Vitamin D and tibial bone density, geometry, and microarchitecture in male military recruits: an observational study and randomized controlled trial.

Vitamin D may mitigate bone stress injuries in military training by modulating changes in bone. This cross-sectional observational study (Study 1) and randomized controlled trial (Study 2) investigated associations between vitamin D metabolites and tibial structure and density, and the effect of vitamin D supplementation on tibial adaptations to military training. A total of 343 (Study 1) and 194 (Study 2) male British Army recruits participated. Circulating vitamin D metabolites (biologically "active" and "inactive") and tibial structure were measured in participants during week 1 and week 12 (Study 2 only) of initial military training. Associations between vitamin D metabolites and HRpQCT outcomes at week 1 were tested in Study 1. Participants in Study 2 were randomly assigned to vitamin D (oral pill or simulated sunlight) or placebo (placebo pill or placebo simulated sunlight) supplementation for 12 wk designed to achieve vitamin D sufficiency. There was no association between total 25(OH)D or vitamin D receptor single-nucleotide polymorphisms and any measure of density, geometry, or microarchitecture (p ≥ .063). Higher 1,25(OH)2D was associated with lower cortical porosity and perimeter (p ≤ .040). Higher total 24,25(OH)2D was associated with higher trabecular number and lower trabecular thickness (p = .016). Higher 25(OH)D:24,25(OH)2D (VMR 1) was associated with higher trabecular thickness, trabecular separation, and cortical porosity (p ≤ .034). Higher 1,25(OH)2D:24,25(OH)2D (VMR 2) was associated with lower trabecular number, and higher trabecular spacing and thickness (p ≤ .035). There was no effect of vitamin D supplementation on any tibial outcome. Training decreased trabecular area (-0.1%), thickness (-4.4%), and separation (-2.1%), and increased cortical thickness (0.8%) and area (0.9%) (p ≤ .042). Vitamin D metabolites and their ratios were associated with tibial size and microarchitecture, but vitamin D supplementation had no impact on the adaptive response to military training.