Conclusions

Low initial fitness of recruits appears to be the principal factor in the development of stress fractures during basic training. A key component of training programs should be to match closely the rate of musculoskeletal adaptation with the participant, in order to avoid interruption of training for cardiovascular and muscular endurance or fitness. In the training program for female soldiers, rapid and excessive increases in exercise habits and abrupt changes in training load may increase the risk of stress fractures of the lower extremities. The subcommittee concludes that muscle mass, strength, and resistance to fatigue with cyclic loading (bone stress created by excessive or rapid incremental skeletal muscle contraction and loading forces) play a critical role in development of stress fracture. To attain an adequate level of fitness, a training program must include a history of sufficient loading and remodeling within bone if stress injuries and fractures are to be prevented during periods of intense training. Proper footwear and appropriate choice of running surfaces also contribute to the prevention of injuries. Currently there may not be sufficient time during basic training to achieve the aerobic fitness level required to avoid musculoskeletal injury.

Recommendations

A more appropriate fitness standard should be achieved by women entering military service either through a structured program prior to their beginning basic training or through an integrated program within basic training. It is recommended that such a program be designed to start women at a lower level of activity and gradually increase their activity as a transition into full-scale basic training. If a prebasic training program is selected, it should utilize training techniques similar to those employed in basic training.

The BCNH subcommittee recommends a program of basic training that encourages and focuses on (1) avoiding training errors by alternating easy and hard days (i.e., substituting low or nonimpact loading for physical routines that lead to cardiopulmonary fitness), (2) gradual building of skeletal muscle mass with selected strength and endurance activities, and (3) identifying specific exercises that may modify the etiology and site distribution of stress fractures among women and provide ones that do not incur an increased risk for developing stress fractures.

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 important 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.

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. Bone stress created by excessive or rapid incremental skeletal muscle contraction and loading forces can cause fractures at specific anatomic sites. However, the major problem for military recruits is likely to be insufficient muscle mass.

Conclusions

It is well recognized that the etiology of stress fracture is multifactorial and that lower bone mineral density is only one contributing factor. Genetics and body mass, specifically muscle mass, are also important determinants in the development of stress fractures. Although current technologies (e.g., dual energy x-ray absorptiometry [DXA], peripheral DXA [pDXA], quantitative computed tomography [QCT], peripheral QCT [pQCT], and ultrasound) may be useful for bone density assessment, which has a wide range of normal values, they cannot be used to screen for stress fracture.

Recommendations

Bone measurements should not be used routinely for screening recruits. Problems with the accuracy of bone mineral content measurements (both specificity and sensitivity) make it difficult to predict stress fractures in military women. Moreover, mean bone mineral density measurements among athletes with stress fracture lie within the normal range.

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 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 (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.

Weight-bearing activity determines the shape and mass of bone. Graded increases in physical activity and resultant increases in the level of musculoskeletal fitness are necessary to ensure sufficient time for loading and remodeling within bone to prevent stress injuries and fractures.

The use of oral contraceptives that contain estrogen with or without progestogens is not considered to have long-term detrimental effects and may benefit bone health. Use of long-acting depot preparations of progestational agents, such as Depro-Provera, has been associated with relative estrogen deficiency. Long-term use of gonadotropin-releasing hormone agonists induces a state of estrogen deficiency and has been associated with bone loss. Cigarette smoking may be a long-term risk factor for the development 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.

Conclusions

Energy intake by military women should be adequate to maintain weight during intense physical fitness training. Training regimens should provide for a gradual increase of load-bearing activities (''ramp-up''). Nutritional modification of diets of incoming recruits cannot effectively prevent stress fractures during the short term of basic training. The use of oral contraceptive agents is not contraindicated. Exogenous estrogen-progestogen hormones may positively affect peak bone mass reached in adulthood, which may be important for future fracture risks in contrast to the use of long-acting progestogens and gonadotropin-releasing hormone agonists.

Recommendations

Implement measures to ensure that energy intakes by military women are consistent and adequate to maintain weight during intense physical fitness training.

Shift emphasis of the program to one of continual physical fitness, which in turn will assist in the maintenance of weight, fat-free mass, and bone mass in all active servicemembers.

The BCNH subcommittee strongly suggests that the Department of Defense (DoD) consider joining with other federal agencies and programs to educate young adults about the importance of physical activity for health and well-being and to identify those individuals who might be at high risk for stress fracture.

This role should be consistent with the DoD's need to have a pool of recruits sufficiently fit for military training.

4. How do caloric restriction and disordered eating patterns affect hormonal balance and the accrual and maintenance of peak bone mineral content?

Caloric restriction or disordered eating may lead to a hormonal disruption that is associated with amenorrhea and an associated estrogen deficiency and loss of bone mineral content (IOM, 1998).

Conclusions

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 body weight is important in preventing the onset of secondary amenorrhea.

Recommendations

In active-duty servicemembers it is recommended that fitness and body composition assessments be performed frequently. At a minimum, body weight and composition should be evaluated more frequently than the current 6 month intervals. This would foster adherence to practices of healthy weight and physical fitness and decrease high risk, or disordered eating behaviors.

The prevalence and underlying causes of oligomenorrhea and amenorrhea should be assessed in women undergoing basic training and advanced training and on active duty. Young women in the military should be provided with information about the associations among the menstrual cycle, estrogen sufficiency (including use of contraceptives), bone health, and energy restriction.

5. How can the military best ensure that the dietary intakes of active-duty military women in training and throughout their military careers do not contribute to an increased incidence of stress fractures and osteoporosis?

Nutrition education programs are key to providing information and direction on the choice and nutrient content of appropriate foods. It is important that education programs for military women be aimed at their meeting requirements for total energy needs as well as for nutrients supportive of optimal bone health. With consumption of appropriately higher energy intakes matched to meet the demands of physical training and fitness, higher intakes of calcium should be promoted.

Women should strive to maintain a stable body weight within weight-range standards appropriate for their service and should refrain from episodes of repetitive dieting and weight loss so as not to disrupt normal hormonal rhythms (IOM, 1998). Weight within standard may be achieved through proper diet, selection of nutrient-dense foods, and participation in weight-bearing exercise activities. These measures will be beneficial for the reduction of stress fracture risk in the short term, as well as for osteoporosis prevention in the long term.

Conclusions

Many predisposing factors can alter the menstrual cycle. It is likely that maintenance of appropriate body weight is important in preventing the onset of secondary amenorrhea. To ensure adequate nutrient intakes, female military personnel must be educated on how to meet both energy and nutrient needs. This education is required to enable women to choose foods of higher nutrient density and to maintain a fitness program that will allow greater energy intake.

Recommendations

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) . . . The subcommittee reinforces the recent efforts of the Army to begin providing 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 (Military Recommended Dietary Allowances) at their usual energy intake. . . .The subcommittee recommends nutritional labeling of all dining hall menu items and provision of food selection guidelines to women in garrison" (p. 162).

The military should develop aggressive education programs for military women aimed at helping them identify and select appropriate foods and fortified food products to increase the number of women meeting their requirements for these nutrients. If nutrition education and counseling sessions fail to promote increased intakes, the use of calcium-fortified products becomes essential. Calcium supplements should be recommended under appropriate guidance by the military to meet women's special needs.

Recommendations for Future Research by the Military

• Research is needed to define the appropriate fitness level that is required to enable a woman to enter and participate in basic training without incurring an increased risk of stress fractures.
• Data on initial fitness levels should be compiled in recruits from all military services by age, gender, and race/ethnicity.
• 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, and steps should be taken to modify their activities in basic training to lower risk.
• Stress fracture incidence statistics should be collected by age, gender, race/ethnicity, and skeletal site, using a gender-independent, standardized definition of stress fracture and a comparable time frame from all military services for both the basic training and posttraining periods.
• Military 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.
• Efforts should be made, particularly in women, to investigate more fully the now-preliminary linkages between low skeletal muscle mass and stress fracture risk. Investigators should attempt to determine if this relationship is due to a low skeletal muscle mass effect per se or an associated factor such as inadequate initial fitness status.
• Research is needed on the effects of implanted or injectable contraceptives, such as Depro-Provera, on bone mineral density and bone strength. Chemical formulation, dosage, and route of administration require further investigation.
• Research is needed that assesses the effect of dietary energy status of military women on the secretion of hormones that affect bone health, particularly in situations of high metabolic stress.
• The military should continue to gather dietary intake data and evidence concerning calcium intakes throughout the soldier's career, as training programs, food choices, and food supply change over time.
• Based on preliminary data from athletes, the potential loss of calcium in sweat due to physical exertion during training and the impact of high levels of activity on calcium requirements needs to be investigated as possible pathophysiological factors in the development of stress fracture.
• More research is needed to evaluate existing technologies for cost-effective assessment of bone mass. These technologies currently include ultrasound, central and peripheral dual-energy x-ray absorptiometry, and central and peripheral quantitative computer tomography. Ultimately, the cost-benefit analysis of all techniques will have to be addressed for specific uses and populations within the military.
• Mechanical models should be developed which link skeletal muscle mass, force/torque, and bone stress in humans, as well as to improve existing in vivo methods of quantifying components of these models.

References

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• IOM. 1998. Assessing Readiness in Military Women: The Relationship to Body Composition, Nutrition, and Health. Committee on Body Composition, Nutrition, and Health of Military Women, Committee on Military Nutrition Research, Food and Nutrition Board. Washington, D.C.: National Academy Press.
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