Background: The female athlete triad (Triad), defined by the American College of Sports Medicine as low energy availability (EA) with or without disordered eating, menstrual dysfunction and low bone mineral density (BMD), is associated with stress fractures and athletes aged 16-17 years are most susceptible. Purpose: To examine whether the Triad increases the risk of stress fractures, athletes were assigned to a "teenage" group and a "20s" group. Methods: This prospective study enrolled 390 elite female athletes and was conducted from 2012 to 2016 at Japan Institute of Sports Sciences. Blood concentrations of various hormones were examined and BMD was measured at the lumbar spine and throughout the whole body using DXA. LEA was defined as body weight = 85% of the ideal body weight for teenage athletes, or BMI = 17.5 for athletes in their 20s. Low BMD was defined as a BMD Z-score of < -1.0 in the lumber spine and the whole body. Results: Among 390 athletes enrolled, 36 developed new stress fractures within 3 months of registration. The risk for stress fractures due to the Triad in teenage athletes was higher than for athletes in their 20s. In teenage female athletes, secondary amenorrhea, low BMD for the whole body, and a low ratio of actual body weight to ideal body weight increased the risk for stress fractures by 12.9 times, 4.5 times and 1.1 times, respectively. Conclusion: To prevent stress fractures in female athletes with the Triad, age of athletes should be taken into consideration
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