| Summary: | Vitamin D is critical for persons with limited exposure to ultraviolet (UV) light, including space travelers. Recent studies indicate that optimal serum 25‐hydroxyvitamin D (25‐OH vit D) should be ≥ 80 nM. This study was designed to define the dose of vitamin D needed to maintain 25‐OH vit D at > 80 nM in persons with limited UV‐B exposure. It was conducted during winter in Antarctica at McMurdo Station, when UV‐B radiation levels are essentially zero. The 66 subjects recruited for this blinded study were randomly divided into 3 groups for vitamin D supplementation: 2000 IU/d (n = 18), 1000 IU/d (n = 18), and 400 IU/d (n = 19). Some subjects (n = 7) did not take supplements or took ones of their own choosing. Blood samples were collected about every 2 mo during the winter. About 5 mo after supplementation started, analysis of samples from subjects (n=9, 5, 11, and 3 for the 2000 IU/d, 1000 IU/d, 400 IU/d, and no pill groups) who completed 4 blood draws showed that 25‐OH vit D increased 63% in the 2000 IU/d group (to 82 ± 28 nM), 61% in the 1000 IU/d group (to 85 ± 36 nM), and 51% in the 400 IU/d group (to 64 ± 13 nM). It decreased 13% (to 35 ± 16 nM) in the group not taking supplements. These data will enable us to provide space crews with evidence‐based recommendations for vitamin D supplementation. The findings also have implications for other persons with limited UV light exposure, including polar workers and the elderly. This study was supported by the NASA Human Research Program.
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