Colorectal cancer and the important role of dietary fiber: A beneficial adjunct for better health

by Mark A. Mahoney, Ph.D., R.D.N.

Colorectal cancer is the third most common cause of cancer in the U.S. 2025 estimates were for 154,270 new cases and 52,900 deaths.

Colon cancer occurs when genetic mutations turn healthy colon cells into cancerous cells. The cancerous cells divide and multiply to form cancerous solon polyps. Over time, cancer grows through the layers of muscle and tissue that makes up your colon wall.  Eventually, the cancer breaks through your colon wall and begins to spread outside your colon.  Medical researchers aren’t sure what triggers the change in cancer polyps but research shows certain factors increase your risk.

Medical researchers aren’t sure what triggers the change in colon polyps.  Research shows certain factors increase your risk. Examples of risk factors include:

Examples of risk factors

• Being 50 or older. Most people with colon cancer are older than 50. But there’s an increasing number of cases in people younger than 50. Medical researchers aren’t sure what drives this change.
• Having a family history of colon cancer. Your risk may be higher if close biological family members have large colon polyps or colorectal cancer. Close family members include your biological parents, siblings and children. Your risk may be higher if any biological family member develops this disease before they turn 45.
• Having many colon polyps. In general, the more polyps in your colon, the greater the chance that one or more may become cancerous.
• Inheriting certain conditions. Conditions like Lynch syndrome (an inherited genetic condition that significantly increases the risk of colorectal and other cancers) and familial adenomatous polyposis (an inherited condition affecting about 1 in 10,000 people that is rare and may lead to colon cancer)
• Having obesity or overweight. Research shows that having a BMI higher than 25 plays a role in this disease
• Smoking cigarettes. One study found that smoking cigarettes increases colon cancer risk.
• Regularly drinking alcohol. Even light to moderate consumption may increase your risk

Role of Fiber
Fiber is a type of carbohydrate that serves as a prebiotic, (or food for the “good” bacteria in the gut) promoting a  healthy microbiome (the full array of microorganisms, the microbiota) that live on and in humans).  Also, rather than being broken down by digestive organs, fiber passes through the body to support healthy bowel movements. There are two types of fiber: soluble and insoluble.

Soluble Fiber
Soluble fiber dissolves in water and creates a gel-like substance that slows the movement of food through the digestive tract. This slower digestion reduces blood sugar spikes, lowers blood cholesterol, and helps you to feel fuller longer.

Some examples of food sources of soluble fiber include:

• Beans of all kinds, including kidney, black, pinto, white, lima, navy, chickpea; edamame; and peas
• Oats
• Tofu
• Avocado
• Brussels sprouts
• Sweet potatoes
• Broccoli

Insoluble Fiber

Insoluble fiber (aka roughage) is found in the skin and structure of fruits and vegetables—it’s what makes these foods crunchy. Instead of dissolving in water, insoluble fiber attracts water into the stool, making it softer and easier to move through the digestive system. This promotes bowel-movement regularity and prevents constipation and hemorrhoids. Also, since these crunchy foods require more chewing, it takes longer to eat a fiber-rich meal, slowing down digestion and helping with portion control.

The best sources of insoluble fiber include:

• Whole wheat products, like 100% wholewheat bread and pasta
• Oat bran and oatmeal
• Legumes: Beans of all kinds, including kidney, black, pinto, white, lima, navy, chickpea; edamame; and peas
• Berries: including blackberries, blueberries, raspberries, and strawberries
• Whole grains, including quinoa, rye, barley, amaranth, brown rice
• Leafy greens, like kale and spinach
• Vegetables, like broccoli, okra, radishes
• Nuts, especially almonds and walnuts
• Fruits with edible skins, like pears and apples
• Avocados
• Sunflower, flax, and chia seeds
• Potatoes and sweet potatoes

Colorectal cancer is one of the most common cancers in the United States.  The good news is that it’s also one of the most preventable.

Eating more fiber helps keep your digestive system moving and may lower your risk for colon cancer

Staying physically active, limiting your alcohol consumption and keeping up with colon cancer screenings can all lower your risk.

Some key takeaways: 

• Fiber promotes a healthy microbiome by feeding the “good” bacteria in the gastrointestinal tract.
• Fiber is not broken down by the digestive organs but instead passes through the body to support healthy bowel movements.
• 95% of Americans are deficient in fiber.
• 19-38 grams of fiber per day is recommended and should be added to every meal.
• Fiber is found in plant-based foods, particularly beans, nuts, fruits, and vegetables.
• Fiber has many health benefits, including reducing risk of cardiovascular disease, type 2 diabetes, and several cancers.
• The American Cancer Society and the U.S. Preventive Task Force recommend average risk individuals start regular screening for colorectal cancer at age 45. This change reflects an increase in colorectal cancer incidence among a younger population.

Colorectal cancer is one of the most common cancers in the United States.  The good news is that it’s also one of the most preventable.

Eating more fiber helps keep your digestive system moving and may lower your risk for colon cancer

Staying physically active, limiting your alcohol consumption and keeping up with colon cancer screenings can all lower your risk.

References/Additional Information
A description on diagnosis and tests related to how doctors diagnose colorectal cancer is available at:

Colon Cancer: Symptoms, Causes & Treatment

Check out the Mayo Clinic’s resource, Dietary Fiber: Essential for a Healthy Diet at:

Dietary fiber: Essential for a healthy diet – Mayo Clinic

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Mark A. Mahoney, Ph.D., R.D.N. served as a Peace Corps Volunteer in Ecuador in the mid-1970s. He has been a Registered Dietitian/Nutritionist for over 40 years completed graduate studies in Nutrition & Public Health at Columbia University and earned a Ph.D. in International/Intercultural Development Education at Florida State University.  He can be reached at marqos69@hotmail.com.