Only lung cancer kills more Americans. The American Cancer Society estimates that about 148,000 people are diagnosed with colorectal cancer each year and about 50,000 people die of this cancer annually in the United States.

But recent research points to the need for screening at an earlier age as a preventive step. The Centers for Disease Control and Prevention (CDC) recommends that anyone age 50 or older have regular screenings for colorectal cancer. The screenings can find and remove small precancerous growths called polyps.

At first, colorectal cancer has few or no symptoms. That’s why it’s so important to get regular screenings, the CDC says. Symptoms include blood in or on the stool; a change in bowel habits; stools that are narrower than usual; general stomach discomfort; frequent gas or pains; and weight loss. See your doctor if you notice any of these symptoms.

The majority of colorectal cancers develop first as polyps, the CDC says. Colorectal polyps are tiny growths inside the colon or rectum that may become cancerous.

Other risk factors include increased age (more common in those 50 and older); a family history of colorectal polyps; and certain diseases of the bowels.

Several tests are available: Fecal occult blood test (FOBT), which checks for blood in the stool. Sigmoidoscopy, an exam that looks at the inside of the rectum and lower portion of the colon through a lighted tube. The doctor may collect samples of tissue or cells for closer examination. Colonoscopy, an exam that looks at the inside of the entire colon through a flexible, lighted instrument called a colonoscope. The doctor may collect samples of tissue or cells for closer examination. Double contrast barium enema, a procedure that involves X-rays of the large intestine taken after a patient is given an enema containing a white dye, or barium, followed by an injection of air. The barium outlines the intestine on the X-ray film.

Researchers have found that people who lead a sedentary lifestyle have higher levels of a substance called prostaglandin E2 in their colons. Cancer experts say this substance appears to boost the risk of colorectal cancer. A recent study found that a daily jog of 30 minutes or a daily walk of an hour reduced the concentration of prostaglandin E2 in the colon by nearly 30 percent.

Obesity, like a sedentary lifestyle, appears to increase the amount of prostaglandin E2 in the colon, and thus the risk of colorectal cancer. A recent study found that when a person’s body mass index—a calculation that takes into account a person’s height and weight—increased from 24.2 to 28.8, prostaglandin E2 levels rose by nearly 30 percent.