Obesity is just one of many factors that influence your risk of cancer, and having obesity doesn’t guarantee you’ll develop cancer. That being said, having obesity does increase the risk of 13 types of cancer, including colorectal cancer (CRC).

CRC affects parts of the large intestine called the colon and rectum. If you’re one of the more than 100 million Americans living with obesity, it’s important to know the facts about how excess weight and CRC risk are connected.

Scientists are still studying how obesity is a factor in CRC and why having obesity raises your risk of cancer. In this article, we’ll dive into the newest research on the link between obesity and CRC.

What Is Colorectal Cancer?

CRC is cancer that starts in the colon or rectum. Both the colon and the rectum are part of your large intestine. The colon is the longest part of the large intestine, where water and some nutrients are absorbed from the food you eat. The rectum is the lower part of the large intestine, where feces (poop) is stored. CRC is the third most common type of cancer worldwide. It is the second leading cause of death from cancer in the United States.

Obesity and Colorectal Cancer Risk

Obesity is one of the risk factors for CRC. Medically, obesity is usually defined as having a body mass index (BMI) over 30. BMI is an estimate of body fat based on your height and weight — it is a useful measurement for doctors, though your BMI does not always reflect your overall health.

Many studies have looked at the connection between carrying excess weight and a person’s risk of CRC. Researchers estimate that people with obesity have about a 30 percent higher risk of CRC than people without obesity. In the last few years, some research has suggested that this number might be an underestimate.

The Effect of Weight Loss on CRC Risk

Cancer research studies show that it takes an average of three to six years to be diagnosed with CRC after the initial cancer starts growing. Part of obesity management is moving toward a healthy weight, so someone’s BMI may change over time. They might have a lower BMI when they receive their cancer diagnosis than they had when the cancer started. Ongoing research is working to understand how BMI affects CRC risk over time by accounting for weight changes and other lifestyle modifications.

Central Obesity and CRC Risk

Another area of current research is how different types of obesity affect someone’s relative risk of CRC. One type of obesity, called central obesity, seems to pose a higher risk of developing cancer. In central obesity, most of the excess fat is carried around the waist. Although BMI is commonly used to diagnose obesity, it can’t tell where the body weight is being carried. Instead, a measurement called the waist-to-hip ratio can be used. The waist-to-hip ratio is calculated by dividing your waist size by your hip size, both of which can be measured using a tape measure. A high waist-to-hip ratio seems to be an even stronger risk factor for CRC than a high BMI. Studies estimate that people with central obesity have a 39 percent to 45 percent higher chance of developing CRC.

Other Risk Factors for CRC

Cancer is a complex disease, and we don’t fully understand all the reasons why some people get it and others don’t. Lifestyle, genetics, family history, and other environmental factors can all play a role.

Besides obesity, CRC has many other known risk factors, including:

• Type 2 diabetes
• Diets high in red meat and processed meats
• Smoking
• Heavy alcohol use

In addition, some inherited risk factors for CRC include:

• Being American Indian, Alaska Native, African American, or Ashkenazi Jewish, according to the American Cancer Society
• Having a history of some large intestine conditions, including adenomas (growths in the colon or rectum) or inflammatory bowel disease
• A family history of CRC or an inherited genetic condition such as Lynch syndrome

Why Does Obesity Increase Colorectal Cancer Risk?

When you carry excess weight, your body experiences changes in inflammation levels, hormones, and metabolism that affect how well it can function. People with obesity have greater amounts of adipose tissue (fat) than people without obesity. Adipose tissue functions as a hormone-releasing organ. Adipose tissue releases hormones and other molecules that increase the inflammation in your body.

Insulin Resistance

Molecules released by adipose tissue interfere with normal organ function. For instance, one type of adipose tissue molecule called an adipokine can increase insulin resistance in the body. This means that your tissues and organs aren’t as good at using sugar from your blood after a meal. Over time, this insulin resistance can lead to the development of type 2 diabetes. Insulin resistance is a risk factor for developing several different kinds of cancer for reasons researchers still don’t fully understand.

DNA Damage

Increased inflammation is stressful for the body. It can cause damage to your DNA and promote cancer-causing mutations. Chronic (long-term) inflammation can also create an environment that makes it easier for cancer cells to grow.

Gut Bacteria Changes

Finally, new research suggests that obesity also changes the gut microbiome in ways that can promote cancer. In people with obesity, gut bacteria may produce molecules that affect cancer cell growth.

Ways To Reduce Your Colorectal Cancer Risk

If you’re following recommended lifestyle changes and a management plan for your obesity, you’re already working to reduce your risk of developing CRC. Studies have shown that dietary changes, including reducing calories and increasing dietary fiber, reduce your overall inflammation and markers of cancer.

Increasing physical activity also reduces inflammation and the cancer-promoting effects of adipose tissue. Finally, medications for obesity and bariatric surgery are being studied as possible ways to reduce the risk of CRC in people with obesity. These treatments appear to reduce insulin resistance and chronic inflammation.

Apart from obesity-related changes, you can also take charge of other risk factors you might have that increase your CRC risk. With your healthcare team’s support, you might consider reducing your alcohol or tobacco use. Getting regular colon cancer screenings and having any unusual colon growths removed are also important for cancer prevention.

Should You Be Screened for Colorectal Cancer?

In the U.S., the American Cancer Society recommends regular colon cancer screening for everyone over age 45. Between the ages of 76 and 85, colon cancer screening is based on personal preferences, screening history, and overall health. After age 85, colon cancer screening is no longer recommended.

There are two different test types for CRC screening: stool-based tests and visual exams.

• Stool-based tests — Stool-based tests involve sending your poop away for lab analysis.
• Visual exams — Visual exams include colonoscopies, virtual colonoscopies, and sigmoidoscopies. All these procedures involve examining the inside of your large intestine.

People with an increased risk of CRC due to previous cancer, family history, and some genetic and digestive conditions may prefer more intensive screening. If you’re concerned about your risk of CRC, talk to your healthcare team about screening options and your personal risk factors.

Talk With Others Who Understand

On MyObesityTeam, the social network for people with obesity and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with obesity.

Are you worried about your cancer risk as a person living with obesity? Have you been diagnosed with colorectal cancer? Share your experience in the comments below or on your Activities page.