According to the Mayo Clinic, excessive bowel movements have many causes, including celiac disease, Crohn's disease, hyperthyroidism, irritable bowel syndrome, side effects of drugs and ulcerative colitis. What constitutes excessive bowel movements varies among individuals, with some healthy individuals having up to three bowel movements per day. Changes in lifestyle such as increased exercise or a diet higher in fiber will harmlessly cause more frequent bowel movements.
The Mayo Clinic states that before frequent bowel movements should be considered excessive or unhealthy, one should make sure that no change in lifestyle can explain the change. Even just increasing water intake or eating high-fiber foods like fruits and vegetables can cause more frequent bowel movements. Even if there is no obvious reason for the change, most of the time there is no cause for concern. Bowel patterns change naturally with age in most people.
However, the Mayo Clinic cautions that one should see a doctor if substantial and sustained changes to bowel movements occur. These include narrow, ribbon-like stool or loose, watery stool. If the excessive bowel movements are accompanied by abdominal pain, this is also a cause for concern. If the stool is accompanied by any other substance, particularly pus or blood, medical care should be sought immediately.