The effect of any dietary compound is influenced by the active bioavailable dose rather than the dose ingested. Depending on the individual predisposition, including genetics and medication, a bioavailable dose may cause different magnitudes of effects in different people. Age might affect the predisposition and thus the requirements for nutrients including phytonutrients (e.g. phytochemicals such as flavonoids, phenolic acids and glucosinolates). These are not essential for growth and development but to maintain body functions and health throughout the adult and later phases of life; they are 'lifespan essentials'. Major mechanisms involved in chronic, age-related diseases include the oxidant/antioxidant balance, but the latest research indicates indirect effects of dietary bioactives in vivo and adaptive responses in addition to direct radical scavenging.