James claims that cholecalicferol consumed in the diet is more metabolically active that than produced in the skin. Joan disagrees. Who is correct and why?
Joan is correct. Regardless of whether it is consumed in the diet or produced in the skin, cholecalciferol must be metabolized further before it can be used. This two-step process occurs in the liver and kidneys. First, cholecalciferol (vitamin D3) is converted to 25-hydroxyvitamin D [25-(OH) D3] in the liver. Then 25-(OH) D3 is circulated in the blood to the kidneys, where it is converted to 1,25-dihydroxyvitamin D [1,25-(OH)2 D3]. It is the 1,25-(OH)2 D3 form, also called calcitriol, that is active in the body. Calcitriol is important for calcium absorption, and conversion of 25-(OH) D3 to calcitriol (1,25-(OH)2 D3) increases when calcium concentration in the blood is low. This increase is stimulated by the actions of parathyroid hormone (PTH) produced in the parathyroid glands when more calcium is needed.
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