Dietary supplement use and colorectal tumors : from prevention to diagnosis

Background: Expert guidelines formulated by the World Cancer Research Fund and the American Institute for Cancer Research (WCRF/AICR) advised no use of dietary supplements for cancer prevention. However, it is unclear whether those recommendations also apply to populations at high risk for incident or recurrent colorectal tumors specifically, since dietary supplement use is ubiquitous in western countries where colorectal tumors are common. Furthermore, before the association between dietary supplement use and recurrence and survival in colorectal cancer patients can be examined, more information about the consistency of dietary supplement use is needed, as it is plausible that use varies over time after colorectal cancer diagnosis. This thesis focuses on the association between dietary supplement use and colorectal tumor risk and recurrence in the general population and in high-risk populations, and describes the consistency of use in patients who were diagnosed with colorectal cancer. Methods and results: First, we conducted a systematic literature review with meta-analyses of observational studies about the association between dietary supplement use and colorectal cancer risk. Our findings suggested inverse associations between multivitamins (use versus no use: RR=0.92; 95% CI 0.87–0.97, calcium supplements (use versus no use: RR=0.86; 95% CI 0.79–0.95) and colorectal cancer risk, while the association for other supplements and colorectal cancer risk was inconsistent. Second, we investigated the role of dietary supplements in recurrence of colorectal adenomas and advanced colorectal adenomas in a prospective cohort study of 565 patients with a history of sporadic colorectal adenomas. Dietary supplement was not associated with total adenoma recurrence (HR=1.03; 95% CI 0.79–1.34). Third, dietary supplement use and colorectal adenoma risk was examined in a prospective cohort study among 470 individuals with Lynch syndrome. No associations were found between dietary supplement use (HR=1.18; 95% CI 0.80–1.73) and colorectal adenoma risk in these individuals. Finally, in an ongoing prospective cohort study among incident colorectal cancer patients we evaluated whether dietary supplement use was consistent over time. Dietary supplement use was extensively assessed with a detailed self-administered questionnaire at diagnosis, six months and two years post-diagnosis. We observed that dietary supplement use among 160 colorectal cancer patients was common at all time points, but use was inconsistent from diagnosis to two years post-diagnosis. Conclusion: The results in this thesis do not point toward a preventive nor a harmful role for dietary supplement use in colorectal tumor risk and recurrence in the general population and in high-risk populations for colorectal cancer. However, dietary supplement use appeared to be inconsistent over time after colorectal cancer diagnosis, and use should be repetitively assessed over time. Since dietary supplement use is rising in countries where colorectal tumors are prevalent and the incidence of colorectal tumors will increase due to screening practices, research on the role of dietary supplement use for primary or tertiary prevention of colorectal tumors should continue in which use should be repetitively and comprehensively assessed.