In a study from the University of Bristol and Imperial College London, researchers have found that a high BMI in mothers before and during pregnancy is not a major cause of offspring’s BMI. This indicates that obesity in children and teens is more likely to be a result of lifestyle factors rather than maternal characteristics or genes. The results were published today in BMC Medicine.
The research team used a method called Mendelian randomization, which measures variation in genes to determine the effect of an exposure on an outcome (in this case mother’s BMI). They looked at birthweight and BMI at age 1 and 4 years for both Children of the 90s (also known as the Avon Longitudinal Study of Parents and Children) participants based at the University of Bristol, and Born in Bradford participants from Bradford Teaching Hospitals NHS Foundation Trust. Follow up data at age 10 and 15 was only available for children from the Children of the 90s study. They found that there was a moderate causal effect between maternal BMI and the birth weight of offspring, however in most older age groups they did not find a strong causal effect.
The team believes that the findings indicate that programs aimed at preventing childhood obesity should focus more on encouraging lifestyle changes rather than focusing solely on women entering pregnancy. “We found that if women are heavier at the start of pregnancy this isn’t a strong cause (of their children) being heavier as teenagers.” says lead author Dr Tom Bond, Senior Research Associate at Imperial College London who conducted the research while at the University of Bristol. “This is important to know.”
The researchers also found that there was no strong causal effect between either fathers’ or both parents’ BMI and the offspring’s weight during childhood and adolescence. They did, however, find a small causal effect on birthweight from mother’s pre-pregnancy BMI in most age groups, which they believe indicates that babies of obese mothers are more likely to be born with a higher birthweight. Lead author Dr Tom Bond notes: “Our results indicate that children of obese women do have a larger weight but these differences only become substantial when looking at young adults where offspring weigh around 3 kg more than those from average weight mothers.” He notes that the study cannot determine why this is the case. “It could be that this is due to genetic influences” says Dr Tom Bond, “or perhaps women who are heavy tend to eat more during pregnancy.”
The team believes that there may have been limitations in the way their results were analysed which may have led to underestimating the effects of maternal pre-pregnancy BMI on offspring’s weight. Lead author Dr Radek Bukowiecki, University of Bristol notes: “Our study only looked at direct effects so neglecting indirect pathways such as parental or offspring behaviours doesn’t allow us to fully understand the mechanisms by which obesity passes from one generation to another.”
Researchers believe studies must now focus more on testing other behavioural factors and characteristics in prospective mothers and fathers during pregnancy to determine the best strategies to combat childhood obesity.