Population-based studies of body mass index, overweight and systolic blood pressure among Swedish young men

Abstract: Overweight and obesity has increased in Sweden and worldwide in recent decades. Lifestyle and environmental changes, genetic susceptibility and interactions between these factors are believed to be important. A theoretical framework often referred to, as the fetal origins hypothesis has been extensively debated over the last 15 years. According to this controversial hypothesis, overweight and other components of the metabolic syndrome - such as high blood pressure - might be long-term consequences of impaired fetal growth. The overall objectives of the studies were to investigate the epidemiology of body mass index and overweight among young men in Sweden and to explore relationships between growth in fetal life on the one hand and overweight and systolic blood pressure in young adulthood on the other. Specific aims were to: 1) study time trends and socio- economic differences in body mass index and overweight among young men over the last decades; 2) study the occurrence of overweight among male international adoptees; and, 3) test the fetal origins hypothesis by analyzing relationships between aspects of fetal growth and overweight, and also between fetal growth and systolic blood pressure among singletons and twins in young adulthood. This thesis is based on nationwide data sets created by record linkages between the Medical Birth Register, the Military Service Conscription Register and several other national registers. The twin studies include additional information collected by a mailed questionnaire. The prevalence of overweight among 18-year-old men increased two-fold over a 25-year period. The results show large socio-economic differentials in overweight, with higher prevalence among young men from loweducated families. Socio- economic differentials did not change over time. The prevalence of overweight was 2-3 times higher among young men adopted from Latin America in early childhood than among non-adopted individuals of the same ages. By contrast, the risk of overweight was not higher among adoptees from the Indian subcontinent or the Far East than among non-adopted individuals. The results show a positive relationship between gestational-age-adjusted birthweight and overweight at 18 years of age among singletons. Clearly, this finding does not support the fetal origins hypothesis. However, the hypothesis was further tested in a population-based twin study. A weak positive association was uncovered for the within-pair effects of birthweight on body mass index at the age of 18 among monozygotic twin pairs. The twin brother with the highest birthweight had a slightly higher risk of overweight at age 18 than his co-twin. No between-pairs effect of birthweight on body mass index was observed for monozygotic pairs of twins. And no relationships were observed among dizygotic twin pairs. Neither of these findings supports the fetal origins hypothesis. When studying fetal growth and systolic blood pressure among singletons, a clear inverse association was found between gestational-age-adjusted birthweight and systolic blood pressure at 18 years of age. That there was an independent inverse association between gestational age per se and systolic blood pressure is a novel finding. For monozygotic twins the results show an inverse between-pairs effect of birthweight on systolic blood pressure at age 18. There was an inverse within-pair effect of birthweight on systolic blood pressure among monozygotic twins, although the effect was not significant. No relationships were found among dizygotic twins. Although the results of the singleton study are in accordance with the fetal origins hypothesis, the twin study does not provide substantial support. In sum, the results show a strong increase in overweight over the last decades among young men, and also demonstrate large socio-economic differentials in overweight. Large and unexplained differences in over-weight among international adoptees compared with non-adopted young men were found. The studies addressing the fetal origins hypothesis showed mixed results with clear support for an inverse association between birthweight and blood pressure in the singleton study, weak support in the twin study, and no clear-cut support in either of the two other studies.