For active girls, healthy bones may require more vitamin D than the typically recommended calcium and milk intake, researchers found in a study of stress fractures.
The highest vitamin D intake cut the risk of stress fractures in half, with a particularly strong effect among girls getting an hour or more of high-impact exercise a day, Kendrin R. Sonneville, ScD, RD, of Children's Hospital Boston, and colleagues reported.
But dairy and calcium intake had no impact on stress fracture risk in the prospective cohort study appearing online in the Archives of Pediatrics & Adolescent Medicine.
Consuming calcium and calcium-rich dairy products is routinely encouraged for optimal bone health, and does likely have long-term benefits, Sonneville noted in an interview with MedPage Today.
"Very low calcium intake is harmful to bone development," she said. "Our findings in no way suggest that calcium isn't important."
Rather these results, along with most other cross-sectional, retrospective, and prospective studies, suggest that additional intake beyond some minimal threshold doesn't help early bone health, her group explained.
Extra vitamin D in the diet, perhaps from supplements, may help though, particularly given that low vitamin D is common in adolescents, the group added.
"Anything that will benefit your bone early is probably going to have lifelong benefits in terms of bone health, and this includes things like fracture risk later in life and osteoporosis," Sonneville told MedPage Today.
Weight-bearing activity boosts peak bone mass during that critical window of bone building in adolescence, but can go too far and lead to overuse injury from stresses that exceed the bones' capacity to heal.
In the Growing Up Today Study, an ongoing cohort study of the adolescent children of women in the Nurses' Health Study, seven years of follow up turned up stress fractures in 4 percent of the 6,712 girls, ages 9 to 15, at baseline.
Nearly all of those fractures (90 percent) occurred in the 30 percent who practiced sports or other high-impact activity at least an hour a day.
While the mean 1,182-mg daily intake of calcium didn't meet the recommended 1,300 mg for girls this age, girls with the highest intake were no less likely than those with the lowest to develop a stress fracture.
Likewise, there was no trend for benefit from higher intake of dairy. Girls who took in three or more servings a day were no less likely to develop a stress fracture than those who ate no dairy.
Daily vitamin D intake also fell below the recommended 600 international units (IU), averaging just 376 IU. But the higher the vitamin D intake, the lower the girls' stress fracture tended to be in the fully adjusted model.
The intermediate intake groups, while still averaging less than 450 IU per day, appeared to be at about one-quarter less stress fracture risk than those with the lowest vitamin D intake averaging about 100 IU per day.
The only statistically significant impact on fracture risk, though, came for the group that met the 600 IU per day recommendations. That top intake quintile, averaging 663 mg per day, were much less likely to suffer a stress fracture compared with the bottom intake group.
Stratifying by activity level, girls getting the most vitamin D while participating in at least an hour of high-impact exercise were at a significant 52 percent lower stress risk than those getting the least vitamin D.
Dairy had no impact in that analysis, and high calcium intake in highly active girls actually tended to increase fracture risk, which the researchers called an unexpected finding that warrants further study.
Adjustment for soda intake had no effect on the results.
The biggest limitation of the study was lack of data on serum vitamin D status, which is only partially dependent on diet, the researchers noted.
The study sample also was not nationally representative, as it included more active and fewer socioeconomically disadvantaged girls.
During the study period, vitamin D supplementation was uncommon, so further studies would be needed to determine whether supplements are as protective as vitamin D in the diet, the researchers added.
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