Causes and Risk Factors of Colon Cancer

Colorectal cancer is the third leading cause of cancer-related deaths in the United States for both men and women. Most cancers in the colon or rectum (the digestive tube located between the colon and anus) develop from polyps, which are growths that form within the inner lining of the colon.

While most polyps do not actually turn into cancer, the ones that are most likely to are called adenomatous polyps or adenomas. Large polyps (greater than 1 centimeter), polyps that contain abnormal cells (called dysplastic polyps), and having two or more polyps within the colon also increases the likelihood for colon cancer.

In terms of risk factors, a person's chance of developing colon cancer increases as they get older, especially after age 50. Furthermore, having type 2 diabetes or inflammatory bowel disease (for example, ulcerative colitis), or a family history of colon cancer also increases a person's risk for developing the disease, as do some modifiable risk factors such as being overweight and eating a diet rich in red and processed meats.

In the end, knowing the causes and risk factors for colon cancer can help you understand the importance of routine screening for colon cancer, as well as learn if you should begin screening at an earlier age.

Common Risk Factors

There are a number of factors that increase the risk for developing colon cancer, some within a person's control (considered modifiable) and some not, such as age, ethnicity, race, and genetics.

Age

Older age is the number one risk factor for colon cancer.

That said, young adults can get colon cancer too. In fact, the incidence of colon cancer in young people between 20 and 39 is increasing, and experts are not sure why. Moreover, contrary to popular thought, most colon cancers in young people are not linked to genetic syndromes, but occur sporadically.

Ethnicity and Race

Ethnicity and race are also factors associated with cancer risk. African Americans are more likely to develop and die from colon cancer than Caucasians. Another high-risk group for getting colon cancer is people of Jewish Eastern European descent.

Being Overweight or Obese

The link between colon cancer and obesity is strong. All told, people who are obese are over 30% more likely to develop this type of cancer than people of normal weight. Regular physical activity can actually protect you from developing colon cancer.

Type 2 Diabetes

Research has consistently shown a link between type 2 diabetes and the development of colon cancer. In addition, the risk of colon cancer increases for people with type 2 diabetes who have been obese for at least four years.

Personal History of Colon Polyps

A colon polyp is an abnormal growth in the lining of the colon. Most commonly, cancers of the colon develop from adenomatous polyps, with adenocarcinoma being the most prevalent type of colorectal cancer. Adenomatous polyps can be villous (frond or leaf-like), raised, or flat.

Virtually all colon cancers develop from adenomatous polyps; having one or more adenomatous polyps increases your risk of developing colon cancer. This risk is higher the larger a polyp is, the more polyps you have, and if a polyp shows dysplasia (contains some abnormal-looking cells).

The upside is that when these polyps are found and removed via colonoscopy, they no longer have the opportunity to become cancerous. 

Personal History of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) includes conditions such as ulcerative colitis and Crohn's disease. Both are associated with the development of colon cancer, and the risk increases the longer a person has IBD.

For example, while the results of different studies vary slightly, according to one analysis, colorectal cancer occurred in 0.4% of people with ulcerative colitis within a 10-year period and in up to 5.3% within 20 years.

In addition to disease duration, people with more extensive colitis (colon inflammation) are at a higher risk. More specifically, people whose entire colon is diseased (called pan-colitis) have the highest risk of developing colon cancer.

Research suggests that people who have ulcerative colitis are almost two and a half times as likely to get colorectal cancer than those without it, and those with extensive colitis have almost a five-fold increased risk. 

It's important to note that IBD should not be confused with irritable bowel syndrome (IBS), which does not increase a person's risk of developing colon cancer.

Radiation

Receiving radiation treatment to the abdomen, pelvis, or spine as a child increases the risk of developing colon cancer. This is why the Children's Oncology Group recommends that “if you were treated with radiation therapy to the abdomen, pelvis, spine, or total body during childhood, adolescence, or young adulthood, you should be screened for colorectal cancer beginning five years after radiation or at age 30, whichever occurs last. These options include stool-based testing every three years or colonoscopy every five years.” 

Research also suggests that men who have received radiation therapy to treat prostate and testicular cancer have a higher rate of colorectal cancer.

Genetics

Research has shown that one in four cases of colon cancer has some sort of genetic link. So if you have a first-degree family member (brother, sister, father, mother, child) with colon cancer or polyps, your risk of developing colon cancer is increased. 

Familial Adenomatous Polyposis (FAP)

This is a family-inherited syndrome that causes the development of hundreds (even thousands) of pre-cancerous polyps in the colon. People with FAP have nearly a 100% chance of developing colorectal cancer, often by age 40. Although fairly rare, people with FAP can be diagnosed with colon cancer in their teens. FAP symptoms may include a change in bowel habits, abdominal pain, or bloody stools (from large polyps).

Hereditary Nonpolyposis Colorectal Cancer (HNPCC)

Also referred to as Lynch syndrome, this is an inherited condition that may increase the risk of developing colon cancer by as much as 80%. There are no outward symptoms of HNPCC, but genetic testing, a family history of colon cancer, and screening exams, such as a colonoscopy, will help your doctor diagnose this syndrome.

Peutz-Jeghers Syndrome (PJS)

This is an inherited condition that causes colon polyps that are more prone to becoming cancerous. PJS is not common: It affects only between one in 25,000 to one in 300,000 people at birth.

PJS can be passed on to a child (50/50 chance) or developed sporadically for unknown reasons. Some symptoms associated with the syndrome that are usually spotted at birth include pigmented dark spotting on the lips or in the mouth, clubbing of the fingers or toenails, and blood in the stool.

Lifestyle Risk Factors

While it's easy to become overwhelmed by the non-modifiable risk factors for developing colon cancer, remember that overweight/obesity—a common factor in the development of colon cancer—is something you can have some influence over. In addition, several other risk factors are also within your control.

Alcohol Consumption

Alcohol is considered one of the major risk factors for colon cancer, and the risk is directly linked to the amount of alcohol consumed. In fact, even moderate alcohol consumption may put a person at risk.

Dietary Factors

Diets high in fat and cholesterol, especially red meats (for example, beef, lamb, and pork), have been linked to colon cancer. There also is research showing that eating more than 1.5 ounces of processed meat per day, such as hot dogs and lunch meat, increases the risk of death due to colon cancer. 

While there are no "set in stone" guidelines for exactly 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 18 ounces per week) and eating very little (if any) processed meat.

Smoking

Smoking is strongly associated with an increased risk of colorectal cancer. According to a review in the American Journal of Gastroenterology, a person's risk of developing colorectal cancer increases proportionately with the number of years they smoke and the intensity of their smoking. As soon as a person quits smoking, however, their risk of colon cancer starts to decrease.

Possible Links

There are many other factors linked to an increased risk of developing colon cancer, though it's important to note the jury is still out on them.

Some of these include:

• Long-term androgen deprivation therapy (ADT), possibly due to insulin resistance as a complication of ADT
• Removal of the gallbladder (cholecystectomy), which has been linked to increased risk of right-sided colon cancer
• Certain medical conditions, like acromegaly or coronary heart disease
• Deficiency in vitamin D, also called the "sunshine vitamin" (your body makes it when exposed to ultraviolet rays)
• Kidney transplant, due to long-term suppression of the immune system

More controversial (meaning the link is even fuzzier) risk factors include:

• Elevated blood levels of C-reactive protein (CRP), a protein made in the liver that increases in response to inflammation in the body
• Chronic constipation and regular use of laxatives, especially non-fiber laxatives
• Infection with certain viruses or bacteria (for example, Helicobacter pylori infection)

Frequently Asked Questions

Is there a specific gene for colon cancer?

Quite a few gene mutations associated with specific inherited syndromes have been implicated in colorectal cancer. These include the APC gene, which is involved in familial adenomatous polyposis, and the STK11 gene, linked to Peutz-Jeghers syndrome. Other genes associated with heightened colon cancer risk include the POLD1 and POLE, GREM1 , MSH3, and NTHL1  genes.

Do all polyps in the colon turn into cancer?

No. Polyps are common in the gastrointestinal tract; most are harmless. The polyps most likely to become malignant are adenomas, although most of these do not become cancerous. Those that do typically are relatively large—more than 1 inch in diameter. It's impossible to tell if a polyp is cancerous or potentially cancerous without removing it and analyzing it in a lab, so most are removed when discovered during a colonoscopy.

What type of diet causes colon cancer?

Colon cancer is not associated so much with a type of diet as it is with certain foods that either increase the risk or lower it. To decrease your risk of colon cancer:

Why does eating processed meat increase the risk of colon cancer?

It's not clear, but there are three main theories:

• Chemicals formed when food is cooked at very high temperatures, such as grilling, may cause mutations in DNA.
• Heme, the component of red meat that gives it color, may cause damage to DNA.
• A molecule called N-glycolylneuraminic acid (Neu5Gc) that occurs in diets that include beef, lamb, and pork may cause colorectal tumors to form and progress.