Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Exercise Science

First Advisor

Russell R Pate

Abstract

The health implications of spending time in sedentary behavior in childhood have not been extensively studied. There is evidence that sedentary behavior increases from early to late childhood, and this increase may be associated with negative health outcomes. Few studies have measured changes in health outcomes during childhood in relation to time spent in sedentary behavior. The overall purpose of this dissertation was to determine if sedentary behavior is independently associated with changes in body mass index (BMI), cardiovascular disease (CVD) risk factors and cardiorespiratory fitness (CRF) during childhood. Three longitudinal studies were used to address the purpose of the dissertation.

In study one, longitudinal quantile regression was used to model the influence of predictors on changes at the 10th, 25th, 50th, 75th and 90th BMI percentiles over time. Objectively measured sedentary behavior was the main predictor and adjustment was made for moderate-to-vigorous physical activity (MVPA), gender, race, maternal education, hours of sleep and healthy eating scores. Spending more time in sedentary behavior (hr/d) was associated with greater increases in BMI at the 90th, 75th and 50th BMI percentiles, independent of MVPA and the other covariates (90th percentile=0.59, 95% CI: 0.19-0.98; 75th percentile=0.48, 95% CI: 0.25-0.72; and 50th percentile=0.19, 95% CI: 0.05-0.33). No associations were observed between sedentary behavior and changes at the 25th and 10th BMI percentiles. It was concluded that sedentary behavior was associated with greater changes in BMI at the 90th, 75th and 50th BMI percentiles between ages 9 and 15, independent of MVPA.

In study 2, longitudinal quantile regression was used to model the influence of predictors on changes at the 10th, 25th, 50th, 75th and 90th CVD risk factor percentiles. Television viewing hours per week was the main predictor, and adjustment was made for maturation, household income, sleep quality, total caloric intake and percent caloric intake from carbohydrates and physical activity levels. In black girls, television viewing (hrs/wk) was positively associated with changes in sum of skinfolds (mm) at the 75th and 90th percentiles (0.22, 99%CI 0.06-0.38 and 0.21, 99% CI 0.05-0.36, respectively), but not at the 10th, 25th or 50th percentiles. In white girls, television viewing (hrs/wk) was positively associated with changes in sum of skinfolds (mm) with the strength of the associations progressively stronger toward the upper tail of the skinfold distribution (10th percentile=0.08, 99% CI 0.03-0.13 and 90th percentile=0.42, 99% CI, 0.24-0.59). No associations were observed between television viewing and changes in systolic blood pressure, triglycerides, or cholesterol levels in black or white girls. It was concluded that spending more time watching television led to greater increases in sum of skinfolds from age 9 to 19 in girls, especially at the upper tail of the skinfold distribution, independent of physical activity levels.

In study three, longitudinal quantile regression was used to model the influence of predictors on changes at the 10th, 25th, 50th, 75th and 90th shuttle run lap percentiles. Screen time was the main predictor, and adjustment was made for BMI, socioeconomic status and self-reported vigorous physical activity (VPA). In boys, more screen time was negatively associated with change in shuttle run laps completed at the 25th, 50th and 75th shuttle run lap percentiles; the strongest association was at the 75th shuttle run percentile (-0.57, 95% CI: -0.93 to -0.21). In girls, more screen time was negatively associated with changes in shuttle run laps completed at the 50th, 75th and 90th shuttle run lap percentiles; the strongest association was at the 90th shuttle run percentile (-0.65, 95% CI: -1.01 to -0.30). Borderline negative associations were found between screen time and changes in shuttle run laps at the 10th shuttle run percentile in boys and girls (-0.28, 95% CI: -0.57 to 0.01 and -0.17, 95% CI: -0.41 to 0.06, respectively). It was concluded that more screen time was associated with fewer shuttle run laps completed between age 11 and 13, independent of VPA. The association was weakest at the lower tail of the shuttle run distribution.

Overall, sedentary behavior was positively associated with changes in BMI and sum of skinfolds during childhood, especially at the upper tails of the BMI and sum of skinfolds distributions. Therefore, reducing sedentary behavior at the population level could reduce the number of children classified as obese. There was evidence that reducing time spent in sedentary behavior could increase CRF levels, but this observation was weakest at the lower tail of the CRF distribution. There was no evidence that reducing time spent in sedentary behavior would lower systolic blood pressure, triglycerides, or cholesterol levels.

Rights

© 2011, Jonathan Mitchell

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