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Background--The presumed protective effect of coffee consumption on colorectal cancer, which is supported by case control studies, has not been confirmed in prospective cohort studies. Cohort studies are few in number however and often suffer from a small number of cases, limited attention to confounding variables, and a low percentage of heavy coffee drinkers.
Methods--We examined data from a large population based cohort of Swedish women who were free from cancer at the start of follow up, with a wide range of coffee consumption, information on many potentially confounding variables, and a larger number of cases than any previous cohort study of coffee consumption and colorectal cancer.
Results--During an average of 9.6 years of follow up of 61 463 women aged 40-74 years, we observed 460 incident cases of colorectal cancer (291 with colon cancer, 159 with rectal cancer, 10 with cancer at both sites). We found no association between coffee consumption and colorectal cancer risk. The risk ratio for drinking four or more cups per day compared with none was 1.04 (95% confidence interval 0.63-1.69; p for trend 0.84). The findings were similar for cancers of the distal and proximal colon and rectum.
Conclusions--The recently published affirmative conclusions regarding the protective effect of coffee consumption may be premature. For patients seeking advice about coffee consumption, the evidence suggests that moderate or even high consumption will probably not influence the risk of colorectal cancer.
Keywords: colorectal neoplasms; cohort studies; coffee drinking
Evidence from the majority of studies of the relation between coffee consumption and the risk of colorectal cancer has led some investigators to conclude that the data convincingly support a protective effect.  As coffee is a common exposure, and colorectal cancer is among the most common diagnosed cancers in developed countries,  this issue is of public health importance. In a recent meta-analysis, case control studies generally showed consistent inverse associations while cohort studies did not support a protective effect. Cohort studies that address this association are few however and often suffer from a small number of cases and limited attention to potentially confounding variables.  Moreover, pooling of results by geographic region showed stronger inverse associations among Northern European studies compared with studies conducted in Southern Europe and in the USA.  This geographic difference is consistent with the higher range of coffee consumption in Scandinavia  and suggests the po ssibility that the range of exposure to coffee may be too narrow in some of the cohort studies to detect a true protective effect.
We examined data from a large population based prospective cohort of women in Sweden with a wide range of coffee consumption, information on many potentially confounding variables, and a larger number of cases than any previous study of coffee consumption and colorectal cancer.
THE SWEDISH MAMMOGRAPHY SCREENING COHORT
From 1987 to 1990, a population based mammography screening programme was introduced in two counties in central Sweden. In Vastmanland county all women born between 1917 and 1948 received a mailed invitation to be screened by mammography between March 1987 and March 1989 (n=41 786) together with a six page questionnaire; 31 735 women (76%) returned completed questionnaires. In Uppsala county all women born between 1914 and 1948 were invited to the screening and received the same questionnaire between January 1988 and December 1990 (n=48 517); 34 916 women (72%) …