The human body can make most of the types of fats it needs from other fats or carbohydrates. That isn’t the case for omega-3 polyunsaturated fatty acids (also called omega-3 fats and n-3 fats). These are essential fats—the body can’t make them from scratch but must get them from food. Foods high in omega-3 include certain fish and seafood, some vegetable oils, nuts (especially walnuts), flax seeds, and leafy vegetables.

What makes omega-3 fats special? They are needed to build cell membranes throughout the body and affect the function of the cell receptors in these membranes. They also provide the starting point for making hormones that regulate blood clotting, contraction and relaxation of artery walls, and inflammation. In addition, they can bind to receptors in cells that regulate genetic function. Due to these effects, omega-3 fats can help prevent heart disease and stroke, may help control lupus, eczema, and rheumatoid arthritis, and may play protective roles in cancer and other conditions. [1]

Types of Omega-3s

There are two main types of omega-3 fats that have essential roles in human health:

  • EPA and DHA: Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) come mainly from cold-water fish, so they are sometimes called marine omega-3s. Salmon, mackerel, tuna, herring, and sardines contain high amounts of EPA/DHA. EPA and DHA can be made from another omega-3 fat called alpha-linoleic acid (ALA), so they are more accurately termed “conditionally essential” fats. But because the conversion from ALA to EPA/DHA may not be sufficiently efficient, EPA/DHA are best obtained directly from food sources.
  • ALA: Alpha-linolenic acid (ALA), the most common omega-3 fatty acid in most Western diets, is found in plant oils (especially canola, soybean, flax), nuts (especially walnuts), chia and flax seeds, leafy vegetables, and some animal fats, especially from grass-fed animals. ALA is a true essential fat because it cannot be made by the body, and is needed for normal human growth and development. It can be converted into EPA and DHA, but the conversion rate is limited so we are still uncertain whether ALA alone can provide optimal intakes of omega-3 fatty acids. [1]
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Omega-3 Fats and Health

The strongest evidence for a beneficial effect of omega-3 fats has to do with heart disease. These fats appear to help the heart beat at a steady clip and not veer into a dangerous or potentially fatal erratic rhythm. [2] Such arrhythmias cause most of the 500,000-plus cardiac deaths that occur each year in the United States. Omega-3 fats also lower blood pressure and heart rate, and improve blood vessel function. At higher doses, they lower triglycerides and may ease inflammation, which plays a role in the development of atherosclerosis. [2]

Given the wide-ranging importance of marine omega-3 fatty acids, it is important to eat fish or other seafood 1-2 times a week, particularly fatty (dark meat) fish that is richer in EPA and DHA. [3] This is especially important for women who are pregnant or hoping to become pregnant and nursing mothers. From the third trimester until the second year of life, a developing child needs a steady supply of DHA to form the brain and other parts of the nervous system as DHA is the most abundant fatty acid in the brain. Many women shy away from eating fish because of concerns that mercury and other possible contaminants might harm their babies, [4] yet the evidence for harm from lack of omega-3 fats is far more consistent, and a balance of benefit vs. risk is easily obtained by limiting intake of the types of fish higher in mercury. (To learn more about the controversy over contaminants in fatty fish, read Fish: Friend or Foe.)

Researchers are also looking at the effects of marine and plant omega-3 fats on prostate cancer. Results from the Health Professionals Follow-up Study and others show that men whose diets are rich in EPA and DHA (mainly from fish and seafood) are less likely to develop advanced prostate cancer than those with low intakes of EPA and DHA. [5] At the same time, some studies show an increase in prostate cancer and advanced prostate cancer among men with high intakes of ALA (mainly from supplements). However, this effect is inconsistent. In the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, for example, there was no link between ALA intake and early, late, or advanced prostate cancer. [6] Also, in a recent analysis, ALA intake was not associated with higher risk of prostate cancer after about 2005. [7] This is consistent with concerns that partial hydrogenation of ALA in vegetable oils was responsible for the increase in prostate cancer; partial hydrogenation was greatly reduced after 2005 as part of efforts to reduce consumption of trans fats.

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Unlike EPA/DHA, there is much less research on the health benefits of ALA. Though part of the health benefits of ALA may be attributed to its conversion to EPA/DHA, ALA alone may offer modest protection against cardiovascular disease and type 2 diabetes. [8] However, more observational studies and clinical trials are needed. ALA is an important source of omega-3 fats for those who have a fish allergy or who eat a vegan diet.

The Omega-3 to Omega-6 Ratio: Separating Claims from the Evidence

Most Americans take in far more of another essential fat—omega-6 fats—than they do omega-3 fats. Like omega-3 fats, omega-6 fats are a critical part of the structure of every cell of our body and are building blocks for hormones that regulate inflammation, narrowing of blood vessels, and blood clotting. Normally, these are important functions that protect the body from injury and infection, but a popular claim is that an excess intake of omega-6 fats can over-stimulate these functions, causing more harm than benefit. In addition, because omega-3 and omega-6 fats compete for the same enzymes to produce other fatty acids, it is believed that eating an excess of one type may interfere with the metabolism of the other, thereby reducing its beneficial effects. [15]

Some researchers have proposed that a higher intake of omega-6 fats compared with omega-3 fats (also referred to as the omega-6/omega-3 ratio) could contribute to the development of chronic health conditions, like cardiovascular disease and cancer, but this has not been supported by evidence in humans. [1,16] Very consistently, carefully controlled feeding studies do not show that omega-6 fats increase inflammatory factors. [17]

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Many studies and trials in humans support cardiovascular benefit of omega-6 fats. In the Health Professionals Follow-up Study, the ratio of omega-6 to omega-3 fats wasn’t linked with heart disease risk because both of these fats were beneficial. [18] In a large prospective study of men and women who were free of cardiovascular disease, cancer, and diabetes at the start of the study, the highest intakes of omega-6 fats were more strongly linked with lower death rates from these diseases than intakes of omega-3 fats. [19]

There is no question that many Americans could benefit from increasing their intake of omega-3 fats, but there is also evidence that omega-6 fats reduce cardiovascular risk factors and heart disease. Thus, the omega-6/omega-3 ratio is not a useful indicator of the healthfulness of a food or diet. Like many essential nutrients, it is possible that too much can cause problems. However in the U.S. diet, we have not been able to find individuals or groups who are consuming excessive amounts of omega-6 fatty acids.

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