Some research suggests that dietary fiber may protect against colorectal cancer. However, more research is needed to better understand the relationship between fiber intake and colorectal cancer.

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Colorectal cancer refers to cancer that occurs in the colon or rectum. While the exact causes of colorectal cancer are unknown, some studies suggest that dietary habits may play a role. In particular, it’s thought that getting enough dietary fiber can help protect against this type of cancer.

However, research on the relationship between fiber and colorectal cancer doesn’t yet offer a clear conclusion.

This article reviews the role that fiber may play in protecting against colorectal cancer, offers ways to get enough fiber, and discusses other tips for cancer prevention.

You’ll notice that the language used to share stats and other data points is pretty binary, fluctuating between the use of “male” and “female” or “men” and “women.”

Although we typically avoid language like this, specificity is key when reporting on research participants and clinical findings.

Unfortunately, the studies and surveys referenced in this article didn’t report data on, or include, participants who were transgender, nonbinary, gender nonconforming, genderqueer, agender, or genderless.

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Over the past few decades, several studies have analyzed the relationship between fiber intake and colorectal cancer risk. Most of these older studies, including studies from 1997 and 1998, found that those with higher intakes of dietary fiber had a lower risk of getting colorectal cancer than those who didn’t eat as much fiber.

How dietary fiber protects against colorectal cancer isn’t fully understood, but a few mechanisms have been proposed. It’s believed that since fiber increases stool bulk and decreases transit time, it helps prevent cancer-causing chemicals from having prolonged contact with the colon and rectum.

Other suggested mechanisms that may have anticancer effects include the bacterial fermentation of fiber in the gut into short chain fatty acids, such as butyrate. The other plant compounds in foods with fiber, such as antioxidants in fruits and vegetables, may also contribute to protection from cancer.

Current research

Research continues to assess whether or not fiber has a protective effect against colorectal cancer, with varying results.

  • A 2018 review of 11 prospective cohort studies concluded that the risk of colon cancer was 14% to 21% lower in the group with the highest dietary fiber intake compared with the group with the lowest intake.
  • Another review of studies that examined nine studies performed in eastern Asian countries found that six studies indicated that dietary fiber has a preventive effect on colon cancer, while three studies didn’t have this conclusion.
  • A review of studies from 2019 concluded that dietary fiber had a significant protective effect against the development of colorectal cancer in Asian adults.

It’s unclear how the type — soluble or insoluble — or source of dietary fiber contributes to the possible anticancer effects. For example, studies offer inconsistent results on whether fiber from one group of foods, such as whole grains and cereals, vegetables, or fruits, is any more protective against colon cancer than fiber from another group.

A 2015 prospective study concluded that fiber from cereals and fruit appeared to be the most protective against colorectal cancer. In addition, a 2020 cohort study found that fiber intake from whole grains, but not other foods, was linked to a lower incidence of colorectal cancer.

However, other research suggests that compounds in specific vegetables, such as carotenoids in green and orange vegetables, are associated with lower colorectal cancer risk even after adjusting for fiber content.

Representation matters

Research has often been limited to certain populations or locations. There’s a need for more studies that include participants from more racial and ethnic backgrounds, age groups, and geographic locations. Additional research on how fiber affects colon cancer risk in males versus females is also needed.

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Keep in mind, also, that studies that show a link between colorectal cancer and dietary fiber intake have primarily been observational. It’s not possible to identify if low fiber intake is actually a cause of this cancer or if it’s just associated with a greater risk.

What qualifies as a “high” fiber intake differs across research, but many studies categorize intake levels in increments of 10 grams per day or a comparable measurement. For example, studies may conclude that every 10 grams of fiber per day reduces the risk of colorectal cancer by a certain percentage.

The American Institute for Cancer Research (AICR) recommends a daily fiber intake of 30 grams per day to reduce cancer risk and suggests that each 10-gram serving of fiber per day decreases colorectal cancer risk by 7%.

This is in line with the U.S. Department of Agriculture (USDA) Dietary Guidelines that recommend an intake of 22 to 34 grams of fiber per day, depending on age and sex, to support overall health.

In addition to the possible protective effects against cancer, getting enough fiber may help reduce the risk of heart disease, diabetes, and obesity.

Meeting fiber needs can be challenging. The USDA estimates that about 90% of women and 97% of men in the United States fall short on consuming enough fiber.

The best sources of dietary fiber include:

  • fruits
  • vegetables
  • whole grains
  • legumes
  • nuts
  • seeds

The two main types of fiber in foods are soluble and insoluble. Some foods are particularly high in one of these types, but most fiber-rich foods offer a mix of the two.

To help you meet the goal of approximately 30 grams of dietary fiber per day, here are some foods to incorporate into your meals:

  • High fiber fruits: raspberries, pears, bananas, apples, avocado, dried fruits (unsweetened)
  • High fiber vegetables: Brussels sprouts, artichokes, broccoli, green peas, celery, acorn squash
  • Whole grains: oats, popcorn, barley, quinoa, bulgur, brown rice
  • Legumes: lentils, chickpeas, black beans, split peas, white beans
  • Nuts and seeds: chia seeds, milled flax seeds, almonds, pistachios, sesame seeds

To increase your fiber intake, try adding these foods to your meals throughout the day. For example, you can have oatmeal with raspberries at breakfast, broccoli with hummus (made with chickpeas) with lunch, and lentil soup for dinner.

If you don’t usually eat a high fiber diet, try to increase intake of fiber-rich foods gradually to prevent side effects like gas and bloating. It’s also helpful to drink a lot of water when you increase fiber intake to help it move through your system.

Finally, you may be wondering if taking fiber supplements is a beneficial way to increase fiber intake. Most experts, including the American Cancer Society, recommend getting the majority of your fiber from whole foods instead. More research is needed on the safety and effectiveness of fiber supplements.

In addition to getting enough fiber from foods, other diet and lifestyle habits may help protect against colorectal cancer.

Lack of physical activity, obesity, and an unbalanced diet, including high salt and red meat intake, have been associated with colorectal and other cancers.

Here are the current recommendations from the American Cancer Society for decreasing your risk of cancer:

  • Maintain a moderate weight.
  • Engage in 150 to 300 minutes of moderate intensity exercise or 75 to 100 minutes of vigorous intensity exercise per week.
  • Limit sedentary behaviors like sitting and watching TV.
  • Eat foods that are nutrient-dense, including fruits, vegetables, legumes and other plant proteins, whole grains, nuts, and seeds.
  • Cut back on or avoid sugar-sweetened beverages, red and processed meats, and highly processed foods like packaged snacks and desserts.
  • Limit alcohol use to no more than one drink per day for people assigned female at birth and no more than two drinks per day for people assigned male at birth, or avoid it completely.

It’s also very important for adults to get regular screening for colorectal cancer. Screening can help prevent cancer or improve the odds of treatment success by locating precancerous polyps or early stage tumors.

The U.S. Preventive Services Task Force and the Centers for Disease Control and Prevention (CDC) recommend that adults ages 45 to 75 get screened for colorectal cancer. Screening is typically needed every 5 to 10 years, depending on the type of test used. Earlier screening may be needed for some people.

Some research suggests that dietary fiber may protect against colorectal cancer. However, more research is needed to better understand the extent of the protection that fiber provides and how much and what type of fiber is best.

Still, getting enough fiber — about 22 to 34 grams per day depending on age and sex — as part of a balanced diet is good for overall health and may help prevent colorectal and other cancers. In addition, regular colorectal cancer screening for adults ages 45 to 75 is very important for prevention and detection.

To meet fiber needs, try to incorporate more fruits, vegetables, legumes, nuts, seeds, and whole grains into your diet using some of the tips in this article.