I take care of a lot of adolescent girls and many of them are involved in very competitive sports. As participation in organized sports and single sports year round has increased so too have overuse injuries.
One of the most common overuse injuries is a stress fracture which occurs when stresses on the bone exceed the bone's capacity to withstand and heal from those forces. Stress fractures have been reported to occur in 3.9% of adolescent girls, and 90% of those stress fractures occurred in girls who participated in at least 1 hour/day of high impact activity.
During my adolescent visits, I have routinely emphasized the importance of healthy diets as well as the need for calcium and calcium rich dairy products. Knowing that adolescence is the most critical period for bone mineral deposition, and therefore has been considered an important window to hopefully prevent osteoporosis later in life.
In a recent study out of Harvard over 6700 girls ages 9 - 15 were followed for 7 years to identify whether calcium, vitamin D, and/or dairy intake was associated with stress fracture.
Surprisingly, there was no evidence that calcium and dairy intakes were protective against developing a stress fracture. But, higher vitamin D intake among girls who participated in at least 1hour/day of high impact activity, was predictive of a lower risk of developing a stress fracture.
So, while a balanced diet including dairy products is important for over all health, vitamin D seems to be protective and lowers the risk of a stress fracture. The study did not look at vitamin D intake above 600 IU/day (which is the current recommended dietary allowance). Further research will be needed to see if even higher amounts of Vitamin D prove to be even more protective.
In the meantime, make sure that your adolescent is getting their recommended daily dose of Vitamin D and keep watching for further studies to determine the mechanism through which Vitamin D may alter stress fracture risk.