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

Description

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.

Overview

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 recent re-analysis of 40 years of research suggests that omega-3 fatty acid supplements do not reduce cancer risk. Evidence is mixed as to whether 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. In one study, fish oils appeared to help improve heart rhythm problems that can cause sudden death. However, they may also increase cholesterol and reduce blood clotting. 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.

How is it promoted for use?

Some people believe that omega-3 fatty acids protect against the spread of solid-tumor cancers (those that form solid masses) 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 released a report stating that eating foods containing omega-3 fatty acids is reasonable and possibly helpful in reducing risk of heart disease. More recent recommendations have suggested eating 2 or more servings of fatty fish per week and eating foods that are high in alpha-linolenic acid such as flaxseed, canola oil, soybeans, and walnuts to reduce cardiovascular disease risk. People who already have cardiovascular disease are recommended to eat more of these products or take supplements.

What is the evidence?

Although some research supports the anti-cancer claims made for omega-3 fatty acids, 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.

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.

A clinical study published in the journal Cancer concluded that omega-3 fatty acids seemed to prolong the survival of cancer patients who were also severely malnourished. An earlier small study looked at patients with advanced pancreatic cancer and severe weight loss. It compared use of EPA mixed with a high-protein, high-calorie supplement to use of the supplement without EPA. After 8 weeks, the study found that EPA did not help the patients gain weight. A more recent review of all research on the use of EPA for cancer-related weight loss found only 5 studies. The reviewers concluded that there was not enough information to determine whether EPA helped people with 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 high blood levels of triglycerides. In one study, fish oils appeared to help improve heart rhythm problems that can cause sudden death. However, they may also increase cholesterol and reduce blood clotting. Alpha-linolenic acid (ALA), the omega-3 fatty acid from vegetable oils, has not shown as strong an effect in studies to date, although it may reduce risk of fatal heart disease.

Research is also focusing on the role of omega-3 fatty acids in relation to omega-6 fatty acids. Omega-6 is another type of essential fatty acid that is found in many vegetable oils (such as corn, safflower, and sunflower oils), cereals, snack foods, and baked goods. Unlike omega-3s, omega-6s are plentiful in the typical American diet. Some researchers believe one cause of Americans' high rates of cardiovascular disease may be an imbalance in the ratio of omega-3 to omega-6 fatty acids. Ideally, the ratio of omega-3 to omega-6 fatty acids in the human body is 1-to-1. However, the typical American diet is low in omega-3s and high in omega-6s. Many people have ten to twenty times more omega-6 fatty acids than omega-3 fatty acids in their systems.

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.

The American Cancer Society's most recent nutrition guidelines recommend eating a balanced diet that includes 5 or more servings a day of vegetables and fruit, and choosing whole grains over processed and refined foods. Limiting intake of red meats and animal fats (including dairy fats) 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, or lamb. The guidelines note that although a diet high in fish can help lower heart disease risk, the clinical evidence regarding cancer is uncertain.

Are there any possible problems or complications?

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 may increase total blood cholesterol and inhibit blood clotting. People who take blood-thinning medications (anticoagulants) or aspirin should not take extra omega-3 because of the risk of excessive bleeding.

The source of some omega-3 fatty acids may also be a health concern. Many larger predatory fish contain toxic chemicals absorbed from pollution. Swordfish, shark, and tilefish (golden bass or golden snapper), for instance, are high in omega-3 fatty acids but may also contain high levels of mercury. King mackerel, a lesser source of omega-3s, may also have high mercury levels. Grouper, red snapper, and fresh or frozen tuna may have more moderate amounts of mercury. Other large fish, such as tuna and salmon, may contain other chemicals such as dioxin and polychlorinated biphenyls or PCBs, although fresh or frozen salmon usually has low mercury levels and large amounts of omega-3 fatty acids. Some studies have shown that farm-raised fish may carry more toxins than fish caught in the wild. Unfortunately, there is no way for a consumer to know what might be present in any particular fish, although some fish are inclined to have higher levels of contamination than others.

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. Mercury poses the greatest risk to young children and unborn babies. Young children and women who are pregnant, trying to get pregnant, or nursing should not eat fish likely to be highly contaminated. They should also limit their intake of moderately contaminated fish.

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.

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. Women who are pregnant or breast-feeding should talk to their doctor 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.

Additional resources

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-ACS-2345).

Guidelines for Using Complementary and Alternative Therapies

Dietary Supplements: How to Know What Is Safe

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

Complementary and Alternative Methods for Cancer Management

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer

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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.