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Phytochemicals are the non-nutrient substances found in plant food that are associated with certain health benefits.

Plants produce phytochemicals for their own protection (e.g., from pathogens, pests, environmental pollution) and for survival (e.g., to attract pollinators and seed dispersers). When humans consume plant foods, these phytochemicals are believed to provide protection for the human body as well.

Phytochemicals may act as antioxidants, affect hormone metabolism, stimulate enzyme activity and prevent potentially harmful DNA replication. These actions are associated with a variety of health benefits, including the potential to reduce the risk of some cancers.

There are thousands of phytochemicals, and many different ways to organize them. In common use, they are often referred to by their potential activity in the body (e.g., antioxidants, phytoestrogens).

Scientists typically categorize phytochemicals into classes based on similar chemical structures. Often, phytochemicals in the same class will have a similar effect on the body. Classes of phytochemicals include:

• Phenols. This includes flavonoids such as resveratrol, catechins, anthocyanins and isoflavones. Phenols also include phenolic acids and lignans.
  
  
• Terpenes. This includes carotenoids such as beta-carotene, lycopene and lutein. Terpenes also include monoterpenes and saponins.
  
  
• Thiols, also known as organosulfur compounds. This includes indoles and isothiocyanates.

Phytochemicals are found in a wide variety of plant foods, including most fruits and vegetables, as well as grains, legumes (peas and beans), nuts, seeds, and tea. Certain phytochemicals may also be available as supplements, although experts generally recommend people receive nutrients and other healthful substances from foods rather than supplements.