The Impacts of Physical (In)Activity on Endothelial Function

Exercise and physical activity are known to have beneficial effects on endothelial function and vascular health. Sedentary behavior and physical inactivity, however, have been shown to be related to poor endothelial function and vascular health. The overall purpose of this dissertation was to further elucidate the physiological links between physical (in)activity and vascular health. In the first study, 12 active, young (18-40 yr) individuals and 12 active, old (55+ yr) individuals were recruited. Participants underwent 5 days of removal of exercise, and had their endothelial function assessed via flow-mediated dilation (FMD) during their normal, habitual exercise state (EX) and during 3 and 5 days of removal of exercise (NOEX). While removal of exercise reduced FMD in both groups in the brachial (%) (Old: EX: 7.20 ± 0.60; NOEX: 5.45 ± 0.91; Young: EX: 5.97 ± 1.14; NOEX: 6.90 ± 0.93; p=0.035) and popliteal (Old: EX: 6.64 ± 0.70; NOEX: 4.84 ± 0.83; Young: EX: 7.39 ± 0.73; NOEX: 6.08 ± 0.58; p=0.014) arteries, the older adults did not have greater reductions in FMD compared to younger adults(p>0.05). To further examine the effects of short-term physical inactivity on endothelial function, the second study employed a systematic review of the literature encompassing all published studies examining the effects of short-term inactivity or removal of exercise interventions on endothelial function. A search was made on two databases (PubMed and Web of Science) for studies employing interventions that reduced physical activity, with variables of interest including FMD, endothelin-1, nitric oxide, contrast-enhanced ultrasound, near-infrared spectroscopy, laser doppler flowmetry, capillary microscopy, and retinal imaging. Several forms of physical inactivity intervention (reduction in daily physical activity/steps, detraining/removal of exercise, bed rest, and immobilization) were included. In healthy individuals, all examined forms of inactivity impaired measures of endothelial function such as decreasing percent FMD or decreasing skin blood flow. While the exact time for participants to demonstrate these impairments is not clear, significant decreases may occur as early as 5 days following reduction of activity, with longer and more severe interventions more consistently demonstrating decreases in endothelial function. The final study of this dissertation determined associations between objectively measured physical activity (steps/day, minutes of moderate-vigorous intensity physical activity, or daily minutes of sedentary time) and endothelial function as assessed by FMD. Additionally, this study examined if reallocating time spent in sedentary behavior with time spent in moderate to vigorous physical activity and light intensity physical activity is associated with a more favorable FMD. Fifty-six healthy, active participants were recruited and had daily physical activity assessed via an Actigraph accelerometer for 7 days. Following 7 days of accelerometer use, participants had both brachial and popliteal artery FMD measured. Multiple regression reduced subset models explained 47% of the variance in popliteal artery %FMD using the variables brachial artery baseline diameter, popliteal artery baseline diameter, daily steps, and race/ethnicity. A significant correlation was determined between popliteal artery %FMD and average daily steps. Multiple regression reduced subset models explained 61% of the variance in brachial artery %FMD using the variables sex, VO2 peak, sedentary time, MVPA time, race/ethnicity, education status, and smoking status. A significant correlation was determined between brachial artery %FMD and sedentary time (R2 = 0.61, p< 0.01), and a trend towards a significant correlation determined between brachial artery %FMD and MVPA (R2 = 0.61, p=0.09). Additionally, isotemporal regression analysis determined that replacing 30 minutes of sedentary time per day with 30 minutes of light intensity physical activity was associated with a 0.80% improvement in brachial artery %FMD. Replacing sedentary time with 30 minutes of moderate-to-vigorous intensity physical activity per day was associated with a 1.12% improvement in brachial artery %FMD, although neither demonstrated an association with popliteal artery %FMD. In this population, these results indicate that changes in brachial artery %FMD may be more related to physical activity intensity, while popliteal artery %FMD changes may be more related to daily steps.