Studies suggest that people who eat more vegetables and fruits, which are rich sources of antioxidants, may have a lower risk for some types of cancer. Because cancer survivors may be at increased risk for second cancers, they should eat a variety of antioxidant-rich foods each day. (Second cancers are new, different cancers, not the same cancer coming back.)
So far, studies of antioxidant vitamin or mineral supplements have not found that they reduce cancer risk. The best advice is to get antioxidants through foods rather than supplements. Many dietary supplements contain levels of antioxidants (such as vitamins C and E) that are much higher than the recommended Dietary Reference Intakes for optimal health.
Many cancer doctors advise against taking high doses of antioxidant supplements during chemotherapy or radiation. There is a concern that the antioxidants might repair the damage to cancer cells that these cancer treatments cause, making the treatments less effective. But others have noted that the possible harm from antioxidants is only in theory. They believe that there may be net benefit in helping to protect normal cells from damage caused by these cancer treatments.
Whether antioxidants or other supplements are helpful or harmful during chemotherapy or radiation treatment is a major question without a clear science- based answer right now. Until more evidence is available, it’s best for cancer survivors getting these treatments to avoid dietary supplements except to treat a known deficiency of a certain nutrient, and to avoid supplements that give more than 100% of the Daily Value for antioxidants.
Although it’s not clear that total fat intake affects cancer outcomes, diets very high in fat tend to be high in calories, too. This can lead to obesity (being very overweight), which is linked to a higher risk of many types of cancer, a higher risk of certain cancers coming back after treatment, and worse survival for many types of cancer. Good sources of fiber are beans, vegetables, whole grains, nuts, and fruits. Eating these foods is recommended because they contain other nutrients that may help reduce cancer risk. They also have other health benefits, such as reduced risk of heart disease. At this time we don’t know if fiber intake can affect cancer risk or survival.
The American Cancer Society guidelines on nutrition and physical activity for cancer prevention recommend limiting your intake of processed and red meats and discourage cooking these and other higher fat sources of protein at high temperatures. Cancer survivors may also want to follow this recommendation for general good health.
There are no studies in humans to show whether organic foods are better than other foods in terms of reducing the risk of cancer, the risk of cancer coming back, or the risk of cancer progression. Phytochemicals are a wide range of compounds made by plants. Some have either antioxidant or hormone-like actions. Only a few studies have looked at the effects that phytochemicals (or the plants that contain them) may have on cancer coming back or getting worse (progressing).
There is strong evidence that a diet rich in vegetables, fruits, and other plant-based foods may reduce the risk of some types of cancer. And some recent studies suggest there may be a helpful effect on recurrence or survival for breast, prostate, and ovarian cancers. But there is no evidence at this time that supplements can provide these benefits. Many healthful compounds are found in vegetables and fruits, and it’s likely that these compounds work together to create these helpful effects. Food is the best source of vitamins and minerals.