Introduction: We assessed the efficacy of vitamin D supplementation in maintaining normal calcium and bone homeostasis in underway submariners deprived of sunlight.
Methods: Serum 25-hydroxycholecalciferol (25(OH)D), 1,25-dihydroxycholecalciferol (1,25(OH)2D), calcium, parathyroid hormone (PTH), phosphate, osteocalcin, bone specific alkaline phosphatase, and urinary levels of N-telopeptide were examined in 51 subjects aboard a submarine. These levels were obtained prior to a 76-d deployment, before and after a 6-d liberty period (deployment day 49 and 55), and on return to homeport. There were 26 subjects who received 400-lU of vitamin D daily supplementation, and 25 who received placebo.
Results: Both groups exhibited significant reductions in 25(OH)D levels in the initial submergence (a decrease from 28.3 +/- 15 ng x ml(-1) to 24.1 +/- 10 ng x ml(-1) in the experimental group and 26.3 +/- 10 ng x ml(-1) to 20.7 +/- 9 ng x ml(-1) in the controls), an increase in 25(OH)D levels not significantly different from baseline during the liberty period, and decrements in 25(OH)D on repeat submergence (22.8 +/- 10 ng x ml(-1) in experimental and 21.4 +/- 10 ng x ml(-1) in controls). Both groups exhibited an increase in post-liberty osteocalcin (20.4 +/- 6 ng x ml(-1) to 24.5 +/- 5 ng x ml(-1) for experimental and 18.3 +/- 6 to 23.5 +/- 7 ng x ml(-1) for controls), and stable serum calcium levels throughout the patrol.
Conclusions: 400-IU daily vitamin D supplementation was insufficient in maintaining serum vitamin D levels in underway submariners, engendering biochemical evidence of bone resorption and turnover. Six d sunlight exposure compensated for 49 d sunlight absence, supporting the enormous capacity of UV-B mediated vitamin D production.