This is extremely embarrassing, but please tell me what causes the shape of bowel movements to change. Sometimes mine will be small and round, and, at other times, long and thin or long and thick. Is this anything to be concerned about? I have an aunt who died of colon cancer and I want to know before it is too late.

M.B.

First of all, don't be embarrassed! Many patients are reluctant to discuss the character and frequency of their bowel movements with their doctors, even though the information may provide important clues to their conditions.

It is not uncommon to experience changes in the character and shape of bowel movements. One simple explanation is changes in your diet. Eating fiber and other bulk-producing foods may increase the size of the stool. Patients who consume low-fiber, high-fat diets may experience constipation and harder, smaller stools. This is especially true if they do not drink much water.

In patients with irritable bowel syndrome, abnormal contractions of the colon can lead to changes in the appearance of the stool. This often occurs in conjunction with intermittent bouts of diarrhea and constipation, accompanied by crampy, lower abdominal pain.

In any patient reporting a change in the character of the stool, doctors must consider the possibility of an obstruction of the colon. This is particularly important because colon obstruction is often caused by cancer. In fact, one of the warning signs of colon cancer listed by the American Cancer Society is a change in bowel habits. In a patient with a family history of colon cancer, this symptom should be given even more weight.

To rule out a colonic obstruction due to cancer, gastroenterologists typically perform a colonoscopy on patients who complain of a significant change in the character of their stools. You should discuss your symptoms and concerns with your doctor.