Lower Stress Fracture Risk in Girls Linked to Vitamin D Intake

Laird Harrison

March 05, 2012

March 5, 2012 — Girls who consumed the most vitamin D had the lowest risk for stress fractures, but dairy consumption did not reduce the risk, and calcium actually increased it, according to results from a new study published online March 5 in the Archives of Pediatrics & Adolescent Medicine.

Stress fractures, a relatively common sports-related injury, occur when stresses on a bone exceed its capacity to withstand those forces and heal from them. This type of injury has been increasing in recent years as more girls participate in high-impact sports, write Kendrin R. Sonneville, ScD, RD, from Children's Hospital Boston in Massachusetts, and colleagues.

However, although calcium and calcium-rich dairy products are routinely encouraged for optimal bone health, the evidence for this recommendation has been challenged, they note.

"Further, while vitamin D deficiency is relatively common among adolescents, data are lacking on the role of vitamin D intake, whether sufficient or in excess of recommended intake, on bone health," the researchers write.

To learn more about the association of these nutrients and stress fractures, Dr. Sonneville and colleagues analyzed data on 6712 preadolescent and adolescent girls (aged 9 - 15 years at baseline) in the Growing Up Today Study. The girls were daughters of women enrolled in the Nurse's Health Study.

Responding to survey questions, 30.4% of the girls said they participated in at least an hour per day of high-impact sports, such as basketball and soccer.

At baseline, the girls consumed a mean of 2.0 servings (±1.3 servings) of dairy foods per day. They took in 1181.5 mg/day (± 478.2 mg/day) of calcium and 376.2 IU/day (±183.0 IU/day) of vitamin D.

In comparison, the US Institute of Medicine's Recommended Dietary Allowances for girls between 9 and 18 years of age are 1300 mg/day of calcium and 600 IU/day of vitamin D.

The researchers divided the girls into quintiles according to their mean vitamin D intakes per day in IU:

  • quintile 1: 107 (±46)

  • quintile 2: 210 (±56)

  • quintile 3: 324 (±43)

  • quintile 4: 433 (±55)

  • quintile 5: 663 (±149)

The multivariable-adjusted hazard ratio (HR) of stress fracture for the highest vs the lowest quintile of vitamin D intake was 0.49 (95% confidence interval [CI], 0.24 - 1.01; P trend = .07), suggesting that the girls who got the most vitamin D were half as likely to get a stress fracture as the girls who got the lowest amount of this nutrient.

Among the girls who got more than an hour a day of high-impact exercise, the association was even stronger (HR, 0.48; 95% CI, 0.22 - 1.02; P trend = .04).

The researchers divided the girls into similar quintiles according to their mean calcium intakes per day, in mg:

  • quintile 1: 541 (±124)

  • quintile 2: 825 (±86)

  • quintile 3: 1111 (±93)

  • quintile 4: 1398 (±93)

  • quintile 5: 1891 (±298)

Among the most highly active girls, those who consumed the most calcium were more likely to get stress fractures (HR, 2.14; 95% CI, 0.98 - 4.69). "This unexpected finding warrants further inquiry," the authors write.

The researchers also divided the girls up according to their dairy consumption, but found no association to fracture risk, even among the highly active girls (HR, 1.15; 95% CI, 0.71 - 1.84).

The researchers obtained these hazard ratios after adjusting for age, body mass index, age at menarche, and maternal history of low bone density or osteoporosis. In addition, they adjusted the vitamin D hazard ratios for mean-centered calcium intake and the calcium ratios for mean-centered vitamin D intake.

The authors conclude that their findings support the Institute of Medicine's recent increase in the recommended dietary allowance for vitamin D for adolescents from 400 IU/day to 600 IU/day.

"Future studies are needed to ascertain whether vitamin D intake from supplements confers a similarly protective effect as vitamin D consumed through dietary intake," they write.

Asked to comment, Letha Griffin, MD, a spokesperson for the American Academy of Orthopaedic Surgeons, told Medscape Medical News that the findings conflict with some of the conventional wisdom on nutrients and bone health.

"It's a confusing situation," said Dr. Griffin, an orthopaedic surgeon in private practice and a team physician for several sports teams at Georgia State University in Atlanta. "Everyone thought calcium was important in exercise, and now this study is saying vitamin D is more important."

Dr. Griffin, who was not associated with the study, pointed out it had the strength of being large, but the potential weakness that the results were self-reported.

There is not enough evidence yet to indicate that athletes should take in different amounts of calcium or vitamin D than nonathletes, she said. Although the study highlights the importance of vitamin D, any changes in the current recommendations will depend on further research.

"We're just putting pieces into the puzzle," she said.

One author of this study disclosed partial salary support from Pfizer/Merck, and another consults for several companies. Dr. Griffin has disclosed no relevant financial relationships.

Arch Pediatr Adolesc Med. Published online March 5, 2012. Abstract

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