People with attention-deficit/hyperactivity disorder, or ADHD, may be more likely to experience a range of gastrointestinal symptoms, according to a systematic review covering more than 1.9 million people.
Researchers screened more than 9,000 records before including 23 studies in their analysis. They found that people with ADHD had higher odds of every gastrointestinal symptom examined, including irritable bowel syndrome, constipation, abdominal pain, diarrhoea, indigestion and encopresis, a condition involving loss of bowel control.
Overall, ADHD was associated with a 48% increase in the odds of experiencing gastrointestinal symptoms. The association was particularly pronounced for constipation, where the odds were almost twice as high, and encopresis, where they were more than four times higher.
Sarah Blake, a medical student at Warwick Medical School and co-lead author of the review, said almost every gastrointestinal symptom examined appeared to be associated with ADHD.
She said the symptoms could have a significant effect on daily life but might be overlooked when attention is focused primarily on ADHD.
The researchers stressed that the findings do not show that ADHD directly causes gastrointestinal problems. Instead, several possible explanations could account for the association.
Behavioural factors may be one contributing factor. Emotional dysregulation and increased sensitivity to stress, both associated with ADHD, could affect gut sensitivity and contribute to functional abdominal pain. Irregular eating habits, skipped meals and binge eating could also influence indigestion, bloating, constipation and abdominal pain.
Food-related sensory sensitivities may also affect eating patterns. Differences in sensitivity to food textures and smells could contribute to selective diets that contain less fibre, potentially affecting bowel function.
The researchers also considered interoception, which is the ability to recognise signals from inside the body. Reduced awareness of the need to use the toilet could potentially contribute to constipation or encopresis.
Medication could be another factor. Stimulant medicines such as methylphenidate can have side effects including reduced appetite, abdominal pain, nausea and constipation. Sleep problems, which are common among people with ADHD, may also affect gastrointestinal movement and sensitivity.
The review also points to the gut microbiome as a possible biological pathway. Previous research has identified differences in gut microbial composition between adults with ADHD and neurotypical people, while separate studies have linked changes in the microbiome with gastrointestinal symptoms.
Researchers said these differences could potentially affect inflammation and communication between the gut and brain. Genetic factors may also play a role.
Saran Shantikumar, associate clinical professor at Warwick Medical School, said researchers could not yet determine whether ADHD causes gastrointestinal problems or what drives the relationship.
He said the pattern was strong enough to warrant greater attention and suggested clinicians caring for people with ADHD consider asking about gastrointestinal symptoms, particularly when they may be affecting quality of life.
