Prediction Model for Stress Fracture in Young Female Recruits During Basic Training. Moran DS et al. Med Sci Sports Exerc, 40(118):S636-644, 2008.
The Association Between Hematological and Inflammatory Factors and Stress Fractures Among Female Military Recruits. Merkel D et al. Med Sci Sports Exerc, 40 (118):S691-697, 2008.
Iron deficiency is common among female athletes, with a reported incidence of between 12-40% in certain groups, with the incidence of anemia between 3-18%. These studies, both done on Israeli military recruits, raise some interesting questions about the association between iron deficiency and the risk for development of stress fractures in females. Military recruits are exposed to similar body stresses as endurance athletes, so there is some applicability to the Sports Medicine population.
In the latter study, Merkel et al looked at 438 new military recruits, 227 female combatants, 83 male combatants and 128 female noncombatants. These subjects had their blood drawn for, among other things, hemoglobin and iron studies, including ferretin, serum iron and serum transferrin at induction, as well as two and four months into training. At induction, 18% of female inductees were anemic (defined as Hemoglobin < 12 g/dL), compared with 8% of males (defined as Hb < 14 g/dL). Iron deficiency, defined as a serum ferretin of 12 or less in females or 20 or less in males, was present in 40% of women, 6% of men. This was clearly a group that tended to iron deficiency, as the average hemoglobin among female combatants was 12.7 – a level that, in the presence of low iron stores probably represents a “relative” anemia (Hemoglobin in the normal range, but lower than the individual subject’s healthy baseline). These subjects were subdivided based on their iron status (low or high) and then followed prospectively (observed as they progressed through their training). No changes in iron status during training were noted. As training progressed, recruits who presented back to the clinic with a diagnosis of stress fracture were identified (diagnosed by x-ray or bone scan). Over the observation period, 12% of female combatants presented with a stress fracture. No males or noncombatant females developed stress fractures. Female combatants with a stress fracture were found to have statistically lower levels of serum iron and iron saturation. Ferretin, which is an indicator of iron stores in the body, did not differ between recruits with or without stress fractures. This may be related to the fact that ferretin tends to increase during times of stress on the body, rising relatively more in the injured group. This study shows an association – not necessarily cause. It may be that poor nutrition leading to decreased body iron stores occurs in parallel with decreased calcium intake, leading to a higher risk for stress fracture. Alternatively, the inflammation associated with the development of stress fractures could suppress iron stores through effects on the liver.
Based on this association, Moran et al. looked at ways of using iron status, among other variables, to predict the risk for development of stress fracture in military recruits during a four-month basic training period. In this study, 227 female and 83 male basic training recruits were assessed at baseline, two months, four months and 16 months after induction. Among variables analyzed were iron status, bone quality, fitness, nutrition, activity patterns, etc. Over one year of training, stress fractures were diagnosed through the use of bone scan or MRI. The researchers then collated the data obtained in these subjects and developed prediction equations to try to predict the risk of development of stress fracture. The final equation developed successfully predicted the development of a stress fracture 76.5% of the time. Risk factors identified as significant for the development of stress fracture over time included being tall, “lean,” feeling “burned out,” having an iron deficiency, with a high normal ferretin. This again speaks to the fact that ferretin levels will often rise in the presence of body injury. Some coaches and clinicians advocate following serum ferretin during the course of the season, but these studies, and others like them, argue against that, as an increase in ferretin over time does not necessarily reflect increasing amounts of body iron stores, but may simply reflect increased stress.
What these studies do suggest, however, is that it is reasonable to check iron status on female endurance athletes, particularly those who are new to their sport or who are increasing the level of their involvement in their sport. Athletes with low iron levels should be supplemented if there is associated anemia (or relative anemia), and also monitored for development of stress fractures, with efforts made at risk reduction before an injury occurs. If you are starting a running program or are considering doing so, consider giving us a call to talk about risk assessment and risk reduction. It might protect you from injury, or if anemia is present, even improve your performance.