One group of female Navy recruits were given 2,000 mg of calcium and 800 IU of vitamin D and the other group received a placebo. Durning a 2 year period the supplemented group got 21% less fractures than the placebo group.
Journal of Bone and Mineral Research
May 2008, Volume 23, Pages 741-749, doi: 10.1359/jbmr.080102
“Calcium and Vitamin D Supplementation Decreases Incidence of Stress Fractures in Female Navy Recruits”