In a prospective study of nearly 90,000 women, Willett et al. found an association between intake of red meat and colon cancer (see Chapter 2, Food Safety 1991, reference 27). Numerous readers have commented on this study (46-53). It was noted that the authors failed to distinguish between cancers of the proximal and distal colon, which are thought to have different pathogenetic mechanisms and whose relative incidence in women varies with age (46). Nor was the possibility of hereditary nonpolyposis colorectal cancer considered in the data analysis. In lumping beef, pork, lamb, and processed meat products such as bacon and sausage into the category “red meat” the authors failed to account for the huge range of fat content and possible role of other constituents, especially in processed meats (47). They also did not discuss other aspects of “healthy” and “unhealthy” lifestyles that tend to be associated with healthy and unhealthy diets (48). A practitioner of family and sports medicine was concerned that active women and girls already at risk for iron deficiency might turn away from the best source of iron available to them, when the only significant risk was associated with consumption of more than a quarter of a pound of red meat daily (49). Estimates of dietary fiber used in this study may have been less than ideal and could have had a major effect on conclusions from the study (50). The possibility of a detection bias was suggested (51). High meat consumption would cause more positive tests for occult blood in the stool, and investigation of these could yield some incidental diagnoses of colon cancer. Willett et al. explained why this was unlikely in their study (54). In response to another comment (52), they explained why they did not analyze total mortality in their relatively young population. Finally, a reader looked at the 2:1 risk ration for colon cancer in meat eaters compared with fish eaters from a different perspective (53). Meat eaters had an incidence of colon cancer of 0.113 %, whereas the incidence in fish eaters was 0.059% difference in these rates is seriously worth worrying about. Willett et al. argued that it is (54), considering the young age of the study population and the short duration of follow-up.
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prostate cancer
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A review by Abraham et al. addressed major topics in the epidemiology of prostate cancer (114). Blacks in the USA have the highest mortality rates in the world for this cancer. They are followed by whites in Norway, Switzerland, and Sweden. The lowest mortality rates occur among Asians in Singapore, Japan, and Hong Kong. However, the incidence of prostate cancer has been rising in Japan, where major dietary changes have occurred in the last 30 y. Further, the incidence of the disease is positively correlated with that of other diet-related cancers and is much higher in Japanese immigrants to Hawaii. Studies on the relationships between prostate cancer and intake of fats and vitamin A and carotenoids are summarized in text and tables. Although evidence for an adverse effect is supported by limited animal data. For effects of β-carotene, other carotenoids, or fruits and vegetables, the data are equivocal. A protective effect of carotenoids is seen principally in the context of a low – fat diet; it is also possible the some fruits contain risk-enhancing factors. Reports of a link between cadmium and cancer risk pertain mostly to occupational exposure. In sufficient data are available on other dietary components.
Rates of prostate cancer are very low in third World countries in populations that maintain a traditional lifestyle, but they rise in urban populations where lifestyles are in transition (115). Information on risk factors for these populations is meager; neither smoking nor alcohol consumption seems to be involved. A case-control study among blacks in Soweto, South Africa, found that high consumption of fat, meat, or eggs increased risk, were as high consumption of fruits or vegetables was protective. Specific foods that afforded protection were carrots, cabbage, and spinach; dietary fiber was also protective. Employment in an occupation that permitted ready access to a Western diet increased risk. No associations were seen for anthropometry, education, social class, smoking, and drinking.
A case-control study in northern Italy identified high milk consumption as a significant risk factor for prostate cancer (116). This agrees with most previous reports. Surprisingly, however, no consistent associations were found for cheese or butter intake, other sources of animal fat, of total fat.