The role of physical activity, obesity and perception of weight status in the prevention of elevated blood pressure in youth: biological and behavioral investigations

Background: Effective interventions to prevent obesity in youth are needed in order to alleviate the public health burden associated with elevated blood pressure (BP). However, interventions designed to prevent obesity through increased physical activity levels have had limited success. Objectives: This thesis has four objectives: 1) to assess the impact of changes in adiposity indicators on increases in systolic blood pressure (SBP) during adolescence; 2) to examine the longitudinal association between declines in physical activity and SBP in youth; 3) to assess whether weight status misperception is associated with exposure to overweight/obese people at home or in school; 4) to examine if weight status misperception is associated with the level of participation in physical activity. Methods: SBP and anthropometry were assessed biannually (1999/2000, 2002, 2004) in a cohort of 1293 Montréal adolescents aged 12-13 years in 1999. Self-report 7-day recalls of physical activity sessions ≥5 minutes were collected every three months over five years. The repeated SBP measurements were analyzed using individual growth models to assess the effect of changes in adiposity indicators on SBP change. Estimates of initial level and rate of decline in number of physical activity sessions per week from individual growth models were used as predictors of BP in linear regression models to assess the effect of declines in physical activity on BP. The hypotheses that youth exposed to obesity at home and in school are more likely to underestimate their weight status, and that those who underestimate their weight status are less likely to engage in physical activity were tested in a provincially representative, school-based sample of 3665 youth (age 9, n = 1267; age 13, n = 1186; age 16, n = 1212). Multi-level linear regression models were used to assess the association between parent and schoolmate BMI, and weight status misperception (i.e., the standardized difference between self-perception of weight status and actual weight status). Poisson regression models were used to assess the association between weight status misperception and physical activity. Results: 1) Despite sex differences in mean SBP changes, the magnitude of the effect of one unit change in each adiposity indicator on SBP change was similar in boys and girls. 2) Physical activity declined during adolescence and a decline of one physical activity session per week with each year of age was associated with higher BP. 3) Majority of overweight youth underestimated their weight status. Higher parent and schoolmate BMI were associated with greater weight status underestimation. 4) Youth who underestimated their weight status were less likely to engage in physical activity. Conclusion: These findings support the need for interventions to prevent excess weight and physical inactivity during adolescence to prevent higher BP in youth. Weight status misperception may reduce motivation among youth to engage in physical activity, and therefore hinder the success of obesity prevention interventions.

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