The large intestine is also known as the colon. It begins at the end of the ileum and extends to the anus, about 59 inches long in the adult. The large intestine is divided into the ascending, transverse, descending, and sigmoid colon. The colon is responsible for mixing of the contents, reabsorption of water, and is the site of much bacterial action upon the intestinal content.
Absorbed molecules are carried to the liver in the hepatic portal system. Much water is removed and the contents are gradually dehydrated as they pass the length of the colon and eventually take on the consistency of feces.
Bacteria and colon cancer connection
Escherichia coli is the most important bacteria found in the colon. Various gram negative bacteria are present in the colon and are passed in the feces. The colon is sterile at birth, but the bacterial flora is established shortly thereafter. The bacteria of the colon are important to digestion and nutrition. While bacteria use some substances like vitamin C and vitamin B12, they make vitamin K and several B vitamins that are absorbed into the blood stream. Intestinal bacteria also seem to be involved in cholesterol metabolism.
Colon cancer is the third most common cancer diagnosed in US adults and accounts for about 20% of all cancer deaths in the US. The incidence of this disease is greatest at the fifth, sixth and seventh decades of life and is common to both men and women.
It tends to run in families and may be associated with dietary factors. High fat, low fiber diets, sedentary lifestyle, obesity and smoking are risk factors for the development of colon caner. Fiber in the diet, cellulose and lignin, seems to be protective, presumably because it hastens transit time throught the colon. Folic acid in a daily multivitamin can lower colorectal cancer risk. Calcium may also lower risk. Currently under study is the use of NSAIDs that, when taken regularly, may radically lower the risk of colorectal cancer.
Some risk factors such as age and heredity cannot be controlled so managing the controllable risk factors takes on more importance. Maintaining a healthy weight, diet and getting physical exercise are things that everyone can do to help prevent colon cancer. Annual tests are recommended for those over 50 or those with a familial history of colon cancer or personal history of polyps or ulcerative colitis.
Symptoms of colon cancer
Most tumors are found in the rectum and sigmoid colon. These are usually slow growing cancers that often are without symptoms for some time. Symptoms include a change in bowel habits, blood in the stool, narrower stools than usual, weight loss without reason, and constant tiredness. Diagnosis is by physical palpation during rectal exam, proctoscopy, and barium enema or other imaging modality.
There are also tests that look for occult blood in the feces or tumor antigens in the blood. Colon cancers may spread to the liver, lungs, bone or lymphatic system. Because these tumors grow slowly they are very curable with early diagnosis and treatment.
Prognosis depends largely on the location of the cancer, degree of spread, and extent of bowel involvement. When found early this cancer is very treatable by surgery, radiation and/or chemotherapy. Indeed the 5-year survival rate is 90% when this cancer is found early, but, unfortunately most are not diagnosed at this early stage. If the cancer has spread, survival rates drop dramatically to 65% if the spread is local and only 8% if distant tissues (liver, lung) are involved.