In the first set of experiments, it was focused on the effects of n-3 polyunsaturated fatty acids (PUFAs) on tunicamycin-, streptozotocin-, and high fat diet (HFD)-induced β-cell damage and dysfunction. Pretreatment with n-3 PUFAs protected RINm5F cells and mouse islets against tunicamycin-induced β-cell damage through suppression of ER stress and apoptosis induction. In multiple low-dose streptozotocin-induced diabetes models, fat-1 mice, which endogenously synthesize n-3 PUFAs from n-6 PUFAs, were fully resistant to the development of diabetes. In HFD-induced diabetes models, fat-1 mice also exhibited improved glucose tolerance and functional β-cell mass. In both diabetes models, ER stress in fat-1 mice was attenuated. Interestingly, n-3 PUFAs attenuated the nuclear translocation of lipogenic transcription factors SREBP-1 and C/EBPβ, induced by tunicamycin or HFD, suggesting that n-3 PUFAs suppress ER stress via modulation of SREBP-1 and C/EBPβ. Together, these results suggest that n-3 PUFAs block ER stress, thus protecting β-cells against diabetogenic insult; therefore, dietary supplementation of n-3 PUFAs has therapeutic potential for the preservation of functional β-cell mass.Diet-induced obesity and consequent insulin resistance are caused, in part, by macrophage polarization and accumulation in peripheral tissues. In the second set of experiments, the effects of endogenously synthesized n-3 PUFAs on macrophage chemotaxis and polarization were examined. Fat-1 mice and WT littermates were fed a HFD for 10 weeks. Bone marrow macrophages (BMMs) from fat-1 and WT mice were used in in vitro chemotaxis assays and macrophage polarization studies. WT mice fed a HFD exhibited glucose intolerance, insulin resistance, and lipid accumulation and macrophage infiltration in liver and adipose tissue. However, these metabolic and inflammatory phenotypes were not observed in HFD-fed fat-1 mice. In flow cytometric analysis, M1 macrophage infiltration into adipose tissue was markedly attenuated in fat-1 mice. Consistently, results from in vitro experiments indicated that n-3 PUFAs prevented adipocyte conditioned medium-mediated macrophage chemotaxis, stimulated M2 polarization, and suppressed M1 polarization. The inhibition of macrophage migration by n-3 PUFAs was associated with suppression of multiple kinases, such as IκB kinase, AKT, and focal adhesion kinase. Our results indicate that n-3 PUFAs play a crucial role in macrophage polarization and chemotaxis, and thus regulate the development of HFD-induced tissue inflammation and metabolic derangements. Taken together, these results suggest that n-3 polyunsaturated fatty acids play a positive role against various metabolic challenges.