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In a new study, girls and young women who consumed lots of vitamin D through diet and supplements were half as likely to suffer a stress fracture as those who didn't get much of the vitamin.
"This study can add to the existing thought that adolescent girls and young women should be particularly cognisant of getting their vitamin D," said Dr 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.
Researchers 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, Dr Sonneville's team asked the girls' mothers whether the girls had been diagnosed with a stress fracture from 1997 on.
Just under 4% 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 online today in the Archives of Pediatrics & Adolescent Medicine.
Stress fractures in teens
While there was no link between dietary calcium and stress fractures, girls and young women 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," Dr Sonneville said. Still, she added, "Our findings in no way suggest that calcium is not important."
Dr Daniel Green, who has studied stress fractures in adolescent athletes at the Hospital for Special Surgery in New York but was not involved in the new study, commented, "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."
"Three or four years ago we rarely asked our patients about their vitamin D intake and rarely checked their vitamin D level," he said. "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 said.
Stress linked to vitamin D intake
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.
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.
Dr 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.
Dr Green recommends 1,000 IU per day to his adolescent patients, and more for those who are very deficient.
(Reuters Health, Genevra Pittman, March 2012)
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