Ingested or skin produced vitamin D is either hydroxylated in the liver to 25-hydroxyvitamin
D (25(OH)D), metabolized and excreted in the urine, or stored in adipose and other tissues.
The capacity for vitamin D storage in adipose tissue is not known, nor the importance of such
storage which may potentially be of vital importance when intake or solar exposure is
limited. In the present study we will include 76 subjects who have participated in an
intervention study with vitamin D (20.000 IU per week) versus placebo for the prevention of
type 2 diabetes, and who have completed the study after 5 years or who have been excluded
because of diagnosed type 2 diabetes or for other reasons. If vitamin D is stored to any
extent in the body the subjects given 20.000 IU vitamin D per week for 2-5 years will have a
considerable amount of stored vitamin D and accordingly, a slow decline in serum 25(OH)D
during the following year without vitamin D substitution, which will be measured in the
present study. If our hypothesis is correct, that vitamin D can be stored in significant
amounts when the supply is abundant; current advice on vitamin D supplementation mainly
during winter should be changed to "year around" in order to build up sufficient stores for
the months without sufficient sun light.