The remarkable international variation in cancer rates, for many of the common cancers, such as breast and prostate, seemed to many to support the traditional view that endogenous hormones could not be the sole or primary cause. The low rates of breast, prostate, and several other cancers in Asian populations and their increase toward Western rates upon migration to the United States or Europe suggested that chemical factors or other environmental agents were the major causes of these cancers. While cigarette smoking was one such obvious chemical carcinogen, which would help to explain the international variation in lung and other smoking-related cancer sites, the most obvious cause of the other cancers seemed to be diet, other lifestyle factors, or unidentified environmental agents. In 1964, an expert committee of the World Health Organization stated that "the categories of cancer that are thus influenced, directly or indirectly, by extrinsic factors . . . collectively account for more than three-quarters of human cancers."6
In their systematic review of cancer causation, Doll and Peto7 placed the majority of the unexplained excess of cancer observed in migrating populations on dietary factors acting directly or indirectly through their potential impact on lifestyle factors (e.g., reproduction, exercise). During the past 20 years, there has been a concerted effort by epidemiologists and experimentalists to verify the role of dietary factors in the etiology of cancer. Much of this effort has been directed toward proving the detrimental effects of dietary fat and the potential protective effect of a wide range of dietary antioxidants.8 Unfortunately, it now seems likely that dietary factors are directly related to only a relatively small number of cancers, primarily, and not surprisingly, those of the digestive tract (esophagus, stomach, and large bowel). At the same time, it appears increasingly likely that the majority, if not all, of the hormone-related cancers have little direct relationship to any particular dietary item.
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