Phytochemicals ("fight-o-chemicals") are substances that are naturally occurring only in plants. Some of them may provide health benefits beyond those provided by essential nutrients (vitamins and minerals). Eating a variety of colorful phytochemical-rich fruits and vegetables has been associated with a lower risk of some chronic diseases such as cancer and heart disease. Phytochemicals may act as antioxidants, protect and regenerate essential nutrients, and/or work to deactivate cancer-causing substances.

It is thought that phytochemicals, vitamins, minerals, and fiber, all present in fruits and vegetables, work synergistically in whole foods to promote health and lower disease risk. For this reason, many authoritative organizations, such as the National Cancer Institute and The American Heart Association, recommend getting phytochemicals from whole foods, such as fruits and vegetables, rather than from supplements.