These results, describing antitumor activity of some of the phytochemicals that have been actively studied, suggest that dietary changes could play a role in decreasing the incidence of a variety of tumors. 13C and the other compounds discussed may well be only prototypes for other as yet unexplored phytochemicals present in the diet. There have been no attempts to explore the possibilities of synergistic action among the various phytochemicals, 13C, limonene, curcumin, epigallocatechin gallate, sulforaphene, or genistein. Mixtures of these compounds might well show potency at lower doses for each of the compounds and show even greater promise than that already demonstrated.