Omega-3 fatty acids don’t help people with preexisting heart disease avoid future cardiovascular trouble, a new study has found. What does this mean for fish oils and our health?

That’s not clear.

Here’s what the study, reported in the Archives of Internal Medicine, did:

Dr. Sang Mi Kwak and a team of S. Korean scientists looked at studies of people with existing heart disease who took EPA or DHA, the kinds of omega-3s found in fatty fish. (Another omega-3, ALA, is found in plant oils and slowly converts in the body to these other kinds.) In all, the 14 studies included 20,485 patients, mostly men.

All of the studies included were the gold-standard kind of clinical trial -- with people assigned at random to either take fish oil or a placebo. The studies ranged in length from one to nearly five years.

The authors detected no reduction in any cardiovascular events, such as heart attacks, sudden death, angina, heart failures, strokes or death, no matter what dose of fish oil used.

The idea that fish oil could be protective against heart disease has its roots back in the 1970s, when scientists hypothesized that a diet heavy in fish and marine food in general might be responsible for low rates of heart disease in the Eskimos in Greenland.

Since then, some studies have reported that EPA and DHA could damp inflammation and arrythmias and reduce plaque formation in the arteries. But though some studies of human populations suggested a benefit, a problem with population studies is that you can’t control what people do as they go about their regular lives. It’s always possible that some behavior other than eating fish is actually responsible for the health effects (scientists can only control for so much).

The answer became muddier when people started conducting real clinical trials, generally in people who already had cardiovascular trouble. Those people are studied because you can get a quicker answer about protection against heart attacks in a group already more prone to have them. Some studies reported benefit. Others did not.

Not all of those studies were well designed -- but this study included only those of the highest quality.

What now? Do we toss our fish oil capsules?

Here’s what researchers Frank Hu and JoAnn Manson of Harvard have to say in a commentary accompanying the piece.

They suggest that maybe these days, drugs such as statins make it hard to detect a benefit from fish oils because the statins and other meds are already helping people so much. This could explain why more recent trials show no omega-3 benefit while older ones did.

They say that this study included trials that were pretty short in duration -- maybe too short to detect a reduced risk of heart attacks and other nasty cardiovascular events.

And they say that maybe there’s a difference in what DHA and EPA can do in people who don’t yet have cardiovascular disease compared with those who already do.

Still they note that there’s “no conclusive evidence” for recommending fish oil to prevent a first or subsequent heart attack.

But that doesn’t mean, they add, that doctors should stop recommending oily fish to their patients.

Fish, if nothing else, could be a good substitute for other forms of protein these scientists think less of, like the famous red meat that got such a drubbing a few weeks back.