Vitamin D hydroxylation in youth and old age.

The effects of the subcutaneous injections of 50 000 i.u. of vitamin D on 25-hydroxy-vitamin D levels were measured in young controls, old people living at home, and in elderly in-patients. Responses were substantially reduced in both groups of old people, particularly in those living at home. This phenomenon may be due to the effects of ageing or disease on 25-hydroxylase activity.