Conclusions and Recommendations
RESPONSE TO TASK QUESTIONS
1. Why is the incidence of stress fractures in military combat training greater for women than for men?
Stress fracture rates among female military trainees during basic training are more than twice those reported for males. This greater incidence appears to be due in part to the initial entry level of fitness of the recruits and specifically the ability of bone to withstand the sudden large increase in physical loading. Some studies that controlled for aerobic fitness were unable to demonstrate a difference in the incidence of injury between males and females when individuals of the same fitness level were compared. Factors such as increased stride length (shorter women having the same stride length as tall men in "co-ed" marching situations) and variations in specific exercise activities (different loading force during drop-knee push-ups) may contribute to the different site distribution of stress fractures in military women compared with men. When training regimens are imposed to deliver the necessary level of physical fitness to meet standards, the resultant stress on the less physically fit (usually women) increases the likelihood of injury. According to military fitness experts, the fitness level of all new recruits has been decreasing over the past years. Reversing the trend in fitness in recruits may require setting higher and more relevant standards for entry. Preventing injury once recruits are in basic training may require reassessing methods used to achieve the desired improvement in fitness. A careful analysis of methods designed to achieve
the desired degree of physical fitness during basic training without incurring an excessive injury rate with its associated loss of training time seems appropriate at this time.
2. What is the relationship of genetics and body composition to bone density and the incidence of stress fractures in women?
Genetics is a determinant of peak bone mass, but it is not known what genes are important, nor is it known how prominent they are in the risk assessment profile for stress fractures. Body mass and composition per se influence bone density. Greater body mass is associated with higher levels of bone mineral mass and density. Hence, individuals who are heavier and have a larger and denser skeleton are at lower risk of bone fracture, particularly osteoporosis-related fractures.
Stress fractures are associated not only with reduced skeletal muscle mass and its concomitant increased fatigability and lower fitness levels but also with an excessive skeletal muscle mass and its enhanced strength. Reduced skeletal muscle mass with associated lower strength and greater fatigability may limit the ability of muscle to decrease bone stress. This relationship between small muscle mass and increased bone fragility may account for the observed association between lower calf circumference and stress fracture risk in women. Bone stress created by excessive or rapid incremental skeletal muscle contraction and loading forces can cause fractures at specific anatomic sites.
In sum, body mass and composition play a substantial role in establishing an individual's skeletal properties including ability to stimulate bone growth and turnover and to withstand physical demands that predispose to stress fracture.
3. What are the effects of diet, physical activity, contraceptive use, and other lifestyle factors (smoking and alcohol) on the accrual of peak bone mineral content, incidence of stress fractures, and development of osteoporosis in military women?
Energy intake by military women should be adequate (2,000–2,800 kcal/d) to maintain weight during moderate and intensive physical fitness training. A diet adequate in calcium, phosphorus, magnesium, and vitamin D (as defined by IOM, 1997) and moderate in sodium and protein (NRC, 1989) should optimize bone health in the short term and theoretically should reduce the long-term risk of developing osteoporosis.
A healthy, active lifestyle should reduce further the long-term risk for osteoporosis. Physical activity of a weight-bearing nature should be introduced in a gradual and progressive manner to minimize risk of stress fractures. The use of oral contraceptives that contain estrogen with or without progestational agents are not considered to have long-term detrimental effects on women's bone health. Some studies show, in fact, that the use of such agents might have a positive impact on the developing young female skeleton. In contrast, the use of long-acting, depot preparations of progestational agents, like medroxyprogestrerone (Depo-Provera) has been associated with relative estrogen deficiency and reduced bone mass.
Gonadotropin-releasing hormone (GnRH) agonists used to treat endometriosis (Lupron) interfere with the hypothalamic-pituitary-ovarian axis, inducing a state of estrogen deficiency. Long-term use of such GnRH agonists is of concern because (1) it mimics the menopausal state of estrogen deficiency, and (2) it has been associated with bone loss.
Agents used to treat the pain and inflammation of injury can include steroids and nonsteroidal anti-inflammatory agents (NSAIDs). Because of the potential of steroids to induce bone loss by the
many mechanisms described earlier in this report, their use should be restricted to special circumstances, short time periods, and the lowest possible doses. An attractive alternative are the NSAIDs, which should not harbor the same potential for negative effects on bone as do the steroidal agents.
Cigarette smoking may be a risk factor for the long-term risk of osteoporosis, whereas excessive alcohol consumption may be a risk factor in the short term for overall injuries. Whether these lifestyle factors are directly related to the development of stress fractures in the short term or are indirectly related through their long-term influence on bone density is not known.
4. How do caloric restriction and disordered eating patterns affect hormonal balance and the accrual and maintenance of peak bone mineral content?
Energy restriction adversely affects the normal hormonal status of military women.. Inadequate energy availability (a negative relationship between energy intake and activity level) changes the pulsatility of luteinizing hormone (LH) and the circulating levels of cortisol and thyroid hormones. Some of these hormonal changes are also associated with amenorrhea. The disruption of LH pulsatility may be caused by a decrease in hypothalamic GnRH pulsatility. The long-term consequence may be anovulation, decreased estrogen secretion, and increased bone resorption. Thus, caloric restriction may directly affect the accrual of peak bone mineral content. Disordered eating, although not well documented in the military, is associated with similar hormonal changes. Inadequate energy availability in military women may result from any of several practices including caloric restriction to attain or maintain body weight standards, disordered eating practices, or rapid increases in exercise levels (IOM, 1998).
Other conditions that induce estrogen deficiency may adversely affect bone health. Secondary amenorrhea may also arise from physical or emotional stress, therapy with GnRH agonists, and use of long-acting progestagens.
5. How can the military best ensure that the dietary intakes of active-duty women in training and throughout their military careers do not contribute to an increased incidence of stress fractures and osteoporosis?
Nutritional surveys of military women, as described in Table 1-6, reveal energy and calcium intakes in a number of different military facilities that are consistently lower than the Military Dietary Recommended Allowances (MRDAs). Thus, it is important that education programs for military women be aimed at meeting requirements for total energy needs as well as for nutrients supportive of optimal bone health. With consumption of appropriately high energy diets matched to meet the demands of physical training and fitness, higher intakes of calcium may be promoted. Obtaining the MRDAs from unfortified food stuffs has the advantage of providing intakes of other beneficial nutrients and food components.
Women should strive to maintain a stable body weight within weight-range and activity standards appropriate for their service and to refrain from episodes of repetitive dieting and weight loss so as not to disrupt normal hormonal rhythms (IOM, 1998). Stable body weight may be achieved through proper diet and participation in weight-bearing aerobic exercise activities, such as running, and walking. These measures will reduce the risk for stress fractures in the short term as well as for osteoporosis in the long term.
Eating fortified food products represents one method by which individuals can increase or maintain intakes without major changes in food habits (IOM, 1997). If women are unable to select foods and/or ration components to meet the MRDA for calcium, then appropriately formulated supplements could be considered.
Stress fractures incurred during military basic training have a significant impact on the physical fitness, training costs, and morbidity of military women. The initial fitness of recruits, both cardiorespiratory and musculoskeletal, appears to be the principal factor in the development of stress fractures during basic training.
As women enter military basic training, there may be insufficient time to achieve the aerobic fitness level required and the musculoskeletal adaptations necessary to avoid injury. Modifications to the basic training program that will promote achievement of the desired level of fitness and minimize training losses due to stress fractures could significantly influence training costs and retention of newly recruited military personnel. Training conditions that produce "optimum" levels of bone mass are important to the longer-term bone health of military women, especially during their later years when the incidence of osteoporosis normally increases.
Muscle mass, strength, and resistance to fatigue with cyclic loading play a critical role in development of stress fracture. Presentations at the workshop, Reducing Stress Fracture in Physically Active Young Servicemembers, indicated that certain training and/or fitness activities might predispose women to risk of stress fracture, especially in the pelvic region. These activities include dropping to the knees for push-ups, use of an unnatural gait or long strides during long marches, and use of incorrectly designed equipment such as footwear. Predisposing physical conditions, such as unequal leg length, have been shown to contribute to increased injury rates. In addition, the rate at which the intensity, frequency, and volume of loading occurs during training activities can increase risk for stress fractures.
Energy intake should be adequate to maintain weight during intensive physical fitness training. The requirements for adequate energy and nutrient intakes to meet the needs of the body at a moderate activity level are in the range of 2,000 to 2,800 kcal/d (AR 40-25, 1985). Nutritional modification of diets for incoming recruits are not effective in preventing stress fractures during the short term of basic training; however, modifications of diet may help to optimize overall bone health in the long term.
Currently, there is insufficient evidence to support the idea that calcium intake above the levels recommended for the general population will prevent stress fractures. In addition, data are too sparse to conclude that intense physical activity increases calcium requirements.
The requirement for military women to meet periodic military weight standards and physical fitness tests may induce inappropriate eating habits and or sudden changes in exercise training regimens, which may adversely affect bone health. Representative data are not available on the prevalence of disordered eating patterns among military women; however, dieting appears prevalent (IOM, 1998).
Conditions that induce estrogen deficiency from any cause (e.g., training regimen, diet, weight loss) may adversely affect the skeleton. It is likely that the maintenance of adequate energy intake is important in preventing the onset of secondary amenorrhea. Exogenous estrogen-progestogen
hormones may positively affect peak bone mass reached in adulthood, which may be important for future fracture risks. Most of the evidence reviewed by the subcommittee indicated that there were no detrimental effects of the use of oral contraceptives on bone health. In contrast, long-acting progestogens may have a detrimental effect on bone mineral density (BMD), and therefore, their use should be discouraged at this time. Caution should be given regarding the long-term use of GnRH agonists to treat endometriosis because of the risk of inducing substantial bone loss.
Additionally, genetics and body mass, specifically skeletal muscle mass, are important determinants in the development of stress fractures. Reduced skeletal muscle mass has been associated with increased fatigability, whereas excessive skeletal muscle mass is associated with increased torque, contributing to fractures at specific anatomic sites. Moreover, except for rare instances where stress fractures result from an underlying metabolic bone disorder, there is currently no evidence that the risk of developing osteoporosis is increased in individuals who previously suffered a stress fracture.
Bone Mass and Bone Health
The subcommittee recommends that bone measurements not be used routinely for screening recruits. This is due to problems with the accuracy (both specificity and sensitivity) of ultrasound bone mineral content (BMC) measurements to predict stress fractures in military women and the fact that mean BMD measurements of athletes with fractures lie within the normal range. The subcommittee acknowledges that other current technologies such as dual energy x-ray absorptiometry (DXA), peripheral DXA (pDXA), quantitative computed tomography (QCT), and peripheral QCT (pQCT) may be useful for bone density assessment, but they cannot be used to screen for stress fractures.
The subcommittee recommends that the military encourage behaviors that are consistent with optimizing bone health. There are data suggesting an association between stress fracture and BMC coupled with the need to optimize bone mass for the prevention of osteoporosis,
Fitness and Training
The subcommittee recommends that a more appropriate fitness standard be achieved by women prior to beginning basic training. The military services should develop and/or adopt a gradual, stratified fitness program that best suits their individual training objectives. A fitness program for individuals who are not prepared to enter military basic training should be designed that starts women at a lower level of activity and gradually increases the activity level to prepare them for entry into basic training. The American College of Sports Medicine is one organization that publishes guidelines adequate for these purposes (ACSM, 1990). This prebasic training program should utilize training techniques similar to those employed in basic training. The overwhelming body of evidence reviewed by the BCNH subcommittee indicated that initial fitness level and the rate at which intensity of activity is increased are critical factors in placing women at risk for stress fractures. Excessive exercise and abrupt changes in training load should be avoided. Although this report addresses the
specific question of lowering the incidence of stress fractures in women, both men and women will benefit and have a lower incidence of stress fractures if they are required to meet an appropriate level of fitness before entering basic training.
Consideration should be given to implementation of a standardized program of military basic training that encourages and focuses on gradual building of skeletal muscle mass with selected strength and endurance activities. The military services should identify and modify activities that produce excessive skeletal muscle force on vulnerable bones; and establish (or modify) exercise habits in women that decrease selected stress fracture risks, similar to those programs and activities that have been proposed by the Naval Health Research Center (Almeida et al., 1997), the U.S. Air Force basic military training (BMT) group (Jaeger, 1996), and the Army's Physical Fitness Training manual (FM 21-20, 1992).
The military should increase the emphasis on physical fitness programs for all active servicemembers. This in turn will assist in the maintenance of weight, fat-free mass, and bone mass. More frequent fitness and body composition assessments would promote continuous adherence to weight and physical fitness programs and decrease high-risk behaviors that result from servicemembers' efforts to pass periodic performance and body composition tests.
The subcommittee strongly suggests that the Department of Defense (DoD) consider joining with other federal agencies and programs to educate the young adult population about the importance of physical activity for health and well-being and to identify those individuals who might be at a high risk for stress fracture. This role should be consistent with the DoD need to have a pool of recruits sufficiently fit for military training.
Reproductive Health and Bone Health
The prevalence and underlying causes of oligomenorrhea and amenorrhea should be assessed in women undergoing basic training, advanced training, and active duty. Because menstrual irregularities can result in bone loss, young women in the military should be provided with information about the associations among the menstrual cycle, estrogen sufficiency (including use of contraceptives), energy restriction, and bone health.
Energy Intake and Bone Health
As recommended in its previous report (IOM, 1998), the BCNH subcommittee ''reinforces the requirement for adequate energy and nutrient intakes to reflect the needs of the body at a moderate activity level (2,000–2,800 kcal/d). Measures should be implemented to ensure that women's energy intakes are consistent and adequate to maintain weight during intensive physical fitness training. To ensure adequate nutrient intakes, female personnel must be educated on how to meet both energy and nutrient needs whether they are deployed and subsisting on operational rations or in garrison. (Many of these principles are outlined in the Army's Physical Fitness Training manual [FM 21-20, 1992].) This education is required to enable women to choose foods of higher nutrient density and to maintain a fitness program that allows for greater energy intake." The education program should be introduced within the training curriculum utilized during the Delayed Entry Program to emphasize the importance and relationship of energy and nutrient
balance to weight control. "The committee reinforces the recent efforts of the Army to provide complete nutritional labeling of all ration components and to include information to enable identification of nutrient-dense components that would help women meet the MRDAs at their usual energy intake. The committee also supports efforts to create ration supplements that would satisfy requirements that may not be readily met through the usual intake of rations. The committee recommends nutritional labeling of all dining hall menu items and provision of food selection guidelines to women in garrison" (IOM, 1998, p. 162).
Aggressive education programs should be aimed at women to help them identify and select appropriate foods, including calcium-fortified food products, thus increasing the number of women meeting their requirement for nutrients essential to bone health. The recommended Adequate Intake for calcium in women aged 19 through 30 years is set at 1,000 mg/d (NRC, 1997), which is consistent with the current MRDAs of 800 to 1,200 mg/d. However, calcium intakes in military women are reported to meet only 75 percent of the MRDA, principally because women often do not consume their full ration components. Data on the intake of vitamin D are not routinely captured in military surveys. Nevertheless, results from military intake studies strongly suggest that because of the lower level of consumption of operational rations and dining hall meals that are based on a 3,600 kcal diet plan, active-duty women are at risk for inadequate intake of several nutrients, particularly calcium. To ensure adequate energy and nutrient intake, some modifications of operational rations may be needed to increase the nutrient density. If adequate intakes of calcium are not being met from operational (field) rations or in garrison, and if nutrition education and counseling sessions fail to promote increased intakes, the use of calcium-fortified products is essential. Calcium supplements should be recommended under appropriate guidance by the military to meet women's special needs.
RECOMMENDATIONS FOR FUTURE RESEARCH
A minimal fitness standard is currently used to allow women to enter basic training. Research is needed to define the appropriate fitness level required for women to enter the military and participate in basic training without incurring an increased risk of stress fractures.
Efforts should be made to compile data from all military services on initial fitness levels of recruits by age, gender, and race/ethnicity.
As recommended in the Report on Injuries in the Military: The Hidden Epidemic (AFEB, 1996), further study is needed to determine the types of activities that may predispose women to stress fractures, especially in the pelvic region and upper leg. Steps should be taken to modify activities in basic training to lower risk.
The subcommittee recommends that the military services collect stress fracture incidence statistics by age, gender, race/ethnicity, and skeletal site, using a gender-independent, standardized definition and collecting data during a comparable time frame from all military services during both the basic-training and post training periods. Although no uniform outpatient surveillance system exists throughout the DoD, the Naval Health Research Center (Almeida et al., 1997) has developed a software application program for the purpose of supporting epidemiological research in musculoskeletal injuries. This may provide the basis for a model for the collection of injury-related data.
Because the military primarily recruits from a population that is accruing its peak bone mass, the BCNH subcommittee recommends that research efforts should contribute to identifying those factors, such as diet, lifestyle, and ethnicity, that may contribute to achieving peak bone mass, as well as components of military programs that may interfere with this process. The Report on Injuries in the Military: The Hidden Epidemic (AFEB, 1996) similarly recommends the identification and surveillance of those risk factors contributing to stress fracture injuries.
Efforts should be made to investigate more fully the now-preliminary linkages between low skeletal muscle mass, particularly in women, and stress fracture risk. Investigators should attempt to determine if this injury risk is a result of low skeletal muscle mass per se or a manifestation of inappropriately designed or enforced training programs.
Most of the evidence reviewed by the BCNH subcommittee indicated no detrimental effects on bone health from the use of oral contraceptives. However, the subcommittee recommends that future research is needed on the effects of implant or injectable contraceptives, such as Depo-Provera, on BMD and bone strength. Chemical formulation, dosage, and route of administration require further investigation.
Research is needed to assess the effect of military women's dietary energy status on the secretion of hormones that affect bone health, particularly in situations of high metabolic stress. Little is known about predisposing factors that alter the menstrual cycle.
The military should continue to gather dietary intake data and evidence concerning calcium intakes throughout a soldier's career as training programs, food choices, and food supply change over time.
Based on preliminary data from athletes, loss of calcium in sweat due to physical exertion during training as a potential pathophysiological factor on the development of stress fracture needs to be investigated. These preliminary data raise a broader question about the impact of high levels of activity on calcium requirement.
More research is needed that evaluates existing technologies of assessing risk of stress fracture, including ultrasound, central and peripheral DXA and central and peripheral QCT. Ultimately the cost-benefit analysis of all techniques will have to be assessed for specific uses and populations within the military.
The DoD should support the development of mechanical models that link skeletal muscle mass, force/torque, and bone stress in humans. As part of this process, efforts should be made to improve existing in vivo methods of quantifying components of these models, including mechanical loads and skeletal muscle mass.