부산대학교 도서관

Adolescence is a period of life involving many cognitive, physical and social changes (DiClemente, William, & Lynn, 1996). Acquisition of autonomy and independence from family are important milestones during this period, which represent an opportunity for setting up healthy rather than risky behaviours (DiClemente et al., 1996). Given that obesity has been commonly linked with lifestyle behaviours and that childhood obesity is likely to follow through to adulthood (Singh, Mulder, Twisk, van Mechelen, & Chinapaw, 2008), understanding its determinants is imperative. In Mexico, 36.3% of adolescents have been classified as obese or overweight (INSP, 2016). Behaviours including diet, physical activity (PA), sleep and sedentary time are poor among Mexican adolescents and it seems that they do not have access to health-promoting environments (Galaviz et al., 2018; INSP, 2016). Using a mixed methods approach, the purpose of this thesis was to: (a) explore some of the determinants of obesity among Mexican adolescents and use these in a needs assessment to (b) design a theoretical and evidence-informed PA intervention prototype ready for testing with Mexican adolescents. Chapter 2 is dedicated to examining the independent relationships between modifiable lifestyle behaviours (diet, sleep duration, physical activity, and screen time) and obesity/overweight among 320 adolescents aged 13 to 15 year old in Mexico City. Participants were far from achieving the recommended guidelines for health and 41.13% were classified as obese or overweight. The lifestyle behaviours studied were not associated with BMI z scores. In line with Self-Determination Theory (SDT), Chapter 3 examined the relationship between psychological need support from physical education teachers, peers and parents and psychological need satisfaction for PA, which in turn, was expected to positively predict autonomous PA motivation, and negatively predict controlled PA motivation. Autonomous PA motivation was expected to positively predict moderate to vigorous physical activity (MVPA) and negatively predict sedentary time, whereas controlled PA motivation was expected to negatively predict MVPA and positively predict sedentary time. Data supported that need support from parents, friends and the physical education teachers positively predicted adolescents' autonomous motivation towards PA, but autonomous motivation did not predict MVPA nor sedentary time. In Chapter 4, 79 adolescents participated in 12 focus groups with the aim to gain understanding of the concepts of PA and healthy eating, as well as perceived barriers to and facilitators of PA and healthy eating. Adolescents' views were grouped into three common themes: a) understanding the health-behaviour link, b) the impact of a restricted life, and c) social factors. Adolescents' perceived exposure to unhealthy food and personal preferences were shown to play a key role in their diet, whereas perceptions of being locked up both in the classroom and at home, lack of time and lack of social support were identified as barriers to PA. Data from Chapters 2-4 was used to inform the development of a PA school-based intervention presented in Chapter 5. Intervention Mapping was used to systematically define the requirements for the intervention, which was then adapted to the context of Mexico. A needs assessment showed PA enjoyment, peer involvement, teachers' support for PA, PA opportunities and school policies as important and changeable PA determinants in the school context among Mexican adolescents. A list of essential (n=5) and desirable (n=6) criteria were set out. From 17 candidate interventions one was nominated for adaptation, which was modified with the addition of two behaviour change techniques and six environmental changes. Chapter 6 summarises the main knowledge contribution of this thesis, as well as its limitations and further directions. Overall, the present work addresses gaps in the literature among a population that has received little or no attention, and which presents high levels of obesity and poor lifestyle health behaviours. In addition, a methodological contribution is provided through the systematic application of Intervention Mapping to design a PA intervention with the potential to help increase PA among Mexican adolescents.