Despite the fact that no causality can be found proving that any single food is capable of causing cancer, there is a great deal of evidence that certain types of food protect against cancer. As a generic food group fruit and vegetables should be eaten at least five times a day. Whilst many American and European doctors recommend five, they do so because they feel that five is a manageable number, yet eight is more realistic and some doctors even recommend eight vegetables and three fruits.
http://healthyfoods.fivestarwayofliving.com/2009/09/the-link-between-fruit-and-vegetables-and-cancer/