Synthetic 1α-hydroxy-vitamin D3 (1α-hydroxycholecalciferol, 1α-H.C.C.) was given orally in a dose of 25 μg. per day to groups of patients in whom calcium malabsorption is a recognised feature. Patients responded with an increase in plasma calcium and phosphate and in calcium absorption irrespective of their pre-treatment values. Net calcium resorption from bone was increased in all patients with normal renal function. In patients with renal failure, however, net bone resorption tended to decrease. The onset of action was rapid and could be seen within 24 hours. On stopping the vitamin the effects were not maintained and their disappearance depended on the duration of treatment. 1α-H.C.C. is very potent in man and will be useful in treatment of patients with calcium malabsorption. The malabsorption of calcium of elderly people is not due to failure of the absorptive mechanism in the small bowel since it responds to 1α-H.C.C. but the dose will need to be less than 25 μg. per day to avoid excessive bone resorption and hypercalcaemia.
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