Maintaining Mg 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 Mg intake and serum Mg in female and male recruits pre- and post-IMT, (2) determine whether serum Mg was related to parameters of bone health pre-IMT, and (3) whether Ca and vitamin D supplementation (Ca/vitamin D) during IMT modified serum Mg. Females (n 62) and males (n 51) consumed 2000 mg of Ca and 25 μg of vitamin D/d or placebo during IMT (12 weeks). Dietary Mg intakes were estimated using FFQ, serum Mg was assessed and peripheral quantitative computed tomography was performed on the tibia. Dietary Mg intakes for females and males pre-IMT were below the estimated average requirement and did not change with training. Serum Mg increased during IMT in females (0·06 ± 0·08 mmol/l) compared with males (–0·02 ± 0·10 mmol/l; P < 0·001) and in those consuming Ca/vitamin D (0·05 ± 0·09 mmol/l) compared with placebo (0·001 ± 0·11 mmol/l; P = 0·015). In females, serum Mg 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 of the polaris axis (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 stress–strain index of the polaris axis (β = 0·314, P = 0·006) at the 66 % diaphyseal site pre-IMT. No significant relationships between serum Mg and bone measures were observed in males. Findings suggest that serum Mg may be modulated by Ca/vitamin D intake and may impact tibial bone health during training in female military recruits.

Alterations in sodium or potassium intake, or both, affect the distribution and excretion of magnesium in man and animals. Experiments were performed on rats to investigate the effect of varying Na, K or Na+K intakes on Na, K and Mg excretion and plasma and tissue concentrations. In Expt 1, increasing the Na intake in a linear fashion produced a significant (P < 0.05) quadratic response in urinary Mg excretion, with a decrease followed by an increase as Na intake rose. No effect was observed on faecal excretion of Mg. Plasma and tissues were sampled at the end of an 18 d collection period; concentrations of aldosterone in plasma and Mg, Na and K in plasma and tissue were determined. Increasing Na intake in a linear fashion produced a significant (P < 0.05) quadratic effect on Mg concentration in heart and muscle, i.e. a decrease followed by an increase as Na intake rose; Na intake did not affect liver or bone Mg concentrations. There were no significant effects of Na intake on plasma Mg, Na or aldosterone but plasma K fell significantly (P < 0.01) as Na intake increased. In Expt 2, constant amounts of four diets supplying adequate or high levels of Na and adequate or high levels of K but a constant intake of Mg were given to rats. The rats fed on the adequate-Na diet had a significantly (P < 0.05) higher urinary Mg excretion than those fed on the high-Na diet; Na intake did not affect faecal Mg excretion.