EFFICACY OF ORAL VITAMIN D SUPPLEMENTATION OR SIMULATED SUNLIGHT ON BONE DURING MILITARY TRAINING

Abstract

25-hydroxyvitamin D (25(OH)D) ≥50 nmol·L-1 is essential for bone health. Vitamin D deficiency during arduous training has implications for increased stress fracture risk. Vitamin D is mainly synthesised from skin exposure to the sun; around 20% of vitamin D is obtained from the diet. PURPOSE: This study compared the efficacy of vitamin D supplementation by oral pill versus simulated sunlight on bone macro- and micro-architecture during a 13-week military training course in winter. METHODS: Eighty male infantry recruits (mean ± SD, age 22 ± 3 years, height 1.78 ± 0.07 m, body mass 77.9 ± 10.7 kg) received oral vitamin D3 (1,000 IU·D-1 for 4 weeks and then 400 IU·d-1 for 8 weeks, n = 21), an oral placebo (n = 19), solar-simulated radiation (SSR, 1.3 × standard erythemal dose in T-shirt and shorts, 3 d·wk-1 for 4 weeks and 1 d·wk-1 for 8 weeks, n = 22), or placebo SSR (n = 18). Total 25(OH)D was measured by LC-MS/MS in week 1, 5 and 13, and bone density and structure was assessed at the ultra-distal dominant tibia by HR-pQCT (Xtreme CTI, Scanco) in week 1 and 13. RESULTS: Oral vitamin D3 and SSR increased 25(OH)D from week 1 (51 ± 22 and 43 ± 21 nmol·L-1) to week 5 (78 ± 23 and 79 ± 15 nmol·L-1) and week 13 (75 ± 15 and 76 ± 12 nmol·L-1 , P < 0.001). The oral and SSR placebos did not change 25(OH)D from week 1 (51 ± 40 and 41 ± 17 nmol·L-1) to week 5 (45 ± 4 and 41 ± 16 nmol·L-1), but 25(OH)D increased in both groups by week 13 (60 ± 4 and 65 ± 15 nmol·L-1, P ≤ 0.010). Supplementation and training did not affect total, trabecular or cortical volumetric bone mineral density, cortical area, trabecular volume, number or thickness, or cortical pore diameter (main effects of time, P ≥ 0.105; group × time interactions, P ≥ 0.258). Training increased cortical thickness (1.30 ± 0.25 to 1.32 ± 0.25 mm), and reduced trabecular area (682 ± 133 to 680 ± 133 mm2) and spacing (360 ± 58 to 345 ± 71 μm, main effects of time, P ≤ 0.041), but supplementation had no effect (group × time interactions, P ≥ 0.181). There was a significant group × time interaction for cortical perimeter (P = 0.033) and porosity (P = 0.049); training had no effect on either measure for any group (P ≥ 0.068). CONCLUSION: Vitamin D supplementation in winter, using strategies to increase total 25(OH)D above 50 nmol·L-1, exerted no effect on bone macro- or micro-architecture beyond the osteogenic effects of exercise training.