What are the links between ADHD and physical ailments in adults? And, where such links exist, how can we tease out where they are due to genetics, shared environment, or unshared environmental influences?
An international research team used the Swedish population and health registers to explore these links in an entire national population. They were able to do this because Sweden has a single-payer national health insurance system, cross-referenced with the population and other national registries through personal identification numbers.
This study identified full-sibling and maternal half-sibling pairs born from 1932 through 1995, through the Population and Multi-Generation Registers. This yielded a total of 4,789,799 individuals - consisting of 3,819,207 full-sibling pairs and 469,244 maternal half-sibling pairs, and 1,841,303family clusters (siblings, parents, cousins, spouses). Roughly half were men, the other half women.
After adjusting for sex and birth year, those with ADHD were at significantly higher risk of a wide range of physical ailments, when compared with individuals without ADHD:
· Over four times as likely to have sleep disorders or develop alcohol-related liver disease;
· Roughly three times as likely to develop the chronic obstructive pulmonary disease, epilepsy, and fatty liver disease;
· Over two and a half times more likely to become obese.
Overall, ADHD was significantly associated with 34 of the 35 physical diseases studied, rheumatoid arthritis being the only exception.
Comparing men with women, women with ADHD were at significantly greater risk of atrial fibrillation, urolithiasis, sleep disorders, and asthma than men with ADHD. Conversely, men with ADHD faced a greater risk of thyroid disorder than women with ADHD.
Between-sibling analyses showed that full siblings of individuals with ADHD were at significantly increased risk for 27 of the 35 physical ailments, suggesting that shared familial factors contributed to the co-occurrence of the conditions. This remained true even after adjusting for the occurrence of ADHD in full siblings.
These associations were generally reduced in maternal half-siblings of individuals with ADHD. The associations between full-siblings were significantly stronger than between maternal half-siblings for type 1 diabetes, obesity, kidney infections, back or spine pain, migraine, sleep disorders, asthma, and chronic obstructive pulmonary disease.
Keep in mind that full-siblings on average share half of their genes, whereas maternal half-siblings share only a quarter of their genes. Maternal (as opposed to paternal) half-siblings were chosen as a basis for comparison because they are typically brought up together in the same family setting, and thus are similar to full-siblings in having a shared family environment. Reduced risk in maternal half-siblings would therefore signal a genetic component to the risk.
Given that ADHD is itself a nervous system disorder, it is unsurprising that it correlated most strongly with other nervous system disorders, with a medium effect size (r=.23). Genetic factors explained over a quarter of the correlation, shared environmental factors over a seventh, and non-shared environmental factors the other three-fifths. The latter could point to environmental risk factors that influence both ADHD and nervous system diseases.
Small-to-medium correlations were found with metabolic, respiratory, and musculoskeletal disease groups, with genetic factors explaining roughly two-thirds of the correlation, and non-shared environmental factors most of the rest.
The authors concluded that "adults with ADHD are at increased risk of a range of physical conditions, across circulatory, metabolic, gastrointestinal, genitourinary, musculoskeletal, nervous system, respiratory, and skin diseases. Most physical conditions showed familial associations with ADHD (mainly from genetic factors). Our findings highlight the need for rigorous medical assessment and care in adult patients with ADHD, and suggest long-term consequences of age-related diseases."
Ebba Du Rietz, Isabell Brickell, AgnieszkaButwicka, Marica Leone, Zheng Chang, Samuele Cortese, Brian M D'Onofrio, Catharina A Hartman, Paul Lichtenstein, Stephen V Faraone, Ralf Kuja-Halkola, Henrik Larsson, "Mapping phenotypic and aetiological associations between ADHD and physical conditions in adulthood in Sweden: a genetically informed register study," Lancet Psychiatry (2021), vol. 8, issue 9, 774-783, published online, https://doi.org/10.1016/S2215-0366(21)00171-1.
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