During and after cancer treatment, there is a probable benefit of taking a standard multiple vitamin and mineral supplement that contains about 100% of the Daily Values because during these times it may be hard to eat a diet with enough of these nutrients. The use of very large doses of vitamins, minerals, and other supplements is not recommended because evidence exists that some high-dose supplements may increase cancer risk.
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 is likely that these compounds work together to create these helpful effects. There are likely to be important, but as of yet unknown, compounds in whole foods that are not in supplements. Food is the best source of vitamins and minerals.