Examining Associations between Timing of Physical Activity and Health Outcomes in Young Adults
Open Access DepositedBackground: To prevent and manage chronic disease, national guidelines call for a minimum of 150-300 min/week of moderate-intensity physical activity, 75-150 min/week of vigorous-intensity physical activity, or an equivalent combination of moderate-to-vigorous-intensity physical activity (MVPA). Yet, little is known about what time of day may be optimal for performing this activity. The timing of physical activity may be particularly important for mitigating health risks in young adults who are vulnerable to social jetlag (i.e., discrepancy between biological rhythms and sleep/wake behaviors), placing them at increased risk for chronic diseases. Objectives: This dissertation examines the cross-sectional and longitudinal relationships between timing of MVPA in young adults with overweight/obesity and health indicators (i.e., BMI [primary outcome], cardiometabolic risk, total weekly MVPA). Methodology: I addressed these objectives by using longitudinal data from the Healthy Body Healthy U (HBHU) trial, which tested a technology-based health messaging weight management intervention for young adults (N=459; 18-35 years) with overweight/obesity. Participants provided objective accelerometry data at baseline and at 6-, 12-, and 18-months. Time-of-day categories were classified by dividing clock hours into quartiles based upon the temporal distribution of MVPA minutes per hour in the sample at baseline: morning (06:00-11:59), afternoon (12:00-15:59), evening (16:00-18:59), and night (19:00-00:59). The proportion (%) of total weekly MVPA accumulated during each time category was used as the predictor of interest in multilevel regression models predicting BMI, cardiometabolic risk, and total weekly MVPA. Adjusted cross-sectional and longitudinal analyses were conducted to examine relationships at baseline, prior to study randomization, and over the 18-month follow-up period, respectively. Models controlled for demographic characteristics and total weekly volume of MVPA (BMI and cardiometabolic risk models only). Moderators (i.e., age, sex, interventions status) and mediators (i.e., total weekly MVPA, insulin) were tested. Key Findings: At baseline, the analytic sample (N=408, mAge=23.8±4.5, mBMI=31.1±4.4) was 79% female and 49% non-Hispanic White, with a mean weekly MVPA of 311±167 minutes. At baseline, MVPA timing was associated with BMI, but not cardiometabolic risk, independent of total weekly MVPA. Specifically, evening MVPA was inversely associated with BMI in 26-35-year-olds (-0.06 [-0.12, -0.01] kg/m2), but not in 18-25-year-olds (0.04 [-0.01, 0.08] kg/m2). Longitudinally, MVPA timing was associated with cardiometabolic risk, but not BMI, independent of total weekly MVPA. Morning MVPA was inversely associated with odds of cardiometabolic risk (OR [95% CI]: 0.99 [0.98, 0.99]) for all sex, age, and study groups. Evening MVPA was inversely associated with cardiometabolic risk for 26-35-year-olds only (0.98 [0.97, 0.99]). Total weekly MVPA and insulin did not mediate relationships between MVPA timing and health outcomes at baseline or over the follow-up period. Conclusion: Although identified relationships had relatively small effect sizes, our results indicate MVPA timing is associated with clinically meaningful health outcomes. For example, cross-sectional data indicate that, for the average 26-35-year-old study participant, accruing a 20-percentage point increase in weekly MVPA in the evening (or an additional 9 minutes per day) would equate to a clinically relevant 1.2 kg/m2 lower BMI (95% CI: -2.4, -0.2). Longitudinally, a 20-percentage point increase in MVPA accumulated during the morning over 18 months was associated with 12% lower odds (OR: 0.88; 95% CI: 0.80, 0.97) of cardiometabolic risk. Contributions to the Field: This dissertation is one of the first studies to examine physical activity timing in young adults, who are biologically and behaviorally different from adult counterparts. Furthermore, to our knowledge, it is the largest study of young adult activity timing, with the longest follow-up period, to-date. This project is the first to test the moderating influence of health messaging on self-selected activity timing and associated outcomes. Finally, this project is the first to test insulin as a mediating factor between physical activity timing and health outcomes. Implications: Data regarding when young adults choose to be active can inform targets for future time-based physical activity interventions among young adults at risk of developing cardiometabolic diseases. Our findings support the theory that there may be incremental health benefits associated with optimizing the timing of activity in young adulthood. Thus, this area of science may lead to the creation of next generation weight management and cardiometabolic disease prevention strategies tailored to the unique biological and behavioral needs of young adults, setting young people on a path towards future health through the lifecycle. Keywords: Physical activity, timing, time-of-day, 24-hour day, circadian, body mass index (BMI), cardiometabolic risk, moderate-to-vigorous physical activity (MVPA), young adult
- All rights reserved
Notice to Authors
If you are the author of this work and you have any questions about the information on this page, please use the Contact form to get in touch with us.