Excellent analysis of recent research reports previously featured on this blog here.
Here's the most important part, but read the whole thing:
PF: The benefits of another popular item, fish oil, have been questioned in a study that recently received a lot of attention in the media. Millions of Americans take fish oil daily. Doctors prescribe it. We have read about so many benefits for cardiovascular health, brain health, and anti-inflammatory benefits. Are there benefits to fish oil, specifically for heart health?
AG: The study you mentioned was a meta-analysis (pooled analysis) of 20 randomized controlled trials (including a total of 68,680 patients) that examined whether supplementing with the omega-3 fatty acids present in fish oil can decrease heart disease-related mortality or deaths due to any cause. In the pooled analysis, omega-3 fatty acids decreased heart disease-related deaths by 9 percent and deaths due to any cause by 4 percent. Because these decreases were not statistically significant, the researchers concluded (erroneously) that omega-3 fatty acids had no effect on cardiac or all-cause mortality. The correct conclusion is that omega-3 fatty acids decreased cardiac deaths by 9 percent and all-cause mortality by 4 percent, but because the effects were not statistically significant, we are less than 95 percent certain that the benefits were real (as opposed to being due to chance). Failure to demonstrate that an effect was statistically significant is not the same as demonstrating there was no effect.
Furthermore, the potential benefit of taking fish oil might be greater than the results of the meta-analysis suggest. That is because some of the studies that showed a strong positive effect were "diluted" by several flawed studies that found no beneficial effect of fish oil. One of the negative studies enrolled 18,645 Japanese patients (27.2 percent of all the patients in the meta-analysis), and therefore had a relatively strong influence on the results of the pooled analysis. The main weakness of that study is that fish consumption is high in Japan. The protective effect of omega-3 fatty acids against heart disease can be obtained with relatively low doses, and little or no additional benefit can be achieved by increasing the dose. Consequently, one would not expect fish oil supplementation to reduce mortality in this population where fish consumption is high. In another negative study included in the meta-analysis, olive oil was used as the "placebo." Olive oil is a known cardioprotective agent, so the fact that fish oil was not more effective than olive oil does not mean fish oil was ineffective.