The influence of lifestyle, menstrual function and oral contraceptive use on bone mass and size in female military cadets

All cadets entering the United States Military Academy (USMA) Class of 2002, West Point, NY were sent a study information packet, including a sample consent form and study protocol, prior to their arrival at the Academy. The study objectives and the associated risks were described in a presentation during their first week at the Academy. Seventy percent of all eligible women in the class agreed to participate and provided written informed consent. There were no differences between those who elected to participate and those who did not with regard to race or age. One hundred and thirty five women consented to participate in a 4-year prospective study examining the determinants of peak bone mass and stress fractures. There are stringent medical requirements for entrance into USMA that led to the exclusion of women with severe polycystic ovarian disease and other diseases that affect BMD, including persons with uncontrolled thyroid disease, epilepsy, gastrointestinal disease or long term steroid use (Army Regulation 40–501, Chapter 2). Race was self-reported on USMA applications. The largest racial group was Caucasian (n = 107). Therefore analyses will focus on this group. However, racial differences in bone density will also be reported. The Institutional Review Board (IRB) of Keller Army Community Hospital (KACH), West Point, NY, approved the study.

Information on exercise, dietary and other lifestyle factors in the year prior to entering USMA was collected by questionnaire, allowing cadets to pick from categories to expedite data collection. Exercise was quantified as the number of exercise hours per week and divided into four categories of 1–3, 4–6, 7–10, or 11 + hours per week. Four questions assessed calcium intake including daily milk consumption (divided into 4 categories: none, <1, 1–2, or 3 or more 8 ounce glasses per day) and weekly servings of yogurt, cheese and high calcium content vegetables (0, 1–3, 4–6 or 7 + serving per week). From these responses, an average daily calcium intake was calculated (mg/day). Daily caffeine intake was divided into three categories: none, 1 to 3 and more than 3 caffeine containing drinks a day. Alcohol intake was categorized as: less than once a month, 1–3 times a month, 1 to 2 times a week, and 3 to 5 times a week. Tobacco use was assessed by type (dip, chew, or cigarettes), dose, and duration. Lifetime history of personal skeletal fracture and site of fracture were determined by questionnaire. Cadets reported their age at menarche, and current and prior use of birth control pills. Prior use was considered any use of oral contraceptives (OC) in the past for greater than 3 months. Current users included individuals taking OC for at least 3 months, including at entry to USMA. Frequency of menstrual cycles in the year prior to entering USMA was categorized as ≤9 cycles per year (amenorrhea/oligomenorrhea), or 10–12 cycles/year (normal) in the past year.

Academy personnel measured each cadet's height and weight at entry in standard athletic clothing in stocking feet and height was measured using a standard scale. Body Mass Index (BMI) was calculated as weight in kilograms divided by height in meters squared. Trained Academy personnel administered the Army Physical Fitness Test (APFT) to all cadets upon entry into USMA. This test measures upper body and abdominal strength and aerobic fitness. It includes the number of full body push-ups and bent leg sit-ups completed in two-minute segments as well as a timed two-mile run. These three events (sit-ups, push-ups and a 2 mile run) are then each converted into an age and gender standardized score ranging from 0–100 [21]. Faster running times, higher numbers of push ups and sit ups earn higher scores. The sum of these three scores represents the overall assessment of fitness with a range of 0–300.

The left total hip and lumbar spine (L1-L4) BMD (gm/cm²) were measured using a mobile DXA scanner (DPX-IQ, General Electric-Lunar, Madison WI). The coefficient of variation (%CV) in vivo was determined by measuring 8 individuals two times with repositioning. The %CV was 1.5% and 1.5% respectively for spine and left total hip in this population. During this study there was no impact of movement of the mobile DXA, based on no significant change in the phantom measurements as part of the daily quality control program for DXA measurements. BMD (g/cm²) of the left calcaneus was assessed using peripheral dual-energy x-ray absorptiometry (pDXA; Pixi, Lunar, Madison, WI). The coefficient of variation in vivo for the calcaneus pDXA was 1.0%. Peripheral QCT (Stratec XCT2000; Germany) was used to image a single slice at the distal tibia. To identify the site, the distal third of the tibia was determined by manual measurement of tibial length between the base of the patella and the styloid process (to the closest centimeter). The pQCT was positioned at the distal site after placing a positioning light of the gantry above the styloid process. The tibia was chosen because it is a major site of stress fractures in military cadets [8]. Bone mineral content (mg per 1 mm slice of bone), cortical thickness (mm), periosteal and endosteal circumferences (mm) were measured. Cortical bone was identified at a threshold above 710 mg/cm³. Cortical thickness was derived using the circular ring model, which calculates a mean cortical thickness from measures of total bone area and cortical bone area. The coefficient of variation was 2.2% for tibial mineral content and 3.2% for cortical thickness.

Student's t-tests were used to examine the differences in BMD among dichotomous lifestyle variables and non-parametric tests were used (Kruskal-Wallis Test) to test differences between variables that are not normally distributed such as menstrual cycle frequency. The relationships between lifestyle determinants and BMD were examined using Pearson's correlation analyses and, where appropriate, with linear regression to control for potential confounders. Effects of categorical lifestyle variables on BMD were also assessed using analysis of variance. The level of significance for alpha was set at 0.05 for these statistical tests. In Caucasian women, a multiple regression model was created using all covariates that had biologic plausibility and were significant in univariate analyses and variables with p < 0.1 were included in the model. All analyses were performed with SPSS statistical software (Version 13.0 for Windows, SPSS Inc., Chicago IL.)

There were 135 women enrolled, including 107 Caucasians, 13 African Americans and 15 Asians. Table 1 provides a summary of the lifestyle variables from the baseline questionnaire for the Caucasian women. Overall, the women cadets in this study had healthy behaviours with 77% reporting they exercised more than 7 hours a week, and reporting an average daily calcium intake approximately equal to the recommended daily intake (RDI) for this age group (1000 mg calcium per day). Total APFT score was more than one standard deviation better than similar aged female recruits that enter basic training, a group that more closely represents the general population [21]. One component of the APFT, the run score equated to times for the two mile run test that ranged from 13 minutes 23 seconds to 22 minutes 39, with 50% of the women running faster than 8.37 pace per mile.

Women on oral contraceptives had a significantly lower run score than women who never took oral contraceptives (p = 0.02). There were no other significant differences in population characteristics between women on oral contraceptives as compared to women who never used oral contraceptives (Table 1). Most women (97%) were tobacco free, and 10% admitted to consumption of more than two alcoholic beverages per week (although alcohol intake may be underestimated in this underage population). Thirty-four% of women reported previous fractures. Approximately 11% of the women currently used oral contraceptives and there was no reported use of Depo-Provera. All women had experienced menarche and the women with oligomenorrhea/amenorrhea reported reaching menarche a year later than those women with normal cycles in the past year (p < 0.05; Table 1). There were no other significant differences between women with normal cycles and women with oligomenorrhea/amenorrhea.

T-scores for the entire population were calculated based on the reference population from Lunar for the spine (average T-score = +0.6) and for the total hip (average T-score = +1.2) using the NHANES database. BMD results for the calcaneus, left total hip, spine (L1-L4) and tibia as well as tibial dimensions are reported by menstrual function category (Figure 1) and oral contraceptive use (Figure 2) and discussed below.

Although there were no significant racial differences in total hip BMD, African American women had slightly higher spine and calcaneus BMD when compared to Caucasians (p < 0.05). However, all races had significantly higher calcaneal, spine and hip BMD than the population average (p < 0.025). African American women had significantly larger periosteal circumference and tibial mineral content when compared to the other races (p < 0.02). However, there were no racial differences seen in cortical thickness.