity. Vitamin A is necessary for the formation and maintenance of cells, particularly the cells of the skin and the respiratory and gastrointestinal mucous membranes. It is involved in the development of cartilage, and is thought to also be involved in the maintenance of cell membranes and cellular division. Vitamin A also appears to be essential for sperm development in males and for normal heat cycles in females. Unlike water-soluble vitamins, which the body cannot store in any significant amounts, excess vitamin A is readily stored primarily in the liver. It is for this reason that vitamin A has a greater potential for causing toxicity and deficiencies tend to develop much more slowly than is the case with water-soluble vitamins. Vitamin A deficiency is rarely seen in dogs and cats. When it occurs, vision can be affected, causing increased sensitivity to changes in light. This eventually leads to night blindness. In young, growing animals, vitamin A deficiency causes abnormal bone growth and nervous system disorders. In adult animals, a deficiency affects reproduction, vision, and the normal functioning of tissue cells. Clinical signs may include appetite loss, eye problems, unsteadiness, skin problems, and multiple disorders in the lungs, salivary glands, and testicles. Vitamin A toxicity is uncommon in animals since the vitamin A precursor beta-carotene is not a toxic substance. However, toxicity can occur in cats fed diets composed exclusively of raw liver or other organ meats. Too much Vitamin A can lead to calcification of the skeleton. In cats, this can result in a disorder called "deforming cervical spondylosis", which is characterized by excessive boney changes on the cervical vertebrae. These changes eventually cause pain, lethargy, reluctance to move, and persistent lameness in one or both front legs. Routine supplementation of a cat's diet with liver, even if it is added to a well-balanced commercial diet, has the potential for causing skeletal problems if the practice is continued over a long period of time. For example, excessive supplementation with cod liver oil, which is very high in vitamin A and D, or other fish oils, can lead to a combined vitamin A and D overdose. Vitamin A is involved in normal growth and development of skin cells. Both an excess and a deficiency of vitamin A can lead to skin diseases in both dogs and cats. Signs include hair loss and a poor hair coat, excessive scaling, and an increased susceptibility to skin infections. Vitamin A and retinoids (natural and synthetic) are recognized as being effective in the treatment of certain skin conditions in dogs, such as "vitamin A-responsive dermatosis", a rare condition characterized by dry and scaly skin that is not due to a vitamin A deficiency. Reprinted with permission from www.animalhealthcare.ca |
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