| This dissertation consists of three studies that were undertaken to better understand 1) the age-related longitudinal trends of physical activity (PA), cardiorespiratory fitness (CRF), and body mass index (BMI) in a group of healthy women, 2) the associations between changes of adiposity and CRF in a group of adult men, and 3) the effects of CRF on aging-glucose trajectory in a group of healthy men. Three manuscripts were written by analyzing the Aerobics Center Longitudinal Study dataset. In study one, Generalized Additive Mixed Models showed the nature of the associations between age and PA, CRF, and BMI differed among 1,467 women. There was a positive curvilinear association between BMI and age. However, the trend of self-reported PA and CRF with aging initially increased and ultimately decreased at 50 and 45 years old, respectively. In study two, Latent Growth Models showed that the trajectories of change in BMI and percent body fat (%BF) were quadratic and depended on the trajectory of change in CRF among 5,143 men. Low fit men at baseline first experienced a decline in BMI, followed with faster rates of increases in BMI over time. Baseline BMI or %BF did not influence the trajectory of changes in CRF over time. In study three, Linear Mixed Models regression analysis showed that fasting glucose increased at a linear rate with aging among 10,092 men. Glucose increased at a yearly rate of 0.17 mg/dL (95% confidence interval (CI):0.16, 0.19). CRF had little influence on the aging glucose trajectory below age 35, but significantly influenced the trend after age 35 (P for interaction <0.001). The aging-related glucose increases in low fit men (0.25 mg/dL per year) was higher than average fit (0.15 mg/dL per year) and high fit (0.13 mg/dL per year) men. In conclusion, adequate CRF levels might help lower the risk of unhealthful weight gain and delay the age-related glucose impairment. More focus should be placed on attaining and maintaining habitual physical activity in order to promote CRF. |
