Why a healthy diet is crucial for reducing risk of colorectal cancer and improving results after diagnosis

UCLA Health experts stress the importance of vitamin D, fiber and other dietary needs.
Pictured is a bowl of broccoli, a cruciferous vegetable that helps boost fiber intake.
Cruciferous vegetables such as broccoli help boost the intake of fiber. Fiber helps reduce constipation, a potential risk factor for colorectal cancer and other diseases. (Photo by Joshua Sudock/UCLA Health)

Although rates overall have been declining thanks to increases in awareness and screening, colorectal cancer remains the fourth-most-common cause of cancer among U.S. adults. Paying attention to lifestyle and diet can help reduce risk and is critical for those who have been diagnosed with colorectal cancer.

“Patients are obviously interested in this when they’ve had their first screening colonoscopy and they’re finding out they have polyps and want to reduce their risk for colon cancer down the road,” said Carl Nordstrom, MD, a gastroenterologist at UCLA Health and associate director of the UCLA GI Fellowship Program. “Or, they’re paying attention to the news and seeing that there’s an increased incidence of colon cancer in younger individuals.” 

Colorectal cancer screening — which for most people should start at age 45 — saves lives by catching cancer early.

Learn more about colorectal cancer screening at UCLA Health

Preventive lifestyle and diet measures

In addition to encouraging regular screening, especially for those who have a family history of colorectal cancer or have a history of polyps, Dr. Nordstrom recommends that people focus on ongoing lifestyle factors, including diet, to help reduce their risk. 

These include:

Paying attention to excess weight and physical activity: Rates of obesity have been going up, Dr. Nordstrom said, especially between early adulthood and midlife. Given the link between excess weight and a range of health risks, including colorectal cancers and several other types of cancers, he recommends that patients make healthy food choices and increase their physical activity to help guard against becoming overweight or obese. Excess weight also increases the risk for diabetes, he noted, which has also been linked to an increased likelihood of developing colorectal cancers.

Curtailing consumption of red meat and processed meat: Both red meat and processed meat have been linked to an increased risk for colorectal cancer. “For the average individual, I don’t say they should never enjoy a steak or hamburger, but it’s not something they should have more than a few times a month,” Dr. Nordstrom said. He also cautioned that high-temperature cooking methods, including blackening or charring the meat, can have carcinogenic effects, and the risk from consuming processed meats, such as bacon or hot dogs, is even higher.  

Increasing fiber intake: Choosing whole grain foods over processed foods and eating more fruits and vegetables – especially cruciferous vegetables, such as broccoli, cauliflower and kale – can help boost overall fiber intake, Dr. Nordstrom noted. Another option to boost fiber intake is taking a daily psyllium husk supplement, he said. 

Fiber helps guard against constipation, which is a risk factor that’s starting to be looked at both for colorectal cancer and for other diseases. “Not having things sit around in the colon is probably a good idea,” he said, with the goal being to eliminate stool daily or every other day. Increased fiber intake also reduces the likelihood of developing adenomatous polyps, which are benign growths that can become cancerous over time, he said.

Obtaining enough vitamin D: “Even in places like Los Angeles where you get a lot of sunshine, there’s still a very high incidence of vitamin D deficiency,” Dr. Nordstrom said. Given that higher levels of vitamin D can inhibit colorectal cancer progression and development of other cancers as well, he recommends that patients be checked for vitamin D deficiency so they can supplement as needed.

Limiting alcohol: Drinking more than two to three alcoholic beverages per day is linked to a significant increase in colorectal cancer, Dr. Nordstrom said. However, having up to one drink a day is far less problematic. “We tend not to be as austere and tell people they have to avoid alcohol completely,” he said.

Avoiding nicotine products: The same can’t be said for nicotine, however. Dr. Nordstrom advises patients to avoid cigarettes and other tobacco products such as vaping completely. “With tobacco, it isn’t even just the nicotine – there are so many other carcinogens that are in there,” he said. In addition, cigarette smoke contains free radicals, which are also considered carcinogenic and are associated with an increased risk of more advanced adenomas, or polyps, in the colon.

“Smoking is a risk factor for more advanced types of adenomas and really should be avoided completely,” Dr. Nordstrom said. “There’s no safe amount.”

Maintaining an approach that’s healthy overall: “Think carefully in terms of patterns over the weeks and months and not just on a day-to-day basis,” Dr. Nordstrom said. “It’s finding the things you can do over the course of a week, like exercising at least three to five days out of the week, and making sure that you’re getting enough fiber. These are important steps that anybody should be able to take.”

Focusing on nutrition remains essential after a colorectal cancer diagnosis

For patients who are diagnosed with colorectal cancer, the same principles still apply but are modified and personalized based on the patient’s situation.

“Once you have cancer, a lot will depend on what stage you have and what the treatment course is going to look like as well as what symptoms you have,” said Nancee Jaffe, RDN, lead dietitian for the UCLA Vatche and Tamar Manoukian Division of Digestive Diseases

In addition to Jaffe’s team of registered dietitians, patients who have colorectal cancer may also work with dietitians at UCLA Health’s radiation centers while they’re undergoing radiation, then return to Jaffe’s team as they proceed to the next stage of their treatment. 

“We’ve seen that when patients have a dietitian as part of their care team, they tend to do better post-surgery and they tend to do better during their adjunct treatments,” Jaffe said. “The dietitian is going to look for things like malnutrition or inadequate intake and can make a huge difference in terms of these patients having good outcomes.”

Pre-surgery, some patients may struggle to get their calorie and protein needs met when they’re undergoing treatment due to nausea, loss of appetite or other factors; in those scenarios, the dietitian working with the patient may recommend supplements such as protein shakes.

Additionally, when patients are going through chemotherapy or radiation, supplements that contain antioxidants may need to be temporarily discontinued because of their potential to interfere with treatment. This may even include supplements containing vitamin C or vitamin E, said Jaffe, who stressed that it’s best for the patient to consult with their care team to determine the best course of action.

Prior to surgery, patients may also be experiencing issues such as constipation if there’s an obstructing tumor in the colon or rectum, Jaffe said. In this scenario, the goal is to help the stool pass through more easily, which might entail boosting fiber intake, but could also mean adding foods or supplements that act as laxatives. 

“Instead of talking about high fiber or low fiber, what we usually talk about is fiber modification,” Jaffe explained. For example, if whole blueberries are causing gastrointestinal issues, the recommendation might be to puree them in a smoothie or bake them in a muffin.

Once a patient undergoes surgery, there may be other issues based on both the healing process and the specific parts of the colon or rectum that were removed. Particularly when large portions of the colon have been removed, ensuring the person is obtaining adequate nutrition – including that it’s being absorbed properly – can be a concern.

If more than half of the colon has been removed, dehydration or issues with bile acid absorption may need to be addressed. A patient who’s lost part of their colon or rectum may notice looser or more frequent bowel movements or may notice a change in color, Jaffe said.

“If a patient is having five or more bowel movements a day or if they’re very loose or watery, that means they’re losing fluid,” she said. “Not only would we want to try to keep the fluid in their body, we may want to have them try oral rehydration solutions or electrolyte drinks.”

For someone who’s had the rectum removed (the chamber where stool is held before it exits the body), Jaffe focuses on dietary changes to help ensure that the stool is formed in a way that’s easier for the person’s changed anatomy to handle. This might mean changing the timing or frequency of meals, adding a fiber supplement, or changing the types of fiber the person consumes, she said.

“After you’ve gone through cancer treatment, the diet is hopefully the same,” Jaffe said. “It’s what we call a ‘plant-forward’ Mediterranean-style diet.” That said, her team may recommend modifications based on the person’s specific situation.

“Every person is unique in how their anatomy settles,” she added, noting that it may take “six months to two years for full settling.”

“Every patient deserves to be looked at as an individual,” said Jaffe, “which is why having their whole care team – their gastroenterologist, their surgeon, their dietitian – communicate with each other and with the patient is paramount.”

Take the Next Step

Learn more about colorectal cancer screening at UCLA Health.