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Omega-3 Fatty Acids

Other common name(s): fish oil, fish oil supplements, marine oil, cod liver oil

Scientific/medical name(s): alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid; also called n-3 fatty acids or n-3 polyunsaturated fatty acids


Omega-3 fatty acids are important nutrients that are involved in many bodily processes. The body cannot make these fatty acids and must obtain them from food sources or from supplements. Three fatty acids compose the omega-3 family: alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid. Alpha-linolenic acid (ALA) is found in English walnuts, in some types of beans, and in canola, soybean, flaxseed/linseed, and olive oils. The other 2, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are found in fish, including fish oil and supplements.


Some studies in animals have found that fish oils rich in omega-3 fatty acids suppress the formation and growth of some types of cancer. Studies in humans have produced conflicting results. A re-analysis of 40 years of research suggests that omega-3 fatty acid supplements do not reduce cancer risk. Evidence so far does not show that fish oil supplements improve cancer-related weight loss.

Studies have shown that the fatty acids in fish oil help protect against heart disease and reduce risk factors such as triglycerides. Fish oils appear to help improve heart rhythm problems that can cause sudden death. The omega-3 fatty acid from vegetable oils (ALA) has not shown as strong an effect in studies to date, although it may reduce risk of fatal heart disease. Lab and animal studies suggest that omega-3 fatty acids can cause bleeding but studies in humans to date have not borne this out.

How is it promoted for use?

Some people believe that omega-3 fatty acids protect against the spread of cancers that are related to hormone production, particularly breast cancer. Some also believe that omega-3 fatty acids inhibit the growth of colon, pancreatic, and prostate cancer. Some people and groups advocate use of omega-3 fatty acids to protect against cardiovascular disease and fatal heart attacks. Others believe that omega-3 fatty acids help rheumatoid arthritis, Crohn's disease, eczema, asthma, kidney failure, depression, and more.

What does it involve?

Oils from some cold-water fish such as sardines, salmon, herring, mackerel, halibut, striped bass, tuna, shark, and cod have high concentrations of the omega-3s fatty acids DHA and EPA. Oil from flaxseed contains more alpha-linolenic acid than any other known plant source (see Flaxseed Oil). Other plant sources of ALA include great northern beans, kidney beans, navy beans, and soybeans (see Soybeans).

Omega-3 supplements, such as fish oil, fish oil capsules, and cod liver oil (also called marine oils), are available at pharmacies and natural food stores.

Some nutritionists recommend eating a diet rich in fish containing omega-3 fatty acids, eating 1 to 2 teaspoons of flaxseed or flaxseed oil daily, or taking daily supplements containing 1 to 2 grams of omega-3s. Omega-3 fatty acids are very unstable and spoil easily, so food manufacturers often remove them from foods to increase shelf life.

What is the history behind it?

Cod liver oil became popular in 19th century England as a vitamin D supplement for sun-deprived children. In the 1950s, a German scientist named Johanna Budwig, PhD, discovered essential fatty acids and developed a diet that she said would fight cancer. Dr. Budwig claimed that many of her patients experienced tumor reduction within 3 months, and she stated that some experienced even more dramatic results. Dr. Budwig has reportedly used omega-3 fatty acids in combination with other nutrients to treat thousands of people with cancer and other diseases.

In 1996, the American Heart Association (AHA) released a report stating that eating foods containing omega-3 fatty acids is reasonable and possibly helpful in reducing risk of heart disease. As of 2012, the AHA recommended eating at least 2 servings of fish, preferably oily fish (salmon, tuna, mackerel, herring and trout), each week to help reduce cardiovascular disease risk. Tofu and other forms of soybeans, canola, walnut, and flaxseed, and their oils are also recommended for their omega-3 fatty acids. The AHA suggests that people who already have cardiovascular disease should eat more foods containing omega-3s and consider talking with their doctors about taking supplements.

What is the evidence?

Prevention. Although some research supports the anti-cancer claims made for omega-3 fatty acids from foods, some does not. Some studies even show an increase in disease when omega-3 supplements are used. The strongest evidence for the health benefits of fatty acids from fish is in the area of heart disease and its risk factors. The relationship between omega-3 fatty acids, cancer, and other diseases is not as well known.

A large study followed more than 34,000 women from 1980 to 1998, observing their fish intake and the ratio of fish fatty acids to omega-6 fatty acids in their diets to determine how the ratio affected their colorectal cancer risk. Women who took in more omega-3 fatty acids did not have a lower colorectal cancer risk, but they had fewer large benign colorectal tumors, or adenomas. This study suggests that omega-3 fatty acids may not reduce colorectal cancer risk but may slow its growth. More research is needed to find out if this holds true.

In 2006, researchers reviewed 38 studies conducted over the past 40 years on the effects of omega-3 fatty acids. Researchers looked at studies that showed positive effects, no effects, and negative effects of omega-3 fatty acids on the development of cancer. In the final analysis, it appeared that there was no effect overall. Researchers concluded that omega-3 supplements are unlikely to prevent cancer.

However, a 2010 study looked at 55 people with familial adenomatous polyposis (FAP), a genetic disorder that causes colon polyps which can progress to cancer. One group was given a special enteric-coated formula of EPA (enteric coating prevents dissolving in the stomach, which might help reduce fishy breath and belching), while the other group got inactive pills. The group that got the special EPA had fewer and smaller polyps after 6 months. This finding deserves further study to learn whether the same thing might happen in other groups.

The American Cancer Society's 2012 nutrition guidelines to help reduce cancer risk recommend choosing foods and beverages in amounts that help you get to and stay at a healthy weight. Eating a balanced diet that includes 2½ cups of vegetables and fruits each day, and choosing whole grains over refined grains and sugar-sweetened products should be part of this plan. Limiting intake of red meats and processed meats such as bacon, sausage, lunch meats, and hot dogs is also recommended in order to help reduce cancer risk. A good way to do this is to choose fish, poultry, or beans for some meals rather than beef, pork, lamb, or processed meats. The guidelines note that although eating fish is linked to a lower risk of heart disease, the evidence regarding cancer in humans is limited.

Cancer-related weight loss. One 2010 study tested EPA and other methods to improve cancer-related weight loss, and reported that EPA when given alone did not help. The patients who got appetite stimulants and thalidomide--a drug with anti-inflammatory effects--along with EPA showed improved body mass, better appetite, and less fatigue. Other studies are still looking at this to see if omega-3 fatty acids might help maintain weight in people with cancer. A 2012 analysis of previous research concluded that EPA supplements do not prevent weight loss in people with advanced pancreatic cancer, and that there was not enough evidence to draw conclusions regarding other types of cancer.

Heart disease. Studies have shown that the fatty acids in fish oil help protect against heart disease and reduce risk factors such as high blood levels of triglycerides. Fish oils also appeared to help improve heart rhythm problems that can cause sudden death.

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

The source of some omega-3 fatty acids may be a health concern. Many large predatory fish contain toxic chemicals absorbed from pollution. Unfortunately, there is no way for a consumer to know what might be present in any particular fish, although some fish tend to have higher levels of contamination than others. Swordfish, shark, king mackerel, and tilefish (golden bass or golden snapper), for instance may contain high levels of mercury. Grouper, red snapper, and fresh or frozen tuna may have smaller amounts.

Mercury poses the greatest risk to young children and unborn babies because of effects on the developing nervous system. Young children and women who are pregnant, trying to get pregnant, or nursing should not eat swordfish, shark, king mackerel, or tilefish. Fish that are lower in mercury such as canned light tuna, salmon, pollock, catfish, and shrimp are safer for children and women of childbearing age, but still should be limited to 2 servings per week. (White or albacore tuna should be eaten only once per week.) The precise risks and benefits of eating these fish are not known at this time. Experts recommend that adults vary the type of fish they eat as part of a healthy, balanced diet to reduce the chances of getting too many contaminants.

For men and middle-aged or older women (after menopause), the benefits of eating fish may outweigh the risks of mercury or other contaminants. Even so, experts suggest limiting intake of the most-contaminated fish to one serving per week. Most refined fish oil supplements have little or none of these contaminants, but it’s best to buy from reputable sources that test their supplements for safety.

Prolonged use of fish oil supplements can cause vitamin E deficiency, which is why vitamin E is added to many supplements. Fish liver oils (such as cod liver oil) can cause toxic levels of vitamins A and D if overused. Supplements may also cause fishy breath odor, belching, or abdominal bloating. They may also increase a tendency toward anemia in menstruating women.

Not enough is known about omega-3 fatty acids to determine whether they are safe in large quantities or when taken with other drugs. Omega-3s appear to affect platelets and inhibit blood clotting in laboratory and animal studies. But so far, studies in people having surgery or other situations have not shown excessive bleeding in those with high levels of omega-3 fatty acids.

Studies of rats that were fed large amounts of omega 3-fatty acid during pregnancy had offspring with poor weight gain, shorter life spans, and hearing problems later in life. Even though human studies have not been done, women who are pregnant or breast-feeding should talk to their doctors before adding extra omega-3 to their diets.

People who are allergic to fish may have serious reactions to fish oil or supplements made from fish and should avoid them. People who are allergic to nuts should avoid supplements that are made of the type of nuts to which they react.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

To learn more

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-227-2345).

Dietary Supplements: What Is Safe?

ACS Guidelines on Nutrition and Physical Activity for Cancer Prevention

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 01/17/2013
Last Revised: 01/17/2013