Eating fish is good for you, but it won't prevent cancer, according to a new study.

It's no reason to stop eating fish rich in the important nutrients called omega-3 fatty acids. Omega-3 fatty acids cut your risk of heart disease and stroke and aid children's early mental development. They seem to do many other good things, too.

They just do not just fight cancer, find RAND researcher Catherine H. MacLean, MD, PhD, and colleagues. As part of a huge, government-funded study, MacLean's team looked for every scrap of evidence on omega-3 fatty acids and cancer risk. They analyzed 38 large studies that evaluated the risk of 11 different cancers in people who consumed different amounts of omega-3 fatty acids.

"Overall these studies -- ranging from 6,000 to 121,000 people, with 3 million person-years of observation, with people from a number of different populations, in a number of different countries -- we see a consistent finding," MacLean tells WebMD. "Over and over again we see that omega-3 fatty acids don't appear to reduce a person's risk of developing cancer."

The findings appear in the Jan. 25 issue of The Journal of the American Medical Association.

Omega-3 Fats Still Recommended

Nutritionist Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh and team nutritionist for the Pittsburgh Steelers, still recommends a diet that gives you plenty of omega-3 fatty acids. She isn't a bit deterred by the finding that the nutrient doesn't prevent cancer.

"It's still important to have omega-3 fatty acids in the diet," Bonci tells WebMD. "For heart disease, the benefit is clear. We know it cuts down on the amount of fat in the blood and reduces risk of stroke. That is very powerful information."

Omega-3 fatty acids are a type of unsaturated fat. People who eat plenty of these healthy fats are significantly less likely to get -- or to die from -- heart disease. Evidence behind this benefit came from an earlier study sponsored by the U.S. Agency for Healthcare Research and Quality (AHRQ), which also sponsored the MacLean study.

"We did find a positive relationship with omega-3 fatty acids and protection against heart disease," AHRQ Director Carolyn M. Clancy, MD, tells WebMD. "The findings are not specific enough to say if you eat salmon once a week and tuna twice a week you are OK. But it can help with heart disease -- and there is no strong evidence of harm."

Omega-3 Evidence Unclear

It's not clear why omega-3 fatty acids have this effect. One reason may be that they keep blood vessels from getting inflamed. Inflamed blood vessels tend to become clogged, leading to heart disease and stroke. But heart patients aren't the only ones who might benefit from omega-3 fatty acids.

"The anti-inflammatory aspect of omega-3 fatty acids may be important for people with psoriasis, arthritis, and asthma," Bonci says.

MacLean, too, praises the benefits of omega-3 fatty acids.

"There is some evidence it may lower blood pressure, reduce the risk of coronary artery reblockage after angioplasty, and increase exercise capacity among people with heart disease," she says. "And it may reduce risk of abnormal heart rhythm. Those are important health benefits. I would not throw away the omega-3 fatty acids. I just would not take them for the benefit of preventing cancer."

Eat fish if you want to get omega-3 fatty acids in your diet. Tuna is the main source of this nutrient in the U.S.

Don't like tuna? You can get omega-3 fatty acids from other fatty fish (such as salmon, mackerel, halibut, sardines, and herring), flaxseeds, flaxseed oil, canola oil, soybeans, pumpkin seeds, and walnuts.

You can also take omega-3 fatty acid supplements. But Bonci says it's better to get it from fish.

"The supplements certainly are going to be much more concentrated than a serving of fish," she says. "The bad news is that for so many products out there, there is concern about purity. And when taking fish-oil supplements, some people get gastrointestinal upset -- and reflux can be an issue. That doesn't usually happen when someone has a piece of salmon. And there are calories involved in taking these supplements. You might as well get the piece of fish along with the calories.”

Scientists Never Say Never

Is the MacLean analysis scientific proof that omega-3 fatty acids can't protect against cancer? No.

Ten of the studies that MacLean's team analyzed did find protection. But each of these positive studies was balanced by one or more studies that found no protection -- or even linked omega-3 fatty acids to increased cancer risk.

But taken together, the studies show no scientific evidence that omega-3 fatty acids protect against cancer. There is no absolute proof of no benefit, but it convincingly shows that there's no evidence of a benefit.

"In science it is hard to ever say never," MacLean says. "Should some research be done that comes up with a plausible reason for why omega-3 fatty acids might have a beneficial effect for a particular type of cancer, then I would say, 'Yes, do more research.' But unless someone comes up with such compelling new evidence, I would not think more research is needed."

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: MacLean, C.H. The Journal of the American Medical Association, Jan. 25, 2006; vol 295: pp 403-415. Catherine H. MacLean, MD, PhD, natural scientist, RAND, Santa Monica, Calif.; and assistant professor of medicine, Greater Los Angeles VA Healthcare System. Leslie Bonci, MPH, RD, director, sports nutrition, University of Pittsburgh Medical Center. Carolyn M. Clancy, MD, director, Agency for Healthcare Research and Quality.