All families of plant food are known to contain phytonutrients, that is, unique substances produced during the natural course of plant growth and development that are specific to each plant's genes and environment. The term phytonutrition refers to the role of these substances in cultural food practices and cuisines worldwide in supporting health. In addition to their phytonutritive role, phytonutrients have a phytotherapeutic role, acting as modifiers of physiological function. The consumption of 30 to 50 mg per day of soy isoflavones in the traditional Japanese diet and the ability of this diet to help lower the incidence of breast cancer among Japanese women is an example of phytotherapeutic effect. The dividing line between phytonutrition and phytotherapy has blurred with the discovery that certain food components can not only modify physiological function but also aid in medical practices such as drug delivery. Naringenin, for example, the 4',5',7-hydroxyflavanone found exclusively in grapefruit, can slow hepatic detoxification of prescription drugs like cyclosporine and thus potentially help prevent rejection of transplanted organs. Natural and synthetic phytonutrients may differ significantly in their effect on physiological function owing to stereoisomeric composition. The potential of specific phytochemicals to promote health must be carefully evaluated with the same risk-benefit model traditionally used to assess concentrated levels of any substance placed into the body.