Pubertal timing – antecedent to risk or resilience?: Epidemiological studies on growth, maturation and health risk behaviours; The Young HUNT study, Nord-Trøndelag, Norway

Background: The timing of pubertal milestones influences the tempo and progression of growth and maturation, which in turn have consequences for boys’ and girls’ body size, shape and composition. Early sexual maturation has been associated with overweight and a shorter adult stature. The biological mechanisms underlying these associations are not clear, but since adipose children tend to mature earlier than leaner children, subsequent changes in height and weight could be attributed to nutrition in childhood rather than to early sexual maturation. The rapid bodily changes and the timing of these changes may also have an impact on boys’ and girls’ self-concepts, their social functioning, roles and behaviours. Early maturation is considered a risk factor for early initiation and advancement in use of alcohol and tobacco in adolescence, but is not clear if these differences persist over time. It has also been unclear whether adolescents’ perceptions of their own pubertal timing play an important role for how they actually act. Most studies concerned with pubertal timing as antecedent to risk or resilience, have been conducted on girls. The main aims of this thesis are: • To estimate the prevalence of advanced (risky) alcohol drinking and cigarette smoking in late adolescence related to the timing of sexual maturation and gender (Paper I and II). • To investigate whether the timing of sexual maturation in both genders, independently or in concert with early central adiposity, affects final height (Paper IV) or the prevalence of overweight (Paper III) as assessed in late adolescence. Material and method: To address these questions we used data from the Young HUNT 1 study (1995-1997) (Paper I,II,III,IV), and the Young HUNT 2 study carried out about four years later (Paper I, III, IV), in addition to data from the Medical Birth Registry of Norway and the adult part of the HUNT 2 study (Paper IV). A total of 9097 students (92%) from all middle and high schools in Nord-Trøndelag county participated in the Young HUNT 1 study and 8950 students completed the questionnaire. Among those, 2399 boys and girls were followed from early (baseline) to late adolescence (follow-up). Both the baseline and follow-up study included questionnaire data and physical examinations of height, weight, and waist and hip circumference. Age at menarche (AAM), the Pubertal Developmental Scale (PDS) and perceived pubertal timing were used to classify the timing of sexual maturation (SM timing). Baseline waist circumference was used to classify central adiposity (Paper III and IV), and body mass index (BMI) was used to classify overweight. Descriptive statistics as well as multivariate logistic regression modelling (Paper I, II and III) and general linear modelling (Paper III and IV) were applied to study the associations between exposures and outcomes. Results: Girls, who had matured early (OR 1.7, CI 1.2-2.4) or late (OR, 1.5, CI 1.1-2.2) were both more likely to report advanced drinking in late adolescence, compared to on time matured counterparts. Late matured boys were less likely (OR 0.5, CI 0.3-0.8) to engage in advanced drinking. Daily smoking was also more common among girls who had matured early (OR 1.5, CI 1.1-2.2), than among girls who were on time (Paper I). Boys and girls who perceived themselves as early matured were more likely to report risky drinking than those who perceived themselves as being on time, both in middle school and high school. A nearly identical pattern was found for smoking (Paper II). Early sexual maturation in girls, but not in boys, was associated with overweight. This association, however, was restricted to girls with high waist circumference (>median) at baseline (OR, 2.7, CI 1.5-4.9). Early matured boys and girls with high waist were on average 5.7 cm and 3.6 cm taller than early maturing boys and girls with low waist circumference. Height was lowest in early matured boys (176.6 cm) and girls (163.8 cm) with a lean body composition at baseline. Height gain during follow-up was closely related to SM timing, but independent of central adiposity. Differences in height and weight related to central adiposity were more pronounced among those who matured early, compared to those who were intermediate or late matured (Paper III and IV). Conclusions: • Early (or rapid) maturation in girls constitutes a risk factor for advanced alcohol drinking and cigarette smoking not only in early adolescence, but also in late adolescence. • Perceptions of being earlier matured than same aged peers may increase the risk of alcohol drinking and cigarette smoking, especially in combination with high pubertal status. • The combination of early age at menarche and central adiposity increases girls’ likelihood of being overweight in late adolescence. • The combination of early sexual maturation and leanness yields short adult stature