The following are commonly proposed mechanisms by which phytonutrients may protect human health. More research is needed to firmly establish the mechanisms of action of the various phytochemicals. Phytonutrients may: serve as antioxidants enhance immune response enhance cell-to-cell communication alter estrogen metabolism convert to vitamin A (beta-carotene is metabolized to vitamin A) cause cancer cells to die (apoptosis) repair DNA damage caused by smoking and other toxic exposures detoxify carcinogens through the activation of the cytocrome P450 and Phase II enzyme systems