The US study, published online by JAMA Internal Medicine, included 77,659 men and women who were assessed by a food frequency questionnaire and categorised into four vegetarian groups (vegan, lacto-ovo vegetarian, pesco-vegetarian, and semi-vegetarian) or a non-vegetarian group. During a mean follow-up of 7.3 years 380 cases of colon cancer and 110 cases of rectal cancer were recorded.
The study, funded by the National Cancer Institute and the World Cancer Research Fund, found that vegetarians had a 22% lower overall risk for colorectal cancers than non-vegetarians (adjusted hazard ratio 0.78 (95% confidence interval 0.64 to 0.95)). Vegetarians had a 19% lower risk for colon cancer and a 29% lower risk for rectal cancer when compared with meat eaters.
Closer analysis showed that the biggest difference was seen in pesco-vegetarians, who eat fish, and they had a 43% lower risk of colorectal cancer than non-vegetarians (0.57 (0.40 to 0.82)). Compared with non-vegetarians, vegans had a 16% lower risk of colorectal cancer, lacto-ovo vegetarians (who eat milk and eggs) had an 18% lower risk, and semi-vegetarians had an 8% lower risk.
Limitations of the study include the assessment of diet only at baseline and the relatively early follow-up. However, the study is continuing, and later follow-up will enhance the statistical power and allow for subgroup analyses.
The findings differed from those of the EPIC-Oxford cohort study – the other major cohort study examining vegetarian dietary patterns – which produced initial results showing about a 50% greater risk of colorectal cancer among UK vegetarians (Key et al., 2009). Later results from that study showed a null association, however (Key et al., 2009). The authors of the Seventh Day Adventist study said that the difference in results could have arisen because the US cohort ate substantially more fruit and vegetables than the British cohort.
Orlich, M.J., Singh, P.N., Sabaté, J., Fan, J., Sveen, L., Bennett, H., Knutsen, S.F., Beeson, W.L., Jaceldo-Siegl, K., Butler, T.L., Herring, R.P. & Fraser, G.E. (2015) Vegetarian Dietary Patterns and the Risk of Colorectal Cancers. JAMA Internal Medicine. Published online March 09, 2015. doi:10.1001/jamainternmed.2015.59.
Key, T.J., Appleby, P.N., Spencer, E.A., Travis, R.C., Roddam, A.W. & Allen, N.E. (2009) Cancer Incidence in Vegetarians: Results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). American Journal of Clinical Nutrition. 89(5), pp.1620S-1626S.
Key, T.J., Appleby, P.N., Spencer, E.A., Travis, R.C., Allen, N.E., Thorogood, N. & Mann, J.I. (2009) Cancer Incidence in British Vegetarians. British Journal of Cancer. 101, pp.192-197.