Phytochemicals can be defined, in the strictest sense, as chemicals produced by plants. However, the term is generally used to describe chemicals from plants that may affect health, but are not essential nutrients. While there is ample evidence to support the health benefits of diets rich in fruits, vegetables, legumes, whole grains and nuts, evidence that these effects are due to specific nutrients or phytochemicals is limited. Because plant-based foods are complex mixtures of bioactive compounds, information on the potential health effects of individual phytochemicals is linked to information on the health effects of foods that contain those phytochemicals.

Phytochemicals are sometimes referred to as phytonutrients; these terms are often used interchangeably. In broad terms, they are said to be any chemical or nutrient derived from a plant source. However, in common usage, they have a more limited definition. They are usually used to refer to compounds found in plants that are not required for normal functioning of the body but that nonetheless have a beneficial effect on health or an active role in the amelioration of disease. Thus, they differ from what are traditionally termed nutrients in that they are not a necessity for normal metabolism, and their absence will not result in a deficiency disease.

Phytonutrients have many and various functions in the body. For example, they may promote the function of the immune system, act directly against bacteria and viruses, reduce inflammation, and be associated with the treatment and/or prevention of cancer, cardiovascular disease and any other malady affecting the health or well-being of an individual.

Many phytochemicals are polyphenol antioxidants that impart bright colors to fruits and vegetables. Lutein makes corn yellow, lycopene makes tomatoes red, carotene makes carrots orange and anthocyanin makes blueberries blue, for example. Both the bright colors and the antioxidant activities are due to alternating single-bonded and double-bonded carbons. There is abundant evidence from epidemiological studies that the phytochemicals in fruits and vegetables can significantly reduce the risk of cancer, probably due to polyphenol antioxidant and anti-inflammatory effects.

Phytochemicals naturally occur in vegetables and fruit.