What’s good for the track team may not be so great for the runner. A new study published in the journal Current Orthopaedic Practice concludes that female runners with lower body weight are both more susceptible to bone injuries and slower to heal.

The idea that thinness equals health is of the biggest health misconceptions out there. It’s certainly among the most harmful. Discrimination against overweight people limits their access to jobs, health insurance, and good medical care, while doctors can overlook illness in seemingly "healthy" thin people.

On top of that, it’s simply not true. Some studies have found little correlation between body mass index (BMI) and health, while others suggest that being overweight may even have a protective effect against certain diseases.

And, as the authors of the current paper discovered, having a low BMI can carry risks of its own. The researchers, all from the Ohio State University Wexner Medical Center, had set out to investigate which factors, if any, could make runners more vulnerable to stress fractures in their bones. For three years, they interviewed and examined every female track and field athlete who arrived in their medical center with a stress fracture. They saw a total of 18 athletes, with 24 fractures among them.

Closeup of a doctor examining a woman's lower leg.
The Ohio State University

Some of those fractures were worse than others. Some took longer to heal. And some athletes seemed more prone to getting injured all over again. Those athletes were the thinnest—specifically, women with a BMI under 19.

The "normal" range for BMI starts at 18.5 and goes to 24.9, which means that women on the low end of "normal" can still face a higher risk.

Coaches consider low weight a good thing because it can increase a runner's speed. But there’s only so much fat a person can burn. After a while, these athlete’s bodies start burning muscle, too. And that, lead author Timothy Miller says, is where the trouble can begin.

"When body mass index is very low and muscle mass is depleted, there is nowhere for the shock of running to be absorbed other than directly into the bones,” he said in a statement.

While BMI may be a notoriously unreliable indicator of health, Miller says it’s "imperative" that women know theirs and "work to maintain a healthy level," he added. "They should also include resistance training in their training regimen to strengthen the lower leg to prevent injury, even if that means adding weight from additional muscle mass," Miller said.