Two new U.S. studies say the nutritional benefits of eating fish outweigh the potential hazards from environmental contaminants. Some environmental and consumer groups dispute the finding.

You may have heard that fish contain mercury and other compounds called PCBs and dioxins that can be harmful physically.  But Harvard University doctors and a separate panel of private experts reporting to the U.S. government say eating fish regularly is very healthy, especially for the heart.

One of the authors of the Harvard study, Dariush Mozaffarian, puts it this way.

"The benefits of eating fish are far greater than the potential risks. If you eat a fish and it has some mercury in it, you might be getting less benefit from that fish than if it did not have mercury in it, but the overall benefit is still positive," he said.

The Harvard team says that benefits are great even for women of childbearing age if they avoid certain fish that are likely to contain mercury levels dangerous to fetuses.  The second study lists them as predatory fish with long lifespans, such as swordfish, shark, and tilefish.

This study is by the U.S. Institute of Medicine, a branch of the National Academy of Sciences that uses private experts to investigate issues for the U.S. government. Like the Harvard study, it is a summary of recent major research on fish consumption.

Both reports note that consumers are faced with conflicting evidence about eating fish. It is a good protein source containing the type of fat considered healthy for the heart. But some species absorb toxins present in the environment, causing confusion about the role of fish in a healthy diet.

So Institute of Medicine committee member David Bellinger, a Harvard nerve specialist not associated with the Harvard study, offers this advice.

"Because of the uncertainties, especially on the risk side, consumers should consume a variety of fish because the fish that contain one contaminant may not be the same fish that contain another contaminant, so that by consuming a variety of species, the benefits can be maximized, but the overall risk profile can be managed," he noted.

A U.S. environmental group disputes the findings. The National Environmental Trust argues that the two studies ignore evidence that chemicals used as flame retardants are also pervasive in the environment and contaminate fish. In addition, the organization's vice-president for marine conservation, Gerald Leape, says boosting fish consumption would strain wild fish populations that already suffer from overfishing and cause expansion of fish farms where contaminants are more prevalent.

"They have left out a true examination of the role of contamination, and there was no effort to take into account the ecological impacts not only of wild fish captures, but also of aquaculture farming," he said.

Another group, the Physicians Committee for Responsible Medicine, warns that fish and shellfish are significant sources of cholesterol, a factor in heart disease. It points out that shellfish in particular have more cholesterol than an equivalent amount of beef.

But Harvard University's Dariush Mozaffarian counters with one of the main findings of his study.

"We found that a modest intake of fish, about one or two servings per week, was enough to reduce the risk of dying from a heart attack by about 35 percent, which is a considerable effect," he explained.