Obesity in young people: a meta-analysis finds cognitive differences without establishing causality

Seven cognitive domains examined

The researchers pooled studies comparing young people under 18 with obesity to normal-weight young people. Their analysis covered inhibition, cognitive flexibility, working memory, verbal fluency, planning, attention and intelligence quotient.

The results were combined using a random-effects model. The abstract specifies neither the total number of participants nor the duration of the studies. It reports high heterogeneity and varying criteria for defining obesity, two important limitations when interpreting the pooled results.

Average differences, not an individual profile

Significantly lower performance is reported in six domains. The standardized differences are largest for working memory, with a Hedges’ g of −0.652, attention, at −0.526, and cognitive flexibility, at −0.499. Differences are also observed in verbal fluency, intelligence quotient and inhibition. No estimate could be made for planning.

These values describe average differences between groups. They represent neither percentages of abilities lost nor predictions of a child’s performance. The abstract does not provide uncertainty intervals for these estimates, limiting assessment of their precision.

Age and sex offer no individual-level conclusions

In separate analyses, school-age children show differences in inhibition, working memory and attention. Among adolescents, the differences concern working memory and attention. However, direct comparisons between these two age groups are not significant: these results therefore do not demonstrate a difference linked to developmental stage.

A higher proportion of girls in the samples is associated with a larger difference in working memory. This is a study-level characteristic, not an individual-level finding that would justify claiming that girls with obesity are more affected.

What this review cannot determine

The authors emphasize that the relationships could be bidirectional. The included studies cannot establish the direction of the relationship between obesity and cognitive performance. Nor do they demonstrate that weight loss would improve test results.

Data on young people who also have attention-deficit/hyperactivity disorder are limited to two studies, examined qualitatively. This review therefore cannot support a firm conclusion for this subgroup.

For you, the key takeaway is the distinction between statistical association and individual assessment: a young person’s weight alone is not enough to infer their cognitive abilities. This study describes an association that needs to be better understood, not a diagnosis or an intervention to apply.

What we can take away

This meta-analysis finds an association between obesity and lower average cognitive performance before age 18. It proves neither that obesity causes these differences nor that weight loss would correct them.