The present invention provides natural fish oils comprising altered fatty acid profiles, which are useful e.g. as a nutritional supplement. In one aspect, the invention relates to a method for obtaining such fish oils with altered fatty acid profile, in particular, oils containing nutritionally important fatty acids such as Arachidonic acid, Eicosapentaenoic acid and Docosahexaenoic acid. The method comprises the steps of (a) feeding fish a composition comprising at least one fatty acid, so as to obtain altered levels of an endogenous and/or non-endogenous fatty acid in said fish, and (b) extracting oil comprising altered levels of the at least one fatty acid from said fish. The invention further relates to a method of purifying a composition comprising at least one fatty acid by feeding a first composition to a fish and extracting a second composition comprising the at least one fatty acid from said fish. Further aspects of the invention relate to a method of preparing a triglyceride, a method of rearing fish, and use of compositions according to the invention for the preparation of a dietary supplement, a nutraceutical and/or as a food/feed additive. The invention also relates to oils obtainable by the methods of the invention, to fish comprising altered levels of Arachidonic acid, Eicosapentaenoic acid and Docosahexaenoic acid, as well as oil from a fish comprising altered levels of these fatty acids.