How do omega 3 fatty acids promote mental health?

Brain tissue is made up of relatively high percentage of lipids (60%), and much of that is polyunsaturated fatty acids. Omega-3's docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are found in their largest concentrations in the brain, and have been implicated in many neuronal processes such as myelination, neurotransmission, and ion channel regulation. On the behavioral side, many of their somewhat contreversial benefits come in the reduction in onset and/or treatment of symptoms in bipolar disorder, depression, schizophrenia, and Alzheimer's disease. Psychiatric illness undoubtedly has multiple neural correlates, but one that has been proposed is impaired cerebral blood flow.

Sinn and Howe propose that fish oil's beneficial effects arise because they act as signaling hormones to reduce the chemical synthesis of some vasoconstricting eicosanoid lipid mediators and promote the synthesis of certain vasodilating eicosanoids. This would improve endothelial function, which is associated with the selectively permeability of the blood brain barrier and perfusion in the brain generally. It might also allow for a more optimal allocation of oxygen, glucose, and nutrients. This makes sense from an evolutionary perspective, because if humans originally adapted to a diet with more polyunsaturated fatty acids (i.e., see the aquatic ape hypothesis), then our baseline levels of performance may make our brains uniquely suited to using omega 3's. Cerebral blood fluid levels can be measured using fMRI, and the authors suggest that further research could attempt to validate their hypothesis by measuring these levels before and after individuals are treated with fish oil pills.

Reference

Sinn N, Howe PR. 2008 Mental health benefits of omega-3 fatty acids may be mediated by improvements in cerebral vascular function. Bioscience Hypotheses 1:103-108.