Depressive symptoms, omega-6:omega-3 fatty acids, and inflammation in older adults.

Depression is the most common psychiatric illness, and both major depression and subthreshold depressive symptoms carry substantial health risks (1, 2). A number of well-controlled prospective studies have linked depressive symptoms with coronary heart disease (CHD), the leading cause of death in the United States (3); moreover, the nearly 60-fold variance in the annual prevalence of major depression across countries is very similar to the pattern for cardiovascular disease, which shows a strong comorbidity for depression (4, 5). Epidemiological studies have demonstrated significant inverse relationships between annual fish consumption and prevalence of major depression (4). Fish oil is the prime source for two key omega-3 (n-3) polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA). Several laboratories have provided evidence that depressed patients have, on average, lower plasma levels of n-3 PUFAs than nondepressed controls; furthermore, there are relationships within these populations between severity of depressive symptoms and lower plasma levels of the n-3 PUFAs (3–6). What is more, all but one of four randomized controlled trials reported significant improvement in the treatment of depression following n-3 PUFA supplementation compared to nonsupplemented controls (6). Significant relationships between lower n-3 PUFA plasma levels and greater negative mood have also been documented in nonpsychiatric populations (7). Omega-6 (n-6) PUFAs are also implicated in depression, with higher n-6:n-3 ratios observed in depressed patients compared to nondepressed controls (3–6). Arachidonic acid (AA) derived n-6 eicosanoids (primarily from refined vegetable oils such as corn, sunflower, and safflower) increase the production of proinflammatory cytokines, operating as precursors of the proinflammatory eicosanoids of the prostaglandin (PG)2-series (5). In contrast, the n-3 PUFAs, found most abundantly in fish, fish oil, walnuts, wheat germ, and flaxseed can curb the production of AA-derived eicosanoids (5, 8). Thus it is not surprising that both higher levels of n-3 PUFAs as well as lower n-6:n-3 ratios are associated with lower proinflammatory cytokine production (9). The fatty acid composition of the modern Western diet has changed dramatically during the last century, and these changes are thought to be related to increases in inflammatory-related diseases, including depression and cardiovascular disease (6, 10). For example, the early hunter-gatherer diet had an n-6:n-3 ratio of 2:1 to 3:1 (11). However, during the last century, the typical Western diet underwent fundamental alterations with the enormous growth in refined vegetable oil use, a central n-6 source that replaced n-3 PUFAs from fish, wild game, nuts, seeds, and green leafy vegetables (5, 11); in the contemporary North American diet, the n-6:n-3 ratio is 15:1 to 17:1 (12, 13). In fact, the heightened n-6:n-3 ratio in the Western diet since 1913 has been suggested as a central stimulus for the sharply increased incidence of major depression (4, 5). The n-6:n-3 ratio is related to serotonergic as well as catecholaminergic neurotransmission (14), providing mechanistic pathways that help explain the relationships with negative mood. Alterations in serotonin (5-HT) receptor numbers and function provoked by changes in PUFAs have been used to link fatty acids with contemporary theories of depression (5, 15). Moreover, immune activation may interact with 5-HT functioning to promote depression (5), because PUFAs influence the synthesis of proinflammatory cytokines. Furthermore, both syndromal depression and depressive symptoms can enhance production of proinflammatory cytokines, including interleukins 1 and 6 (IL-1 and IL-6) and tumor necrosis factor alpha (TNF-α) (2, 16–21). Brief laboratory stressors can also provoke transient increases in proinflammatory cytokines (1, 22, 23). Moreover, stress and depression may effectively “prime” inflammatory responses, promoting larger cytokine increases in reaction to subsequent stressors and/or pathogens (19, 24–27). Thus, chronic or longer-term stress and depression can elevate both acute and chronic proinflammatory cytokine production (16, 20, 23, 28). Importantly, these are bi-directional relationships; cytokines have substantial effects on the central nervous system (CNS), producing and enhancing negative moods, as well as physical symptoms such as lethargy and fatigue (18). Indeed, there is evidence that cytokines play a role in the neuroendocrine and behavioral features of depressive disorders (18). In a provocative study relevant to the present investigation, students who had higher n-6:n-3 ratios (above the mean) before exams demonstrated greater TNF-α production by lipopolysaccharide (LPS) and mitogen-stimulated peripheral blood leukocytes (PBLs) during exams than those with lower ratios (29). These data suggest that the n-6:n-3 ratio may influence the proinflammatory response to stressors; because TNF-α and IL-6 are produced by a variety of types of cells, serum cytokine levels may better reflect the overall inflammatory profile than stimulated PBLs or ex vivo production (8). Indeed, the aging and depression literatures have focused on proinflammatory cytokine levels assessed in serum (2, 30). Accordingly, we were interested in how depressive symptoms and the n-6:n-3 ratio would be related to serum TNF-α, IL-6, and the IL-6 soluble receptor (sIL-6r). We studied an older population because proinflammatory cytokine production is elevated following menopause or andropause even in the absence of infection, trauma, or stress (31). Both higher n-6:n-3 ratios and higher levels of depressive symptoms have been associated with greater production of proinflammatory cytokines; however, in accord with the student examination study (29), we expected that higher n-6:n-3 ratios and higher levels of depressive symptoms would predict higher levels of TNF-α, IL-6, and sIL-6r than either alone.