There is a fair bit of misinformation in this thread, and that's not really too surprising. Fatty acid biochemistry is rather complex, and poorly understood, even by doctors. I'll try to cover all the points of concern I noted in this one post.

Fish oil contains a number of fatty acids, but two in particular that are rare to nonexistent in other food stuffs. Those are DHA and EPA, both in the omega-3 family. A number of vegetable sources have a different omega-3 fatty acid, called alpha-linolenic acid (ALNA, or sometimes ALA). They are not interchangeable. We do have enzymes which have some very limited capacity to elongate and desaturate ALNA to form EPA, and theoretically then onwards to DHA, but recent experiments have shown that dietary ALNA is converted to EPA at no better than 5%, and virtually none goes on to form DHA. To obtain EPA and DHA, you need to consume fish oil, or other marine oils (seal, krill, whale, etc.), or their original source, algae. There are a couple of algal-based DHA supps, but they are very expensive.

EPA is essential for signalling purposes, regulating inflammation and signal transduction. DHA is essential to the proper functioning of neuronal membranes, and receptor function. Fish oil benefits not only the brain, but especially the cardiovascular system. Not everyone notices the cognitive and psych effects of fish oil, but the improvement in cardiac and circulatory parameters will definitely be noticeable by bloodwork. So, even if you don't notice a difference, take it for your heart. In my own case, I find I have more resiliency. More balance. Life is easier, with fish oil. And my cholesterol and triglycerides are superb.

The typical Western diet is grossly deficient in DHA and EPA, and massively over-supplied with omega-6 fatty acids from vegetable oils. The imbalance leads to excess inflammatory activity, and increased blood clotting risk. Think heart attack and ischemic stroke. I don't recommend obtaining omega-3s from e.g. flax oil for two reasons: a) you're not going to get the DHA and EPA your body craves; and, b) the high proportion of omega-6 fatty acids opposes the benefit you might otherwise obtain, due to competition at enzyme active sites, and their pro-inflammatory nature.

There is a risk of fishy burps, as has been noted here. You can reduce the likelihood by only taking fish oil immediately following your largest meal of the day, preferably one with some other fat sources. The food in your stomach protects the fish oil from direct exposure to stomach acid which decomposes it, and the other fat (or protein) in the meal inhances gall bladder secretions which enhance uptake. Don't ever take fish oil on an empty stomach. Also, because fish oil is a perishable commodity, you have to have fresh supplies. I recently bought a couple of large bottles of capsules that had a three year expiry, but they were already rancid. If they weren't handled properly before you got them, it happens.

Because of the perishable nature of these fats, you have to consider your antioxidant intake. They are just as perishable in your body as they are outside it. You should take 100-200 IU mixed tocopherols (natural vitamin E) and 100 mg vitamin C for every 1000 mg of fish oil, averaged over a week. (I.e. you don't need to take them at the same time, and your diet may provide enough antioxidants.....).

Fish may indeed contain mercury, but fish oil has none. Mercury binds to protein, and there is no protein in food-grade fish oil. Nonetheless, I still think whole fish is superior to fish oil, as it has other nutrients that improve brain function. Modest mercury exposure can be protected against with selenium intake. Mercury preferentially binds to selenium, forming a totally insoluble non-reactive complex. In any case, smaller fish (herring, menhadden, anchovies) have much less mercury than do e.g. their predators (tuna, salmon). So, proper fish selection goes a long way in the avoidance of bioaccumulating toxins.

Evening primrose oil (EPO) is not selected for its omega-9 content. The only omega-9 fat commonly found in from plants in the genus Olea (olive), and it's called oleic acid, a mono-unsaturate. Instead, EPO is an excellent source of gamma-linolenic acid (GLA). Even though GLA is an omega-6, it is anti-inflammatory. Its effect is enhanced in the presence of fish oil, also. Borage oil has more GLA than does EPO, but it's likely that the latter has other beneficial compounds not found in the former. Black currant oil also has a good deal of GLA.

There are a few newer food products that have enhanced DHA content. I've seen yoghurt, for example, and milk and bread. Omega-enhanced eggs are not from genetically modified birds, however. It's just that their diet includes flax seeds, and we benefit from the elongation and desaturation enzymes of the hen by modest enhancement of the EPA and DHA content of the yolk. There's also a boost in vitamin E.

Historical dietary analysis indicates that we evolved with roughly equal intake of omega-6 and omega-3 fats. Current intake is between 30 and 60:1 excess of omega-6. I think everyone should supplement with fish oil. It takes 18-24 months of daily supplementation to fully stock your body with the optimal proportion of long-chain omega-3 fats. Every cell wants some. So, it's best thought of as a lifestyle adaptation, rather than a short-term treatment.

Questions welcome.

Lar