A recent study published by the Journal of Clinical Endocrinology and Metabolism discovered that after 3-5 years of supplementation, vitamin D stored in adipose tissue may contribute to serum vitamin D levels during times of no vitamin D input.
Researchers from this study included a total of 74 individuals from a randomized controlled trial that was exploring the preventative effect of vitamin D supplementation on type II diabetes. Participants were divided into an intervention group, who received 20,000 IU of vitamin D3 weekly, and a placebo group, who were administered a placebo pill weekly. This weekly dose of vitamin D or placebo continued for a total of five years. Patients who were still involved in the study and who had not developed type II diabetes over the five year period were invited to join this study.
In this follow-up study, 41 individuals were included from the vitamin D supplement group, and 33 individuals were included from the placebo group. The researchers required that all those included in this analysis refrain from supplementing with over 400 IU vitamin D for the duration of the study period. All participants had their vitamin D levels measured at baseline and at months 1, 3, 6, 9 and 12.
Of the participants, 29 individuals volunteered for abdominal fat biopsies in order to determine the 25(OH)D content of adipose tissue. At each meeting, anthropometric data was measured and participants answered questions about dietary intake and sun exposure patterns to adjust for outside sources of vitamin D.
This is what the researchers found:
- Average vitamin D levels were 48.8 ng/ml (122 nmol/l) and 28.4 ng/ml (71 nmol/l) in the vitamin D and placebo groups, respectively.
- Average vitamin D levels were 33.8 ng/ml (84.5 nmo/l) and 29.2 ng/ml (73.1 nmol/l) in the vitamin D and placebo groups, respectively, after 12 months.
- Vitamin D levels were significantly higher in the supplement group than in the placebo group during the entirety of the 12-month follow-up period (p < 0.01).
- For those who completed the fat biopsy, adipose vitamin D status decreased by approximately 52% over the 12 month period in both study groups.
The researchers concluded,
“…Our study strongly indicates that vitamin D stored in adipose and possibly other tissues is physiologically important and may partly explain why serum 25(OH)D levels do not fall to critically low levels during the winter.”
Martinaityte, I. et al. Vitamin D Stored in Fat Tissue During a 5-Year Intervention Affects Serum 25-Hydroxyvitamin D Levels the Following Year. Journal of Clinical Endocrinology and Metabolism, 2017.