You can read an excellent summary of a review that looked at this data here:
Omega-3 Polyunsaturated Fatty Acids and Cardiovascular Disease Protection (American College of Cardiology CME)
One small area of concern about the research cited is that quite a lot of it was done with a prescription (i.e. expensive, proprietary) form of fish oil called Lovaza made by drug kingpin GlaxoSmithKline. You can read the official Prescribing Information about Lovaza HERE.
Lovaza costs around $100 for 60 1 gm capsules and in the studies cited 4 gram daily doses were used. The fish oil you buy at the pharmacy is about $15 for 200 capsules and you can often find "buy one get one free" offers.
This issue is important because the above review notes that re a major Lovaza study "Other studies, including a recent underpowered OMEGA trial, have not demonstrated such benefits." The explanation could be that Lovaza is worth the obscene price or that GSK used the usual big pharma study design dodges and statistical tricks to skew the results. My guess is the latter.
Subtract the drug company studies, though, and there is still a lot of evidence pointing to fish oil as being helpful in inflammatory contexts. You can read a older full-text review that cites many observational studies HERE.
A very recent study examined the impact of fish oil on gene expression, comparing the effects on genes in human subjects of six month supplementation with fish oil against supplementation with sunflower oil.
Fish-oil supplementation induces antiinflammatory gene expression profiles in human blood mononuclear cells. Bouwens M et al. Am J Clin Nutr 2009 Aug;90(2):415-24. Epub 2009 Jun 10.
The finding of the gene study was that
A+DHA intake resulted in a decreased expression of genes involved in inflammatory- and atherogenic-related pathways, such as nuclear transcription factor kappaB signaling, eicosanoid synthesis, scavenger receptor activity, adipogenesis, and hypoxia signaling.The dose in the above study was 1.8 g a day of fish oil. It is not mentioned if the study used a branded version.
This suggests that fish oil would be a worthwhile supplement for people with diabetes because heart disease is now known to be an inflammatory condition. Some people with Type 2 also turn out to have autoimmune factors at work in attacking their beta cells.
But as good as fish oil might be for you there is one huge caveat. Thanks to the heedless pollution of our environment by coal burning plants releasing mercury in the air, fish has become an extremely toxic way to get your fish oil.
The data commonly cited about the amount of mercury in fish is between twenty and thirty years old and what little evidence we have the current levels of mercury in fish is far, far higher.
Here is the FDA listing of mercury levels in fish:
FDA Mercury Levels in Commercial Fish and Seafood
Note that the date of most of the mercury concentration data cited for many of the fish is "1990-1994" and for some, like shrimp and mackerel, it is 1978.
I personally know two people who consumed fish several times a week believing in its health benefits who were diagnosed with toxic blood mercury levels by mainstream doctors and given chelation therapy.
In a recent book, Experimental Man, by David Ewing Duncan, one that is otherwise not worth reading, the author ate a fish and then went to the lab and had his blood analyzed for mercury. The reading reported was far higher than what the FDA lists of mercury in that particular fish would suggest.
So this suggests that eating fish is not a good way to get the benefits of fish oil.
Capsules are better, though they may, in fact, contain very small amounts of mercury, the amounts are dramatically lower than that found in fish.
Here is a study that gives you some idea of how much mercury might really be in fish oil capsules. Many are free of it, some of it do have small amounts:
Measurement of Mercury Levels in Concentrated Over-the-Counter Fish Oil Preparations: Is Fish Oil Healthier Than Fish? Stacy E. Foran et al. Archives of Pathology and Laboratory Medicine, Vol. 127, No. 12, pp. 1603–1605.
One VERY important warning. Over the year that I have been buying fish oil capsules, I have found the ingredients of the capsules changing from bottle to bottle within the same brand. So does the origin of the fish oil which may be listed as "sardines" in one bottle and not mentioned in another, suggesting it comes from larger, more mercury-polluted fish. I found this true for several different brands. So when a manufacturer cites test values those values may have been done months before and may be entirely different from results you'd see if the tests were performed now.
Because mercury levels in fish roughly correspond to the size of the fish, you'd do best with fish oil made from very small fish, like sardines. However, small fish taken from highly polluted waters may be more toxic than you'd expect. I also see soy oil increasingly making its way into capsules, a concern for those who have problems with soy proteins. So when you buy your fish oil, examine the label carefully each time.
Another very important point: Buy "Enteric coated" fish oil. Unlike the plain kind, it will not cause you to burp up disgusting bits of fish oil all day.