IntroductionThe human intestinal tract harbors trillions of bacteria referred to as the gut microbiota, which play an important role in digestion and host metabolism 1 . The change in the composition of gut microbiota has been implicated in the development of metabolic diseases, including type-2 diabetes and obesity 2 . The composition of gut microbiota is influenced by the intestinal environment, including the type of food digested by the host, digestive fluid volume, and digestive fluid quality 3 . Numerous studies have evaluated the composition of gut microbiota and its metabolites in experimental animals fed high-fat or low-fiber diets 4, 5 . However, few reports have focused on the effects of different types of dietary fat sources on gut microbiota and its

IntroductionFish oil is a rich source of n-3 polyunsaturated fatty acids PUFA , such as eicosapentaenoic acid EPA and docosahexaenoic acid DHA , and has health-promoting functions in humans. For example, n-3PUFA intake has proven beneficial against cardiovascular diseases 1 , hyperlipidemia 2 , Alzheimer s disease 3 , inflammation 4 , and cancer 5 . Accordingly, various organizations recommend the intake of n-3PUFA, especially EPA and DHA 6 .Most prior studies investigated the health-promoting effects of dietary n-3PUFA employed commercially available fish oil, which is a mixture of EPA and DHA. However, the molecular structures and physiological functions of EPA and DHA are different. EPA C20:5n-3 has a shorter carbon chain 20 vs. 22 and fewer double bonds 5 vs. 6 per molecule than DHA C22:6n-3 . DHA has a greater influence on membrane fluidity and the activity of membrane