Stress fractures occur more frequently in young female athletes than in boys. Vitamin D plays an important role in keeping girls healthy and their bones strong.
“Vitamin D may boost muscle performance and helps the immune system function as well,” said Julie Kerr, MD, with Akron Children’s Sports Medicine. “But the No. 1 benefit is that vitamin D helps the body build and maintain strong bones. If your body isn’t absorbing calcium to strengthen your bones, you’ll be at greater risk for stress fractures.”
How can a female athlete tell if her vitamin D level is low? “In addition to stress fractures, signs of vitamin D insufficiency may include chronic muscle fatigue, feeling of muscle weakness or a slow-healing injury,” Dr. Kerr explained. “The best way to tell how much vitamin D your body has is a ‘25-hydroxy’ vitamin D level blood test.”
The 25-hydroxy test is usually measured in nanograms per milliliter (ng/mL). “If a girl is vitamin D deficient or close to it, our goal is to get her up to 50 ng/mL,” Dr. Kerr said. “We accomplish that in a variety of ways, including vitamin D supplements. Your body produces vitamin D when your skin is exposed to the sun’s UV rays, so 5 to 15 minutes a day is all you need. Sunshine can be hard to come by in the winter months here in northeastern Ohio, so we may look at vitamin D supplementation.”
Diet plays a key role in helping female athletes maintain healthy levels of vitamin D. “We see a lot of female patients who place dietary restrictions on themselves. They don’t drink milk, or they avoid carbs in the interest of losing weight,” Dr. Kerr said. “What they don’t realize is that they could be missing important nutrients by not eating a healthy, balanced diet.
“You can increase vitamin D intake by eating foods such as salmon and egg yolks, along with drinking milk or juice fortified with vitamin D. The process of absorbing dietary vitamin D is only about 50%, however, so you lose much of the nutrient value during digestion.”
The amount of exercise girls get also plays a role in boosting vitamin D and reducing the risk of stress fractures. “Exercise helps load your bone healthfully, resulting in good bone density, but you have to be careful not to do too much,” Dr. Kerr offered. “Research shows that girls who train rigorously for 8 or more hours a week are at 2 times’ greater risk for stress fractures than girls who train 4 hours a week.”
Vitamin D intake is critical to girls’ long-term bone health, too. “We want them to be on track to achieve optimal bone health by age 30,” Dr. Kerr said. “When bone density starts to decrease naturally over age 50, the decline will be less severe in women who start with good bone mass.”