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NCCIH Clinical Digest

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Omega-3 Fatty Acids:
What the Science Says

August 2013
Gel pills

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Heart Disease

Strength of Evidence

There has been a substantial amount of research on omega-3 supplements and heart disease. Experts agree that seafood 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.

Research Results

A Diet Rich in Seafood

  • Epidemiological studies done more than 30 years ago noted relatively low death rates due to cardiovascular disease in Eskimo populations with high seafood consumption. Since then, much research has been done on seafood and heart disease. The results provide moderate, though not conclusive evidence that people who eat seafood at least once a week are less likely to die of heart disease than those who rarely or never eat seafood.
  • The Dietary Guidelines for Americans, 2010 (2.9MB PDF) 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.

Omega-3 Supplements and Risk of Heart Disease

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 have been inconsistent; some showed evidence 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.

What We Know About Eating Seafood and Prevention of Heart Disease

  • Eating seafood rich in omega-3s a few times a week might provide enough omega-3s to protect the heart; more may not be better. A recent meta-analysis showed 1 to 4 servings of seafood per week was associated with a significant reduction in risk of heart disease mortality. However, consuming more than 5 servings of seafood per week was associated with only a marginal, additional protective effect on heart disease mortality.
  • Some of the benefits of a diet that includes regular seafood consumption may be related to factors other than its omega-3 content, or result from people eating seafood in place of less healthful foods.
  • There is also evidence that people who eat seafood have generally healthier lifestyles, and these other lifestyle characteristics may be responsible for the lower incidence of cardiovascular disease.

Rheumatoid Arthritis

Strength of Evidence

  • Much research has been conducted on the effects of omega-3s on the symptoms of rheumatoid arthritis.

Research Results

  • A 2012 systematic review concluded that the types of omega-3s found in seafood 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.

Infant Development

Strength of Evidence

  • Many studies have been conducted on the effects of omega-3 fatty acids and infant health, such as brain and eye development.

Research Results

  • 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, 2010 recommends that women who are pregnant or breastfeeding should consume 8 to 12 ounces of seafood per week from a variety of seafood types that are low in methyl mercury as part of a healthy eating pattern and while staying within their calorie needs.

Diseases of the Brain and the Eye

Strength of Evidence

Research on omega-3 fatty acids and diseases of the brain and eye are ongoing, but currently there is not enough evidence to draw conclusions about the effectiveness of omega-3s for these conditions.

Research Results

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 were investigated in a large National Institutes of Health (NIH)-sponsored study called Age-Related Eye Disease Study 2 (AREDS2), which showed that adding omega-3 fatty acids to a supplement combination of vitamin C, vitamin E, beta-carotene, and zinc did not improve the effectiveness of the supplement combination.
  • 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.


  • 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.
  • Whether people with fish or shellfish allergies can safely consume fish oil supplements has not been well studied.
  • 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 non-steroidal 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.
  • 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.
  • There is conflicting evidence on how DHA might affect the risk of prostate cancer:
    • Three large prospective studies now suggest concern that omega-3 fatty acids/DHA supplements may be associated with an increased risk of prostate cancer. A 2013 study showed an increased risk of prostate cancer among men with high blood concentrations of omega-3 fatty acids. 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.
    • On the other hand, a combined analysis of data from multiple studies showed that eating seafood, which is a good source of DHA, is associated with a lower likelihood of dying from prostate cancer.
Scientific Evidence on Omega-3s for Certain Conditions
  Some Evidence of Potential Benefit Insufficient, Mixed, or No Evidence of Potential Benefit
Cardiovascular Disease    
Diet Rich in Seafood X  
Omega-3 supplements Lowering Heart Disease Risk   X
Rheumatoid Arthritis X  
Infant Development X  
Eye Diseases    
Age-Related Macular Degeneration   X
Brain Diseases    
Cognitive Decline   X
Multiple Sclerosis   X
Mental and Behavioral Health Problems   X


NCCIH Clinical Digest is a service of the National Center for Complementary and Integrative Health, NIH, DHHS. NCCIH Clinical Digest, a monthly e-newsletter, offers evidence-based information on complementary health approaches, including scientific literature searches, summaries of NCCIH-funded research, fact sheets for patients, and more.

The National Center for Complementary and Integrative Health is dedicated to exploring complementary health products and practices in the context of rigorous science, training complementary health researchers, and disseminating authoritative information to the public and professionals. For additional information, call NCCIH's Clearinghouse toll-free at 1-888-644-6226, or visit the NCCIH Web site at NCCIH is 1 of 27 institutes and centers at the National Institutes of Health, the Federal focal point for medical research in the United States.


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This page last modified August 27, 2013