Body size and physical activity : epidemiological studies on children and young adults

Sammanfattning: The overall aim of this thesis was to increase the knowledge about different aspects of body size and physical activity among Swedish children and young adults. Specific aims were: (i) to investigate how BMI-distributions (BMI=Body Mass Index) have changed among Swedish children over a 12 year period, (ii) to study effects of pre- and postnatal growth on body size and body composition in adolescence, (iii) to study associations between parents and children s physical activity, and (iv) to investigate genetic contributions to physical activity behavior. The study of changes in BMI-distributions was based on a comparison of two population-based cohorts; one of 3,650 children born in 1973-75 and a one of 2,591 children born in 1985-87 - the COMPASS study. The comparison showed marked differences in the BMI-distributions from 5-6 years of age with higher BMI-values in the younger cohort. No differences were observed for the children at 2-5 years of age. Differences started to appear at the 25th percentile and became increasingly pronounced in the upper parts of the distributions, i.e. the heavy children became heavier over the 12 year period. The COMPASS study was also used for the study of associations of pre- and postnatal growth with body size and body composition at age 15. Prenatal growth, measured with birth weight z-score, was positively associated with BMI, fat free mass, fat mass and waist circumference. For one unit increase in birth weight z-score, fat free mass increased 1.66 kg among boys and 1.05 kg among girls. Further, there was a positive relation between prenatal growth and fat mass as percentage of total body mass among boys, but not among girls. Postnatal growth, measured by change in weight z-score during first year of life, showed strong, positive associations with all measures of body composition in adolescence. For one unit increase in weight z-score during first year of life, fat free mass increased 2.30 kg among boys and 1.64 kg among girls. Postnatal growth had stronger effect than prenatal growth on all measures of body composition. The study of associations between parents and children s physical activity was based on the PITCH study. The study population comprised 1,124 12-year-old children and their parents. Parents physical activity was strongly associated with their children s participation in sport and vigorous activities. With two parents active in sport the odds ratio for their children to participate in sport was 3.9 (95% CI, 2.2; 6.9) (girls) and 8.8 (4.3; 18.0) (boys) compared with having two inactive parents. The Swedish Young Male Twins Study was used to investigate the contribution of genetic factors to physical activity behavior. The study population comprised 2,044 male twins born 1973-1979. There were moderate to high genetic contributions to physical activity. Heritability estimates were in the range of 0.40 to 0.65, depending on physical activity dimension. Only additive genetic and non-shared environmental factors seemed to be of importance for the variation in physical activity in this young adult male population.