Twenty different cohort studies were identified. These ranged in size from 6,000 to 121,000 participants with 9,000 to 1.5 million person-years of observation, with a total of over 700,000 participants and 3 million person-years of observation.
All of the included cohort studies were of variable quality. Qualitative and visual evaluation suggests that there were no differences in reported risks based on sample size.
Breast cancer (8 studies, n=412,134): one study found that there was an increased risk of breast cancer for women in the highest quartile of fish consumption compared with women in the lowest quartile (incidence RR 1.47, 95% confidence interval, CI: 1.10, 1.98). Two studies found that there was a reduced risk for women consuming larger amounts of fish or marine omega-3 than for women with lower consumption: the RRs were 0.77 (95% CI: 0.60, 0.98) and 0.72 (95% CI: 0.53, 0.98), respectively. None of the other studies found associations between breast cancer and omega-3 consumption. One study reported a reduced risk of breast cancer for women in the highest quintile of ALA consumption compared with women in the lowest quintile (RR 0.70, 95% CI: 0.51, 0.97), but no associations between EPA or DHA consumption and breast cancer risk.
Colorectal cancer (9 studies (7 cohorts), n=391,376): one study suggested that there was a reduced risk of colorectal cancer for those in the highest quartile of fish intake compared with those in the lowest quartile (RR 0.49, 95% CI: 0.27, 0.89). One study suggested a trend for a reduction in risk for those with higher consumption of omega-3 when adjusting for age. None of the other studies demonstrated associations between colorectal cancer risk and omega-3 consumption.
Lung cancer (n=151,987): of the 3 studies that evaluated the relationship between lung cancer and omega-3 intake through fish consumption, one suggested an increased risk of lung cancer (incidence RR 3.0, 95% CI: 1.2, 7.3), one suggested a reduced risk of lung cancer (RR 0.32, 95% CI: 0.13, 0.76), and the third showed no association.
Prostate cancer (7 studies (5 cohorts), at least n=129,255): one study found that there was a reduced risk of prostate cancer for men that never or seldom ate fish compared with those with moderate consumption (RR 2.3, 95% CI: 1.2, 4.5), one study showed a trend towards a similar effect, and two found no association. One study reported an increased risk of prostate cancer for men in the highest quintile of ALA consumption compared with men in the lowest quintile (RR 1.98, 95% CI: 1.34, 2.93). No association was found between prostate cancer risk and EPA, DHA or marine omega-3 consumption in the same study.
Skin cancer (n=64,779): one study was identified. This demonstrated an increase in the risk of basal cell carcinoma for those in the highest quartile of omega-3 fat consumption compared with those in the lowest quartile of consumption (RR 1.13, 95% CI: 1.01, 1.27).
No significant associations between omega-3 fatty acids and cancer risk were found with regards to aerodigestive cancer (n=8,006), bladder cancer (n=8,006), lymphoma (n=163,537), ovarian cancer (n=183,163), pancreatic cancer (n=148,811) or stomach cancer (n=13,250).