The Association between Bone Mineral Density, Lifestyle Factors, and Body Composition in a Fit College Population

The aim of this dissertation was to explore the determinants of bone mineral density and weight change in a fit, college-aged population. Specifically, this dissertation is a series of four papers that examined the determinants of bone mineral density (BMD) at multiple skeletal sites in men and women at college entrance, BMD differences related to prior participation in sports, and lastly, weight changes in women during four years at university. The subjects were 891 students, 755 males and 136 females of various racial backgrounds, entering one class at the United States Military Academy (USMA) at West Point. This was a unique population because these college students are healthier, fitter and engage in more positive health behaviors than other college populations. The data for these papers came from a larger Department of Defense funded prospective study examining longitudinal changes in BMD and the risk factors for stress fractures. Upon arrival at USMA, a baseline questionnaire assessed prior exercise frequency, consumption of milk and other high calcium foods, caffeine and alcohol consumption, as well as tobacco and oral contraceptive use. Annual surveys assessed diet, menstrual function and contraceptive use. Academy staff measured height, weight and fitness annually. Varsity level sport specific information was collected from high school applications to assess skeletal differences in BMD associated with prior sport participation. Calcaneal BMD was measured by peripheral dual energy x-ray absorptiometry (pDXA). Peripheral-quantitative computed tomography (pQCT) was used to measure tibial bone density, circumference and cortical thickness. Spine and hip BMD were measured in all women and a subset of male cadets. Body composition was assessed using bio-electrical impedance. The Eating Disorder Inventory-2 was given to all participants in their final year of university to assess eating behaviors. Baseline BMD was approximately one standard deviation above young normal at the calcaneus and hip. There were significant gender and racial differences in baseline BMD at multiple skeletal sites. African American men had significantly higher hip, spine and heel BMD and greater tibial mineral content and cortical thickness than Caucasians and Asians men. Similarly, African American women had significantly higher calcaneal and spine BMD than Caucasians. Higher caffeine intake in men had a deleterious effect on BMD. Oral contraceptive use in women was associated with reduced BMD and bone size. Women who had approximately normal menstrual cycles evidenced higher BMD at all sites, greater tibial mineral content and tibial cortical thickness as compared to those who had 9 or less menstrual cycles in the year prior to entry. Sport specific differences in BMD were apparent. Prior participants of high loading sports (football) had significantly more BMD at multiple sites while participants in non-loading sports (swimming) had less BMD as compared to participants in other sports, even after controlling for body mass index (BMI). During their four years at university, the Caucasian women studied had small but significant weight, body fat and BMI increases, while fitness scores also significantly increased. Younger age of menarche was associated with increased body fat at graduation. The use of depot medroxyprogesterone acetate was positively associated with a change in body fat at graduation. A number of measures of eating disorders, including a sense of ineffectiveness, body dissatisfaction, interpersonal distrust and maturity fears, were associated with graduation weight, body composition, BMI and changes in these variables during the four years at university. The most significant predictors of graduation weight and change in weight were better performance on the standardized fitness test and entry weight. There was a small subset of women studied who gained weight, but not body fat. This study on weight change demonstrated that weight gain is a complex social, physical and psychological issue that can impact college-aged women. Both osteoporosis and obesity are life course diseases that may be influenced by existing behaviors in youth and those acquired in university. Therefore, studying the determinants of BMD and weight change in this population may help public health educators determine strategies that could positively influence the current obesity and osteoporosis epidemics