Phenolic extracts of apple and carrot juices and apple and carrot tissue, and selected pure compounds were examined for their ability to decrease the viability of human colon cancer cells (HT29 and LoVo) in vitro. In addition, apple and carrot juices were supplemented with phloretin and chlorogenic acid, respectively, to simulate genetically modified products with enhanced levels of these phytochemicals and screened as described above. Amongst the pure compounds tested, phloretin was shown to have the greatest effect on the viability of both cell lines. Here we report that phloretin has a marked effect on the survival of colon cancer cells at concentrations as low as 50 μmol/L, and therefore shows significant anticarcinogenic potential. With regard to the effect of the phenolic extracts of apple and carrot tissue, that from transgenic carrot decreased cell viability by as much as 20% at the highest concentration (200 µg/mL). Phenolic extracts of control carrot and control apple tissue had little effect. However similar extracts of apple juice supplemented with either 500 or 1000 μmol/L phloretin reduced the survival of LoVo cells to 10% and showed marked activity at 100 µmol/L. Apple juice alone did not reduce cell viability, probably because the concentration of phloretin was too low. This information supports the recommendation that consumers should be encouraged to eat more fruit and vegetables, especially those containing compounds such as phloretin. There is also potential for food producers and manufacturers to develop products with enhanced flavonoid content, either by conventional breeding, transgenically or by supplementation.