I order Carlson's from Amazon. I suppose there's a chance it could be heat-damaged, but I haven't noticed any adverse effects from taking it the past couple years. I generally try to get my O3s from fresh fish, though.
Dietary intake of both Omega-3 and Omega-6 contribute to something called HUFA - highly unsaturated fatty acids. HUFA is stored in your cell membranes, and the proportion of Omega-3 to Omega-6 intake determines the composition of your tissue HUFA. If you eat a ton of vegetable oil and little fish, your tissue HUFA will be mostly Omega-6.
Eicosanoids are signaling molecules in the body. We use eicosanoids mostly to control inflammation and immunity, and we create eicosanoids from tissue HUFA. If your HUFA is Omega-6 dominant, the eicosanoids created will be highly inflammatory and blood clot-promoting; Omega-3 heavy HUFA creates less inflammatory eicosanoids. IL-6 is a highly inflammatory cytokine that's effective in small doses, but
high O-6 HUFA leads to excessive amounts of IL-6 in response to inflammatory signaling.
Too much IL-6 may
induce leptin resistance (by upregulating production of leptin receptor-binding SOCS-3), and too much tissue Omega-6 may
increase heart disease. As Stephan shows, groups with the most Omega-3 HUFA (Japanese, Inuit) have the greatest heart health, while groups with more O-6 HUFA (like the USA) have the worst.
Plus, there's the fact that linoleic acid is pretty unstable, on the shelf and in the frying pan, so there's a good chance that whatever we ingest is usually already oxidized and damaged.