Monday, 19 November 2012



Omega-3 fatty acids are a group of polyunsaturated fatty acids that are important for a number of functions in the body. They are found in foods such as fatty fish and certain vegetable oils and are also available in dietary supplements. This fact sheet provides basic information about omega-3 fatty acids (omega-3s)—with a focus on dietary supplements, summarizes scientific research on effectiveness and safety, and suggests sources for additional information.

Key Points

  • There has been a substantial amount of research on omega-3 supplements and heart disease. The findings of individual studies have been inconsistent. In 2012, two combined analyses of the results of these studies did not find convincing evidence that omega-3s protect against heart disease.
  • There is some evidence that omega-3s are modestly helpful in relieving symptoms in rheumatoid arthritis. Omega-3s may also be helpful for age-related muscular degeneration (AMD; an eye disease that can cause loss of vision in older people). For most other conditions for which omega-3s have been studied, definitive conclusions cannot yet be reached.
  • Omega-3 supplements may interact with drugs that affect blood clotting.
  • It is uncertain whether people with fish or shellfish allergies can safely consume fish oil supplements.
  • Fish liver oils (which are not the same as fish oils) contain vitamins A and D as well as omega-3 fatty acids; these vitamins can be toxic in high doses.
  • Tell all your health care providers about any complementary health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.

About Omega-3 Fatty Acids

The three principal omega-3 fatty acids are alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). The main sources of ALA in the U.S. diet are vegetable oils, particularly canola and soybean oils; flaxseed oil is richer in ALA than soybean and canola oils but is not commonly consumed. ALA can be converted, usually in small amounts, into EPA and DHA in the body. Fatty fish, such as salmon, mackerel, herring, sardines, and tuna, are rich sources of EPA and DHA; leaner types of fish and shellfish provide smaller amounts.
Commonly used dietary supplements that contain omega-3s include fish oil (which provides EPA and DHA) and flaxseed oil (which provides ALA). Algae oils are a vegetarian source of DHA.
Omega-3 fatty acids are important for a number of bodily functions, including muscle activity, blood clotting, digestion, fertility, and cell division and growth. DHA is important for brain development and function. ALA is an “essential” fatty acid, meaning that people must obtain it from food or supplements because the human body cannot manufacture it.

Safety

  • Omega-3 fatty acid supplements usually do not have negative side effects. When side effects do occur, they typically consist of minor gastrointestinal symptoms, such as belching, indigestion, or diarrhea.
  • It is uncertain whether people with fish or shellfish allergies can safely consume fish oil supplements.
  • Omega-3 supplements may extend bleeding time (the time it takes for a cut to stop bleeding). People who take drugs that affect bleeding time, such as anticoagulants (“blood thinners”) or nonsteroidal anti-inflammatory drugs (NSAIDs), should discuss the use of omega-3 fatty acid supplements with a health care provider.
  • Fish liver oils, such as cod liver oil, are not the same as fish oil. Fish liver oils contain vitamins A and D as well as omega-3 fatty acids. Both of these vitamins can be toxic in large doses. The amounts of vitamins in fish liver oil supplements vary from one product to another.
  • There is conflicting evidence on how DHA might affect the risk of prostate cancer. A 2011 study found that men with higher blood levels of DHA (which reflect higher intakes of this fatty acid) had an increased risk of high-grade (more aggressive) prostate cancer, and an earlier study found increased risks of both low-grade and high-grade prostate cancer in men with higher blood levels of DHA. However, other studies found no relationship or the opposite relationship between blood levels of DHA and prostate cancer risk, and a combined analysis of data from multiple studies showed that eating fish, which is a good source of DHA, is associated with a lower likelihood of dying from prostate cancer.

Use of Omega-3 Supplements in the United States

According to the 2007 National Health Interview Survey, which included a comprehensive survey on the use of complementary health approaches by Americans, fish oil/omega-3/DHA supplements are the nonvitamin/nonmineral natural product most commonly taken by adults, and the second most commonly taken by children. Among survey participants who had used selected natural products in the last 30 days, about 37 percent of adults (10.9 million) and 31 percent of children (441,000) had taken an omega-3 supplement for health reasons.

Cardiovascular Disease

Experts agree that fish rich in omega-3 fatty acids should be included in a heart-healthy diet. However, omega-3s in supplement form have not been shown to protect against heart disease.
  • Epidemiological studies done more than 30 years ago noted relatively low death rates due to cardiovascular disease in Eskimo populations with high fish consumption. Since then, much research has been done on fish and heart disease. The results provide strong, though not conclusive evidence that people who eat fish at least once a week are less likely to die of heart disease than those who rarely or never eat fish.
  • The 2010 edition of the Federal Government’s Dietary Guidelines for Americans includes a new recommendation that adults eat 8 or more ounces of seafood (fish or shellfish) per week because it provides a range of nutrients, including omega-3 fatty acids. (Smaller amounts are recommended for young children.) The Dietary Guidelines report notes that eating the recommended amount of seafood is associated with reduced heart disease deaths.
  • Many studies have evaluated the effects of omega-3 supplements on heart disease risk. In these studies, researchers compared the number of cardiovascular events (such as heart attacks or strokes) or the number of deaths in people who were given omega-3 supplements with those in people who were given inactive substances (placebos) or standard care. Most of these studies involved people who already had evidence of heart disease. A smaller number of studies included people with no history of heart disease. The results of individual studies were inconsistent; some indicated that omega-3s were protective, but others did not. In 2012, two groups of scientists conducted meta-analyses of these studies; one group analyzed only studies in people with a history of heart disease, and the other group analyzed studies in people both with and without a history of heart disease. Neither meta-analysis found convincing evidence of a protective effect of omega-3s.
  • There are several reasons why omega-3 supplements may not help to prevent heart disease even though a diet rich in fish apparently does. Eating fish a few times a week might provide enough omega-3s to protect the heart; more may not be better. Some of the benefits of fish may result from people eating it in place of less healthful foods. There is also evidence that people who eat fish have generally healthier lifestyles, and these other lifestyle characteristics may be responsible for the lower incidence of cardiovascular disease.

A 2012 systematic review concluded that the types of omega-3s found in fish and fish oil are modestly helpful in relieving symptoms of rheumatoid arthritis. In the studies included in the review, many of the participants reported that when they were taking fish oil they had briefer morning stiffness, less joint swelling and pain, and less need for anti-inflammatory drugs to control their symptoms.Rheumatoid Arthritis

Infant Development

A moderate amount of evidence suggests that consuming adequate amounts of omega-3 fatty acids, especially DHA, during pregnancy and breastfeeding is associated with improvements in infant health, such as brain and eye development. The Dietary Guidelines for Americans recommend that women who are pregnant or breastfeeding consume at least 8 ounces but no more than 12 ounces of seafood each week and not eat certain types of fish that are high in mercury—a toxin that can harm the nervous system of a fetus or young child.

Diseases of the Brain and the Eye

The results of research on omega-3 fatty acids and diseases of the brain and eye have been promising, but there is not enough evidence to draw conclusions about the effectiveness of omega-3s for these conditions.
  • DHA plays important roles in the functioning of the brain and the eye. Researchers are actively investigating the possible benefits of omega-3 fatty acids in preventing or treating a variety of brain- and eye-related conditions, including the following:
    • Diseases of the eye, such as age-related macular degeneration (AMD; an eye disease that can cause vision loss in older people) and dry eye syndrome. The effects of omega-3s on AMD are being investigated in a large National Institutes of Health (NIH)-sponsored study called Age-Related Eye Disease Study 2 (AREDS2). Previous studies have shown that people who eat diets rich in fish are less likely to develop the advanced stage of this eye disease. AREDS2 is designed to determine whether an omega-3 fatty acid supplement will reduce the likelihood that people who already have the intermediate stage of AMD will progress to the advanced stage.
    • Diseases of the brain or nervous system, such as cognitive decline and multiple sclerosis.
    • Mental and behavioral health problems, such as depression, attention-deficit hyperactivity disorder, autism, bipolar disorder, borderline personality disorder, and schizophrenia.
  • NCCAM and other institutes and centers of NIH have supported studies on the use of omega-3s in several of these conditions, including AMD, depression, and multiple sclerosis.

Other Conditions

Omega-3s also have been studied for preventing or treating a variety of other conditions such as allergies, asthma, cachexia (severe weight loss) associated with advanced cancer, Crohn’s disease, cystic fibrosis, diabetes, kidney disease, lupus, menstrual cramps, obesity, osteoporosis, and ulcerative colitis, as well as organ transplantation outcomes (e.g., decreasing the likelihood of rejection). No conclusions can be drawn about whether omega-3s are helpful for these conditions based on currently available evidence.

If You Are Considering Omega-3 Supplements

  • Do not use omega-3 supplements to replace conventional care or as a reason to postpone seeing a health care provider about a medical problem.
  • Consult your health care provider before using omega-3 supplements. If you are pregnant or nursing a child, if you take medicine that affects blood clotting, if you are allergic to fish or shellfish, or if you are considering giving a child an omega-3 supplement, it is especially important to consult your (or your child’s) health care provider.
  • Look for published research studies on omega-3 supplements for the health condition that interests you. Information on evidence-based studies is available from NCCAM atnccam.nih.gov/health/omega3.
  • Tell all your health care providers about any complementary health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care. For tips about talking with your health care providers about complementary health approaches, see NCCAM’s Time to Talk campaign.

NCCAM-Funded Research

Recent NCCAM-sponsored studies have been investigating the effects of omega-3s/fish oil on conditions including:
  • Adolescent depression
  • Autism spectrum disorders
  • Brain injury
  • Complications of HIV infection, including bone loss
  • Depression during pregnancy and postpartum depression
  • Treatment-resistant epilepsy.
Other NIH research includes studies on the effects of omega-3s/fish oil on many different conditions, including:
  • Autoimmune diseases
  • Alzheimer’s disease
  • Diabetic kidney disease
  • Pregnancy outcome, including infant health and development
  • Sudden cardiac death.
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