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==What is Vitamin A (Retinol)?==

==What is Vitamin A (Retinol)?==

'''Vitamin A is a general term for the group of [[Nutrition Glossary#Essential Nutrients|essential]] fat-soluble retinoids'''. Retinol is the alcohol form of Vitamin A alcohol found in circulation, while retinal, retinyl esters, and β-carotene are more commonly found in foods. Dietary retinal and retinyl esters are found in animal tissues and are hydrolysed by pancreatic and intestinal enzymes in the intestinal lumen, incorporated into mixed micelles with dietary fat, and absorbed by diffusion across the mucosal surface of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. β-carotene is synthesized by plants and is considered a pro-Vitamin A molecule comprised of 2 retinol subunits. β-carotene is absorbed across the small intestine intact and in dogs is cleaved into two retinal molecules by β-carotene 15,15 dioxygenase. Cats lack this enzyme and require preformed Vitamin A in the diet<ref>Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptation. Nutr Res Rev 2002;15:153-168.</ref>.  

'''Vitamin A is a general term for the group of [[Nutrition Glossary#Essential Nutrients|essential]] fat-soluble retinoids'''. Retinol is the alcohol form of Vitamin A found in circulation, while retinal, retinyl esters, and β-carotene are more commonly found in foods. Dietary retinal and retinyl esters are found in animal tissues and are hydrolysed by pancreatic and intestinal enzymes in the intestinal lumen, incorporated into mixed micelles with dietary fat, and absorbed by diffusion across the mucosal surface of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. β-carotene is synthesized by plants and is considered a pro-Vitamin A molecule comprised of 2 retinol subunits. β-carotene is absorbed across the small intestine intact and in dogs is cleaved into two retinal molecules by β-carotene 15,15 dioxygenase. Cats lack this enzyme and require preformed Vitamin A in the diet<ref>Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptation. Nutr Res Rev 2002;15:153-168.</ref>.  

After absorption, retinol is released into the [[Lymphatic System Overview - Anatomy & Physiology|lymphatics]] with chylomicrons and transported to the [[Liver - Anatomy & Physiology|liver]] where it is bound to retinol-binding protein (RBP). '''Liver is the primary storage site of Vitamin A'''. Retinol bound to RBP is released into portal circulation and is transported to target tissues. Retinoic acid is the primary active retinol metabolite within the cell.

After absorption, retinol is released into the [[Lymphatic System Overview - Anatomy & Physiology|lymphatics]] with chylomicrons and transported to the [[Liver - Anatomy & Physiology|liver]] where it is bound to retinol-binding protein (RBP). '''Liver is the primary storage site of Vitamin A'''. Retinol bound to RBP is released into portal circulation and is transported to target tissues. Retinoic acid is the primary active retinol metabolite within the cell.

==Consequences of Vitamin A Deficiency==

==Consequences of Vitamin A Deficiency==

Vitamin A deficiencies can occur due to low dietary intake of retinol, with concurrent diseases affecting absorption of dietary fat (e.g., protein-losing enteropathy) or with abnormal storage and transport of retinol (i.e., [[:Category:Liver - General Pathology|liver disease]]).

Vitamin A deficiencies can occur due to low dietary intake of retinol, with concurrent diseases affecting absorption of dietary fat (e.g. [[Protein Losing Enteropathy|protein-losing enteropathy]]) or with abnormal storage and transport of retinol (i.e.[[:Category:Liver - General Pathology|liver disease]]).

====Dogs:====  

====Dogs:====  

Puppies fed Vitamin A deficient diets have poor growth rates and develop defective remodelling of bone during active growth<ref name="NRC"/>, especially the bones of the cranium, which can result in [[Ear - Anatomy & Physiology#Deafness|deafness]]<ref>Mallenby E. The experimental production if deafness in young animals by diet. J Physiol 1938;94:380-398.</ref>. Adult dogs fed Vitamin A deficient diets experience '''anorexia, weight loss, squamous metaplasia of mucus secreting cells that can result in xerophthalmia (dry eye) and increased susceptibility to [[Pneumonia Overview|pneumonia]]'''<ref>Russell WC and Morris ML. Vitamin A deficiency in the dog. Experimental production of vitamin A deficient condition. JAVMA 1939;95:316-320.</ref>. Cutaneous hyperkeratosis and skin lesions are also associated with Vitamin A deficiency in adult dogs<ref>Ihrke PJ and Goldschmidt MH. Vitamin A-responsive dermatosis in the dog. JAVMA 1983;182:687-690.</ref>. Adult females fed inadequate levels of Vitamin A during gestation have an increased risk of foetal abnormalities<ref name="NRC"/>.  

Puppies fed Vitamin A deficient diets have poor growth rates and develop defective [[Bone & Cartilage Development - Anatomy & Physiology|remodelling of bone]] during active growth<ref name="NRC"/>, especially the bones of the cranium, which can result in [[Ear - Anatomy & Physiology#Deafness|deafness]]<ref>Mallenby E. The experimental production if deafness in young animals by diet. J Physiol 1938;94:380-398.</ref>. Adult dogs fed Vitamin A deficient diets experience '''anorexia, weight loss, squamous metaplasia of mucus secreting cells that can result in xerophthalmia (dry eye) and increased susceptibility to [[Pneumonia Overview|pneumonia]]'''<ref>Russell WC and Morris ML. Vitamin A deficiency in the dog. Experimental production of vitamin A deficient condition. JAVMA 1939;95:316-320.</ref>. Cutaneous hyperkeratosis and skin lesions are also associated with Vitamin A deficiency in adult dogs<ref>Ihrke PJ and Goldschmidt MH. Vitamin A-responsive dermatosis in the dog. JAVMA 1983;182:687-690.</ref>. Adult females fed inadequate levels of Vitamin A during gestation have an increased risk of foetal abnormalities<ref name="NRC"/>.

 

====Cats:====  

====Cats:====  

Feeding Vitamin A deficient diets to pregnant queens can result in increased risk of '''spontaneous abortions, foetal resorptions, and [[Cleft Palate|cleft palate]] in kittens'''<ref name="NRC"/>. Adult cats fed Vitamin A deficient diets will also experience weight loss and squamous metaplasia of mucous secreting cells<ref name="NRC"/>, with xerosis and associated corneal keratinization and vascularization.

Feeding Vitamin A deficient diets to pregnant queens can result in increased risk of '''spontaneous abortions, foetal resorptions, and [[Cleft Palate|cleft palate]] in kittens'''<ref name="NRC"/>. Adult cats fed Vitamin A deficient diets will also experience weight loss and squamous metaplasia of mucous secreting cells<ref name="NRC"/>, with xerosis and associated corneal keratinization and vascularization.

==References==

==References==

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[[Category:Vitamins]]

[[Category:Vitamins]]