Does beingoverweight/obese directly contribute to risk of ADHD in offspring?

An international team of researchers combined a meta-analysis with a nationwide cohort study to evaluate the added risk of ADHD in children of overweight and obese mothers.

The meta-analysis encompassed over 780,000 mother-child pairs, or over one and a half million individuals, in the United States, Spain, Sweden, and Denmark. It found that overweight mothers were 30 percent more likely, and obese mothers were 90 percent more likely, to have a child with ADHD than normal-weight mothers (underweight mothers were excluded).

There was negligible heterogeneity among studies and no sign of publication bias. The studies were rated to be of overall high quality. Excluding Sweden from the results (because of the overlap with the national cohort study) had no significant effect on the outcome.

The researchers then explored the effect of potential confounding variables by conducting a cohort study of over 1.2 million births using the Swedish national registers. Excluding severe birth defects, deaths, emigration, and individuals with missing information reduced the study population to just over 970,000. As in the meta-analysis, overweight mothers were 30 percent more likely, and obese mothers were 90percent more likely, to have a child with ADHD than normal-weight mothers.

Next, the team adjusted for previously identified confounding variables, including offspring sex, birth order, and year of birth; mother’s country of birth; maternal education; maternal age at delivery; smoking during pregnancy; and cohabitation with child's father at childbirth. This reduced the adjusted risk of ADHD offspring for overweight mothers to 20 percent and obese mothers to 60 percent.

But that still left out the effect of unmeasured confounders, including parental ADHD. To address this, the researchers used the Swedish registers to identify over 155,000 maternal first cousins and 460,000 full biological siblings within the overall study population.

Looking only at maternal first cousins and adjusting for the same measured confounders further reduced the adjusted extra risk of ADHD offspring for overweight mothers to 10 percent and obese mothers to 44 percent. Doing the same with biological siblings removed the extra risk for offspring of overweight mothers altogether, and reduced it to a non-significant 10 percent for obese mothers. For the most obese (class III) mothers, the risk descended from an extra 180 percent to an extra 70percent, but the result was again non-significant, although barely so, and may have been due to the small sample size in this category.

Among the limitations to keep in mind, while sample sizes were very large, they were confined to prosperous countries in Europe and North America. Results elsewhere in the world remain unmeasured.

The authors concluded, “there is an association between maternal-pregnancy overweight/obesity and ADHD in offspring, but this association is largely ascribable to unmeasured familial confounding and not a strong causal relationship. Our findings highlight the importance of accounting for unmeasured familial confounders in risk-factor studies of ADHD in offspring.”