© 2020 Elsevier Ltd Phytochemicals in fruits and vegetables produce health benefits, but questions remain regarding their bioavailability, molecular targets, and mechanism of action. Here, we address these issues by considering the prebiotic and biological properties of phytochemicals. A fraction of phytochemicals consumed orally passes through the gut lumen, where it modulates the composition of the gut microbiota and maintains intestinal integrity. Phytochemicals and microbiota-derived metabolites that are absorbed by the organism comprise compounds that, at low doses, induce stress resistance mechanisms, including autophagy, DNA repair, and expression of detoxifying and antioxidant enzymes. We propose that these mechanisms improve cellular and organ function and can account for the promiscuous bioactivities of phytochemicals, despite their limited bioavailability and extremely varied chemical structures.