Omega-3 fatty acids are thought to have immunologic benefits, and studies have suggested that persons consuming diets high in these acids may be less susceptible to some types of cancer. MacLean and associates reviewed studies that analyzed the associations between high omega-3 fatty acid intake and cancer incidence.

The authors included only prospective cohort design studies. Of 1,228 articles retrieved, 38 met the inclusion criteria. These studies reported the effects of omega-3 fatty acids on 11 types of cancer, predominantly breast, colorectal, and prostate cancers. The pooled cohorts yielded more than 700,000 participants. Most of the studies provided only baseline information about self-reported omega-3 fatty acid intake and equivalents (i.e., fish consumption).

The authors determined that the study results were inconclusive. No association between fish consumption and incidence of aerodigestive cancer was found. There was no significant effect of fish consumption on the incidence of bladder cancer. With regards to breast cancer, one study demonstrated a greater risk in the highest quartile of fish eaters, another reported a reduced risk, and two others showed no association. Studies analyzing total omega-3 fatty acids found no association with breast cancer, although one detected a reduced risk in the highest quartile of omega-3 consumption when compared with the lowest quartile. Menopausal status did not impact any association between breast cancer risk and fish consumption. When analyzing colorectal cancer, four studies showed no association between fish consumption and cancer, and one showed a reduced risk associated with higher fish intake. However, after adjusting for multiple variables, there was no association between fish intake and colorectal cancer. No significant association was found between fish intake and incidence of lung cancer. There also was no association between fish intake and non-Hodgkin’s lymphoma or ovarian, stomach, and pancreatic cancers. One study of prostate cancer incidence found a favorable effect with reduced fish consumption and one showed a trend towards favorable effect with increased fish consumption. Two other trials showed no effect of fish consumption on prostate cancer. However, one trial showed an increased risk of advanced prostate cancer with greater linolenic acid consumption. One study showed that consumption of higher amounts of omega-3 fatty acid increased the risk of basal cell carcinoma.

The authors concluded that only 10 out of 65 estimations of association showed a statistically significant impact of omega-3 fatty acid consumption and its proxies on cancer incidence. Even in studies where significance was found, increased and decreased risks were reported. Therefore, there was no overall impact of omega-3 fatty acid consumption on cancer risk, and it appears unlikely that this dietary component has any positive or negative effect on any one type of cancer. Because most studies surveyed omega-3 fatty acid consumption at only one point in the study, it cannot be said whether this represented the participants’ actual consumption for the duration of the study.