Vegetarians differ from non-vegetarians in many ways besides diet. Differences between the two groups may account for observed differences in cancer mortality or incidence, rather than differences in diet itself. Several aspects of lifestyle must be considered when evaluating the health and mortality experience of vegetarian and non-vegetarian populations.
Smoking: The reduction in cancer mortality (and incidence) in vegetarians appears to be stronger in men than women and the possibility that differences in alcohol and tobacco consumption in vegetarians vs. non-vegetarians must be considered as an explanation for this finding. The use of tobacco, generally lower in vegetarian populations,12 is clearly related to cancer risk. A recent review indicated that 38% of cancer deaths among males in the U.S. could be attributed to cigarette smoking, while among women, 23% of all cancer deaths are due to cigarettes.63 These estimates do not include the impact of cigar, pipe, or smokeless tobacco, nor do they include the influence of environmental tobacco smoke. Cancer sites that have been associated with cigarette smoking include lung, oral cavity, esophagus, larynx, bladder, pancreas, kidney, and cervix. Recent data have suggested that colon cancer may be associated with cigarette smoking (but only after a long latency period).64-65 The more relevant question today may be which cancer sites are not associated with tobacco; the list appears to be quite short and currently includes prostate, endometrium, and breast cancer.
Alcoholic beverage consumption is also associated with increase in cancer risk for several types of cancer. Cancer of the oral cavity, pharynx, larynx, esophagus, and liver are associated with excess alcohol consumption and, in some instances, alcohol may act synergistically with tobacco in cancer initiation.66
Many of the epidemiologic studies reviewed, however, adjusted for smoking history or compared risk of cancer death only to a non-smoking reference population.20 In general, those studies in which adjustments such as these were made found the reduced risk of cancer death in non-meat eaters to persist, suggesting that the reduction in cancer risk that vegetarians experience does not seem to be due to differences in tobacco use.
Screening Bias: For some forms of cancer, screening is effective in early detection. Specifically, some of the female cancers such as cervix and breast, as well as prostate cancer in men may be detected more frequently in populations that have a greater than normal degree of access to screening programs. For example, in the Adventist Health Study in which cancer incidence was monitored among SDA between 1976 and 1982, elevated incidence of skin cancer, prostate, and cervix cancers was observed.28 These findings may not be a reflection of an actual increased biologic risk of these forms of cancer in SDAs, but an artifact of more intense screening for cancers among SDAs than in the general population. There is a strong focus on healthy lifestyles among SDAs, as well as greater access to health services among SDAs than the general public. The same phenomenon may act in the opposite direction in regard to mortality, since screening most likely is associated with early detection and better survival for many forms of cancer, which would impact mortality rates in populations with different levels of cancer screening. Specifically, if vegetarians were to enjoy greater access to screening programs than non-vegetarians, the numbers of observed cancer deaths would be lower than otherwise, which would tend to produce lower SMRs for vegetarians.
Selection: Another phenomenon that must be considered when evaluating cancer risk in vegetarian societies or religious organizations that promote a vegetarian lifestyle is the possibility of selection bias. Only a very small percentage of the general population choose to join religious groups that advocate lifestyle changes, yet a large proportion (about 50%) of the SDA church, for example, is composed of adult converts. It is possible that people who join these organizations as adults already experience lower cancer risk due to educational or socioeconomic status and lifestyle practices, and it is these conditions that explain their lowered cancer risk rather than membership in a vegetarian organization.21
Religiosity: Those people who are observant of a religious faith (regardless of which particular sect or denomination) enjoy lower incidence and mortality than the general population.67 Regular attendance at church services has been associated with lowered mortality from several chronic diseases.68 The possibility that some aspect of spiritual life or some other lifestyle highly correlated with spirituality explains the lower cancer risk in religious denominations that espouse vegetarianism must be considered.
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Among the evils which a vitiated appetite has fastened upon mankind, those that arise from the use of Tobacco hold a prominent place, and call loudly for reform. We pity the poor Chinese, who stupifies body and mind with opium, and the wretched Hindoo, who is under a similar slavery to his favorite plant, the Betel but we present the humiliating spectacle of an enlightened and christian nation, wasting annually more than twenty-five millions of dollars, and destroying the health and the lives of thousands, by a practice not at all less degrading than that of the Chinese or Hindoo.