Regular weight-bearing exercise is recommended for healthy young adults to optimize bone mineral density (BMD) (Physical Activity Guidelines Advisory Committee, 2008). BMD alone however, is not a significant predictor of osteoporosis or fracture risk. While bone turnover markers are used clinically to assess efficacy of pharmacological intervention in individuals diagnosed with osteoporosis, they may also have a role in evaluating acute responses to exercise. The purpose of this dissertation was to compare two groups of premenopausal women, those which are sedentary (low fitness) versus those that are well-trained (high fitness), by BMD and bone turnover markers at rest and the acute response of these bone turnover markers to a single bout of 60 min of aerobic jogging (or brisk walking).
The first study included 21 trained and 14 untrained premenopausal women, mean VO2 Max 42.2±5.3mL/kg/min and 27.9±3 3mL/kg/min respectively, who performed 60 min of jogging or brisk walking at an aerobic intensity, 70-80% maximum heart rate. Fasting venous blood samples were drawn at pre-exercise, post-exercise and 30 min post-exercise. All markers of bone turnover and hormones were adjusted for plasma volume shifting. Mean serum C-telopeptide of type 1 collagen (sCTx) was higher in the trained group, but not significantly different between groups. Procollagen type 1 N-terminal propeptide (P1NP) was significantly higher in the trained group compared to the untrained group at post-exercise (76.0±5.0 vs 47.2±5.3μg/L, p<.001) and 30 min post-exercise (57.4±3.7 vs 42.6±5.0μg/L, p=.01). In both groups, P1NP significantly increased from pre-exercise to post-exercise and at 30mins post-exercise P1NP returned to pre-exercise levels. In both groups, total serum calcium (Ca2+) and mean intact parathyroid hormone (iPTH) increased from pre-exercise to post exercise and at 30mins post-exercise total Ca2+ and iPTH returned to pre-exercise levels. 25-OH Vitamin D was significantly higher in the trained group compared to the untrained group at pre-exercise, post-exercise and 30mins post-exercise. In both groups, 25-OH Vitamin D significantly increased from pre-exercise to post-exercise.
The second study included 23 trained and 19 untrained premenopausal women, mean VO2 Max 42.4±5.2mL/kg/min and 28.2±3.3mL/kg/min respectively. Age-matched fitness was determined by reference tables published by the American College of Sports Medicine. BMD was measured by DXA scan for arms, legs, pelvis, spine and total body. Fasting venous blood samples were drawn to measure sCTx, P1NP, total Ca2+, iPTH, 25-OH Vitamin D and Cortisol. BMD was analyzed by independent t-test, adjusted for differences in body fat percentage and BMI. Spearman's correlations were calculated for age-matched fitness and hormones and markers of bone turnover. There were no significant differences in BMD at any of the body regions (p>.05). Age-matched fitness significantly correlated with P1NP (.393, p=.01), iPTH (-.316, p=.04), 25-OH Vitamin D (.429, p=.01), and Ca2+ (.447, p=.003).
Trained premenopausal women have a different bone turnover marker profile and greater response to exercise than untrained premenopausal women. Despite finding no significant differences in BMD between the groups, bone turnover markers of type 1 collagen were elevated in the trained group at rest and greater increases were observed post-exercise. Furthermore, age-matched fitness was significantly correlated with P1NP, iPTH, total Ca2+ and 25-OH Vitamin D. This highlights the inherent value of weight-bearing aerobic exercise in promoting bone remodeling and osteogenic responses to the skeleton in premenopausal women, but with greater responses in individuals with higher fitness levels.