Vitamin A
My-Dog.info > Dogs, Diet & Food
Vitamin 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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