At the start of one study, researchers measured dairy, calcium and vitamin D levels in the diets of 6,712 girls, aged nine to 15. After seven years of follow-up, girls who consumed the most dairy products and calcium—also mostly from dairy—had no added protection from bone fracture. Among the most active girls, compared to those who got less calcium, girls who got the most calcium were twice as likely to have a stress fracture, a surprising finding suggesting more study. However, for vitamin D, among the most physically active girls who participated in high-impact activity for at least one hour per day, those who got the most vitamin D were half as likely to have a stress fracture as girls who got less vitamin D.
Doctors said the findings support the recent increase in the U.S. RDA for vitamin D from 400 IU to 600 IU per day.