VITAMIN D DEFICIENCY (RICKETS)
Rather than a vitamin, vitamin D is now considered as a prohormone to the active form 1,25-dihydroxycholecalciferol, which is synthesized endogenously, secreted under the control of another endocrinal product—parathormone (PTH) and plays a vital role in mineral metabolism.
Physiology: Vitamin D is a fat-soluble vitamin, essential for normal mineralization of growing bones (Table 6.17).
RDA for vitamin D varies from 400 IU in infants and 600 IU in older children and adolescents.
Sources: Cholecalciferol (vitamin D3) is the natural form of vitamin D, present as a preformed vitamin in animal
TABLE 6.17: Vitamin D at a glance
RDA: 400-600 IU#8725;day
Sources:
• Endogenous synthesis under sunlight (see text)
• Animal sources: Animal fats and fish liver oils
• Fortified foods
Functions:
• Gut absorption of calcium and phosphorus
• Bone mineralization
• Renal reabsorption of phosphates and ? calcium Deficiency states:
• Rickets (in children)
• Osteomalacia (in adults)
Hypervitaminosis D (develops after 1-3 months)
• Renal: Polyuria, polydipsia, renal failure
• GIT: Anorexia, vomiting
• CNS: Irritability, hypotonia
• Metastatic: Soft-tissue calcifications, osteopetrosis
sources, e.g. fats and fish liver oils. However lt;10% of the daily requirement is fulfilled by the diet.
Major bulk of the vitamin D is synthesized endogenously on exposure to ultraviolet rays in sunlight, from a natural pro-vitamin (7-dehydrocholesterol), present under the human skin. Plants do not contain vitamin D3, though another variant (calciferol or vitamin D2) may be derived by irradiation of a plant sterol - ergosterol.
Cholecalciferol, itself an inactive substance, needs to be sequentially hydroxylated in the liver and kidneys, to convert into either an active metabolite 1,25-dihydroxycholecalciferol (1,25(OH)2D3) or relatively inactive metabolite 24,25-dihydroxycholecalciferol (25(OH)D3), depending on the needs (Fig. 6.9). Active metabolite promotes calcium and phosphorus absorption from gut and kidneys as well as bone mineralization.
Functions: Vitamin D plays an important role in mineralization of bones and absorption of calcium and phosphates from gut and reabsorption from renal tubules.
6.5.1