The biological functions associated with consumption of omega-3 fatty acids suggest that it may have some impact on cancer risk (Larsson, 2004). Available evidence comes primarily from observational studies rather than randomized controlled trials (Terry, 2003; MacLean, 2006) (see Appendix Table B-2i; see also MacLean et al., 2005b, AHRQ Report No. 113). A small number of these studies show some protection for certain types of cancer (i.e., breast, colorectal, and lung), whereas others support an increase in risk (e.g., breast). The majority of the studies, however, conclude there is no significant effect on risk for cancer associated with seafood consumption or intake of other sources of EPA/DHA. Overall, the consumption of seafood, ALA, or EPA/DHA from all sources does not appear to decrease cancer risk (MacLean, 2006).
Consumption of EPA/DHA, specifically from seafood consumption, may provide some protection in terms of age-related cognitive decline as well as risk for Alzheimer’s and other neurological diseases (Kalmijn et al., 1997; Gharirian et al., 1998; Barberger-Gateau et al., 2002; Morris et al., 2003). It should be noted that, as discussed above for cancer, evidence for reduced risk for these diseases comes primarily from observational studies. The beneficial effects appear to be more closely related to the consumption of seafood and/or global intake of DHA rather than EPA or ALA. Overall, the evidence is tenuous and counterbalanced by a number of studies that did not find significant benefits (see Appendix Table B-2j; see also MacLean et al., 2005a, AHRQ Report No. 114).
Results from individual studies are not consistent and results from critical reviews are not clearly supportive of a cardioprotective effect of EPA/DHA. Furthermore, evidence for an effect on other adult chronic disease is controversial. Tables that summarize the committee’s assessment of levels of evidence and reports from individual studies are shown in Table 3-2 and Appendix Tables B-1 through B-2. The level of evidence identified as “Contradictory or insufficient evidence to base recommendations” includes outcomes where a large body of literature exist, but leads to contradictory conclusions, as well as outcomes where the body of literature is too small to lead to recommendations. The committee’s assessment of level of evidence summarized in Table 3-2 has its limitations. The Oxford Centre for Evidence-based Medicine’s levels of evidence may not be ideal to assess nutrition studies. The quality of the various studies cannot be summarized