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

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

'''Vitamin A is a general term for the group of 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. 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 Overview - Anatomy & Physiology|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 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. 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 Overview - Anatomy & Physiology|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 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 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.