Have you ever heard of that people with ADHD are more likely to be obese than those without? This has been supported by two separately conducted meta-analyses in 2016,1,2 where the researchers combined results from previous independent studies using a statistical technique to provide more precise estimates.
In most studies to date, obesity was defined solely by body mass index (BMI) ≥30 kg/m2, based on the World Health Organization classification. In our recent study (published on Psychological Medicine),3 we attempted to revisit the association between ADHD and obesity in 2.5 million individuals identified from the Swedish national registers. Both ADHD and obesity were assessed via clinical diagnosis. We speculate that clinically diagnosed obesity may better reflect the pathological aspect of body fat deposition. Not surprisingly, we observed that people with ADHD were more likely than those without to receive a clinical diagnosis of ADHD during their adolescence and young adulthood.
Genetic alterations common to both ADHD and obesity have been proposed as one of the plausible mechanisms underlying the association. To test this hypothesis, we conducted analysis in the relatives of the 2.5 million individuals. We compared the relatives of those affected by ADHD with the relatives of those unaffected. Here come the findings: first, relatives of individuals with ADHD were at higher risk for obesity diagnosis; second, the association was stronger in full siblings than in half siblings or full cousins; third, the association did not differ much between maternal half siblings and paternal half siblings. The first and the second findings just confirm that ADHD and obesity may run together in families, while the third one actually suggests that such familial co-aggregation is primarily due to the genetic sharing between family members. Here is the reason behind: the degree of genetic sharing between maternal half siblings is similar to that between paternal half siblings, whereas the maternal half siblings in our study were likely to share more of environmental factors than the paternal half siblings. This is because that children tend to live with their mothers following parental separation during the study period in Sweden. Nonetheless, the difference in environmental sharing did not seem to make the association stronger in maternal half siblings than in paternal half siblings.
More evidence for the genetic influence on the association is from our subsequent quantitative genetic analysis. The method is commonly applied to data from identical and fraternal twins to estimate the relative genetic and environmental contributions to the covariance between two traits. We applied the method to data from full and half siblings instead and found that the covariance between ADHD and obesity could be predominantly attributed to genetic factors. Environmental factors seem to play only a limited role in the covariance.
Since ADHD predicts obesity in adolescence and young adulthood, it might be a good idea to monitor children with ADHD for weight gain. Interventions, such as organized physical activity, tailored to those on a suboptimal trajectory might help prevent obesity later in life. Hopefully, clinically actionable genetic variants can be discovered and benefit people suffering from both conditions.
Dr. Qi Chen is a research coordinator in the department of Medical Epidemiology and Biostatistics at Karolinska Institutet. Her research is supported by the CoCA project.
1 Cortese S, Moreira-Maia CR, St Fleur D, Morcillo-Penalver C, Rohde LA, Faraone SV. Association Between ADHD and Obesity: A Systematic Review and Meta-Analysis. Am J Psychiatry 2016; 173: 34-43.
2 Nigg JT, Johnstone JM, Musser ED, Long HG, Willoughby M, Shannon J. Attention-deficit/hyperactivity disorder (ADHD) and being overweight/obesity: New data and meta-analysis. Clinical psychology review 2016; 43: 67-79.
3 Chen Q, Hartman CA, Kuja-Halkola R, Faraone SV, Almqvist C, Larsson H. Attention-deficit/hyperactivity disorder and clinically diagnosed obesity in adolescence and young adulthood: a register-based study in Sweden. Psychol Med 2018 Sep 17: 1-9.