Study casts doubt on causal link between cognitive ability and obesity

The well-replicated associations between cognitive ability and body mass index (BMI) may largely reflect confounding by other factors related to family background, according to a new study published April 13^th in the open access journal PLOS Medicine by Liam Wright of University College London, UK, and colleagues.

Obesity is a major contributor to the global disease burden and its prevalence is expected to continue rising. Existing studies have found links between cognitive ability and obesity, with a lower cognitive ability in childhood or adolescence associated with a higher BMI or higher rate of obesity in later adulthood.

In the new study, researchers used data on 12,250 siblings from 5,602 households followed from adolescence to age 62 as part of four separate United States youth population cohort studies. By comparing the association between cognitive ability and BMI within families, the team could account for unobserved factors related to family background.

When comparing unrelated individuals in the dataset, the researchers found that moving from the 25^th to 75^th percentile of adolescent cognitive ability was associated with an estimated 0.61 kg/m² decrease in BMI (95% CI -0.90 to -0.33) when adjusted for family socioeconomic position. When comparing siblings, however, moving from the 25^th to 75^th percentile of adolescent cognitive ability was associated with only a 0.06 kg/m² decrease in BMI (95% CI -0.35 to 0.23).

“The results suggest that existing findings on the link between cognitive ability and BMI are biased by shared family factors,” the authors say. “Given that associations between cognitive ability and other health outcomes have been found using similar observational research designs, sibling data may be useful for assessing potential bias for these health outcomes too.”

Wright adds, “Does higher cognitive ability (intelligence) help one to avoid gaining too much weight? Lots of studies have found an association between the two, but our study suggests that these links may not be causal in nature.”

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In your coverage, please use this URL to provide access to the freely available paper in PLOS Medicine:

http://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1004207

Citation: Wright L, Davies NM, Bann D (2023) The association between cognitive ability and body mass index: A sibling-comparison analysis in four longitudinal studies. PLoS Med 20(4): e1004207. https://doi.org/10.1371/journal.pmed.1004207

Author Countries: United Kingdom, Norway

Funding: DB is supported by the Economic and Social Research Council (http://www.esrc.ac.uk; ES/M001660/1); DB and LW by the Medical Research Council (http://www.mrc.ac.uk; MR/V002147/1). NMD works in a unit that receives support from the University of Bristol and the UK Medical Research Council (MC_UU_00011/1) and is supported by a Norwegian Research Council Grant number 295989. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.