Lewy body diseases (LBD)-including Parkinson's disease (PD) and dementia with Lewy bodies (DLB)-are associated significant gastrointestinal (GI) symptoms, particularly constipation. Epidemiological and pathological studies have revealed that LBD can start with constipation alone (referred to as Lewy body constipation), primarily due to neuronal loss and Lewy bodies' appearance in the myenteric plexus. Neuronal lesions may begin at the bowel (constipation), spread to the central nervous system via the vagal nerve, and manifest as PD, further spreading to the cortical LBD. Neuroimaging such as metaiodobenzylguanidine (MIBG) scintigraphy and dopamine transporter (DAT) scan provide a way to detect the early stage of Lewy body constipation. Arriving at a diagnosis is not always easy; collaboration between gastroenterologists and neurologists is highly recommended to maximize patients' quality of life.