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==What is Vitamin D (Cholecalciferol)?==

==What is Vitamin D (Cholecalciferol)?==

Vitamin D is an '''essential fat-soluble vitamin'''. It is found in the diet in one of two forms: '''animal-derived vitamin D₃ (cholecalciferol) or plant-derived vitamin D₂ (ergocalciferol)'''. Both forms are incorporated into mixed micelles with dietary [[Fat Overview - Nutrition|fat]] and absorbed by [[Diffusion - Physiology|diffusion]] across the mucosal surface of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. Absorbed vitamin D is released into the [[Lymphatic System Overview - Anatomy & Physiology|lymphatics]] for transport to the [[Liver - Anatomy & Physiology|liver]], where it undergoes the first of two hydroxylation steps. From the liver, vitamin D is then transported to the [[Renal Anatomy - Anatomy & Physiology|kidney]] for the second and final hydroxylation step to form calcitriol. '''Calcitriol is the biologically active form of vitamin D that facilitates [[Calcium - Nutrition|calcium]] balance in the body'''.

Vitamin D is an '''[[Nutrition Glossary#Essential Nutrients|essential]] fat-soluble vitamin'''. It is found in the diet in one of two forms: '''animal-derived vitamin D₃ (cholecalciferol) or plant-derived vitamin D₂ (ergocalciferol)'''. Both forms are incorporated into mixed micelles with dietary [[Fat Overview - Nutrition|fat]] and absorbed by [[Diffusion - Physiology|diffusion]] across the mucosal surface of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. Absorbed vitamin D is released into the [[Lymphatic System Overview - Anatomy & Physiology|lymphatics]] for transport to the [[Liver - Anatomy & Physiology|liver]], where it undergoes the first of two hydroxylation steps. From the liver, vitamin D is then transported to the [[Renal Anatomy - Anatomy & Physiology|kidney]] for the second and final hydroxylation step to form calcitriol. '''Calcitriol is the biologically active form of vitamin D that facilitates [[Calcium - Nutrition|calcium]] balance in the body'''.

Humans are able to synthesise vitamin D by conversion of 7-dehydrocalciferol (a precursor to cholesterol) to cholecalciferol when skin is exposed to ultraviolet (UV) radiation, but neither dogs or cats are able to synthesise adequate levels of vitamin D with UV exposure<ref>How KL, et al. Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. Gen Comp Endocrinol 1994;96:12-18.</ref> due to high 7-dehydrocalciferol-Δ7 reductase activity<ref>Morris JG. Ineffective vitamin D synthesis in cats is reversed by an inhibitor of 7-dehydrocalciferol-Δ7 reductase. J Nutr 1999;129:903-909.</ref>.  

Humans are able to synthesise vitamin D by conversion of 7-dehydrocalciferol (a precursor to cholesterol) to cholecalciferol when skin is exposed to ultraviolet (UV) radiation, but neither dogs or cats are able to synthesise adequate levels of vitamin D with UV exposure<ref>How KL, et al. Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. Gen Comp Endocrinol 1994;96:12-18.</ref> due to high 7-dehydrocalciferol-Δ7 reductase activity<ref>Morris JG. Ineffective vitamin D synthesis in cats is reversed by an inhibitor of 7-dehydrocalciferol-Δ7 reductase. J Nutr 1999;129:903-909.</ref>.

==Why is it Important?==

==Why is it Important?==