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A new study predicts stress fracture risk in female athletes with devastating accuracy.Getty Images/iStockphoto

Top athletes have an iron will, but their bones are made of the usual imperfect collection of minerals.

That's the reality that confronted junior tennis star Bianca Andreescu, who was sidelined for six months by a stress fracture in her foot last year. Similarly, Canadian marathon record-holder Lanni Marchant's career was nearly derailed by a series of stress fractures in college.

The endlessly repetitive impacts of high-level training – running, jumping, pivoting, cutting – often make such injuries seem like an inevitable occupational hazard for athletes. But a new study in the American Journal of Sports Medicine, published by researchers at Stanford University, offers an important reminder that training isn't the only risk factor: Eating patterns, and the broader cluster of conditions known as the "female athlete triad" predict stress fracture risk in female athletes with devastating accuracy.

The study followed 239 female student athletes at Stanford University, using data from preparticipation health questionnaires and bone-density scans to classify each of them as having a low, moderate or high risk of suffering a "bone stress injury" – a category that includes the hairline bone cracks known as stress fractures as well as less-severe precursors called stress reactions.

The risk assessment was calculated with an algorithm developed by a group of international experts on the female athlete triad, including Jenna Gibbs of the University of Waterloo and Marion Olmsted of the University of Toronto, and published in the British Journal of Sports Medicine in 2014.

The female athlete triad refers to the relationships between energy availability, menstrual function and bone mineral density. In athletes whose food intake doesn't provide enough calories – after the demands of training are accounted for – to support necessary physiological needs, both menstrual function and bone health are compromised. The condition exists along a spectrum, and even mild problems in one of the areas may signal hidden or impending problems in the other two.

How common the problem is depends on who you ask. Endurance, aesthetic and weight-class sports tend to be particularly vulnerable because of the emphasis on low body weight. Studies have found that between a quarter and a third of elite female athletes in these sports have clinical eating disorders.

But triad-related problems can occur long before an eating disorder is present. The triad risk-assessment tool assesses six different markers to get an overall picture, assigning each one either 0, 1 or 2 points: low energy availability, low body-mass index, delayed age of first period, irregular or absent periods, low bone mineral density and previous history of stress fracture.

Over all, just more than 70 per cent of the Stanford athletes were considered low risk, while 26 per cent were moderate risk and 4 per cent were high risk.

During the study period, 25 of the 239 athletes suffered a bone stress injury within an average of about a year – and, as predicted, the moderate-risk athletes were twice as likely to suffer an injury, and the high-risk athletes were four times as likely.

The results confirm for the first time that the risk-assessment algorithm does a good job of identifying which athletes are likely to suffer problems, according to Adam Tenforde, the study's lead author.

"Our study shows the risks we identified are not theoretical," says Tenforde, an assistant professor at Harvard Medical School and a sports medicine physician at the Spaulding National Running Center. "The high rate of injury is very concerning and may be motivating to an athlete to engage in treatments to optimize nutrition, menstrual function and bone health."

The results were particularly stark for the study's 47 cross-country runners, whose sport combines repetitive pounding with a focus on minimizing weight. Half of the 16 moderate-risk runners and five of the seven high-risk runners suffered stress fractures.

The sport with the second-highest rate of stress fractures, at 22 per cent, was basketball – even though all the basketball players in the study were classified as low risk.

The differences between the two sports are worth noting. Basketball players, as well as other athletes classified as low risk, tended to suffer stress fractures in their feet and lower legs, presumably as a direct result of training impacts.

Many of the moderate- and high-risk runners, in contrast, suffered fractures in their pelvis and hip regions, where the bone structure is different and thought to be more vulnerable to the effects of low bone mineral density. As a result, Tenforde says, any fracture in those areas should trigger further investigation by health providers to see if other elements of the triad are present.

Identifying the problem, of course, is only part of the battle. Treating eating disorders is notoriously difficult and requires professional help.

But for people at the mild end of the triad spectrum, the new data may serve as a powerful counterargument to the seductive but misleading belief that restricting calories will boost your athletic performance – especially if your odds of a season-ending injury, like the Stanford cross-country runners, end up being worse than a coin toss.

Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience