Vitamins come in two different types--water-soluble and fat-soluble. Water-soluble vitamins can't be stored in the body in excessive amounts because they dissolve in the water of the body and are flushed out. Fat-soluble vitamins, like vitamin A, can get stored in body fat. If too much is consumed, these vitamins can build up to toxic levels, even in dogs.
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Vitamin A
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Vitamin A is made with three components--retinol, retinal and retinoic acid. Vitamin A comes from carotenoids, which are yellow and orange pigments synthesized by plant cells.
Foods
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Vitamin A is absorbed by the body through foods such as carrots, sweet potatoes and dark green vegetables, all of which contain large amounts of beta-carotene, the most active carotenoid. Other food sources that do not contain carotenoids but still provide the body with vitamin A include milk, egg yolks and liver--especially fish liver oils.
Function
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Vitamin A helps a dog's body with its vision, tissue health, tooth and skeletal development and reproduction issues.
Toxicity
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Because vitamin A is fat-soluble, excess vitamin A can become stored up in the liver quite easily and can potentially become toxic if you don't watch your dog's diet. It is unlikely that your dog will reach toxic levels of vitamin A because beta-carotene, the largest carotenoid, is not a toxic substance in any amount. If your dog's diet is very high in raw liver and other organ meats, however, toxicity can occur.
Effects
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If vitamin A levels do reach toxic levels in your dog, the effects include things such as calcification of the skeleton and skin diseases. Calcification of the skeleton manifests itself in what is called "deforming cervical spondylosis," which causes pain and lethargy as well as a reluctance to move. Skin diseases will show effects such as hair loss or a poor looking coat, scaling and a susceptibility to skin infections.
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