Vitamin D in pre-teen girls protects from stress fractures and bone injuries: Study

By Dr. Ananya Mandal, MDMar 5 2012

A new study shows that girls and young women who got lots of vitamin D through their diet and supplements were half as likely to suffer a stress fracture as those who didn't.

“This study can add to the existing thought that adolescent girls and young women should be particularly cognizant of getting their vitamin D,” said Kendrin Sonneville, from Children's Hospital Boston, who worked on the study.

Stress fractures are defined as small cracks in the bone that typically affect people who do lots of high-impact exercise, like running or gymnastics. And they're especially concerning in teen girls because bone strength at that age is tied to the risk of osteoporosis and more serious injuries later in life.

It was believed that high dietary calcium was linked to protection against stress fractures. But in the new study, it was higher levels of vitamin D that were tied to fewer injuries -- not calcium. Still, the findings can't prove that the vitamin prevents fractures, since it's possible there were other differences between girls who ate high- and low-vitamin D diets that the researchers couldn't measure.

For this study, they followed 6,721 girls ages 9 to 15 who were daughters of women participating in the long-term Nurses' Health Study. The researchers surveyed them every year or so between 1996 and 2001 about their typical eating habits and use of vitamin supplements. From that information the scientists calculated how much vitamin D each girl got in a typical day.

Then, in 2004, Sonneville's team asked the girls' mothers whether their daughter had been diagnosed with a stress fracture from 1997 on. They found that just under four percent of the girls had had a stress fracture, with a much higher risk seen among those who did high-impact exercise for at least an hour a day, according to findings published Monday in the Archives of Pediatrics & Adolescent Medicine.

Surprisingly, high calcium intake was associated with a doubling of the risk of stress fracture. However, the authors, from Children's Hospital Boston, said that this “unexpected finding” should be further investigated. Soda intake did not alter the fracture. The risk was also unchanged when calcium and vitamin D from food only (excluding supplements) were considered.

“We know that calcium is important for bone health, so we were surprised to find that vitamin D was only found to be protective,” Sonneville told Reuters Health. Still, she added, “Our findings in no way suggest that calcium is not important.” Vitamin D is necessary for calcium absorption, she explained.

Dr. Daniel Green, who has studied stress fractures in adolescent athletes at the Hospital for Special Surgery in New York said, “Often the kids who are taking a longer time to heal or the kids who didn't heal their fractures predictably, we're finding end up having low vitamin D levels in their blood.” Green wasn't involved in the new study. “Three or four years ago we rarely asked our patients about their vitamin D intake and rarely checked their vitamin D level,” he told Reuters Health. “Now, that conversation is happening on a daily basis.” Green recommends 1,000 IU per day to his adolescent patients, and more for those who are very deficient in the vitamin.

Dr. Zeev Harel, who studies adolescent bone health at the Warren Alpert Medical School of Brown University in Providence, welcomed the new study. “We have something to tell our teenagers, because in the past we didn't really have a lot to tell them,” he told Reuters Health. Harel, who didn't participate in the new research, pointed out that most of the girls in the study were white - almost 94 percent - and African Americans and Hispanics tend to have lower levels of vitamin D.

The new study was funded by Children's Hospital and the National Institutes of Health, and some of the researchers report relationships with pharmaceutical and medical device companies.