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Omega-3 in fish: How eating fish helps your heart

By Mayo Clinic staff

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Omega-3 in fish: How eating fish helps your heart

The omega-3 fatty acids in fish are good for your heart. Find out how the heart-health benefits of eating fish usually outweigh any risks.

By Mayo Clinic staff

If you're worried about heart disease — whether you want to avoid it, or you already have it and want to get healthier — eating one to two servings of fish a week could reduce your risk of dying of a heart attack by a third or more.

Doctors have long recognized that the unsaturated fats in fish, called omega-3 fatty acids, appear to reduce your risk of dying of heart disease. For many years, the American Heart Association has recommended that people eat fish rich in omega-3 fatty acids at least twice a week.

But some people are still concerned about mercury or other contaminants in fish outweighing its heart-healthy benefits. However, when it comes to a healthier heart, the benefits of eating fish usually outweigh the possible risks of exposure to contaminants. Find out how to balance these concerns with adding a healthy amount of fish to your diet.

What are omega-3 fatty acids, and why are they good for your heart?

Fish contain unsaturated fatty acids, which, when substituted for saturated fatty acids such as those in meat, may lower your cholesterol. But the main beneficial nutrient appears to be omega-3 fatty acids in fatty fish. Omega-3 fatty acids are a type of unsaturated fatty acid that's thought to reduce inflammation throughout the body.

Omega-3 fatty acids are also believed to improve learning ability in children, decrease triglycerides, lower blood pressure, reduce blood clotting, enhance immune function and improve arthritis symptoms. Consuming one to two servings a week of fish, particularly fish that's rich in omega-3 fatty acids, appears to reduce the risk of heart disease, particularly sudden cardiac death.

Does it matter what kind of fish you eat?

Fatty fish, such as salmon, herring, and to a lesser extent tuna, contain the most omega-3 fatty acids and therefore the most benefit, but many types of seafood contain small amounts of omega-3 fatty acids. Most freshwater fish have less omega-3 fatty acids than do fatty fish from the sea. Some varieties of trout have relatively high levels of omega-3 fatty acids.

Are there any kinds of fish you should avoid?

Some fish, such as tilapia and catfish, have low levels of omega-3 fatty acids and high levels of arachidonic acid, a type of omega-6 fatty acid that's also found in red meat and egg yolks. Eating too much arachidonic acid can increase your risk of heart disease because it can cause inflammation, which can contribute to the buildup of plaques in your arteries. This can cause coronary artery disease, a heart attack or stroke.

How much fish should you eat?

For adults, at least two servings of omega-3-rich fish a week are recommended. A serving size is 3 ounces (85 grams), or about the size of a deck of cards. Women who are pregnant or plan to become pregnant and children under age 12 should limit the amount of fish they eat because they're most susceptible to the potential effects of toxins in fish.

Does mercury contamination outweigh the health benefits of eating fish?

The risk of getting too much mercury or other contaminants from fish is generally outweighed by the health benefits that omega-3 fatty acids have. The main types of toxins in fish are mercury, dioxins and polychlorinated biphenyls (PCBs). The amount of toxins depends on the type of fish and where it's caught.

Mercury occurs naturally in small amounts in the environment. But industrial pollution can produce mercury that accumulates in lakes, rivers and oceans. Microorganisms in these bodies of water convert the mercury to a highly toxic form, called methyl mercury. When fish eat these microorganisms, methyl mercury builds up in the bodies of the fish.

Large fish that are higher in the food chain — such as shark, tilefish, swordfish and king mackerel — tend to have higher levels of methyl mercury than do smaller fish. Larger fish eat the smaller fish, gaining higher concentrations of the toxin. The longer a fish lives, the larger it grows and the more mercury it can collect.

Pay attention to the type of fish you eat, how much you eat, and other information such as state advisories. Each state issues advisories regarding the safe amount of locally caught fish that can be consumed.

Should any groups of people avoid eating fish because of the concerns over mercury or other contaminants?

If you eat enough fish containing methyl mercury, the toxin can accumulate in your body. It can take weeks, months or even a year for your body to remove these toxins. Methyl mercury is particularly harmful to the development of the brain and nervous system of an unborn child and young children. For the average adult, however, it's unlikely that methyl mercury would cause any health concerns.

Still, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) recommend that women who are pregnant or trying to become pregnant, nursing mothers, and children under age 12 need to limit the amount of fish they eat. Pregnant women and children can still get the heart-health benefits of fish by eating fish that's typically low in mercury, such as salmon, and limiting the amount they eat to:

  • No more than 12 ounces (340 grams) of fish in total a week
  • No more than 6 ounces (170 grams) of canned tuna a week
  • No amount of any fish that's typically high in mercury (shark, swordfish, king mackerel and tilefish)

Can you get the same heart-health benefits by eating other foods that contain omega-3 fatty acids, or by taking omega-3 fatty acid supplements?

The evidence is stronger for the benefits of eating fish rich in omega-3 fatty acids compared with using supplements. However, people who have heart disease may benefit from supplements of omega-3 fatty acids and should discuss this with their doctors.

Other nonfish food options that do contain some omega-3 fatty acids include flaxseed, flaxseed oil, walnuts, canola oil, soybeans and soybean oil. However, similar to supplements, the evidence of heart-healthy benefits from eating these foods isn't as strong as it is from eating fish. More research on the effects of supplements is necessary.

References
  1. Effects of omega-3 fatty acids on cardiovascular disease. Rockville, Md.: Agency for Healthcare Research and Quality. http://www.ahrq.gov/downloads/pub/evidence/pdf/o3cardio/o3cardio.pdf. Accessed Nov. 24, 2008.
  2. Hooper L, et al. Risks and benefits of omega-3 fats for mortality, cardiovascular disease, and cancer: Systematic review. British Medical Journal. 2006;332:752.
  3. Fish and omega-3 fatty acids. American Heart Association. http://www.americanheart.org/presenter.jhtml?identifier=4632. Accessed Nov. 24, 2008.
  4. Weaver KL, et al. The content of favorable and unfavorable polyunsaturated fatty acids found in commonly eaten fish. Journal of the American Dietetic Association. 2008;108:1178.
  5. Wang C, et al. n-3 Fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies: A systematic review. American Journal of Clinical Nutrition. 2006;84:5.
  6. Lichtenstein AH, et al. Diet and lifestyle recommendations revision 2006: A scientific statement from the American Heart Association Nutrition Committee. Circulation. 2006;114:82.
  7. What you need to know about mercury in fish and shellfish. U.S. Food and Drug Administration and Environmental Protection Agencies. http://www.epa.gov/waterscience/fish/files/MethylmercuryBrochure.pdf. Accessed Dec. 1, 2008.
  8. Philibert, A, et al. Fish intake and serum fatty acid profiles from freshwater fish. American journal of clinical nutrition. 2006; 84:1299.

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Dec. 31, 2008

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