Stress fractures are 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.

"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.

Researchers have wondered whether eating a high-calcium diet with lots of dairy products might protect girls 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 close to 7,000 girls who were daughters of women participating in the long-term Nurses' Health Study.

Starting when the girls were between nine and 15 years old, 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.

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.

While there was no link between how much calcium girls and young women got in their diets and their chance of getting injured, those with the greatest daily vitamin D intake were half as likely to have a stress fracture as those who got the least.

"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.

Doctors are paying more attention when it comes to vitamin D levels as they relate to fracture risk and healing, according to Dr. Daniel Green, who has studied stress fractures in adolescent athletes at the Hospital for Special Surgery in New York.

"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," said Green, who 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."

Dr. Zeev Harel, who studies adolescent bone health at the Warren Alpert Medical School of Brown University in Providence, said the new study is a step forward in clearing up the previously murky link between vitamin D and stress fractures.

"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.

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.

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.

Vitamin D is naturally present in fatty fish, but is also added to dairy products like milk and yogurt. Still, Sonneville said, doctors typically recommend girls and young women take a supplement that includes vitamin D, because it's not always easy to get enough through food.

The Institute of Medicine recommends kids and adults get 600 international units (IU) of vitamin D daily. Girls in the study were getting an average 376 IU per day.

Sonneville said that although many adolescent girls are deficient in vitamin D, it's hard to know based on her team's study if levels above those recommendations might translate to an even lower risk of stress fractures.

Green recommends 1,000 IU per day to his adolescent patients, and more for those who are very deficient in the vitamin.

SOURCE: bit.ly/pD1ZHL Archives of Pediatrics & Adolescent Medicine, online March 5, 2012.

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