Fish oils are said to be the most marvellous of things, they are widely praised for their cardio-protective properties.

A cohort study of 70 495 people from Washington State, followed up for four to six years, suggests that they are even better at preventing cancer (Bell et al., 2014). The research noted that a higher combined intake of eicosapentaenoic acid and docosahexaenoic acid from diet and supplements was associated with a decreased risk of total mortality (hazard ratio 0.82, 95% CI 0.73 to 0.93) and death from cancer (0.77, 0.64 to 0.92) but only a small reduction in risk of death from cardiovascular disease (0.87, 0.68 to 1.10).

However, it depends on what cancer you are referring to.

Fish oils may actually promote cancer of the endometrium, according to the same VITamins And Lifestyle cohort study (Brasky et al., 2014). Women in the highest compared with the lowest fifth of dietary eicosapentaenoic acid and docosahexaenoic acid intake had a 79% increased risk of endometrial cancer (95% CI 16% to 175%; P trend=0.026).

There is a general consensus that all foods either prevent or cause cancer, often according to the week you read about them.

References

Bell, G.A., Kantor, E.D., Lampe, J.W., Kristal, A.R., Heckbert, S.R. & White, E. (2014) Intake of Long-Chain ω-3 Fatty Acids From Diet and Supplements in Relation to Mortality. American Journal of Epidemiology. Advanced Access: doi: 10.1093/aje/kwt326.

Brasky, T.M., Neuhouser, M.L., Cohn, D.E. & White, E. (2014) Associations of long-chain ω-3 fatty acids and fish intake with endometrial cancer risk in the VITamins And Lifestyle cohort. American Journal of Clinical Nutrition. Advanced Access: doi:10.3945/ajcn.070524.