Omega-3 fatty acids (more properly called "n-3 fatty acids") are a group of naturally occurring fat molecules. They are found mainly in fish and other marine-derived oils, but some can also be extracted from plants. Omega-3's are currently very popular, but the evidence for their usefulness isn't so clear. A recent study failed to show any benefit in preventing dementia. A new study out of the Netherlands looked at omega-3's in heart disease. In order to understand the study, we need to back up for a moment.
We already know that some dietary fats have a strong effect on heart disease: certain types of cholesterol are elevated in heart disease, and altering the levels of these fats reduces the risk of heart disease. The treatment of cholesterol problems is a mainstay of heart disease treatment and prevention. The idea that omega-3-fatty acids may help prevent heart disease is a few decades old. Like many interesting hypotheses, it was based on observations. In this case, investigators noted that Greenland Inuit had a much lower rate of heart attack than "Westerners". Researches posited that it was the high levels of dietary fish oils that protected Inuit from heart disease.
We've already discussed some of the ways scientists evaluate risk and benefit; in this case, much of the evidence for the benefit of fish oil in heart disease is based on observational studies rather than randomized controlled trials. Many of these studies looked at the dietary choices of subjects to assess their omega-3 consumption. There is evidence for a role for omega-3's in the primary prevention of heart disease, that is, preventing someone who doesn't have heart disease from getting it, but the evidence isn't terribly strong. The evidence for secondary prevention is a bit meatier. One large Italian study gave omega-3 supplements (rather than guessing at dietary consumption) to people who had had a recent heart attack. The results were very encouraging, with a large reduction in the risk of death from another heart attack. But---there's always a 'but'--the study was done at a time when many of the effective medications (such as statins) weren't yet in wide use, so it's unclear what effect there would be in a more modern context.
A newer study out of the Netherlands took an interesting approach. Rather than simply guessing at diet or giving a pill, the researchers developed margarines supplemented with omega-3's. They gave the margarines to people who already had heart disease and observed them for a period of time. The doses of omega-3's were relatively low, but when they compared the regular margarine group to the omega-3 margarine group, there were no significant differences in the rates of major heart attacks.
This isn't the final word on omega-3's for heart disease-prevention. Part of the problem in studying the question is that the current treatment of heart disease is impressively effective. Any intervention that doesn't have a dramatic effect is going to be hard to evaluate. Given the epidemiologic evidence, and the questions still out there, this study isn't going to end the question of omega-3's in heart disease.
Kromhout, D., Giltay, E., & Geleijnse, J. (2010). n–3 Fatty Acids and Cardiovascular Events after Myocardial Infarction New England Journal of Medicine, 363 (21), 2015-2026 DOI: 10.1056/NEJMoa1003603