Some polyphenols such as lignans, flavonoids, and flavonolignans, phytosterols, and non-digestible oligo- and polysaccharides were shown to have in vitro and in vivo chemopreventive and other positive physiological effects on the epithelium and metabolic processes in the gastrointestinal tract (GIT) of humans. Most of these substances are metabolized in the colon; some of them stimulate bifidogenic bacteria and Lactobacilli. It was proved that sufficient daily intake of these phytochemicals shows beneficial effects on cancerogenesis in stomach and colon. Functional foods and dietary supplements that contain these substances may play an important role at present when the incidence of cancer diseases of GIT increases.