Rickets still occurs in dark¬skinned infants who are exclusively fed on breast milk.
Cod-liver oil and sunshine exposure were recognised as the cures for rickets in the late 19th century.
There are two types of physiologically important vitamin D: cholecalci¬ferol (D3) and ergocalciferol (D2). D3 is synthesised in the skin from 7-dehydrocholesterol in cell membranes upon exposure to UVB (290-320 nrn), while D2 is plant and yeast derived and produced exogenously by UV irradiation of ergostero]
Vitamin D in the circulation is metabolised to 25-hydroxyvitamin D (25(OH)D) in the liver and further metabolised to the active metabolite, 1,25-dihydroxyvita¬min D (l,25(OH2D), in the kidney. The concentration of 1,25(OH2D is highly regulated by a variety of factors including serum parathyroid hormone and P.
The majority of circulating 25(OH)D and l,25(OH2D is bound to vitamin D binding protein and Albumin ( 10-20 %), and a small fraction is free.
The majority of circulating 25(OH)D and 1,25(OHhD is bound to vitamin D binding protein (DBP) (80-90 %) and albumin (10-20 %), while a small fraction of both 25(OH)D
Vitamin D-DBP complex is taken up by proximal tubules through the endocytic receptor megalin, after which DBP is proteolytically degraded, leaving the vitamin D metabolites for physiological action or metabolism,
The half-lives of vitamin D, 25(OH)D and 1,25(OH2D are approximately 24 h, 3 weeks and 4h respectively
Liver production of 25(OH)D is not significantly regulated and is primarily dependent on the availability of vitamin D.
Measuring the total levels of serum 25(OH)D is considered the best estimate of vitamin D nutritional status.
Vitamin D may reduces the risk of bone fracture, falls, autoimmune diseases, type 2 diabetes, CVD and cancer.
There is still epidemic vitamin D insufficiency especially among individuals living at high latitudes or with dark skin.
Serum levels of 25-hydroxyvitamin D’ (25(OH)D) are considered the best biomarker of vitamin D nutritional status. ‘Appropriate’ sunshine exposure or oral supplementation is necessary to maintain sufficient vitamin D status, which is generally accepted as serum 25(OH)D > 75 nmol/l.

Wang 2009 Epidemiology of vitamin D in health and disease . Nutrition Research Reviews vol 22 ;188-203

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