While physical activity strengthens bones, high-impact activity can markedly increase the risk of stress fracture among adolescent girls, recent findings suggest.

“As with many things, too much of a good thing can be bad for you,” Dr. Alison E. Field commented.

In a study published in the medical journal Pediatrics, Field and colleagues at Harvard Medical School, in Boston, examined data from 5461 girls enrolled in the Growing Up Today Study.

The girls self-reported their weight and height, whether they had started having periods, physical activity, dietary intake, and disordered eating habits on annual surveys. Their mothers reported their daughters’ histories of stress fractures on their 1998 annual questionnaire.

The average age of the girls in 1998 was 14 years and 68 percent of them had started menstruating. Three percent of the girls had Eating disorders, defined as fasting, diet pill or laxative use, or Vomiting to control weight.

Sixteen percent of the girls participated in moderate to vigorous activity, and 2.7 percent had a history of stress fracture.

“Girls who participated in 16 or more hours per week of physical activity were at the highest risk for a stress fracture,” Field told Reuters Health. “They were 88 percent more likely than girls who engaged in less than 4 hours per week of activity to have a history of stress fracture,” she said.

“Running, cheerleading, and gymnastics were the activities that were the highest risk,” Field pointed out

Eating disorders were more common among those who participated in at least 16 hours per week of activity, but were not independently linked to stress fracture.

“Physical activity should be promoted for young girls, especially for bone health, but those who participate at the higher end may not only be at increased risk of developing a significant bone injury (stress fracture) but also of developing unhealthful habits (such as using vomiting or laxatives to control their weight) that, ironically, jeopardize their bone health, increasing their long-term risk of Osteoporosis,” Field said.

“The results should not be interpreted to mean that physical activity is bad for young girls,” the investigator emphasized.

“By making doctors, parents, and coaches more aware of the problem and the potential risk factors, hopefully we can help prevent some young girls from developing stress fractures,” she concluded.

SOURCE: Pediatrics, April 2005.