Calcium and vitamin D supplementation maintains parathyroid hormone and improves bone density during initial military training: A randomized, double-blind, placebo controlled trial

Calcium and vitamin D are essential nutrients for bone health. Periods of activity with repetitive mechanical loading, such as military training, may result in increases in parathyroid hormone (PTH), a key regulator of Ca metabolism, and may be linked to the development of stress fractures. Previous studies indicate that consumption of a Ca and vitamin D supplement may reduce stress fracture risk in female military personnel during initial military training, but circulating markers of Ca and bone metabolism and measures of bone density and strength have not been determined. This randomized, double-blind, placebo-controlled trial sought to determine the effects of providing supplemental Ca and vitamin D (Ca + Vit D, 2000 mg and 1000 IU/d, respectively), delivered as 2 snack bars per day throughout 9 weeks of Army initial military training (or basic combat training, BCT) on PTH, vitamin D status, and measures of bone density and strength in personnel undergoing BCT, as well as independent effects of BCT on bone parameters. A total of 156 men and 87 women enrolled in Army BCT (Fort Sill, OK; 34.7°N latitude) volunteered for this study. Anthropometric, biochemical, and dietary intake data were collected pre- and post-BCT. In addition, peripheral quantitative computed tomography was utilized to assess tibia bone density and strength in a subset of volunteers (n = 46). Consumption of supplemental Ca + Vit D increased circulating ionized Ca (group-by-time, P = 0.022), maintained PTH (group-by-time, P = 0.032), and increased the osteoprotegerin:RANKL ratio (group-by-time, P = 0.006). Consistent with the biochemical markers, Ca + Vit D improved vBMD (group-by-time, P = 0.024) at the 4% site and cortical BMC (group-by-time, P = 0.028) and thickness (group-by-time, P = 0.013) at the 14% site compared to placebo. These data demonstrate the benefit of supplemental Ca and vitamin D for maintaining bone health during periods of elevated bone turnover, such as initial military training.This trial was registered with ClincialTrials.gov, NCT01617109.