Omega-3 fatty acids
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Introduction
Several lines of reasoning have led to the hypothesis that consumption of some, or any omega-3 fatty acids (DHA and EPA, found primarily in fish, and ALA, found in plants) will reduce the risk of dementia (and can even improve cognition in youth). The evidence is mixed, and the effects on ApoE-ε4 carriers appear to be especially weak, or non-existent. Recent studies, as well as new reviews, have been more positive however.
Effectiveness
Supplemental DHA is probably somewhat helpful as a preventive measure for ε4(-). Until recently, the effect was thought not to obtain in ε4-carriers. Once Alzheimer's has been diagnosed, fish oil seems likely ineffective regardless of ε4 status. Other omega-3 fatty acids are not as well researched.
A recent paper, however, concluded the following:
- Conclusions and Relevance. In cross-sectional analyses, moderate seafood consumption was correlated with lesser Alzheimer disease neuropathology. Although seafood consumption was also correlated with higher brain levels of mercury, these levels were not correlated with brain neuropathology. [1]
Note, a few researchers suspect that the reason DHA has not been effective in ε4 carriers is that the negative consequences of DHA consumption (DHA oxidation) overwhelm potential benefits, and that consumption of sufficient antioxidants along with the DHA, or, alternatively, consuming it in a natural form, where it would be less subject to oxidation, would permit it to be beneficial for ε4 carriers. For a summary, see "Omega-3s, ApoE Genotype and Cognitive Decline". This theory has not been tested however, and the epidemiological evidence offered in support of the theory is mostly from societies that consume the omega-3 fatty acid found in plant-based diets, ALA (which, itself, may help ε4 carriers, but more research is needed).
Other researchers contend that the amount of DHA used in humans trials (often 2-3 grams/day) is far too high (see, for example PubMed ID:21045096.