How does colon cancer cause constipation?

Obstruction: Cancer can obstruct the flow of fecal matter in the colon and the obstruction may become severe enough to cause obstruction. The cancer may be ring shaped and encircle the colon or may form a large mass and block the flow of intestinal contents. At your age, constipation is unlikely to be due to colon cancer. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.