How much fiber do you really need—and can it reduce your risk of colon cancer?

Colon cancer rates are increasing among young people and it’s now the third most common cancer worldwide. At the same time, most Americans are only getting about half the fiber we need each day—and scientists think the two may be connected. 

"Strong evidence shows that diets high in dietary fiber reduce risk of colon cancer," says Karen Collins, a registered dietitian and the nutrition advisor to the American Institute for Cancer Research. 

(Colon cancer is rising among young adults. Here are signs to watch out for.)

Fiber is also associated with a host of other health advantages. "Fiber supports digestive health, lowers LDL cholesterol for improved heart health, helps regulate blood sugar levels for reduced diabetes risk, aids in weight management by promoting fullness, and feeds beneficial gut microbes that live symbiotically in our gastrointestinal tract," says Jen Messer, a registered dietitian and president of the New Hampshire Academy of Nutrition and Dietetics.

Here's why fiber is such a powerhouse nutrient and how it keeps your body running—plus how much you really need to stay healthy. 

What fiber does for the body

Fiber is a type of indigestible carbohydrate that remains in the colon after other components of the food become absorbed in the body, says Alice Lichtenstein, director of the cardiovascular nutrition team at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. 

There are two types of fiber—soluble and insoluble—which have different functions and benefits.

Soluble fiber’s primary role is forming “bulk” for our stool. "In this way, it works like a gentle brush, moving through our intestines to help pull waste out more efficiently," says Abbie McLellan, a clinical dietitian with Stanford Health Care. As soluble fiber does this, she explains, it binds with bile acids, which are made of cholesterol, and removes them from the body—thereby reducing LDL cholesterol levels, also known as "bad" cholesterol. The fiber-added bulk improves stool softness and size as well, reducing bloating and constipation. 

(Here's why women are more likely to have IBS.)

Insoluble fiber absorbs fluid, creating a gel-like substance that promotes feelings of fullness by slowing digestion down. This increased satiety can curb hunger pangs and help you keep your weight in check.

Throughout this process, insoluble fiber also boosts insulin sensitivity, thereby improving blood sugar levels and preventing the kind of excessive glucose spiking that’s associated with diabetes and prediabetes, McLellan explains. 

Soluble fiber is found in foods like bananas, apples, citrus fruits, peas, carrots, black beans, lima beans, brussels sprouts, barley, oats, and avocados. Insoluble fiber is found in whole-wheat flour, nuts, seeds, wheat bran, green beans, cauliflower, and potatoes. 

Both types of fiber have been shown to improve cardiovascular health and reduce inflammation and associated conditions such as heart disease and arthritis. 

Each fiber type is also a major food source for beneficial microorganisms in our gut, "which have a wide range of important functions such as hormone production and improved immune health,” says Lichtenstein.

Can fiber prevent colon cancer? 

Another top advantage of getting enough fiber is reducing your risk of colon cancer. In 2003, a major European study found that people with the highest fiber intake have a significantly lower risk of colorectal cancer. Meta-analysis of more recent studies has consistently shown the same.

One way fiber accomplishes this is by feeding beneficial gut bacteria. This produces metabolites—tiny molecules that are the result of the body breaking down food or chemicals—that reduce inflammation and thereby protect cells from becoming cancerous, explains Neil Iyengar, a medical oncologist who studies the relationship between diet, metabolism, and cancer at Memorial Sloan Kettering Cancer Center in New York. 

(Your gut health can affect the rest of your body. Here's why.)

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Why this type of carb is so good for your gut health

"These metabolites can also activate our immune system and increase our anti-cancer immunity," he adds. And they can stimulate the production of a layer of mucus in the colon, which strengthens the bond between cell connections, reducing leakage. “A leaky colon increases overall risk of cancer-causing inflammation in the body,” Iyengar explains, "so keeping these cell junctions tight is generally recognized as a protective feature of fiber intake."

Metabolites also “produce short-chain fatty acids that nourish the inner lining of the colon and further help protect cells from damage," says Messer. 

Research shows that fiber also reduces colon cancer risk by moving stool more quickly through the body, "thereby reducing the amount of time waste products come into contact with colonic cells," explains Carolyn Newberry, an attending physician, nutrition scientist, and gastroenterologist at Weill Cornell Medical Center in New York City. This is important because stool can contain cancer-causing carcinogens from red meat and ultra-processed foods.

Fiber’s role in weight management is also helpful because “obesity and excess weight gain are strongly linked to greater colon cancer risk," says Collins. 

For people who already have cancer, fiber could also aid with treatment because of its effects on the gut microbiome. Research shows that this can make chemotherapy and immunotherapy more effective and reduce their side effects, Messer explains. 

How much fiber do you need to stay healthy?

The amount of fiber you need varies, but the suggested daily fiber intake is 14 grams per 1,000 calories consumed. In other words, for a typical 2,000-calorie diet, you need 28 grams of fiber a day. The American Institute for Cancer Research takes it a step further by recommending that adults consume at least 30 grams of dietary fiber a day to lower cancer risk. 

Despite such recommendations, "nine out of 10 Americans do not meet their daily fiber needs," says Newberry.

But when it comes to preventing cancer, the evidence is still out on the best sources of fiber, says Eric Rimm, a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health.

(You may be overlooking this nutritional powerhouse. You shouldn't.)

For instance, researchers are still trying to better understand how gut microorganisms metabolize soluble and insoluble fiber differently, so "a variety of sources is still what is recommended to maximize fiber in the diet,” Rimm explains. 

Newberry agrees, noting that "the exact amount, type, and formulation of fiber that has maximal health benefits, including reduction in cancer risk, is still being studied." Several variables such as genetics and lifestyle choices like diet and exercise patterns also need to be more thoroughly researched to account for the cancer-related benefits associated with fiber consumption, Iyengar says.

It's also not known if fiber from supplements is as effective in reducing cancer risk as fiber from foods, though Rimm says it's likely that "the greatest benefit comes when fiber is consumed in foods because of the many other vitamins and minerals and other healthy compounds (such as antioxidants) that come along with high-fiber foods." 

Tips for getting more

To reap associated benefits, it's essential for most people to increase the amount of fiber they consume daily.

This starts with swapping out your diet staples for more fiber-rich options, such as pasta or breads made with whole-grain flour instead of refined flour and choosing brown rice over white rice. 

(Here's how to find the most effective whole grains.)

When it comes to the best foods to add to your diet, "the age-old recommendation from dietitians to 'eat the rainbow' really holds up," says McLellan. This means eating a variety of colors of fruits, vegetables, whole grains, nuts, and seeds such as quinoa. Iyengar says that beans and other legumes are especially beneficial sources of fiber.  

As you increase your fiber intake, it's important to also drink more water and to increase fiber consumption gradually, "rather than trying to double or triple your fiber intake in a day or two," cautions Rimm. "Adding too much fiber in the diet too quickly may lead to some gastrointestinal symptoms that could be unpleasant." 

To avoid this and to increase fiber consumption in a realistic manner, Newberry suggests adding it little by little to each meal or choosing a small handful of nuts or an apple to be enjoyed as snacks throughout the day.

“Many of us have work to do to reach our daily fiber needs, but the overall health benefits and lower cancer risk doesn’t have to be all-or-nothing,” says Collins. “Each increase in daily fiber consumption helps.”