Our diet contains a great variety of natural substances that cause genetic changes (mutagens) as well as substances capable of causing cancer (carcinogens) in living tissue. Many of these mutagens and carcinogens may act through the generation of free oxygen radicals, which play a major role in the initiation of degenerative processes, such as DNA damage and promotion that may be related to cancer, heart disease, and premature aging
Fortunately, our food contains many antimutagens and anticarcinogens that reduce the rate DNA changes, thus lowering our cancer risks. But what foods have these protective ingredients? Unfortunately, these beneficial substances are not found in our Standard American Diet (the only nice thing about it is the SAD acronym). The substances that I am referring to are found only in the organic plant foods. Yes, organic because the plants develop those ingredients (antioxidants) in response to pests. If the plants get sprayed with pesticides, herbicides, or fungicides which protect them form pests, plants have no incentive to develop these protective ingredients. Nature always trys to conserve energy.
Dietary intake of natural antioxidants becomes an important aspect of our body's defense mechanism against these DNA damaging agents. Many antioxidants are being identified as anticarcinogens. Optimizing such defense systems is an important part of a strategy of minimizing cancer and other age-related diseases. (http://www.ncbi.nlm.nih.gov/pubmed/6351251)