A new review suggests that ADHD is associated with a substantial burden of functional gastrointestinal symptoms — problems affecting how the digestive system works without necessarily reflecting an identifiable structural disease.Researchers screened over 9,000 records and included 23 studies, and found that ADHD was associated with higher odds of every gastrointestinal symptom examined — including irritable bowel syndrome, constipation, abdominal pain, diarrhoea, indigestion and encopresis.”Almost every gut symptom we looked at … appeared to be associated with ADHD,” said co-lead author Sarah Blake, a Medical Student at Warwick Medical School. “These symptoms can have a real impact on daily life, but they could easily be overlooked if the focus is on ADHD itself.” The researchers found a 48% increase in the odds of experiencing gastrointestinal symptoms overall among people with ADHD. The association was particularly strong for constipation, where the odds were almost twice as high, and encopresis — loss of bowel control — where they were more than four times as high.Why the link?The researchers say there are several possible explanations, but the study does not establish that ADHD itself causes gastrointestinal problems.One possibility is behaviour. Emotional dysregulation and increased sensitivity to stress, both associated with ADHD, may affect gut sensitivity and contribute to functional abdominal pain. Irregular eating patterns, skipped meals and binge eating may also influence symptoms such as indigestion, bloating, constipation and abdominal pain.Food-related sensory sensitivities could play a role, too. People with ADHD may experience differences in sensitivity to food textures and smells, potentially contributing to selective diets that are lower in fibre and could affect bowel habits.The researchers also point to differences in interoception — the ability to recognise internal bodily signals — as a possible factor. Reduced awareness of the need to have a bowel movement could potentially contribute to constipation or encopresis.Medication may be another contributor. Stimulants such as methylphenidate are known to cause side effects including appetite suppression, abdominal pain, nausea and constipation. Sleep disturbances, which are common among people with ADHD, may also affect gastrointestinal motility and sensitivity.The gut-brain connectionThe researchers also highlight the gut microbiome as a possible biological pathway.Previous research has identified differences in gut microbial composition between adults with ADHD and neurotypical controls. Other studies have linked changes in the gut microbiome with gastrointestinal symptoms.One hypothesis is that these differences could influence inflammation and communication between the gut and brain. Genetic factors may also contribute. “We can’t yet say ADHD causes these gut problems, or exactly what is driving the link, but the pattern is striking enough to warrant attention,” Saran Shantikumar, associate clinical professor at Warwick Medical School, said. “Clinicians caring for people with ADHD may want to consider asking about gastrointestinal symptoms, particularly if they may be affecting quality of life.”