Fiber and Cancer Prevention: Why It Matters
Should you be eating more fiber to help prevent cancer? And how much do you really need to reduce your risk?
It could be time for a fiber tune-up of your diet. Most American adults get only half the recommended daily dose, according to the Centers for Disease Control and Prevention (CDC).
Clinical dietitian-nutritionist Cara Anselmo, MS, RDN, is an expert in breast cancer nutrition and is a certified specialist in obesity and weight management.
“Eating more fiber is often overlooked as one of the healthiest things we can do for ourselves,” Anselmo says.
Fiber not only improves heart health, reduces diabetes risk, and aids in weight control, it may reduce the risk of several common cancers such as colorectal cancer.
How Does Fiber Reduce the Risk of Colorectal Cancer?
Dietary fiber has long been linked to a lower risk of certain types of cancer, particularly colorectal cancer. Colon cancer rates are increasing among young adults. Because less than 10% of adults are getting enough fiber, scientists think this trend may be connected.
One of the main ways that fiber helps to protect against colon cancer, according to Anselmo, is by fostering a healthy population of bacteria in the colon, known as the microbiome. Fiber remains in the colon after other components of the diet become absorbed by the body.
“When fiber is fermented by the bacteria in our colon,” Anselmo says, “it produces metabolites, which reduce inflammation and protect the colon cells from becoming cancerous. These metabolites can also activate our immune system and increase anti-cancer immunity.”
Fiber also has several other effects that improve our metabolic health, which indirectly lowers the risk of several cancers. For example, fiber can increase the speed at which food passes through the gut, which can lower absorption and exposure time to less healthy foods we may have consumed. Fiber can also lower cholesterol levels.
What Is a High-Fiber Diet?
A high-fiber diet means you eat foods that are rich in this type of carbohydrate, found mainly in plant foods such as fruits, vegetables, whole grains, beans, peas, and legumes. Consuming more than 25 to 30 grams of fiber per day is generally considered to be a high-fiber diet.
What Are the Kinds of Fiber?
There are two main types of dietary fiber — soluble and insoluble. Both types of fiber are healthy and can lower cancer risk.
Soluble fiber attracts water into the intestines and becomes a gel. It can help lower blood sugar and cholesterol levels. Foods higher in soluble fiber include:
• Oats
• Beans
• Citrus fruits
• Peas
Insoluble fiber doesn’t dissolve in water. It can help move food through your digestive system, absorb water to provide bulk, and may help prevent constipation. Foods higher in insoluble fiber include:
• Whole grains
• Some vegetables
• Nuts and seeds
How Much Fiber Do You Need?
The MSK registered dietitian-nutritionist team recommends that most people eat 25 grams of fiber per day. “Consume half of your fiber from whole foods like whole grains, vegetables, and beans,” Anselmo advises. “Fiber-supplemented products are not as effective as whole foods and may not have additional nutrients that help you reach your optimal health.”
Anselmo also stresses the importance of staying well hydrated when increasing dietary fiber in order to minimize potential symptoms such as bloating and abdominal discomfort as your gastrointestinal tract adapts to new changes. She reminds patients to always ask their cancer care team if they should follow any special diet before, during, or after treatment.
How Is MSK Researching the Role of Fiber and Cancer?
Studies are underway to help determine a person’s optimal dose of fiber to prevent cancer, based on their biology, genetics, and lifestyle.
MSK is also investigating which bacteria are involved in fermenting fiber into the metabolites that protect against various cancers, including breast, skin, prostate, colon, and rectal cancer. If scientists can identify a specific bacterial signature, other interventions such as bacterial transplants may work to reduce cancer risks.
Finally, there are ongoing studies testing whether fiber intake and certain probiotic compounds might enhance the effectiveness of chemotherapy and immunotherapy. (Source: Memorial Sloan Kettering News- 2025, Issue 1)
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