Pathways contributing to childhood weight development and overweight in Norway

Abstract

Background: Obesity is currently one of the world’s largest threats to health. Non-communicable diseases such as obesity, diabetes and hypertension - all leading to early cardiovascular death - are increasing all over the world, and the World Health Organization (WHO) and United Nations (UN) point to the importance of fighting the obesity epidemic. If this epidemic continues to increase, major problems will arise both for people’s health and for society as a whole, as the burden of treating its complications will eventually ruin state and country budgets. Obesity is a leading public health problem also facing children today1. The obesity epidemic affects all age groups, and children with a higher body mass index (BMI) are more likely to become obese later on in life2-5. The intrauterine environment is thought to affect many aspects of health, but there is only limited evidence for the involvement of uterine risk factors in childhood obesity. A growing body of epidemiological and experimental evidence indicates that the prenatal period may be critical for the development of childhood obesity4,6. Body weight is regulated by numerous physiological mechanisms, and short sleep duration can associate with a hormonal imbalance caused by hunger and appetite7,8. Therefore, identifying factors during critical periods in early life that are predictive of obesity later on in life could guide public health interventions in childhood obesity, and hopefully contribute to future research that will give us a better understanding of the complex aetiology of childhood weight gain and obesity.<br><br> Aims: The overall aim of this thesis was to use a causal model for childhood weight development / overweight to explore pathways contributing to childhood weight gain and obesity.<br> The purpose of Paper I was to estimate the association between maternal pre-pregnancy BMI, maternal weight change during pregnancy and offspring birth weight using a BMI classification developed by WHO and adopted by the Institute of Medicine (IOM) in 2009.<br> The purpose of Paper II was to estimate the association between maternal pre-pregnancy BMI or gestational weight change (GWC) during pregnancy and offspring BMI at 3 years of age, while taking several pre-and postnatal factors into account.<br> Lastly, in Paper III, we estimated the association between sleep duration in infancy and BMI of the child at 3 years of age to test the proposed hypothesis that impaired sleep during early childhood might increase the weight of the child.<br><br> Material and methods: This thesis relies on data from the Norwegian Mother and Child Cohort Study (MoBa), a prospective population-based pregnancy cohort study conducted by the Norwegian Institute of Public Health9,10. Participants were recruited from all over Norway from 1999-2008, and 38.5% of invited women consented to participate in this study. The cohort now includes 108 000 children, 90 700 mothers and 71 500 fathers. Blood samples were obtained from both parents during pregnancy and from the mother and child (umbilical cord) at birth. Follow up was conducted by questionnaires distributed at regular intervals and by linkage to national health registries. Several sub-studies also collected additional data and biological materials for analysis. Different approaches were used to describe the causal pathways leading to weight gain and obesity, and before analysing data, we drew causal diagrams to evaluate possible associations.<br><br> Main results: Offspring birth weight increased with increasing maternal pre-pregnancy BMI and with maternal weight gain during pregnancy in all six categories of maternal pre-pregnancy BMI. We also found a positive association between maternal pre-pregnancy BMI and GWC and mean offspring BMI at 3 years of age. Pre-pregnancy BMI and GWC also interacted, and the strength of this interaction associated with the increase in offspring BMI among mothers who gained the most weight during pregnancy and had the highest pre-pregnancy BMI. Furthermore, we have estimated the association between sleep duration in infancy and BMI of the child at 3 years of age. This study indicated that sleep duration in infancy does not associate with BMI at 3 years of age.<br><br> Conclusions and future perspectives: Results from the first two studies support the theory that the prenatal period associates with offspring weight and BMI. In the first study we showed offspring birth weight to increase with increasing maternal pre-pregnancy BMI and with maternal weight gain during pregnancy in all six categories of maternal pre-pregnancy BMI. In the second study we showed that both maternal pre-pregnancy BMI and GWC were positively associated with mean offspring BMI at 3 years of age. In the third study, sleep duration in infancy did not associate with BMI at 3 years of age, and thus did not support the hypothesis that less sleep during infancy increases BMI at 3 years of age. Preventing maternal overweight and obesity before pregnancy and encouraging woman to maintain a healthy weight during pregnancy should be made a public health priority.