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Colon Cancer Causes and Risk Factors

Colon cancer (commonly referred to as colorectal cancer) is a type of cancer that occurs in the large intestine (colon) or the rectum (end of the colon). Most colon cancers develop from small benign tumors called polyps that form in the wall of the intestine.

Colon cancer locations

Over time, some of these polyps may turn into colon cancer if left untreated or not removed properly. For this reason, regular screenings are recommended to help prevent colon cancer by identifying and removing polyps before they turn into cancer.

Colon cancer is the third most common cancer in the U.S., affecting both men and women. One in 22 men and one in 24 women will be diagnosed with colon cancer in their lifetime.

According to the American Cancer Society, more than 130,000 Americans are diagnosed with colorectal cancer in 2018 and about 50,000 deaths during the year.

Types of Colon Cancer

Adenocarcinomas are the major colon cancer type, accounting for more than 95% of all the cancer cases. These cancers develop in the lining of the colon or rectum, which make up the large intestine. They tend to start in the inner lining and then spread to other layers.

The other less common types include:

  • Carcinoid tumors, which start in hormone-producing cells in the intestines.
  • Gastrointestinal stromal tumors (GISTs), which start in the small intestine but can occur anywhere in the gastrointestinal (GI) tract.
  • Lymphomas, which are cancers of the immune system that usually start in the lymph nodes but can start in the colon.

Colon Cancer Causes and Risk Factors

Almost all colon cancers begin as noncancerous (benign) polyps, which slowly develop into cancer. The exact causes are unknown, but colon cancer has several potential risk factors.


Your risk of colorectal cancer increases as you age. Around 90% of colorectal cancers are diagnosed after age 50, but it can occur at any age.

Being overweight or obese

Being overweight is clearly linked to an overall increased risk of colon cancer and an increased risk of dying of colon cancer when compared to those who have a normal weight [1].


Research has consistently shown a link between type 2 diabetes and the development of colon cancer.

A large scale study published in 2011 indicated that people with type 2 diabetes had a 38% higher risk of developing colon cancer than those without diabetes [2].

Family history of colon cancer

If you have family members with colon cancer, you have an increased risk for colon cancer, especially if more than one family member has colon cancer or rectal cancer.


People who smoke may have an increased risk of colon cancer. Moreover, a person’s risk of developing colon cancer increases proportionately with the number of years they smoke.

The good news is that as soon as a person quits smoking, their personal risk of colon cancer starts to decrease. Check out these tips to help you quit smoking.

Alcohol consumption

Alcohol is now considered one of the major risk factors for colon cancer, and the risk is directly linked to the amount of alcohol consumed. In fact, a meta-analysis of 57 studies that examined the association between alcohol consumption and colorectal cancer risk showed that people who have 3 1/2 drinks per day (about 50 grams of alcohol) have a 50 percent increased risk of developing colon cancer compared to occasional drinkers or non-drinkers [3].

The American Institute for Cancer Research recommends limiting alcohol consumption to no more than one drink per day for women and two drinks per day for men.

High-fat, low-fiber diet

A diet high in fat, such as red meats (beef, pork, lamb, or liver) and processed meats (ham, bacon, sausages, hotdogs, and luncheon meats) increases your risk of colon cancer [4].

While there are no exact guidelines for how much red or processed meat you can consume to avoid increasing your colon cancer risk, the World Cancer Research Fund recommends consuming less than 500 grams of red meat per week (equivalent to about 17.5 ounces per week) and eating very little (if any) processed meats.

The American Cancer Society also recommends limiting red and processed meats and eating more fiber foods, such as fruits, vegetables, and whole grains, to lower your risk for getting colon cancer.

Inherited gene changes (mutations)

Patients who have inherited the hereditary colon cancer syndrome genes are at risk of developing colon polyps, usually at young ages, and are at higher risk of developing colon cancer early in life. They also are at risk of developing other types of cancers.

Some of the inherited syndromes that can increase the risk of developing colorectal cancer include:

  • Familial Adenomatous Polyposis Syndrome (FAP)
    This is a family-inherited syndrome that causes the development of hundreds or even thousands of polyps in the lining of your colon. Individuals with FAP have a greatly increased risk of developing colon cancer before age 40.

    These patients are also at risk of developing cancers in the thyroid gland, stomach, and the ampulla (part of the bile duct where it drains into the small intestine from the liver). FAP symptoms may include abdominal pain, a change in bowel habits, or bloody stools (from large polyps).

  • Hereditary Nonpolyposis Colorectal Cancer Syndrome (HNPCC)
    HNPCC, also referred to as Lynch syndrome, is the most common form of hereditary colorectal cancer. In most cases, this syndrome is caused by an inherited defect in either the MLH1 or MSH2 gene, but changes in other genes can also cause Lynch syndrome. These genes normally help repair DNA that has been damaged.

    Patients with HNPCC are also at risk of developing cancers of the stomach, small intestine, liver, bile ducts, brain and skin. Furthermore, women with this condition have a high risk of developing cancers of the uterus and ovaries.

  • Peutz-Jeghers Syndrome (PJS)
    PJS is a rare genetic condition characterized by multiple polyps in the gastrointestinal tract and pigmented spots on the skin. People with Peutz-Jeghers syndrome often develop hamartomatous polyp in the digestive tract. It is a growth of normal-appearing tissue that builds up into a benign (noncancerous) tumor.

    Peutz-Jeghers syndrome is also associated with other types of cancers, including breast, pancreatic, stomach, ovarian, lung and small intestine cancers. Symptoms of this syndrome include pigmented dark spotting on the lips or in the mouth, clubbing of the fingers or toenails, and blood in the stool.

  • MYH-associated polyposis (MAP)
    People with this syndrome develop many colon polyps and they almost always become cancer if not found early with regular colonoscopies. These people also have an increased risk of other cancers, such as gastrointestinal (GI) cancer and thyroid cancer. This syndrome is caused by mutations in the MYH gene and often leads to cancer at a younger age.

Radiation therapy

Several studies have suggested that men who had radiation therapy to treat prostate and testicular cancer have a higher risk of getting colon and rectal cancer.

Radiation therapy directed at the abdomen or pelvis to treat previous cancers also increases the risk of colon cancer.

Inflammatory bowel disease (IBD)

If you have inflammatory bowel disease (IBD), you are at a higher risk of getting colon cancer.
Inflammatory bowel disease (IBD), a condition in which the lining of the digestive tract becomes inflamed, causing sores and bleeding. The two most common types of IBD include ulcerative colitis and Crohn’s disease.

It’s important to note that inflammatory bowel disease (IBD) is different from irritable bowel syndrome (IBS), which does not increase a person’s risk of developing colon cancer.

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