Parental genes rather than pregnancy weight influence childhood body mass index

The link between parents' body mass index (BMI) and their children's BMI in childhood is driven largely by genetic inheritance, rather than by any direct biological effects of parental weight during pregnancy, a new study suggests. The collaborative international study by the universities of Bristol and Queensland and the Norwegian Institute of Public Health is published in PLOS Medicine today [23 June].

Higher parental BMI is consistently associated with higher childhood BMI. It has been difficult for researchers to disentangle how much of this association is due to genetics and how much is due to biological effects of maternal weight during pregnancy. This may have implications for interventions that aim to control childhood BMI by targeting pre-conception parental weight.

In the new study, researchers analysed data from the Norwegian Mother, Father and Child Cohort Study, a prospective birth cohort of children born between 1999 and 2009. Data on 86,000 children, including their birth weight and BMI from six months to eight years of age, as well as appetite-related eating behaviours at age eight, was available in the dataset. The researchers looked at twin, sibling, and half-sibling relationships across multiple generations to directly quantify how much of the parent-child BMI association could be attributed to genetic confounding.

Maternal BMI was more strongly associated with offspring birth weight than paternal BMI, consistent with an effect of maternal body weight on birthweight through the environment inside the uterus. However, after birth the associations of maternal and paternal BMI with offspring BMI were broadly similar from age two to eight. Models showed that genetic effects explained an estimated 79% of the statistical association between a mother's BMI and her child's BMI at age 8, and 94% of the association for fathers. Higher parental BMI was also associated with obesity-related eating behaviours in children, including greater food responsiveness and emotional overeating, although the study was able to conclusively determine how much of this was genetically driven.

Dr Tom Bond, Senior Research Associate in the Bristol Medical School: Population Health Sciences (PHS), who carried out the research while at the University of Queensland, said: "Obesity runs in families, but it is difficult to work out why this is. Our results suggest that the link between a mother's or father's body mass index (BMI) and their children's BMI up to age 8 is mostly due to inherited genes. Expectant parents should be encouraged to maintain a healthy weight, but this may not be enough to ensure that their children also have a healthy weight."

We were interested in examining whether obesity in mothers during pregnancy might also have adverse effects on the risk of obesity in their offspring when the children get older. We found that whilst maternal body mass index during pregnancy was likely to adversely affect offspring birthweight, it didn't appear to have large effects on risk of offspring obesity in later life beyond that explained through the transmission of genes from mothers to their offspring."

David Evans, NHMRC Leadership Fellow and Professor of Statistical Genetics, University of Queensland

Alexandra Havdahl, Scientific Director of the MoBa cohort and Centre Director of the PsychGen Center for Genetic Epidemiology and Mental Health at the Norwegian Institute of Public Health, explained: "Our findings suggest that the link between parents' and children's body mass index is driven largely by shared genes rather than by the intrauterine environment or parenting behavior."

The research team cautions that these findings do not support the idea that childhood obesity is inevitable for children of heavier parents. Children who inherit a genetic predisposition to higher BMI may still express those genes differently depending on their environment. The results also do not argue against the importance of maternal health in pregnancy, the authors say. Maternal obesity is well established to increase risk of adverse perinatal outcomes for both mother and child.

"Our results may have important public health implications, when considered alongside prior evidence," they said. "Maternal BMI may be unlikely to have a large causal effect on child BMI beyond birth… and any causal effect of paternal BMI on offspring childhood BMI is likely to be similar to or smaller than that of maternal BMI. Consequently, reductions in the BMI of either parent before pregnancy may be unlikely to cause large reductions in childhood adiposity."