Summary: The associations between cognitive ability and body mass index (BMI) may largely reflect confounding by other factors related to family background. The study suggests that previous findings on the link between cognitive ability and BMI may be biased by shared family factors, rather than being causal.
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 13th 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 25th to 75th 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 25th to 75th 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.”
About this obesity and cognition research news
Author: Claire Turner Source: PLOS Contact: Claire Turner – PLOS Image: The image is in the public domain
The association between cognitive ability and body mass index: A sibling-comparison analysis in four longitudinal studies
Body mass index (BMI) and obesity rates have increased sharply since the 1980s. While multiple epidemiologic studies have found that higher adolescent cognitive ability is associated with lower adult BMI, residual and unobserved confounding due to family background may explain these associations. We used a sibling design to test this association accounting for confounding factors shared within households.
Methods and findings
We used data from four United States general youth population cohort studies: the National Longitudinal Study of Youth 1979 (NLSY-79), the NLSY-79 Children and Young Adult, the NLSY 1997 (NLSY-97), and the Wisconsin Longitudinal Study (WLS); a total of 12,250 siblings from 5,602 households followed from adolescence up to age 62. We used random effects within-between (REWB) and residualized quantile regression (RQR) models to compare between- and within-family estimates of the association between adolescent cognitive ability and adult BMI (20 to 64 years).
In REWB models, moving from the 25th to 75th percentile of adolescent cognitive ability was associated with −0.95 kg/m2 (95% CI = −1.21, −0.69) lower BMI between families. Adjusting for family socioeconomic position reduced the association to −0.61 kg/m2 (−0.90, −0.33). However, within families, the association was just −0.06 kg/m2 (−0.35, 0.23).
This pattern of results was found across multiple specifications, including analyses conducted in separate cohorts, models examining age-differences in association, and in RQR models examining the association across the distribution of BMI. Limitations include the possibility that within-family estimates are biased due to measurement error of the exposure, confounding via non-shared factors, and carryover effects.
The association between high adolescent cognitive ability and low adult BMI was substantially smaller in within-family compared with between-family analysis. The well-replicated associations between cognitive ability and subsequent BMI may largely reflect confounding by family background factors.