The prevalence of obesity has increased dramatically in developed countries since the 1980s, with significant consequences for public health and national economies. Despite numerous policy efforts, obesity remains a leading cause of disability and mortality.
While diet changes and reductions in physical activity are considered primary factors, individual factors also play a role. One such factor is cognitive ability, which is associated with better health outcomes. In this context, individuals with higher cognitive ability are likely to have greater access to healthier and more varied diets and to live in safer, more walkable neighborhoods.
Additionally, cognitive ability may increase an individual’s ability to understand and use nutritional and other health-relevant information. Numerous studies have examined the association between cognitive ability and BMI, with most finding that individuals with higher cognitive ability have lower BMI and obesity rates in adulthood.
However, these associations could be explained by unobserved or residual confounding. To address potential confounding factors, this study employs a sibling comparison design to investigate the relationship between cognitive abilities during adolescence and body mass index (BMI) in adulthood within families.
A recent study published in PLOS Medicine has found a significant difference between the between-family and within-family estimates of the association between adolescent cognitive ability and adult body mass index (BMI). While higher cognitive ability was associated with lower BMI in between-family analysis, the effect sizes in the within-family analysis were small, and the confidence intervals overlapped the null.
The results suggest that shared factors between siblings may bias the association between cognitive ability and BMI observed in previous studies. These factors may include childhood socioeconomic position (SEP), parenting practices, early neighborhood environments, and shared genetic factors that influence cognition and BMI.
While the within-family estimates still suggest a causal effect of cognitive ability on BMI, the small effect sizes are surprising, given the importance of health literacy and SEP in predicting obesity. The study suggests that non-volitional factors, such as appetite and dietary norms, may be of greater importance than conscious, reflective decision-making for eating and physical activity. The study also suggests that parental cognitive ability may be necessary through its influence on family SEP and parenting practices.
The study’s strengths include using multiple samples and the longitudinal design. In contrast, limitations include reliance on self-reported height and weight measurements, non-shared factors that may still bias associations, and substantial cohort attrition. The study’s results were based on data from a single country (the US) and may not apply to other study contexts.
The study’s results suggest that shared factors between siblings may bias the association between cognitive ability and BMI and that non-volitional factors may be of greater importance in predicting obesity than conscious decision-making. The study highlights the importance of considering shared factors when studying the relationship between cognitive ability and BMI.