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What to Know About Omega-3s and Fish

Experts explain which fish are best for omega-3s, and which you should limit due to mercury.

From the WebMD Archives

Fish and omega-3 fatty acids. If you keep up with the latest nutrition news, you may have a pretty good sense of what they offer. But, if you're like many people, you still can't tell your omega-3s from your omega-6s -- and you sure as heck can't pronounce eicosapentaenoic acid. That's OK. Our fishing expedition turned up some interesting facts to share about omega-3 fatty acids and fish.

What Are Omega-3 Fatty Acids?

Omega-3 fatty acids are found in fatty layers of cold-water fish and shellfish, plant and nut oils, English walnuts, flaxseed, algae oils, and fortified foods. You can also get omega-3s as supplements. Food and supplement sources of these fatty acids differ in the forms and amounts they contain.

There are the two main types of omega-3 fatty acids:

  • Long-chain omega-3 fatty acids are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). These are plentiful in fish and shellfish. Algae often provides only DHA.
  • Short-chain omega-3 fatty acids are ALA (alpha-linolenic acid). These are found in plants, such as flaxseed. Though beneficial, ALA omega-3 fatty acids have less potent health benefits than EPA and DHA. You'd have to eat a lot to gain the same benefits as you do from fish.

Fishing for Facts: What Studies Reveal About Omega-3s and Fish

In addition to omega-3s, fish is high in protein, vitamins, and minerals. And, it's low in saturated fat.

Hundreds of studies suggest that omega-3s may provide some benefits to a wide range of diseases: cancer, asthma, depression, cardiovascular disease, ADHD, and autoimmune diseases, such as rheumatoid arthritis.

How could fatty acids be so beneficial for so many different conditions?

"All these diseases have a common genesis in inflammation," says Joseph C. Maroon, MD, professor and vice chairman of the department of neurological surgery at the University of Pittsburgh School of Medicine. Co-author of Fish Oil: The Natural Anti-Inflammatory, Maroon says that in large enough amounts omega-3's reduce the inflammatory process that leads to many chronic conditions.

For these and other reasons, the Department of Health and Human Services (HHS), the U.S. Department of Agriculture (USDA), the American Heart Association, and the American Dietetic Association recommend eating two 8-ounce servings of fish each week.

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The Other Fatty Acid: Omega-6

Unfortunately, the American diet is swimming in omega-6s instead, says Jeffrey Bost, PAC, clinical instructor in the department of neurosurgery at the University of Pittsburgh Medical Center and also co-author of Fish Oil: The Natural Anti-Inflammatory.

"It's in almost everything we eat," he says. "Our diet has shifted away from fresh veggies and fish to foods high in omega-6s, such as crackers, cookies, and corn-fed beef."

Before the introduction of grains, fats, and artificial substances, says Maroon, the ratio of omega-6s to omega-3s was two to one. Today, we consume at least 20 times more omega-6s than omega-3s. The problem is that excessive amounts of omega-6 fatty acids can promote inflammation, a key step in many chronic diseases.

Powerful Health Benefits of Omega-3s

Many studies documenting the benefits of omega-3s have been conducted with supplemental daily dosages between 2 and 5 grams of EPA and DHA, more than you could get in 2 servings of fish a week. But that doesn't mean eating fish is an exercise in futility. Many studies document its benefits. For example, a 2003 National Eye Institute study showed that 60- to 80-year-olds eating fish more than twice a week were half as likely to develop macular degeneration as those who ate no fish at all.

Here's a sample of other recent studies on omega-3s and fish.

Omega-3 Fatty Acids as Brain Food

DHA is one of the most prevalent fatty acids in the brain. This could partly explain why our brains do better with a greater supply. A Rush Institute for Healthy Aging study analyzed fish-eating patterns of more than 800 men and women, ages 65 to 94. Those eating fish at least once a week were much less likely to develop Alzheimer's disease than those who turned up their nose at it.

Another study of more than 2,000 Norwegians, ages 70 to 74, used food-frequency questionnaires to evaluate consumption of five different types of fish. The researchers then conducted cognitive tests. Those who ate fish of any kind were two to three times less likely to perform poorly on the tests.

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Investigators at the University of Kuopia, Finland, and at Harvard Medical School looked at the incidence of silent brain damage in about 3,500 people age 65 or older. Eating tuna or other non-fried fish was associated with a 25% lower risk of these abnormalities, which are linked to higher rates of stroke and cognitive decline.

While omega-3 fatty acids have a number of benefits, these studies do not prove a cause and effect, only that there is an association between eating fatty acids in fish and the risks of Alzheimer's disease or the risks of dementia.

Cancer Prevention With Omega-3s

Among 1,300 Swedish men, those who ate salmon and similar fish, such as herring or mackerel, had a much lower risk of developing prostate cancer than those who didn't eat fish. Those eating five or more servings a week had a 64% lower risk of the disease.

Omega-3s for Healthier Arteries

Following postmenopausal women in Finland and the United States, investigators found that those eating two or more servings of fish each week had healthier arteries than women who ate less than two servings. Benefits were even greater in those eating tuna or another type of dark fish at least once a week.

To Fish or not to Fish: Weighing the Benefits and Risks

Mercury and polychlorinated biphenyls (PCBs) are common toxins in seafood. Although the U.S. banned the use of PCBs and DDT in 1976, these and other chemicals are still used in half the world's commercial chemical processes. Substances like these can hang around in the air, soil, and water for many years. They end up in the bodies of fish and animals.

The higher on the food chain, the greater the accumulation of toxins. Fish that eat plants are less contaminated than those that eat other fish. That's why it's better, in general, to eat smaller fish lower on the food chain or smaller portions of fish that may be contaminated.

The FDA released an advisory about fish. The alert wasn't meant for everyone. It was directed at women who were planning to become pregnant, were already pregnant, or were nursing a young child. For this group of people, the FDA advised against eating shark, swordfish, king mackerel, or tilefish -- which contain high levels of mercury.

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The FDA didn't throw all the proverbial fish back in the water, though. It recommended eating two meals, or up to 12 ounces a week, of a variety of fish and shellfish containing lower amounts of mercury. Safer sources cited were:

  • canned light tuna
  • catfish
  • pollock
  • salmon, especially wild salmon
  • shrimp

The FDA also made similar recommendations for feeding fish and shellfish to young children, but in smaller portions. It recommended checking local advisories for information about locally caught fish.

Questioning Conventional Wisdom About Fish

Muddying the waters, though, are research results outside the U.S. Some of these studies challenge U.S. assumptions and advice about fish consumption by pregnant women.

In a U.K. study, children of mothers who ate more than 12 ounces a week actually scored better on tests of verbal I.Q., social behavior, and development and communication than children of mothers who ate none. In the Seychelles Islands, where people average 12 fish meals -- not ounces -- a week, there are no reports of links between mercury exposure and poor outcomes in children. These studies suggest that eating less than 12 ounces of fish each week could do more harm to a child's developing neurological system than mercury poisoning.

Unfortunately, fears about mercury and other pollutants may have caused Americans to start eating less fish. Following the FDA's advisory, the Center for Food, Nutrition, and Agriculture Policy at the University of Maryland took an opinion poll of more than 1,000 Americans. What they found was this:

  • A little more than a third ate fish occasionally.
  • More than 1 in 10 were eating less fish and feeding less to their children than before the advisory came out.
  • Most people didn't realize the FDA aimed its advisory at only certain groups: women who are pregnant, nursing babies, or planning to get pregnant soon.

Reaping the Best Benefits of Fish and Omega-3s

You can take several steps to get the best benefits from fish.

Deep-six the omega-6s. Foods high in polyunsaturated fats, such as soybean, corn, sunflower, or safflower oils hurt you in more ways than one. Apparently these omega-6 fatty acids, when eaten in excess, can reduce your body's ability to metabolize the beneficial omega-3 fatty acids.

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Cast about for healthy canned tuna. Think all tuna is created equal? Think again. Choose canned light tuna instead of tuna steaks or albacore tuna. It tends to have less mercury. Albacore may contain three times the mercury of chunk light tuna. Check fish guides for the latest information about foods low in toxins but high in omega-3. Two good online sources are:

  • Monterey Bay Aquarium's Seafood Watch web site
  • Environmental Defense Fund web site

Do the sniff test. Buy the freshest fish you can find. The longer a fish is exposed to oxygen, says Bost, the more it loses some of its omega-3 benefits.

Cook it up right. You can't remove toxins by cooking, but you minimize exposure to PCBs by removing fish skin and surface fat before eating.

WebMD Feature Reviewed by Brunilda Nazario, MD on October 17, 2011

Sources

SOURCES:

Fats of Life web site.

Maroon, J. and Bost, J. Fish Oil: The Natural Anti-Inflammatory, Basic Health Publications Inc., 2006.

David K. Becker, MD, MPH, assistant clinical professor, department of pediatrics, University of California, San Francisco.

FDA: "FDA and EPA Announce the Revised Consumer Advisory on Methylmercury in Fish."

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