Date of Degree
PhD (Doctor of Philosophy)
Kathleen F. Janz
The goal of this dissertation research was to better understand relationships among physical activity (PA), cardiorespiratory fitness, adiposity, and cardiovascular (CV) health in children and adolescents. The aim of the first paper was to examine whether fitness and adiposity are independently associated with CV risk factors during puberty. Study participants were 126 prepubertal Caucasian children participating in a longitudinal four-year follow-up study. Fitness level was determined by VO2 max (L/min) obtained from maximal graded exercise testing and adiposity level was determined by the sum of skinfolds. Gender-specific individual growth curve models, including both VO2 max and the sum of skinfolds simultaneously, were fit to predict CV risk factor variables. Models also included covariates such as age, height, weight, and pubertal stage by the Tanner criteria. In both boys and girls, total cholesterol, triglyceride, LDL-C, and systolic blood pressure percentile were positively associated with the sum of skinfolds (P < 0.05), but not with VO2 max (P > 0.05). In conclusion, fitness was not associated with CV risk factors, after adjusting for adiposity, among healthy adolescents. This study suggests that adiposity may play a role in the mechanism underlying the effect of fitness on CV health during puberty. The aim of the second paper was to examine whether early adiposity level is inversely associated with subsequent PA behaviors in childhood. Study participants were 326 children participating in the Iowa Bone Development Study. PA and fat mass were measured using accelerometers and dual energy X-ray absorptiometry (DXA) at approximately 5, 8, and 11 years of age. Gender-specific generalized linear models were fit to examine the association between percent body fat (BF%) at age 8 and intensity-weighted moderate- to vigorous-intensity PA (IW-MVPA) at age 11. After adjusting for IW-MVPA at age 8, an interval between the age 5 and 8 examinations, residualized change scores of BF% and IW-MVPA from age 5 to 8 and mother's education level, BF% at age 8 was inversely associated with IW-MVPA at age 11 among boys (P < 0.05). After adjusting for IW-MVPA at age 8, physical maturity, and family income, BF% at age 8 was inversely associated with IW-MVPA at age 11 among girls (P < 0.05). Categorical analysis also showed that the odd of being in the lowest quartile relative to the highest quartile of IW-MVPA at three-year follow-up for boys and girls with high BF% was approximately four times higher than the odd for those with low BF% (P < 0.05). This study suggests that adiposity levels may be a determinant of PA behavior. Specific intervention strategies for overweight children may be needed to promote PA. The aim of the third paper was to examine whether accelerometer-measured daily light-intensity PA is inversely associated with DXA-derived body fat mass during childhood. The study sample was 577 children participating in the longitudinal Iowa Bone Development Study. Fat mass and PA were measured at about 5, 8, and 11 years of age. Two PA indicators were used, applying two accelerometer count cut-points: the daily sum of accelerometer counts during light-intensity PA (IW-LPA) and the daily sums of accelerometer counts during high-light-intensity PA (IW-HLPA). Measurement time point- and gender-specific multivariable linear regression models were fit to predict fat mass based on IW-LPA and IW-HLPA, including covariates, such as age, birth weight, fat-free mass, height, IW-MVPA and maturity (only for girls). Among boys, both IW-LPA and IW-HLPA were inversely associated with fat mass at age 11 (P < 0.05), but not at ages 5 and 8. Among girls, both LPA variables were inversely associated with fat mass at ages 8 and 11 (P < 0.10 for LPA at age 11, P < 0.05 for others), but not at age 5. In conclusion, this study suggests that light-intensity PA may have a preventive effect against adiposity among older children.
Adolescents, Cardiorespiratory fitness, Cardiovascular health, Children, Obesity, Physical activity
Copyright 2010 Soyang Kwon