Colon, Rectal and Anal Cancer: Frequently Asked Nutrition Questions

Author: Katrina VB. Claghorn, M.S., R.D., L.D.N.
Updated by Carly Roop, RD, CSO, MA
Last Reviewed: February 06, 2023

Should I follow a special diet during chemotherapy or radiation treatment?

During cancer treatment, it is best to follow a healthy diet that will nourish your body. By eating well you will get the energy and strength you need. Make sure to include these foods and nutrients:

  • Protein from lean protein foods such as fish, chicken, low-fat dairy foods, and dried beans
  • Choose from the "healthier" fats such as monounsaturated fats (found in olive, canola, and peanut oils) and omega 3 fatty acids (found in fish, seeds, and nuts).
  • Eat whole-grain foods such as oatmeal, brown rice, and whole wheat bread
  • Eat lots of fruits and vegetables
  • Drink plenty of water and/or other decaffeinated fluids

The side effects of treatment may make it hard to follow a "perfect" diet. When you lose your appetite, it is important to eat the foods that work for you. For more information read our Nutrition During Treatment Overview.

What about weight loss during treatment?

Losing weight during treatment is not encouraged. If you lose weight during treatment, it means you are not getting enough calories. If you do not get enough calories, you are also not getting enough protein, vitamins, and minerals. This will affect your strength and ability to tolerate treatment. It may also affect your response to treatment and slow your recovery. Cancer treatment can often increase your nutritional needs. During treatment, it is important to watch your weight and nutrition intake.

Should I choose a high fiber diet during treatment?

Chemotherapy and radiation therapy can cause diarrhea. If you experience diarrhea, you should follow a low fiber diet. Although a high fiber diet may reduce the risk of developing cancer of the lower gastrointestinal tract, it is not recommended during treatment. Since low fiber diets do not stimulate the bowel to move, they will not cause diarrhea. Also, low fiber diets are easier to digest. To learn more, read the fact sheet on Low Fiber Diet for Diarrhea.

If you do have diarrhea, avoid:

  • Raw fruits and vegetables.
  • Whole-grain breads and cereals.
  • Dried beans and legumes.
  • Corn.
  • Seeds.
  • Nuts - however, smooth peanut butter or smooth nut butters are OK.

Is it safe to take vitamins or herbal supplements during chemotherapy or radiation treatment?

It is recommended that you not take extra antioxidant vitamin supplements during treatment. High levels of antioxidants may interact with cancer treatment. It is okay for you to take a standard multivitamin that provides 100% of the RDI (recommended daily intake) for vitamins and minerals. Herbal supplements should also be avoided, since they may contain antioxidants and other compounds that may interact with cancer treatments. The research on vitamins and herbals during treatment is not yet conclusive. There are several studies showing that vitamin supplements such as Vitamins C, E, and A can interfere with the effectiveness of cancer treatment. The theory is that since antioxidants protect the body's cells from damage, taking excess amounts may actually protect the cancerous cells from the chemotherapy or radiation treatment. This will decrease the effectiveness of the treatment. On the other hand, there have also been studies showing benefit from combining antioxidants and chemotherapy. Because there is not enough research in this area, we recommend that you avoid extra supplements.

Please discuss any supplements you are taking or considering taking with your oncologist.

Is it true that sugar "feeds" cancer, and so I should avoid all sugar in my diet?

Recent studies suggest that a diet high in simple sugar (such as white sugar, sodas, and sweets) may increase the risk of developing cancer. While this association has been noted, we are still not sure of the cause. It may be that a diet high in simple sugar has less cancer fighting nutrients. It may be that simple sugars cause a surge in insulin, which increases the growth of cancer cells. Despite the lack of research, a number of books and websites prematurely advise people with cancer to eliminate sugar as part of a cancer prevention diet. However, it would be unhealthy to avoid all food sources of simple sugars, since this would include fruits, vegetables, and grains. These foods are the primary source of nutrients that have been shown to reduce cancer risk. The way to prevent surges in insulin is to reduce your intake of foods that are very high in simple sugar. This is done by avoiding foods such as sweets and sodas. Also, by eating foods with simple sugars with foods that have fiber, protein, and fat this will slow the absorption of sugar and avoid a surge in insulin. Based on research, the American Cancer Society makes the following recommendations to reduce cancer risk: Eat 5 or more servings of a variety of vegetables and fruits each day. Eat other foods from plant sources several times each day (such as whole grains, beans, nuts). Limit the amount of simple sugar (including pastries, sweetened cereals, soft drinks, and sugar) you eat. Limit alcohol consumption.

Do I need to buy all organic fruits and vegetables to protect myself against pesticides?

Overall, the research shows that the benefits of eating produce outweigh the risk of pesticide exposure. Therefore, there is no need to spend extra money on organic produce. If you choose organic anyway, look for produce that carries the "Certified Organic" label. Also, remember to wash all produce with water. Washing will remove most of the remaining pesticides. Special fruit or vegetable rinses are not necessary. A good washing with water alone is just as effective. Many studies have been done looking at pesticide exposure from commercially grown fruits and vegetables. These studies show very little proof of cancer risk from pesticides and other chemical residues in foods. Frightening headlines or websites stating that pesticides cause cancer are based solely on animal studies using doses of pesticides high above those normally found in foods. Additionally, the United States regulates the use of pesticides to keep amounts at safe levels.

What are the recommendations for a cancer-protective diet after cancer treatment?

Research has shown that there are diet and lifestyle factors that, in general, seem to be protective against cancer. However, there is little research on the effect of diet in preventing the recurrence of colorectal cancers. Recommendations for diet and nutrition after treatment for colon, rectal, and anal cancers are based on what has been shown to prevent cancer from developing the first time.

These recommendations include:

  • Eat a high fiber diet. Choose whole grains, dried beans, fruits, and vegetables. A minimum of 25 gm of dietary fiber a day is recommended.
  • Eat plenty of fruits and vegetables. A minimum of 5 servings a day is best.
  • Reduce saturated fats which are mostly from animal foods. Choose low-fat dairy products. Limit red meat and processed meats.
  • Include food sources of calcium.
  • Eat dairy foods, calcium-fortified juices, broccoli, canned salmon, and sardines (with bones). Aim for about 1000 mg of calcium daily. Take a calcium supplement if needed.
  • Eat foods high in folate (folic acid) such as orange juice, dried beans, spinach, broccoli, and fortified cereals and breads. A daily multivitamin will provide 100% of the Daily Value.
  • Limit alcohol.
  • Maintain a healthy weight.
  • Exercise!


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