Q. After a bad auto accident, I had several feet of my small and large intestines removed. Since then, I experience abdominal cramps and diarrhea daily. I have to run to the bathroom several times a day because of this problem.

I've had a bunch of tests to check for infections, diverticulitis, adhesions and other possibilities. When nothing was found, I was told that I have a spastic colon and that I should change my diet. So far, no changes in what I eat have made any difference.

I've taken anti-spasm and anti-diarrhea medications to no avail. But recently, I've been taking an antibiotic for a tooth infection, and for once I feel great and secure. Could this be a clue as to what's going on with me? What could cause persistent diarrhea like this?

A. Your response to antibiotics probably is a clue to your problem. It sounds like you have what's called bacterial overgrowth. This is one of the many causes of persistent diarrhea. Although you may not be able to cure your problem, there are some things you can do to keep it under control.

Bacterial overgrowth means you have developed excess amounts of bacteria in your small intestine. This overgrowth leads to problems absorbing food, or malabsorption, which in turn leads to abdominal cramps and diarrhea.

Overgrowth of bacteria usually starts after something damages your intestines, either changing their structure or how they function.

In your case, surgery removed the part of your intestines where the small intestine joins the large intestine, or colon. Because of this, you lost the portion that has a valve that keeps bacteria from flowing backward from the colon into the small intestine. This led to an overgrowth of bacteria in your small intestine and, subsequently, diarrhea. When you took antibiotics for a while, it cut down on the bacteria, decreased the malabsorption and improved the diarrhea.

Any intestinal surgery that alters normal function can lead to overgrowth of bacteria; stomach surgery or lack of stomach acid can as well.

In addition, intestinal diseases such as Crohn's, which alter the structure of the intestines, also promote overgrowth. Other diseases that can cause the problem include diabetes, hypothyroidism and scleroderma.

Besides bacterial overgrowth, there are many other causes of malabsorption. Some of these stem from having insufficient digestive juices, which can occur if you have a pancreatic problem, such as chronic pancreatitis or cystic fibrosis. Others occur if your liver isn't working properly.

If you've had many feet of intestines removed, you may not have enough to absorb everything you eat. This condition is known as the short gut syndrome. In fact, this was an early surgical treatment for obesity--removing or bypassing enough intestine to interfere with proper absorption of foods, leading to diarrhea and other problems.

Other diseases of the intestines can cause malabsorption, including Whipple's disease, celiac disease and lymphoma involving the small intestines. Even heart disease can lead to malabsorption and diarrhea by affecting the blood vessels to the intestines, impairing blood flow and proper function.

Bacterial overgrowth and malabsorption can be tricky to diagnose. There are special tests to tell whether you have malabsorption or not. If so, there are other tests to see what's causing it. Most causes of malabsorption and persistent diarrhea can be treated once they are found.

For treatment of bacterial overgrowth, antibiotics may help. They may initially give you relief, but you may have to take them regularly. Ask your doctor for further advice, including what other specialists you may need to see. By the way, because vitamin B-12 is absorbed in the portion of intestines you had removed, you run the risk of vitamin B-12 deficiency and pernicious anemia. Ask your doctor to check you for vitamin B-12 deficiency.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, DC 20071. Questions cannot be answered personally.