Vitamin D Stored in Fat Tissue During a 5-Year Intervention Affects Serum 25-Hydroxyvitamin D Levels the Following Year.

Context: Vitamin D and 25-hydroxyvitamin D [25(OH)D] are stored in adipose tissue, but the clinical relevance is uncertain.

Objective: To evaluate changes in serum 25(OH)D and adipose tissue vitamin D levels after stopping vitamin D supplementation.

Design: A prospective, double-blind cohort follow-up study.

Setting: Clinical Research Unit at University Hospital of North Norway.

Patients: Seventy-six subjects were included after participation in a 3- to 5-year prevention of type 2 diabetes study and were administered 20,000 IU of vitamin D or placebo per week.

Intervention: During the 12-month follow-up period, blood samples were drawn at the beginning and after 1, 3, 6, 9, and 12 months. Fat biopsies were taken at the start and end.

Main Outcome Measures: Changes in 25(OH)D level in serum and 25(OH)D and vitamin D levels in adipose tissue.

Results: Forty-one of 42 subjects who were given vitamin D and 33 of 34 subjects who were given placebo completed the study. At the inclusion, mean serum 25(OH)D levels were 122 and 71 nmol/L in the vitamin D and placebo groups, respectively. Serum 25(OH)D levels were significantly higher in the vitamin D group than in the placebo group throughout and were 84.5 and 73.1 nmol/L, respectively, after 12 months. In the vitamin D group, adipose tissue vitamin D levels decreased by 52% over 12 months.

Conclusion: Vitamin D and 25(OH)D stored in adipose tissue after 3 to 5 years of vitamin D supplementation may have a clinically relevant effect on serum 25(OH)D level the following year.