Divergent effects of sex and calcium/vitamin D supplementation on serum magnesium and markers of bone structure and function during initial military training.

Maintaining magnesium status may be important for military recruits, a population that experiences high rates of stress fracture during initial military training (IMT). The objectives of this secondary analysis were to 1) compare dietary magnesium intake and serum magnesium in female and male recruits pre- and post- IMT, 2) determine whether serum magnesium was related to parameters of bone health pre-IMT, and 3) whether calcium and vitamin D supplementation (Ca/vitamin D) during IMT modified serum magnesium. Females (n=62) and males (n=51) consumed 2,000 mg calcium and 1,000 IU vitamin D/d or placebo during IMT (12 weeks). Dietary magnesium intakes were estimated using FFQ, serum magnesium was assessed, and pQCT was performed on the tibia. Dietary magnesium intakes for females and males pre-IMT were below the estimated average requirement and did not change with training. Serum magnesium increased during IMT in females (0.06±0.08 mmol/L) compared to males (-0.02±0.10 mmol/L; P<0.001) and in those consuming Ca/vitamin D (0.05±0.09 mmol/L) compared to placebo (0.001±0.11 mmol/L; P=0.015). In females, serum magnesium was associated with total bone mineral content (BMC, β=0.367, P=0.004) and robustness (β=0.393, P=0.006) at the distal 4% site, stress strain index (SSIp, β=0.334, P=0.009) and robustness (β=0.420, P=0.004) at the 14% diaphyseal site, and BMC (β=0.309, P=0.009) and SSIp (β=0.314, P=0.006) at the 66% diaphyseal site pre-IMT. No significant relationships between serum magnesium and bone measures were observed in males. Findings suggest that serum magnesium may be modulated by Ca/vitamin D intake and may impact tibial bone health during training in female military recruits.