A complex plant polyphenolic preparation (PP) was produced from chokeberry, raspberry, wild strawberry, peach, bilberry, apricot, cranberry, and parsley, using ultrafiltration and C18 preparative chromatography. Thirty main compounds were identified in PP (LC-MS), with the highest contribution of cyanidin 3-O-glucoside, p-coumaroyl glucoside, chlorogenic acid, neochlorogenic acid, and isoquercetin. PP was used (at 0.16% m/m) for the production of a sourdough bread (based on rye flour, water, and salt), followed by in vitro digestion. Fluid obtained after PP-enriched bread digestion (EBD fluid) was tested in terms of cytotoxicity, growth inhibition, antioxidant activity, and morphological changes in cancerous intestinal epithelial cells (HT-29) and normal (CCD 841 CoTr). Results show that EBD fluid concentration over 125 μg/mL significantly decreased activity of succinate dehydrogenase in HT-29 cells and reduced their viability of 25%. At this concentration of EBD fluid, modification in cellular morphology was also observed. DPPH analysis revealed that the highest antioxidant activity was observed at concentration of 75 μg/mL, both PP and EBD fluid. Our results show that an introduction of PP into relatively low-polyphenolic, baking products should be carefully considered because polyphenols still retain its biological activity. Antioxidant activity of polyphenols is one of the mechanisms that explains the observed effect of inhibiting the growth of colon cancer cells.