The multiple-step model of carcinogenesis discussed here identifies the two major stages of initiation and promotion. A more recent research development proposes that oncogenes present in chromosomes are activated by viral, chemical, or physical agents and cause cancer. A great variety of natural mutagens and carcinogens find their way into the modern US diet. Excessive fat and alcohol consumption have been studied in relation to many kinds of malignancies. Dietary anticarcinogens include vitamins A, C, and E, although under certain conditions some generally inhibitive substances can actually enhance carcinogenesis. A provocative hypothesis argues that a high-fiber diet can substantially reduce the likelihood of carcinoma of the colon.
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