Context. The role of magnesium (Mg) as a determinant of bone mass has not been extensively explored. Limited studies suggest that dietary Mg intake and bone mineral density are correlated in adults, but no data from interventional studies in children and adolescents are available.

Objective. To determine if Mg supplementation in peri-adolescent girls enhances accrual of bone mass.

Design. Prospective, placebo-controlled, randomized, one-year double-blind trial of Mg supplementation.

Setting. Clinical Research Center in University Medical School.

Patients or Other Participants. Healthy 8-14 yr-old Caucasian girls were recruited from community pediatricians' offices. Dietary diaries from over 120 volunteers were analyzed and those with dietary Mg intake of less than 220 mg/day were invited to participate in the intervention.

Intervention. Magnesium (300 mg elemental Mg per day in 2 divided doses) or placebo, given orally, for 12 months.

Main Outcome Measure. The primary outcome measure was interval change in bone mineral content (BMC) of the total hip, femoral neck, Ward's area and lumbar spine (L1-L4) after 12 months of Mg supplementation.

Results. Significantly increased accrual (P = 0.05) in integrated hip BMC occurred in the Mg-supplemented vs. placebo group. Trends for a positive Mg effect were evident in the pre- and early puberty and in mid-late puberty. Lumbar spinal BMC accrual was slightly (but not significantly) greater in the Mg-treated group. Compliance was excellent; 73% of capsules were ingested as inferred by pill counts. Serum mineral levels, calciotropic hormones, and bone markers were similar between groups.

Conclusions. Oral Mg oxide capsules are safe and well-tolerated. A positive effect of Mg supplementation on integrated hip BMC was evident in this small cohort.