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Stress Fractures in Female Army Recruits: Implications of Bone Density, Calcium Intake, and Exercise

Department of Food Science and Human Nutrition, Colorado State University, Ft. Collins

Address reprint requests to: Alana D. Cline, PhD, RD, Pennington Biomedical Research Center, Louisiana State University, 6400 Perkins Road, Baton Rouge, LA 70808-4124

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Objective: To identify characteristics and factors associated with increased risk for stress fractures in military women.

Design: Case-control study to retrospectively examine physical activity, prior calcium intake, and bone density as predictors of stress fractures.

Setting: A military training installation which incorporates physical training for women.

Subjects: Forty-nine female soldiers with confirmed stress fractures (cases) and 78 female soldiers with no orthopedic injuries (controls), aged 18 to 33 years.

Measures: Retrospective self-reports of habitual exercise, sports participation, and food intake; current height, weight, and body mass index (BMI); demographic variables (age, ethnicity, menstrual patterns, smoking habits); and bone density on radiologically defined stress fractures.

Results: Cases and controls were similar in height, weight, and BMI. Measurements of bone density (g/cm²) at the trochanter (cases, 0.77±0.09; controls, 0.77±0.08); femoral neck (cases, 0.94±0.10; controls, 0.94±0.09); Ward’s triangle (cases, 0.91±0.11; controls, 0.93±0.11); lumbar spine (cases, 1.21±0.12; controls, 1.24±0.10); and radius shaft (cases, 0.67±0.09; controls, 0.68±0.05) were not different between groups. Calcium intake was not different between groups (cases, 1154±751 mg/day; controls, 944±513 mg/day) and did not correlate with bone density (r=0.01 to -0.06 at four sites). Sports participation positively correlated with bone density in the hip (r=0.49). Leisure activity energy expenditure (kcal/day) tended toward association with lower stress fracture risk as expenditure level increased (p=0.06).

Conclusion: Stress fracture in female Army recruits was not correlated with bone density or calcium intake during adolescence, although a weak relationship to prior physical activity was observed.

Key words: stress fracture, bone density, physical activity, calcium, exercise

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Stress fracture has long been considered a hazard of military life. This type of injury occurs in a normal bone and is attributed to repeated stress from unaccustomed submaximal exercise, usually weight-bearing. In military basic training, the stress fracture is a common orthopedic problem, causing loss of manpower, loss of training time, expense of medical care, and recycling or discharge of affected soldiers [1]. Prevention of even a small number of these injuries would yield large benefits, both monetarily and in troop productivity. This condition, however, is not confined only to military personnel. With increased emphasis on physical conditioning and participation in sports by the general population, it is quite possible that the incidence of stress fractures will increase in the future.

Since the first description of stress fracture in a soldier, many extensive clinical surveys of stress fractures in military recruits in various geographical areas have been reported. The majority of these studies have included only men, but as more women enter the services, several recent studies have included the latter as well [1–5]. No studies have identified nutritional predictors of stress fracture or low bone density in military personnel.

Women entering the military have a much higher incidence of stress fracture injury during basic training than men, with approximately 1 to 3% and 10 to 12% of new male and female recruits, respectively, being affected [4,6]. Although studies of female athletes [7,8] and ballerinas [9,10] have provided some useful information investigating the relationships of bone density, calcium intake, or amenorrhea to stress fractures, there are no studies which have investigated these relationships in female military recruits. Several military studies have considered the relationship of physical activity to stress fractures [4,5,11]. However, there is no current consensus on what factors play the greatest role in incidence of stress fracture.

With the rising recruitment of women into the armed forces, data are needed on their response to physical training and on any physical differences that may limit their performance. At the same time, the number of women engaged in sports is increasing. As a result, there has been a significant increase in the number of lower extremity stress fractures in women. In the few studies that have compared men and women, it has been shown that women have a higher incidence of stress fractures than men while participating in the same physical activities. The distribution patterns among civilian runners and joggers are very similar to those stated previously for military trainees [3,6,12].

The underlying objective of this study was to identify those characteristics and factors associated with increased risk for stress fractures in military women. The hypotheses tested were that female soldiers who experience stress fractures (cases) compared to female soldiers without stress fractures (controls) have: a) decreased bone density, b) a history of less physical activity during high school than those with no stress fractures, and c) a history of lower dietary calcium consumption. The correlation between bone density and body mass index (BMI) was also investigated.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Subjects
The research design was a case-control study of female Army recruits who had recently completed their 12-week basic training course. Volunteers were asked to complete evaluation procedures, which included anthropometric data, diet and exercise questionnaires, and bone density measurement. A total of 141 women (63 cases, 78 controls) volunteered to participate, of which 51 (38 cases, 13 controls) received bone density evaluation, and 127 (49 cases, 78 controls) completed questionnaires. There was a disparity between number of women completing questionnaires and those receiving bone density evaluation due to high drop-out rates by controls for density evaluation. Study volunteers were reluctant to miss additional training time for non-medical appointments for density evaluation so withdrew from further participation in the study after completing questionnaires. Women with stress fractures (cases) were identified upon medical examination during evaluation and treatment of pain. A small group of women (cases) with incomplete questionnaires were included in the study because valuable data on physical characteristics and bone density were available. All stress fractures were confirmed by bone scan, and patients were referred to the principal investigator for recruitment into the study. Control subjects were randomly selected from women reporting to the troop medical clinic for non-orthopedic, minor health problems. All subjects gave their informed consent, and the Committee on Human Research at Colorado State University and the Brooke Army Medical Center Clinical Investigation Committee approved the research plan.

Demographic information collected included age, race, geographic location during adolescence, date of menarche, menstrual patterns, use of oral contraceptives, and smoking habits; anthropometric data collected were height, weight, and BMI (kg/m²).

Food Frequency Questionnaire
To evaluate the nutrient intake of participants, an abbreviated version of the Health Habits and Diet Questionnaire by Block et al [13] was used, with a slight modification in the instructions to ask participants to recall food habits during adolescence. The questionnaire began with the following statement: "This section is about your usual eating habits. Thinking back over your TEENAGE (ages 13 to 18) years, how often did you usually eat the foods listed on the next page?" An additional question asked if participants had limited intake of dairy products for any reason during the same time period. The 60-item questionnaire evaluates 18 major nutrients and includes foods representing approximately 93% of total United States caloric consumption.

Validation of use of a food frequency questionnaire (FFQ) to recall diet 4 years in the past has been reported by Willett et al [14], indicating that a self-administered FFQ can be used to provide information about previous diet. Reasonable correlations with diet 10 to 15 years previous were obtained by interview with a FFQ for a variety of nutrients by Sobell et al [15], and Frazier et al [16] reported reproducibility with use of a FFQ to measure adolescent diet recall in women aged 40 to 65, suggesting that recall is reasonably reproducible.

Nutrient estimates for dietary assessment were provided by computer software and are based on the NHANES II nutrient content database [17]. Questionnaires were designed to be self-administered; they were returned directly to the study dietitian who conferred individually with each subject to verify information before coding for computer analysis.

Exercise Questionnaire
Exercise and energy expenditure from leisure and sports activities in the year immediately prior to entry into the Army were evaluated by a modification of the Minnesota Leisure Activities Questionnaire [18], which is designed to assess leisure activities during the past year. Use of this modified questionnaire has been validated and reported by Jones et al [19] on numerous studies with Army recruits. Energy expenditure from activities during the prior year was calculated using reference formulas and intensity codes [20,21]. Participation in organized or varsity sports during high school, and the individual’s perception of her physical activity level (prior year) on a scale of 1 (inactive) to 4 (very active) were also reported.

Bone Density Evaluation
Single-photon absorptiometry (LUNAR SP2, Lunar Radiation Co., Madison, WI) was used to determine bone mineral content and mineral density of the radius [22]. The site of measurement was one-third of the distance from the ulnar styloid process to the olecranon on the non-dominant arm, which contains approximately 95% cortical bone. Measurement was also made on the ulna and radius distal to the site of juxtaposition of the radius and ulna at the wrist sites, which contain predominantly trabecular bone. The coefficient of variation (CV) of repeated measurements at these two sites using this technique was approximately 2.1 and 2.3%.

Bone mineral measurements of the lumbar spine and hip, both predominantly trabecular bone, were made using dual-photon absorptiometry (LUNAR DP3, Lunar Radiation Co., Madison, WI) which permits a quantitative assessment of skeletal bone mineral by use of a gadolinium-153 radionuclide source [23]. Precision error (CV) of this technique was approximately 2.0% for the spine and 2.7% for the hip. All scans were performed by the same experienced technician over a 1-month period; standardized calibration techniques were used for quality control at the start of each day.

Data Analysis
Statistical Package for the Social Sciences (SPSS, New York:McGraw-Hill, 1983) Release 4.0 was used to analyze data. The Student’s t-test was first used to determine if there were significant group differences in historical calcium intake, exercise levels, intake of other nutrients, bone density, and physical characteristics. Statistical significance was determined using a two-tailed test and a 5% level of significance for each test.

Relationships between variables were examined using simple correlation procedures, and stepwise multiple regression analysis was used to identify major predictors of bone density. The stepwise procedure was used so that independent variable(s) remaining in the model would only be those that were statistically significant. Chi square analysis was used to examine differences between stress fracture and control subjects when examined by race, smoking, menstrual status, oral contraceptive use, activity level, calcium, and exercise energy expenditure [24].

Case and control groups were initially divided into non-blacks and blacks for statistical analysis; Hispanics were included in the non-black group after analysis revealed no differences between groups with or without their inclusion. Of the 29 black study participants, only seven received bone density evaluation. They did not differ from other participants in bone density, calcium intake, or physical activity. Because of the small sample size, no attempt was made to evaluate the influence of race on outcome variables in the study. Therefore, cases and controls were considered as single groups without dividing by race.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Table 1 shows no significant differences in age, height, weight, and BMI between injured (case) and control soldiers. Because of the narrow age range for enlistment into the military, there was little between group heterogeneity in age.

Thirty-seven percent of all subjects reported involvement in varsity sports while in school, with no differences between cases and controls. Correlations were found between bone mineral density of the trochanter and height (r=0.36, p<0.01), weight (r=0.29, p<0.05), and high school organized or varsity sports participation (r=0.49, p<0.01).

Stepwise multiple regression analysis was used to examine possible relationships between independent and dependent variables. The dependent variables of bone mineral density of the radius (BMDR), bone mineral density of L2-L4 spine (BMDL2L4), bone mineral density of the femoral neck (BMDFN), bone mineral density of Ward’s triangle (BMDWT), and bone mineral density of the trochanter (BMDTR) were compared with the independent variables of all nutrients, physical characteristics, demographic characteristics, and stress fracture occurrence. Weak associations at three sites, negative for BMDWT with percent fat calories eaten (r=0.43, P<0.05), positive BMDTR with sports participation (r=0.47, p<0.05), and negative BMDL2L4 with stress fracture (r=0.44, p<0.05) were most likely due to chance and are not considered to be informative. No independent variables tested were significantly associated with bone mineral density at any skeletal site.

Calcium and Nutrient Intake vs. Bone Density
Reported intakes of protein, carbohydrate, fat, and energy during adolescence were not significantly different between injured and control subjects (Table 4). The intake level of most nutrients approximated or exceeded Recommended Dietary Allowances (RDA) levels [26]. Intakes of vitamins, minerals, and fiber and their percentages of RDA also were not different between groups (Table 4). Regular vitamin supplementation (multivitamin plus iron) was reported by 31% of cases and 27% of controls. Additionally, four subjects (controls) reported taking supplemental vitamins A, C, and E; six cases and six controls, supplemental iron; and one subject (control), calcium.

Servings of dairy products consumed per week were higher in the stress fracture (case) group (14.5±3.1) than in the control group (11.1±8.4) (p<0.05). Assessment of the relationship of patterns of dairy consumption to bone density at each of the measured sites indicated that individuals who had at least three servings of dairy products per day (i.e., a dairy serving with each meal) (n=10) had no significant difference in bone density when compared with those who reported eating dairy products less frequently (n=26).

No significant correlations were found in the relationship of individual nutrients (calcium, phosphorus, iron, sodium, potassium, and dietary fiber) with bone density measurements. Statistical regressions of bone density measures BMDFN, BMDR, BMDL2L4, BMDTR, and BMDWT, on calcium intake were not statistically significant (p>0.05), with correlation coefficients ranging from 0.009 to 0.055.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Stress Fractures vs. Bone Density
Weight, height, and BMI of this study population were not different from comparable values in other studies of young normal women in similar age groups for these variables [23,27]. Based on this finding, it appears that this sample of women enlisting in the Army had physical characteristics similar to a cross-section of the general population of women the same age. Women who enter the military are screened for medical problems, so they are likely to be in better health than their civilian counterparts in this age group, in general. Although the mean height of this study group was comparable to ballerinas with stress fractures [9], the ballerinas weighed less. Height and weight of this study group were comparable to values in other studies of military females with stress fractures [3–5]. Jones et al [19], in a study on basic trainees, reported similar data for height, weight, and BMI. In the current study, chi-square analysis revealed no significant relationship between race, age, or height and stress fracture. These findings differ from those of Brudvig et al [1], who reported that race and age were factors related to the development of stress fractures. Findings of this study are consistent with those of Reinker and Ozburn [3], and Hopson et al [2], who found that these factors were unrelated to stress fracture risk.

Bone density values in the current study are lower than those of white young women as reported by Mazess and Barden [25] (Table 2), but not significantly different. One individual had a vertebral mineral density below the fracture threshold as defined by Riggs et al [27], of 0.965 g per square centimeter, and 12 (10 cases) were in the range of -1 to -2.5 standard deviations (SD) for osteopenia. Table 2 indicates percentages of subjects with BMD more than 1 SD below reference value for each site measured.

Comparison of bone density values from the current study group with ballerinas who experienced stress fractures [9] showed there were no significant differences between stress fracture and non-stress fracture groups in both studies. Ballerinas had slightly lower bone density values than the soldiers, which may be a reflection of the body build of those girls most likely to participate in ballet.

These results differed from a stress fracture study on athletes in South Africa [28], in which athletes with stress fractures had significantly lower bone mineral densities than controls; the lower density was in both trabecular and cortical bone.

Winters et al [29] reported lumbar bone mineral densities similar to those in the present study in comparisons of trained runners and moderately active controls. However, half of the runners reported stress fractures, compared to one incidence in the controls, and they had significantly lower total bone calcium per kg of soft lean tissue obtained from DEXA scans.

Physical Activity and Amenorrhea vs. Bone Density
The few studies investigating stress fractures in female athletes reported an association between amenorrhea and stress fracture [28,9]. The association most frequently discussed is that of amenorrhea and lower bone density values, which then appears to result in more stress fractures in amenorrheic athletes. The absence of menses reported by the current study participants was temporary, and occurred only during their basic training for a duration of approximately 2 to 3 months. The short duration may explain why no relationship was evident between bone density values and stress fractures in amenorrheic soldiers. This finding supports a study by Hetland et al [30], which reported that women who began a running program experienced irregularities in their menstrual cycles, but found no statistically significant relation between running activity and bone mineral measurements or bone turnover. Previous studies on athletes reported amenorrhea that lasted from 1 to 7 years or longer resulting in disturbances of estrogen and other sex-steroid hormone levels.

Recently it has been reported that ovulatory disturbances which are common during exercise training may be detrimental to bone health, even in women who have apparently normal menstrual cycles. Physically active women with shortened luteal phases or anovulatory cycles have shown lower BMD than physically active women with normal cycles [29,31,32]. Because almost one-fourth of the subjects in this study had reported recent menstrual irregularities, incidence equally distributed between the two groups, ovulatory disturbances associated with bone loss may have been present which were not measured in this study.

Energy expenditure for leisure activity of over 600 kcal/day in both groups of the current study is comparable with athlete values reported in a study by Lewis [33] that compared athletes and non-athletes. This suggests that women who enlist in the Army tend to be physically active prior to entry. Because exercise participation was equally high in both groups, the effect of exercise on bone may explain bone density values that are similar in both groups.

The findings of this study agree with those of previous studies [2,4,11] that individuals who are less physically active prior to entering the military are more likely to get stress fractures. This differs from the finding of Ozburn et al [5], that physical conditioning is not a factor.

Exercise has a positive effect on bone density [34–36], which may be reflected in this study by correlations between varsity or organized sports participation and bone density, both requiring some degree of physical training. An exception to this is a negative correlation between training and bone mineral density of the spine and femur in all subjects reported by Myburgh [28]. Cross-sectional studies demonstrated that exercise history in individuals indicated a greater bone mass, but the difference between adults who exercised and those who did not was less than 10% [34,37].

The only correlates in this study that appear to have a relationship with bone density or stress fracture are high school sports with bone density in the hip, and a trend toward significance of leisure activity energy expenditure with stress fracture.

Calcium and Nutrient Intake vs. Bone Density
When comparing retrospective adolescent food consumption data of current study participants with the report by McKoy [38] on adolescents from the South, and with NHANES III [39,40], intake of current study participants was higher in some nutrients, but differences were not large. Diet analysis in other studies utilized either a 3-day diary or a 7-day diary, with the exception of the study by Frusztajer et al [9], which used a food frequency questionnaire. Current study participants were asked to recall diet habits from 1 to approximately 18 years in the past, which has been reported as being valid and a reasonable way of retrospectively assessing diet [14,15]. Consensus is that correlation between past diet and recall of that diet is greater than the impact of the current diet on recall. Good recall has been seen in milk products, whole milk, fish, and alcohol, most likely because of the dominant role they play in consumption patterns. Also to be considered is that adolescent diets may have associated memories that enable better recall, and that meal consumption in the family situation may be more rigid than adult self-selection. Although written instructions are provided with the questionnaires, it remains important for follow-up instructions and interviews with respondents to clarify answers and review questionable responses. This became apparent in the present study as questionnaires were reviewed with volunteers upon completion.

Energy intake in the current study was, on the average, approximately 20% higher than the National Food Consumption Survey [41], NHANES III [39,40], and Southern adolescents [38,42]. Higher energy consumption may be explained by the fact that subjects in this study were physically active in high school and required more energy to participate in sports activities.

Myburgh et al [28] reported that subjects with stress fractures had lower intakes of dietary calcium and dairy products than did control subjects, and that there was a significant positive correlation between calcium intake and bone mineral density in the weight-bearing bones. In the current study, although stress fracture subjects reported a higher intake of dairy products than controls, calcium intake was not significantly different between groups.

Bone density was not found to differ between injured soldiers and control soldiers, nor was it correlated significantly with calcium consumption during adolescence. This differs from previously reported studies that showed a positive influence of calcium intake on bone mass [43,44]. However, it agrees with the report by Ulrich et al [45] that no significant associations were found between any measures of milk consumption or calcium intake and BMD.

Although calcium intake during training was not assessed in this group, recent studies on nutritional intake of women in basic training have reported mean calcium intakes of 907 and 728 mg [46,47], which, if similar to our study population, would reflect a significantly lower intake of calcium than that reported prior to training. Also unknown is how quickly a change in bone density occurs if calcium intake decreases in combination with increased exercise, or what effect a decrease of calcium may have had on bone remodeling during increased stress on bone.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
The occurrence of stress fractures in female Army recruits was not associated with decreased bone density measured at five different sites, with reported calcium intake during adolescence, or with adolescent sports participation. Leisure activity energy expenditure (kcal/day) tended toward association with lower stress fracture risk as expenditure level increased. Bone density at several sites was positively related to adolescent sports participation, but was unrelated to reported adolescent calcium intake. Although no relationship was seen in this study between diet or bone density with stress fracture, numerous questions must still be answered regarding the high stress fracture incidence in young women entering the military.

	ACKNOWLEDGMENTS

 
The authors thank the staff of the Brooke Army Medical Center Nuclear Medicine Clinic for providing bone density evaluations for study volunteers.

	FOOTNOTES

 
Disclaimer: "The opinions or assertations contained herein are the private views of the author(s) and are not to be construed as official or as reflecting the views of the U.S. Army or the Department of Defense."

Received May 1, 1997. Accepted October 1, 1997.

	REFERENCES

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